Merge branch 'main' into improve-platform-error

.github/workflows/ci_zig.yml

@@ -85,18 +85,25 @@ jobs:
         include:
           - os: macos-15-intel
             cpu_flag: -Dcpu=x86_64_v3
+            target_flag: ''
           - os: macos-15
             cpu_flag: ''
+            target_flag: ''
           - os: ubuntu-22.04
             cpu_flag: -Dcpu=x86_64_v3
+            target_flag: -Dtarget=x86_64-linux-musl
           - os: ubuntu-24.04-arm
             cpu_flag: ''
+            target_flag: -Dtarget=aarch64-linux-musl
           - os: windows-2022
             cpu_flag: -Dcpu=x86_64_v3
+            target_flag: ''
           - os: windows-2025
             cpu_flag: -Dcpu=x86_64_v3
+            target_flag: ''
           - os: windows-11-arm
             cpu_flag: ''
+            target_flag: ''
 
     steps:
       - name: Checkout
@@ -119,7 +126,7 @@ jobs:
       - name: build roc + repro executables
         uses: ./.github/actions/flaky-retry
         with:
-          command: "zig build -Dfuzz -Dsystem-afl=false -Doptimize=ReleaseFast ${{ matrix.cpu_flag }}"
+          command: "zig build -Dfuzz -Dsystem-afl=false -Doptimize=ReleaseFast ${{ matrix.cpu_flag }} ${{ matrix.target_flag }}"
           error_string_contains: "EndOfStream"
           retry_count: 3
 
@@ -136,7 +143,7 @@ jobs:
       - name: Run Test Platforms (Unix)
         if: runner.os != 'Windows'
         run: |
-          zig build test-cli
+          zig build test-cli ${{ matrix.target_flag }}
 
       - name: Setup MSVC (Windows)
         if: runner.os == 'Windows'
@@ -165,13 +172,13 @@ jobs:
           zig-out\bin\roc.exe check ./src/PROFILING/bench_repeated_check.roc
 
       - name: zig snapshot tests
-        run: zig build snapshot -- --debug
+        run: zig build snapshot ${{ matrix.target_flag }} -- --debug
 
       # 1) in debug mode
       - name: build and execute tests, build repro executables
         uses: ./.github/actions/flaky-retry
         with:
-          command: "zig build test -Dfuzz -Dsystem-afl=false"
+          command: "zig build test -Dfuzz -Dsystem-afl=false ${{ matrix.target_flag }}"
           error_string_contains: "double roundtrip bundle"
           retry_count: 3
 
@@ -179,7 +186,7 @@ jobs:
       - name: Build and execute tests, build repro executables. All in release mode.
         uses: ./.github/actions/flaky-retry
         with:
-          command: "zig build test -Doptimize=ReleaseFast -Dfuzz -Dsystem-afl=false ${{ matrix.cpu_flag }}"
+          command: "zig build test -Doptimize=ReleaseFast -Dfuzz -Dsystem-afl=false ${{ matrix.cpu_flag }} ${{ matrix.target_flag }}"
           error_string_contains: "double roundtrip bundle"
           retry_count: 3
 
@@ -208,6 +215,8 @@ jobs:
         # We can re-evaluate as new version of zig/valgrind come out.
         if: ${{ matrix.os == 'ubuntu-22.04' }}
         run: |
+          # Install libc6-dbg which is required for Valgrind's function redirections
+          sudo apt-get update && sudo apt-get install -y libc6-dbg
           sudo snap install valgrind --classic
           valgrind --version
           ./ci/custom_valgrind.sh ./zig-out/bin/snapshot --debug --verbose
@@ -248,7 +257,7 @@ jobs:
         run: |
           git clean -fdx
           git reset --hard HEAD
-          nix develop ./src/ -c zig build && zig build snapshot && zig build test
+          nix develop ./src/ -c zig build ${{ matrix.target_flag }} && zig build snapshot ${{ matrix.target_flag }} && zig build test ${{ matrix.target_flag }}
 
   zig-cross-compile:
     needs: check-once

build.zig

@@ -292,6 +292,156 @@ const CheckTypeCheckerPatternsStep = struct {
     }
 };
 
+/// Build step that checks for @enumFromInt(0) usage in all .zig files.
+///
+/// We forbid @enumFromInt(0) because it hides bugs and makes them harder to debug.
+/// If we need a placeholder value that we believe will never be read, we should
+/// use `undefined` instead - that way our intent is clear, and it can fail in a
+/// more obvious way if our assumption is incorrect.
+const CheckEnumFromIntZeroStep = struct {
+    step: Step,
+
+    fn create(b: *std.Build) *CheckEnumFromIntZeroStep {
+        const self = b.allocator.create(CheckEnumFromIntZeroStep) catch @panic("OOM");
+        self.* = .{
+            .step = Step.init(.{
+                .id = Step.Id.custom,
+                .name = "check-enum-from-int-zero",
+                .owner = b,
+                .makeFn = make,
+            }),
+        };
+        return self;
+    }
+
+    fn make(step: *Step, options: Step.MakeOptions) !void {
+        _ = options;
+        const b = step.owner;
+        const allocator = b.allocator;
+
+        var violations = std.ArrayList(Violation).empty;
+        defer violations.deinit(allocator);
+
+        // Recursively scan src/ for .zig files
+        var dir = std.fs.cwd().openDir("src", .{ .iterate = true }) catch |err| {
+            return step.fail("Failed to open src directory: {}", .{err});
+        };
+        defer dir.close();
+
+        try scanDirectoryForEnumFromIntZero(allocator, dir, "src", &violations);
+
+        if (violations.items.len > 0) {
+            std.debug.print("\n", .{});
+            std.debug.print("=" ** 80 ++ "\n", .{});
+            std.debug.print("FORBIDDEN PATTERN: @enumFromInt(0)\n", .{});
+            std.debug.print("=" ** 80 ++ "\n\n", .{});
+
+            std.debug.print(
+                \\Using @enumFromInt(0) is forbidden in this codebase.
+                \\
+                \\WHY THIS RULE EXISTS:
+                \\  @enumFromInt(0) hides bugs and makes them harder to debug. It creates
+                \\  a "valid-looking" value that can silently propagate through the code
+                \\  when something goes wrong.
+                \\
+                \\WHAT TO DO INSTEAD:
+                \\  If you need a placeholder value that you believe will never be read,
+                \\  use `undefined` instead. This makes your intent clear, and if your
+                \\  assumption is wrong and the value IS read, it will fail more obviously.
+                \\
+                \\  When using `undefined`, add a comment explaining why it's correct there
+                \\  (e.g., where it will be overwritten before being read).
+                \\
+                \\  Example - WRONG:
+                \\    .anno = @enumFromInt(0), // placeholder - will be replaced
+                \\
+                \\  Example - RIGHT:
+                \\    .anno = undefined, // overwritten in Phase 1.7 before use
+                \\
+                \\VIOLATIONS FOUND:
+                \\
+            , .{});
+
+            for (violations.items) |violation| {
+                std.debug.print("  {s}:{d}: {s}\n", .{
+                    violation.file_path,
+                    violation.line_number,
+                    violation.line_content,
+                });
+            }
+
+            std.debug.print("\n" ++ "=" ** 80 ++ "\n", .{});
+
+            return step.fail(
+                "Found {d} uses of @enumFromInt(0). Using placeholder values like this has consistently led to bugs in this code base. " ++
+                    "Do not use @enumFromInt(0) and also do not uncritically replace it with another placeholder like .first or something like that. " ++
+                    "If you want it to be uninitialized and are very confident it will be overwritten before it is ever read, then use `undefined`. " ++
+                    "Otherwise, take a step back and rethink how this code works; there should be a way to implement this in a way that does not use hardcoded placeholder indices like 0! " ++
+                    "See above for details.",
+                .{violations.items.len},
+            );
+        }
+    }
+
+    const Violation = struct {
+        file_path: []const u8,
+        line_number: usize,
+        line_content: []const u8,
+    };
+
+    fn scanDirectoryForEnumFromIntZero(
+        allocator: std.mem.Allocator,
+        dir: std.fs.Dir,
+        path_prefix: []const u8,
+        violations: *std.ArrayList(Violation),
+    ) !void {
+        var walker = try dir.walk(allocator);
+        defer walker.deinit();
+
+        while (try walker.next()) |entry| {
+            if (entry.kind != .file) continue;
+            if (!std.mem.endsWith(u8, entry.path, ".zig")) continue;
+
+            const full_path = try std.fmt.allocPrint(allocator, "{s}/{s}", .{ path_prefix, entry.path });
+
+            const file = dir.openFile(entry.path, .{}) catch continue;
+            defer file.close();
+
+            const content = file.readToEndAlloc(allocator, 10 * 1024 * 1024) catch continue;
+            defer allocator.free(content);
+
+            var line_number: usize = 1;
+            var line_start: usize = 0;
+
+            for (content, 0..) |char, i| {
+                if (char == '\n') {
+                    const line = content[line_start..i];
+
+                    const trimmed = std.mem.trim(u8, line, " \t");
+                    // Skip comments
+                    if (std.mem.startsWith(u8, trimmed, "//")) {
+                        line_number += 1;
+                        line_start = i + 1;
+                        continue;
+                    }
+
+                    // Check for @enumFromInt(0) usage
+                    if (std.mem.indexOf(u8, line, "@enumFromInt(0)") != null) {
+                        try violations.append(allocator, .{
+                            .file_path = full_path,
+                            .line_number = line_number,
+                            .line_content = try allocator.dupe(u8, trimmed),
+                        });
+                    }
+
+                    line_number += 1;
+                    line_start = i + 1;
+                }
+            }
+        }
+    }
+};
+
 /// Build step that checks for unused variable suppression patterns.
 ///
 /// In this codebase, we don't use `_ = variable;` to suppress unused variable warnings.
@@ -1419,6 +1569,10 @@ pub fn build(b: *std.Build) void {
     const check_patterns = CheckTypeCheckerPatternsStep.create(b);
     test_step.dependOn(&check_patterns.step);
 
+    // Add check for @enumFromInt(0) usage
+    const check_enum_from_int = CheckEnumFromIntZeroStep.create(b);
+    test_step.dependOn(&check_enum_from_int.step);
+
     // Add check for unused variable suppression patterns
     const check_unused = CheckUnusedSuppressionStep.create(b);
     test_step.dependOn(&check_unused.step);
@@ -1496,6 +1650,8 @@ pub fn build(b: *std.Build) void {
         }
         // Ensure host library is copied before running the test
         run_fx_platform_test.step.dependOn(&copy_test_fx_host.step);
+        // Ensure roc binary is built before running the test (tests invoke roc CLI)
+        run_fx_platform_test.step.dependOn(roc_step);
         tests_summary.addRun(&run_fx_platform_test.step);
     }
 
@@ -2050,9 +2206,8 @@ fn addStaticLlvmOptionsToModule(mod: *std.Build.Module) !void {
     mod.linkSystemLibrary("z", link_static);
 
     if (mod.resolved_target.?.result.os.tag != .windows or mod.resolved_target.?.result.abi != .msvc) {
-        // TODO: Can this just be `mod.link_libcpp = true`? Does that make a difference?
-        // This means we rely on clang-or-zig-built LLVM, Clang, LLD libraries.
-        mod.linkSystemLibrary("c++", .{});
+        // Use Zig's bundled static libc++ to keep the binary statically linked
+        mod.link_libcpp = true;
     }
 
     if (mod.resolved_target.?.result.os.tag == .windows) {

src/base/mod.zig

@@ -11,6 +11,7 @@ pub const parallel = @import("parallel.zig");
 pub const SmallStringInterner = @import("SmallStringInterner.zig");
 
 pub const safe_memory = @import("safe_memory.zig");
+pub const stack_overflow = @import("stack_overflow.zig");
 
 pub const target = @import("target.zig");
 pub const DataSpan = @import("DataSpan.zig").DataSpan;
@@ -158,6 +159,7 @@ test "base tests" {
     std.testing.refAllDecls(@import("Scratch.zig"));
     std.testing.refAllDecls(@import("SExprTree.zig"));
     std.testing.refAllDecls(@import("SmallStringInterner.zig"));
+    std.testing.refAllDecls(@import("stack_overflow.zig"));
     std.testing.refAllDecls(@import("StringLiteral.zig"));
     std.testing.refAllDecls(@import("target.zig"));
 }

src/base/stack_overflow.zig

@@ -0,0 +1,263 @@
+//! Signal handling for the Roc compiler (stack overflow, segfault, division by zero).
+//!
+//! This module provides a thin wrapper around the generic signal handlers in
+//! builtins.handlers, configured with compiler-specific error messages.
+//!
+//! On POSIX systems (Linux, macOS), we use sigaltstack to set up an alternate
+//! signal stack and install handlers for SIGSEGV, SIGBUS, and SIGFPE.
+//!
+//! On Windows, we use SetUnhandledExceptionFilter to catch various exceptions.
+//!
+//! WASI is not currently supported (no signal handling available).
+
+const std = @import("std");
+const builtin = @import("builtin");
+const handlers = @import("builtins").handlers;
+const posix = if (builtin.os.tag != .windows and builtin.os.tag != .wasi) std.posix else undefined;
+
+/// Error message to display on stack overflow
+const STACK_OVERFLOW_MESSAGE = "\nThe Roc compiler overflowed its stack memory and had to exit.\n\n";
+
+/// Callback for stack overflow in the compiler
+fn handleStackOverflow() noreturn {
+    if (comptime builtin.os.tag == .windows) {
+        // Windows: use WriteFile for signal-safe output
+        const DWORD = u32;
+        const HANDLE = ?*anyopaque;
+        const STD_ERROR_HANDLE: DWORD = @bitCast(@as(i32, -12));
+
+        const kernel32 = struct {
+            extern "kernel32" fn GetStdHandle(nStdHandle: DWORD) callconv(.winapi) HANDLE;
+            extern "kernel32" fn WriteFile(hFile: HANDLE, lpBuffer: [*]const u8, nNumberOfBytesToWrite: DWORD, lpNumberOfBytesWritten: ?*DWORD, lpOverlapped: ?*anyopaque) callconv(.winapi) i32;
+            extern "kernel32" fn ExitProcess(uExitCode: c_uint) callconv(.winapi) noreturn;
+        };
+
+        const stderr_handle = kernel32.GetStdHandle(STD_ERROR_HANDLE);
+        var bytes_written: DWORD = 0;
+        _ = kernel32.WriteFile(stderr_handle, STACK_OVERFLOW_MESSAGE.ptr, STACK_OVERFLOW_MESSAGE.len, &bytes_written, null);
+        kernel32.ExitProcess(134);
+    } else if (comptime builtin.os.tag != .wasi) {
+        // POSIX: use direct write syscall for signal-safety
+        _ = posix.write(posix.STDERR_FILENO, STACK_OVERFLOW_MESSAGE) catch {};
+        posix.exit(134);
+    } else {
+        // WASI fallback
+        std.process.exit(134);
+    }
+}
+
+/// Error message to display on arithmetic error (division by zero, etc.)
+const ARITHMETIC_ERROR_MESSAGE = "\nThe Roc compiler divided by zero and had to exit.\n\n";
+
+/// Callback for arithmetic errors (division by zero) in the compiler
+fn handleArithmeticError() noreturn {
+    if (comptime builtin.os.tag == .windows) {
+        const DWORD = u32;
+        const HANDLE = ?*anyopaque;
+        const STD_ERROR_HANDLE: DWORD = @bitCast(@as(i32, -12));
+
+        const kernel32 = struct {
+            extern "kernel32" fn GetStdHandle(nStdHandle: DWORD) callconv(.winapi) HANDLE;
+            extern "kernel32" fn WriteFile(hFile: HANDLE, lpBuffer: [*]const u8, nNumberOfBytesToWrite: DWORD, lpNumberOfBytesWritten: ?*DWORD, lpOverlapped: ?*anyopaque) callconv(.winapi) i32;
+            extern "kernel32" fn ExitProcess(uExitCode: c_uint) callconv(.winapi) noreturn;
+        };
+
+        const stderr_handle = kernel32.GetStdHandle(STD_ERROR_HANDLE);
+        var bytes_written: DWORD = 0;
+        _ = kernel32.WriteFile(stderr_handle, ARITHMETIC_ERROR_MESSAGE.ptr, ARITHMETIC_ERROR_MESSAGE.len, &bytes_written, null);
+        kernel32.ExitProcess(136);
+    } else if (comptime builtin.os.tag != .wasi) {
+        _ = posix.write(posix.STDERR_FILENO, ARITHMETIC_ERROR_MESSAGE) catch {};
+        posix.exit(136); // 128 + 8 (SIGFPE)
+    } else {
+        std.process.exit(136);
+    }
+}
+
+/// Callback for access violation in the compiler
+fn handleAccessViolation(fault_addr: usize) noreturn {
+    if (comptime builtin.os.tag == .windows) {
+        const DWORD = u32;
+        const HANDLE = ?*anyopaque;
+        const STD_ERROR_HANDLE: DWORD = @bitCast(@as(i32, -12));
+
+        const kernel32 = struct {
+            extern "kernel32" fn GetStdHandle(nStdHandle: DWORD) callconv(.winapi) HANDLE;
+            extern "kernel32" fn WriteFile(hFile: HANDLE, lpBuffer: [*]const u8, nNumberOfBytesToWrite: DWORD, lpNumberOfBytesWritten: ?*DWORD, lpOverlapped: ?*anyopaque) callconv(.winapi) i32;
+            extern "kernel32" fn ExitProcess(uExitCode: c_uint) callconv(.winapi) noreturn;
+        };
+
+        var addr_buf: [18]u8 = undefined;
+        const addr_str = handlers.formatHex(fault_addr, &addr_buf);
+
+        const msg1 = "\nAccess violation in the Roc compiler.\nFault address: ";
+        const msg2 = "\n\nPlease report this issue at: https://github.com/roc-lang/roc/issues\n\n";
+        const stderr_handle = kernel32.GetStdHandle(STD_ERROR_HANDLE);
+        var bytes_written: DWORD = 0;
+        _ = kernel32.WriteFile(stderr_handle, msg1.ptr, msg1.len, &bytes_written, null);
+        _ = kernel32.WriteFile(stderr_handle, addr_str.ptr, @intCast(addr_str.len), &bytes_written, null);
+        _ = kernel32.WriteFile(stderr_handle, msg2.ptr, msg2.len, &bytes_written, null);
+        kernel32.ExitProcess(139);
+    } else {
+        // POSIX (and WASI fallback): use direct write syscall for signal-safety
+        const generic_msg = "\nSegmentation fault (SIGSEGV) in the Roc compiler.\nFault address: ";
+        _ = posix.write(posix.STDERR_FILENO, generic_msg) catch {};
+
+        // Write the fault address as hex
+        var addr_buf: [18]u8 = undefined;
+        const addr_str = handlers.formatHex(fault_addr, &addr_buf);
+        _ = posix.write(posix.STDERR_FILENO, addr_str) catch {};
+        _ = posix.write(posix.STDERR_FILENO, "\n\nPlease report this issue at: https://github.com/roc-lang/roc/issues\n\n") catch {};
+        posix.exit(139);
+    }
+}
+
+/// Install signal handlers for stack overflow, segfault, and division by zero.
+/// This should be called early in main() before any significant work is done.
+/// Returns true if the handlers were installed successfully, false otherwise.
+pub fn install() bool {
+    return handlers.install(handleStackOverflow, handleAccessViolation, handleArithmeticError);
+}
+
+/// Test function that intentionally causes a stack overflow.
+/// This is used to verify the handler works correctly.
+pub fn triggerStackOverflowForTest() noreturn {
+    // Use a recursive function that can't be tail-call optimized
+    const S = struct {
+        fn recurse(n: usize) usize {
+            // Prevent tail-call optimization by doing work after the recursive call
+            var buf: [1024]u8 = undefined;
+            buf[0] = @truncate(n);
+            const result = if (n == 0) 0 else recurse(n + 1);
+            // Use the buffer to prevent it from being optimized away
+            return result + buf[0];
+        }
+    };
+
+    // This will recurse until stack overflow
+    const result = S.recurse(1);
+
+    // This should never be reached
+    std.debug.print("Unexpected result: {}\n", .{result});
+    std.process.exit(1);
+}
+
+test "formatHex" {
+    var buf: [18]u8 = undefined;
+
+    const zero = handlers.formatHex(0, &buf);
+    try std.testing.expectEqualStrings("0x0", zero);
+
+    const small = handlers.formatHex(0xff, &buf);
+    try std.testing.expectEqualStrings("0xff", small);
+
+    const medium = handlers.formatHex(0xdeadbeef, &buf);
+    try std.testing.expectEqualStrings("0xdeadbeef", medium);
+}
+
+/// Check if we're being run as a subprocess to trigger stack overflow.
+/// This is called by tests to create a child process that will crash.
+/// Returns true if we should trigger the overflow (and not return).
+pub fn checkAndTriggerIfSubprocess() bool {
+    // Check for the special environment variable that signals we should crash
+    const env_val = std.process.getEnvVarOwned(std.heap.page_allocator, "ROC_TEST_TRIGGER_STACK_OVERFLOW") catch return false;
+    defer std.heap.page_allocator.free(env_val);
+
+    if (std.mem.eql(u8, env_val, "1")) {
+        // Install handler and trigger overflow
+        _ = install();
+        triggerStackOverflowForTest();
+        // Never returns
+    }
+    return false;
+}
+
+test "stack overflow handler produces helpful error message" {
+    // Skip on WASI - no process spawning or signal handling
+    if (comptime builtin.os.tag == .wasi) {
+        return error.SkipZigTest;
+    }
+
+    if (comptime builtin.os.tag == .windows) {
+        // Windows test would need subprocess spawning which is more complex
+        // The handler is installed and works, but testing it is harder
+        // For now, just verify the handler installs successfully
+        if (install()) {
+            return; // Success - handler installed
+        }
+        return error.SkipZigTest;
+    }
+
+    try testStackOverflowPosix();
+}
+
+fn testStackOverflowPosix() !void {
+    // Create a pipe to capture stderr from the child
+    const pipe_fds = try posix.pipe();
+    const pipe_read = pipe_fds[0];
+    const pipe_write = pipe_fds[1];
+
+    const fork_result = posix.fork() catch {
+        posix.close(pipe_read);
+        posix.close(pipe_write);
+        return error.ForkFailed;
+    };
+
+    if (fork_result == 0) {
+        // Child process
+        posix.close(pipe_read);
+
+        // Redirect stderr to the pipe
+        posix.dup2(pipe_write, posix.STDERR_FILENO) catch posix.exit(99);
+        posix.close(pipe_write);
+
+        // Install the handler and trigger stack overflow
+        _ = install();
+        triggerStackOverflowForTest();
+        // Should never reach here
+        unreachable;
+    } else {
+        // Parent process
+        posix.close(pipe_write);
+
+        // Wait for child to exit
+        const wait_result = posix.waitpid(fork_result, 0);
+        const status = wait_result.status;
+
+        // Parse the wait status (Unix encoding)
+        const exited_normally = (status & 0x7f) == 0;
+        const exit_code: u8 = @truncate((status >> 8) & 0xff);
+        const termination_signal: u8 = @truncate(status & 0x7f);
+
+        // Read stderr output from child
+        var stderr_buf: [4096]u8 = undefined;
+        const bytes_read = posix.read(pipe_read, &stderr_buf) catch 0;
+        posix.close(pipe_read);
+
+        const stderr_output = stderr_buf[0..bytes_read];
+
+        try verifyHandlerOutput(exited_normally, exit_code, termination_signal, stderr_output);
+    }
+}
+
+fn verifyHandlerOutput(exited_normally: bool, exit_code: u8, termination_signal: u8, stderr_output: []const u8) !void {
+    // Exit code 134 = stack overflow detected
+    // Exit code 139 = generic segfault (handler caught it but didn't classify as stack overflow)
+    if (exited_normally and (exit_code == 134 or exit_code == 139)) {
+        // Check that our handler message was printed
+        const has_stack_overflow_msg = std.mem.indexOf(u8, stderr_output, "overflowed its stack memory") != null;
+        const has_segfault_msg = std.mem.indexOf(u8, stderr_output, "Segmentation fault") != null;
+
+        // Handler should have printed EITHER stack overflow message OR segfault message
+        try std.testing.expect(has_stack_overflow_msg or has_segfault_msg);
+    } else if (!exited_normally and (termination_signal == posix.SIG.SEGV or termination_signal == posix.SIG.BUS)) {
+        // The handler might not have caught it - this can happen on some systems
+        // where the signal delivery is different. Just warn and skip.
+        std.debug.print("Warning: Stack overflow was not caught by handler (signal {})\n", .{termination_signal});
+        return error.SkipZigTest;
+    } else {
+        std.debug.print("Unexpected exit status: exited={}, code={}, signal={}\n", .{ exited_normally, exit_code, termination_signal });
+        std.debug.print("Stderr: {s}\n", .{stderr_output});
+        return error.TestUnexpectedResult;
+    }
+}

src/build/modules.zig

@@ -307,7 +307,7 @@ pub const ModuleType = enum {
             .fs => &.{},
             .tracy => &.{ .build_options, .builtins },
             .collections => &.{},
-            .base => &.{.collections},
+            .base => &.{ .collections, .builtins },
             .roc_src => &.{},
             .types => &.{ .base, .collections },
             .reporting => &.{ .collections, .base },

src/build/roc/Builtin.roc

@@ -119,6 +119,19 @@ Builtin :: [].{
 					},
 			)
 
+		count_if : List(a), (a -> Bool) -> U64
+		count_if = |list, predicate|
+			List.fold(
+				list,
+				0,
+				|acc, elem|
+					if predicate(elem) {
+						acc + 1
+					} else {
+						acc
+					},
+			)
+
 		fold : List(item), state, (state, item -> state) -> state
 		fold = |list, init, step| {
 			var $state = init
@@ -331,6 +344,18 @@ Builtin :: [].{
 			from_numeral : Numeral -> Try(U8, [InvalidNumeral(Str), ..others])
 			from_str : Str -> Try(U8, [BadNumStr, ..others])
 
+			# # List of integers beginning with this `U8` and ending with the other `U8`.
+			# # (Use [until] instead to end with the other `U8` minus one.)
+			# # Returns an empty list if this `U8` is greater than the other.
+			to : U8, U8 -> List(U8)
+			to = |start, end| range_to(start, end)
+
+			# # List of integers beginning with this `U8` and ending with the other `U8` minus one.
+			# # (Use [to] instead to end with the other `U8` exactly, instead of minus one.)
+			# # Returns an empty list if this `U8` is greater than or equal to the other.
+			until : U8, U8 -> List(U8)
+			until = |start, end| range_until(start, end)
+
 			# Conversions to signed integers (I8 is lossy, others are safe)
 			to_i8_wrap : U8 -> I8
 			to_i8_try : U8 -> Try(I8, [OutOfRange, ..others])
@@ -977,8 +1002,29 @@ Builtin :: [].{
 	}
 }
 
-# Private top-level function for unsafe list access
-# This is a low-level operation that gets replaced by the compiler
+range_to = |var $current, end| {
+	var $answer = [] # Not bothering with List.with_capacity because this will become an iterator once those exist.
+
+	while $current <= end {
+		$answer = $answer.append($current)
+		$current = $current + 1
+	}
+
+	$answer
+}
+
+range_until = |var $current, end| {
+	var $answer = [] # Not bothering with List.with_capacity because this will become an iterator once those exist.
+
+	while $current < end {
+		$answer = $answer.append($current)
+		$current = $current + 1
+	}
+
+	$answer
+}
+
+# Implemented by the compiler, does not perform bounds checks
 list_get_unsafe : List(item), U64 -> item
 
 # Unsafe conversion functions - these return simple records instead of Try types

src/builtins/handlers.zig

@@ -0,0 +1,338 @@
+//! Generic signal handlers for stack overflow, access violation, and arithmetic errors.
+//!
+//! This module provides a mechanism to catch runtime errors like stack overflows,
+//! access violations, and division by zero, handling them with custom callbacks
+//! instead of crashing with a raw signal.
+//!
+//! On POSIX systems (Linux, macOS), we use sigaltstack to set up an alternate
+//! signal stack and install handlers for SIGSEGV, SIGBUS, and SIGFPE.
+//!
+//! On Windows, we use SetUnhandledExceptionFilter to catch various exceptions.
+//!
+//! WASI is not currently supported (no signal handling available).
+
+const std = @import("std");
+const builtin = @import("builtin");
+const posix = if (builtin.os.tag != .windows and builtin.os.tag != .wasi) std.posix else undefined;
+
+// Windows types and constants
+const DWORD = u32;
+const LONG = i32;
+const ULONG_PTR = usize;
+const PVOID = ?*anyopaque;
+const HANDLE = ?*anyopaque;
+const BOOL = i32;
+
+const EXCEPTION_STACK_OVERFLOW: DWORD = 0xC00000FD;
+const EXCEPTION_ACCESS_VIOLATION: DWORD = 0xC0000005;
+const EXCEPTION_INT_DIVIDE_BY_ZERO: DWORD = 0xC0000094;
+const EXCEPTION_INT_OVERFLOW: DWORD = 0xC0000095;
+const EXCEPTION_CONTINUE_SEARCH: LONG = 0;
+const STD_ERROR_HANDLE: DWORD = @bitCast(@as(i32, -12));
+const INVALID_HANDLE_VALUE: HANDLE = @ptrFromInt(std.math.maxInt(usize));
+
+const EXCEPTION_RECORD = extern struct {
+    ExceptionCode: DWORD,
+    ExceptionFlags: DWORD,
+    ExceptionRecord: ?*EXCEPTION_RECORD,
+    ExceptionAddress: PVOID,
+    NumberParameters: DWORD,
+    ExceptionInformation: [15]ULONG_PTR,
+};
+
+const CONTEXT = extern struct {
+    // We don't need the full context, just enough to make the struct valid
+    data: [1232]u8, // Size varies by arch, this is x64 size
+};
+
+const EXCEPTION_POINTERS = extern struct {
+    ExceptionRecord: *EXCEPTION_RECORD,
+    ContextRecord: *CONTEXT,
+};
+
+const LPTOP_LEVEL_EXCEPTION_FILTER = ?*const fn (*EXCEPTION_POINTERS) callconv(.winapi) LONG;
+
+// Windows API imports
+extern "kernel32" fn SetUnhandledExceptionFilter(lpTopLevelExceptionFilter: LPTOP_LEVEL_EXCEPTION_FILTER) callconv(.winapi) LPTOP_LEVEL_EXCEPTION_FILTER;
+extern "kernel32" fn GetStdHandle(nStdHandle: DWORD) callconv(.winapi) HANDLE;
+extern "kernel32" fn WriteFile(hFile: HANDLE, lpBuffer: [*]const u8, nNumberOfBytesToWrite: DWORD, lpNumberOfBytesWritten: ?*DWORD, lpOverlapped: ?*anyopaque) callconv(.winapi) BOOL;
+extern "kernel32" fn ExitProcess(uExitCode: c_uint) callconv(.winapi) noreturn;
+
+/// Size of the alternate signal stack (64KB should be plenty for the handler)
+const ALT_STACK_SIZE = 64 * 1024;
+
+/// Storage for the alternate signal stack (POSIX only)
+var alt_stack_storage: [ALT_STACK_SIZE]u8 align(16) = undefined;
+
+/// Whether the handler has been installed
+var handler_installed = false;
+
+/// Callback function type for handling stack overflow
+pub const StackOverflowCallback = *const fn () noreturn;
+
+/// Callback function type for handling access violation/segfault
+pub const AccessViolationCallback = *const fn (fault_addr: usize) noreturn;
+
+/// Callback function type for handling division by zero (and other arithmetic errors)
+pub const ArithmeticErrorCallback = *const fn () noreturn;
+
+/// Stored callbacks (set during install)
+var stack_overflow_callback: ?StackOverflowCallback = null;
+var access_violation_callback: ?AccessViolationCallback = null;
+var arithmetic_error_callback: ?ArithmeticErrorCallback = null;
+
+/// Install signal handlers with custom callbacks.
+///
+/// Parameters:
+/// - on_stack_overflow: Called when a stack overflow is detected. Must not return.
+/// - on_access_violation: Called for other memory access violations (segfaults).
+///   Receives the fault address. Must not return.
+/// - on_arithmetic_error: Called for arithmetic errors like division by zero. Must not return.
+///
+/// Returns true if the handlers were installed successfully, false otherwise.
+pub fn install(
+    on_stack_overflow: StackOverflowCallback,
+    on_access_violation: AccessViolationCallback,
+    on_arithmetic_error: ArithmeticErrorCallback,
+) bool {
+    if (handler_installed) return true;
+
+    stack_overflow_callback = on_stack_overflow;
+    access_violation_callback = on_access_violation;
+    arithmetic_error_callback = on_arithmetic_error;
+
+    if (comptime builtin.os.tag == .windows) {
+        return installWindows();
+    }
+
+    if (comptime builtin.os.tag == .wasi) {
+        // WASI doesn't support signal handling
+        return false;
+    }
+
+    return installPosix();
+}
+
+fn installPosix() bool {
+    // Set up the alternate signal stack
+    var alt_stack = posix.stack_t{
+        .sp = &alt_stack_storage,
+        .flags = 0,
+        .size = ALT_STACK_SIZE,
+    };
+
+    posix.sigaltstack(&alt_stack, null) catch {
+        return false;
+    };
+
+    // Install the SIGSEGV handler for stack overflow and access violations
+    const segv_action = posix.Sigaction{
+        .handler = .{ .sigaction = handleSegvSignal },
+        .mask = posix.sigemptyset(),
+        .flags = posix.SA.SIGINFO | posix.SA.ONSTACK,
+    };
+
+    posix.sigaction(posix.SIG.SEGV, &segv_action, null);
+
+    // Also catch SIGBUS which can occur on some systems for stack overflow
+    posix.sigaction(posix.SIG.BUS, &segv_action, null);
+
+    // Install the SIGFPE handler for division by zero and other arithmetic errors
+    const fpe_action = posix.Sigaction{
+        .handler = .{ .sigaction = handleFpeSignal },
+        .mask = posix.sigemptyset(),
+        .flags = posix.SA.SIGINFO | posix.SA.ONSTACK,
+    };
+
+    posix.sigaction(posix.SIG.FPE, &fpe_action, null);
+
+    handler_installed = true;
+    return true;
+}
+
+fn installWindows() bool {
+    _ = SetUnhandledExceptionFilter(handleExceptionWindows);
+    handler_installed = true;
+    return true;
+}
+
+/// Windows exception handler function
+fn handleExceptionWindows(exception_info: *EXCEPTION_POINTERS) callconv(.winapi) LONG {
+    const exception_code = exception_info.ExceptionRecord.ExceptionCode;
+
+    // Check if this is a known exception type
+    const is_stack_overflow = (exception_code == EXCEPTION_STACK_OVERFLOW);
+    const is_access_violation = (exception_code == EXCEPTION_ACCESS_VIOLATION);
+    const is_divide_by_zero = (exception_code == EXCEPTION_INT_DIVIDE_BY_ZERO);
+    const is_int_overflow = (exception_code == EXCEPTION_INT_OVERFLOW);
+    const is_arithmetic_error = is_divide_by_zero or is_int_overflow;
+
+    if (!is_stack_overflow and !is_access_violation and !is_arithmetic_error) {
+        // Let other handlers deal with this exception
+        return EXCEPTION_CONTINUE_SEARCH;
+    }
+
+    if (is_stack_overflow) {
+        if (stack_overflow_callback) |callback| {
+            callback();
+        }
+        ExitProcess(134);
+    } else if (is_arithmetic_error) {
+        if (arithmetic_error_callback) |callback| {
+            callback();
+        }
+        ExitProcess(136); // 128 + 8 (SIGFPE)
+    } else {
+        if (access_violation_callback) |callback| {
+            // Get fault address from ExceptionInformation[1] for access violations
+            const fault_addr = exception_info.ExceptionRecord.ExceptionInformation[1];
+            callback(fault_addr);
+        }
+        ExitProcess(139);
+    }
+}
+
+/// The POSIX SIGSEGV/SIGBUS signal handler function
+fn handleSegvSignal(_: i32, info: *const posix.siginfo_t, _: ?*anyopaque) callconv(.c) void {
+    // Get the fault address - access differs by platform
+    const fault_addr: usize = getFaultAddress(info);
+
+    // Get the current stack pointer to help determine if this is a stack overflow
+    var current_sp: usize = 0;
+    asm volatile (""
+        : [sp] "={sp}" (current_sp),
+    );
+
+    // A stack overflow typically occurs when the fault address is near the stack pointer
+    // or below the stack (stacks grow downward on most architectures)
+    const likely_stack_overflow = isLikelyStackOverflow(fault_addr, current_sp);
+
+    if (likely_stack_overflow) {
+        if (stack_overflow_callback) |callback| {
+            callback();
+        }
+    } else {
+        if (access_violation_callback) |callback| {
+            callback(fault_addr);
+        }
+    }
+
+    // If no callback was set, exit with appropriate code
+    if (likely_stack_overflow) {
+        posix.exit(134); // 128 + 6 (SIGABRT-like)
+    } else {
+        posix.exit(139); // 128 + 11 (SIGSEGV)
+    }
+}
+
+/// The POSIX SIGFPE signal handler function (division by zero, etc.)
+fn handleFpeSignal(_: i32, _: *const posix.siginfo_t, _: ?*anyopaque) callconv(.c) void {
+    if (arithmetic_error_callback) |callback| {
+        callback();
+    }
+
+    // If no callback was set, exit with SIGFPE code
+    posix.exit(136); // 128 + 8 (SIGFPE)
+}
+
+/// Get the fault address from siginfo_t (platform-specific)
+fn getFaultAddress(info: *const posix.siginfo_t) usize {
+    // The siginfo_t structure varies by platform
+    if (comptime builtin.os.tag == .linux) {
+        // Linux: fault address is in fields.sigfault.addr
+        return @intFromPtr(info.fields.sigfault.addr);
+    } else if (comptime builtin.os.tag == .macos or
+        builtin.os.tag == .ios or
+        builtin.os.tag == .tvos or
+        builtin.os.tag == .watchos or
+        builtin.os.tag == .visionos or
+        builtin.os.tag == .freebsd or
+        builtin.os.tag == .dragonfly or
+        builtin.os.tag == .netbsd or
+        builtin.os.tag == .openbsd)
+    {
+        // macOS/iOS/BSD: fault address is in addr field
+        return @intFromPtr(info.addr);
+    } else {
+        // Fallback: return 0 if we can't determine the address
+        return 0;
+    }
+}
+
+/// Heuristic to determine if a fault is likely a stack overflow
+fn isLikelyStackOverflow(fault_addr: usize, current_sp: usize) bool {
+    // If fault address is 0 or very low, it's likely a null pointer dereference
+    if (fault_addr < 4096) return false;
+
+    // If the fault address is close to the current stack pointer (within 16MB),
+    // it's very likely a stack overflow. The signal handler runs on an alternate
+    // stack, but the fault address should still be near where the stack was.
+    const sp_distance = if (fault_addr < current_sp) current_sp - fault_addr else fault_addr - current_sp;
+    if (sp_distance < 16 * 1024 * 1024) { // Within 16MB of stack pointer
+        return true;
+    }
+
+    // On 64-bit systems, stacks are typically placed in high memory.
+    // On macOS, the stack is around 0x16XXXXXXXX (about 6GB mark).
+    // On Linux, it's typically near 0x7FFFFFFFFFFF.
+    // If the fault address is in the upper half of the address space,
+    // it's more likely to be a stack-related issue.
+    if (comptime @sizeOf(usize) == 8) {
+        // 64-bit: check if address is in upper portion of address space
+        // On macOS, stacks start around 0x100000000 (4GB) and go up
+        // On Linux, stacks are near 0x7FFFFFFFFFFF
+        const lower_bound: usize = 0x100000000; // 4GB
+        if (fault_addr > lower_bound) {
+            // This is in the region where stacks typically are on 64-bit systems
+            // Default to assuming it's a stack overflow for addresses in this range
+            return true;
+        }
+    } else {
+        // 32-bit: stacks are typically in the upper portion of the 4GB space
+        const lower_bound: usize = 0x40000000; // 1GB
+        if (fault_addr > lower_bound) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+/// Format a usize as hexadecimal (for use in callbacks)
+pub fn formatHex(value: usize, buf: []u8) []const u8 {
+    const hex_chars = "0123456789abcdef";
+    var i: usize = buf.len;
+
+    if (value == 0) {
+        i -= 1;
+        buf[i] = '0';
+    } else {
+        var v = value;
+        while (v > 0 and i > 2) {
+            i -= 1;
+            buf[i] = hex_chars[v & 0xf];
+            v >>= 4;
+        }
+    }
+
+    // Add 0x prefix
+    i -= 1;
+    buf[i] = 'x';
+    i -= 1;
+    buf[i] = '0';
+
+    return buf[i..];
+}
+
+test "formatHex" {
+    var buf: [18]u8 = undefined;
+
+    const zero = formatHex(0, &buf);
+    try std.testing.expectEqualStrings("0x0", zero);
+
+    const small = formatHex(0xff, &buf);
+    try std.testing.expectEqualStrings("0xff", small);
+
+    const medium = formatHex(0xdeadbeef, &buf);
+    try std.testing.expectEqualStrings("0xdeadbeef", medium);
+}

src/builtins/mod.zig

@@ -3,6 +3,7 @@ const std = @import("std");
 
 pub const host_abi = @import("host_abi.zig");
 pub const dec = @import("dec.zig");
+pub const handlers = @import("handlers.zig");
 pub const hash = @import("hash.zig");
 pub const list = @import("list.zig");
 pub const num = @import("num.zig");
@@ -12,6 +13,7 @@ pub const utils = @import("utils.zig");
 
 test "builtins tests" {
     std.testing.refAllDecls(@import("dec.zig"));
+    std.testing.refAllDecls(@import("handlers.zig"));
     std.testing.refAllDecls(@import("hash.zig"));
     std.testing.refAllDecls(@import("host_abi.zig"));
     std.testing.refAllDecls(@import("list.zig"));

src/canonicalize/CIR.zig

@@ -719,7 +719,10 @@ pub fn fromF64(f: f64) ?RocDec {
 
 /// Represents an import statement in a module
 pub const Import = struct {
-    pub const Idx = enum(u32) { _ };
+    pub const Idx = enum(u32) {
+        first = 0,
+        _,
+    };
 
     /// Sentinel value indicating unresolved import (max u32)
     pub const UNRESOLVED_MODULE: u32 = std.math.maxInt(u32);

src/canonicalize/Can.zig

@@ -51,8 +51,10 @@ in_statement_position: bool = true,
 scopes: std.ArrayList(Scope) = .{},
 /// Special scope for rigid type variables in annotations
 type_vars_scope: base.Scratch(TypeVarScope),
-/// Special scope for tracking exposed items from module header
-exposed_scope: Scope = undefined,
+/// Set of identifiers exposed from this module header (values not used)
+exposed_idents: std.AutoHashMapUnmanaged(Ident.Idx, void) = .{},
+/// Set of types exposed from this module header (values not used)
+exposed_types: std.AutoHashMapUnmanaged(Ident.Idx, void) = .{},
 /// Track exposed identifiers by text to handle changing indices
 exposed_ident_texts: std.StringHashMapUnmanaged(Region) = .{},
 /// Track exposed types by text to handle changing indices
@@ -180,7 +182,8 @@ pub fn deinit(
     const gpa = self.env.gpa;
 
     self.type_vars_scope.deinit();
-    self.exposed_scope.deinit(gpa);
+    self.exposed_idents.deinit(gpa);
+    self.exposed_types.deinit(gpa);
     self.exposed_ident_texts.deinit(gpa);
     self.exposed_type_texts.deinit(gpa);
     self.placeholder_idents.deinit(gpa);
@@ -234,7 +237,6 @@ pub fn init(
         .scratch_record_fields = try base.Scratch(types.RecordField).init(gpa),
         .scratch_seen_record_fields = try base.Scratch(SeenRecordField).init(gpa),
         .type_vars_scope = try base.Scratch(TypeVarScope).init(gpa),
-        .exposed_scope = Scope.init(false),
         .scratch_tags = try base.Scratch(types.Tag).init(gpa),
         .scratch_free_vars = try base.Scratch(Pattern.Idx).init(gpa),
         .scratch_captures = try base.Scratch(Pattern.Idx).init(gpa),
@@ -458,8 +460,8 @@ fn processTypeDeclFirstPass(
         // Type was already introduced - check if it's a placeholder (anno = 0) or a real declaration
         const existing_stmt = self.env.store.getStatement(existing_stmt_idx);
         const is_placeholder = switch (existing_stmt) {
-            .s_alias_decl => |alias| @intFromEnum(alias.anno) == 0,
-            .s_nominal_decl => |nominal| @intFromEnum(nominal.anno) == 0,
+            .s_alias_decl => |alias| alias.anno == .placeholder,
+            .s_nominal_decl => |nominal| nominal.anno == .placeholder,
             else => false,
         };
 
@@ -483,13 +485,13 @@ fn processTypeDeclFirstPass(
                 .alias => Statement{
                     .s_alias_decl = .{
                         .header = final_header_idx,
-                        .anno = @enumFromInt(0), // placeholder - will be replaced below
+                        .anno = .placeholder, // placeholder, will be overwritten
                     },
                 },
                 .nominal, .@"opaque" => Statement{
                     .s_nominal_decl = .{
                         .header = final_header_idx,
-                        .anno = @enumFromInt(0), // placeholder - will be replaced below
+                        .anno = .placeholder, // placeholder, will be overwritten
                         .is_opaque = type_decl.kind == .@"opaque",
                     },
                 },
@@ -503,13 +505,13 @@ fn processTypeDeclFirstPass(
             .alias => Statement{
                 .s_alias_decl = .{
                     .header = final_header_idx,
-                    .anno = @enumFromInt(0), // placeholder - will be replaced
+                    .anno = .placeholder, // placeholder, will be overwritten
                 },
             },
             .nominal, .@"opaque" => Statement{
                 .s_nominal_decl = .{
                     .header = final_header_idx,
-                    .anno = @enumFromInt(0), // placeholder - will be replaced
+                    .anno = .placeholder, // placeholder, will be overwritten
                     .is_opaque = type_decl.kind == .@"opaque",
                 },
             },
@@ -636,13 +638,13 @@ fn introduceTypeNameOnly(
         .alias => Statement{
             .s_alias_decl = .{
                 .header = header_idx,
-                .anno = @enumFromInt(0), // placeholder - will be updated in Phase 1.7
+                .anno = .placeholder, // placeholder, overwritten in Phase 1.7
             },
         },
         .nominal, .@"opaque" => Statement{
             .s_nominal_decl = .{
                 .header = header_idx,
-                .anno = @enumFromInt(0), // placeholder - will be updated in Phase 1.7
+                .anno = .placeholder, // placeholder, overwritten in Phase 1.7
                 .is_opaque = type_decl.kind == .@"opaque",
             },
         },
@@ -1210,7 +1212,6 @@ fn processAssociatedItemsSecondPass(
                     const parent_text = self.env.getIdent(parent_name);
                     const name_text = self.env.getIdent(name_ident);
                     const qualified_idx = try self.env.insertQualifiedIdent(parent_text, name_text);
-
                     // Create anno-only def with the qualified name
                     const def_idx = try self.createAnnoOnlyDef(qualified_idx, type_anno_idx, where_clauses, region);
 
@@ -1747,7 +1748,7 @@ pub fn canonicalizeFile(
 
     // canonicalize_header_packages();
 
-    // First, process the header to create exposed_scope and set module_kind
+    // First, process the header to populate exposed_idents/exposed_types and set module_kind
     const header = self.parse_ir.store.getHeader(file.header);
     switch (header) {
         .module => |h| {
@@ -2552,11 +2553,9 @@ fn createExposedScope(
     self: *Self,
     exposes: AST.Collection.Idx,
 ) std.mem.Allocator.Error!void {
-    const gpa = self.env.gpa;
-
-    // Reset exposed_scope (already initialized in init)
-    self.exposed_scope.deinit(gpa);
-    self.exposed_scope = Scope.init(false);
+    // Clear exposed sets (they're already initialized with default values)
+    self.exposed_idents.clearRetainingCapacity();
+    self.exposed_types.clearRetainingCapacity();
 
     try self.addToExposedScope(exposes);
 }
@@ -2595,9 +2594,8 @@ fn addToExposedScope(
                     // Add to exposed_items for permanent storage (unconditionally)
                     try self.env.addExposedById(ident_idx);
 
-                    // Use a dummy pattern index - we just need to track that it's exposed
-                    const dummy_idx = @as(Pattern.Idx, @enumFromInt(0));
-                    try self.exposed_scope.put(gpa, .ident, ident_idx, dummy_idx);
+                    // Just track that this identifier is exposed
+                    try self.exposed_idents.put(gpa, ident_idx, {});
                 }
 
                 // Store by text in a temporary hash map, since indices may change
@@ -2628,9 +2626,8 @@ fn addToExposedScope(
                     // Don't add types to exposed_items - types are not values
                     // Only add to type_bindings for type resolution
 
-                    // Use a dummy statement index - we just need to track that it's exposed
-                    const dummy_idx = @as(Statement.Idx, @enumFromInt(0));
-                    try self.exposed_scope.type_bindings.put(gpa, ident_idx, Scope.TypeBinding{ .local_nominal = dummy_idx });
+                    // Just track that this type is exposed
+                    try self.exposed_types.put(gpa, ident_idx, {});
                 }
 
                 // Store by text in a temporary hash map, since indices may change
@@ -2661,9 +2658,8 @@ fn addToExposedScope(
                     // Don't add types to exposed_items - types are not values
                     // Only add to type_bindings for type resolution
 
-                    // Use a dummy statement index - we just need to track that it's exposed
-                    const dummy_idx = @as(Statement.Idx, @enumFromInt(0));
-                    try self.exposed_scope.type_bindings.put(gpa, ident_idx, Scope.TypeBinding{ .local_nominal = dummy_idx });
+                    // Just track that this type is exposed
+                    try self.exposed_types.put(gpa, ident_idx, {});
                 }
 
                 // Store by text in a temporary hash map, since indices may change
@@ -2711,9 +2707,8 @@ fn addPlatformProvidesItems(
             // Add to exposed_items for permanent storage
             try self.env.addExposedById(ident_idx);
 
-            // Add to exposed_scope so it becomes an export
-            const dummy_idx = @as(Pattern.Idx, @enumFromInt(0));
-            try self.exposed_scope.put(gpa, .ident, ident_idx, dummy_idx);
+            // Track that this identifier is exposed (for exports)
+            try self.exposed_idents.put(gpa, ident_idx, {});
 
             // Also track in exposed_ident_texts
             const token_region = self.parse_ir.tokens.resolve(@intCast(field.name));
@@ -2815,7 +2810,7 @@ fn populateExports(self: *Self) std.mem.Allocator.Error!void {
     const defs_slice = self.env.store.sliceDefs(self.env.all_defs);
 
     // Check each definition to see if it corresponds to an exposed item.
-    // We check exposed_scope.idents which only contains items from the exposing clause,
+    // We check exposed_idents which only contains items from the exposing clause,
     // not associated items like "Color.as_str" which are registered separately.
     for (defs_slice) |def_idx| {
         const def = self.env.store.getDef(def_idx);
@@ -2823,7 +2818,7 @@ fn populateExports(self: *Self) std.mem.Allocator.Error!void {
 
         if (pattern == .assign) {
             // Check if this identifier was explicitly exposed in the module header
-            if (self.exposed_scope.idents.contains(pattern.assign.ident)) {
+            if (self.exposed_idents.contains(pattern.assign.ident)) {
                 try self.env.store.addScratchDef(def_idx);
             }
         }
@@ -4122,15 +4117,6 @@ pub fn canonicalizeExpr(
                                     break :blk_qualified;
                                 }
 
-                                // Check if this is a package-qualified import (e.g., "pf.Stdout")
-                                // These are cross-package imports resolved by the workspace resolver
-                                const is_pkg_qualified = if (module_info) |info| info.is_package_qualified else false;
-                                if (is_pkg_qualified) {
-                                    // Package-qualified import - member resolution happens via the resolver
-                                    // Fall through to normal identifier lookup
-                                    break :blk_qualified;
-                                }
-
                                 // Generate a more helpful error for auto-imported types (List, Bool, Try, etc.)
                                 const is_auto_imported_type = if (self.module_envs) |envs_map|
                                     envs_map.contains(module_name)
@@ -5185,7 +5171,7 @@ pub fn canonicalizeExpr(
                         .patterns = ok_branch_pat_span,
                         .value = ok_lookup_idx,
                         .guard = null,
-                        .redundant = @enumFromInt(0),
+                        .redundant = try self.env.types.fresh(),
                     },
                     region,
                 );
@@ -5259,7 +5245,7 @@ pub fn canonicalizeExpr(
                         .patterns = err_branch_pat_span,
                         .value = return_expr_idx,
                         .guard = null,
-                        .redundant = @enumFromInt(0),
+                        .redundant = try self.env.types.fresh(),
                     },
                     region,
                 );
@@ -5273,7 +5259,7 @@ pub fn canonicalizeExpr(
             const match_expr = Expr.Match{
                 .cond = can_cond.idx,
                 .branches = branches_span,
-                .exhaustive = @enumFromInt(0), // Will be set during type checking
+                .exhaustive = try self.env.types.fresh(),
             };
             const expr_idx = try self.env.addExpr(CIR.Expr{ .e_match = match_expr }, region);
 
@@ -5650,7 +5636,7 @@ pub fn canonicalizeExpr(
                         .patterns = branch_pat_span,
                         .value = value_idx,
                         .guard = null,
-                        .redundant = @enumFromInt(0), // TODO
+                        .redundant = try self.env.types.fresh(),
                     },
                     region,
                 );
@@ -5670,7 +5656,7 @@ pub fn canonicalizeExpr(
             const match_expr = Expr.Match{
                 .cond = can_cond.idx,
                 .branches = branches_span,
-                .exhaustive = @enumFromInt(0), // Will be set during type checking
+                .exhaustive = try self.env.types.fresh(),
             };
             const expr_idx = try self.env.addExpr(CIR.Expr{ .e_match = match_expr }, region);
 
@@ -6447,6 +6433,28 @@ fn canonicalizePattern(
                 return malformed_idx;
             }
         },
+        .var_ident => |e| {
+            // Mutable variable binding in a pattern (e.g., `|var $x, y|`)
+            const region = self.parse_ir.tokenizedRegionToRegion(e.region);
+            if (self.parse_ir.tokens.resolveIdentifier(e.ident_tok)) |ident_idx| {
+                // Create a Pattern node for our mutable identifier
+                const pattern_idx = try self.env.addPattern(Pattern{ .assign = .{
+                    .ident = ident_idx,
+                } }, region);
+
+                // Introduce the var with function boundary tracking (using scopeIntroduceVar)
+                _ = try self.scopeIntroduceVar(ident_idx, pattern_idx, region, true, Pattern.Idx);
+
+                return pattern_idx;
+            } else {
+                const feature = try self.env.insertString("report an error when unable to resolve identifier");
+                const malformed_idx = try self.env.pushMalformed(Pattern.Idx, Diagnostic{ .not_implemented = .{
+                    .feature = feature,
+                    .region = Region.zero(),
+                } });
+                return malformed_idx;
+            }
+        },
         .underscore => |p| {
             const region = self.parse_ir.tokenizedRegionToRegion(p.region);
             const underscore_pattern = Pattern{
@@ -7655,8 +7663,8 @@ fn processCollectedTypeVars(self: *Self) std.mem.Allocator.Error!void {
 
         // Collect problems for this type variable
         const is_single_use = !found_another;
-        // Use a dummy AST annotation index since we don't have the context
-        try collectTypeVarProblems(first_ident, is_single_use, @enumFromInt(0), &self.scratch_type_var_problems);
+        // Use undefined AST annotation index since we don't have the context here
+        try collectTypeVarProblems(first_ident, is_single_use, undefined, &self.scratch_type_var_problems);
     }
 
     // Report any problems we found
@@ -10871,14 +10879,154 @@ fn tryModuleQualifiedLookup(self: *Self, field_access: AST.BinOp) std.mem.Alloca
         return null;
     };
 
-    // This is a module-qualified lookup
+    // This IS a module-qualified lookup - we must handle it completely here.
+    // After this point, returning null would cause incorrect fallback to regular field access.
     const right_expr = self.parse_ir.store.getExpr(field_access.right);
-    if (right_expr != .ident) return null;
+    const region = self.parse_ir.tokenizedRegionToRegion(field_access.region);
+
+    // Handle method calls on module-qualified types (e.g., Stdout.line!(...))
+    if (right_expr == .apply) {
+        const apply = right_expr.apply;
+        const method_expr = self.parse_ir.store.getExpr(apply.@"fn");
+        if (method_expr != .ident) {
+            // Module-qualified call with non-ident function (e.g., Module.(complex_expr)(...))
+            // This is malformed - report error
+            return try self.env.pushMalformed(Expr.Idx, Diagnostic{ .expr_not_canonicalized = .{
+                .region = region,
+            } });
+        }
+
+        const method_ident = method_expr.ident;
+        const method_name = self.parse_ir.tokens.resolveIdentifier(method_ident.token) orelse {
+            // Couldn't resolve method name token
+            return try self.env.pushMalformed(Expr.Idx, Diagnostic{ .expr_not_canonicalized = .{
+                .region = region,
+            } });
+        };
+
+        // Check if this is a type module (like Stdout) - look up the qualified method name directly
+        if (self.module_envs) |envs_map| {
+            if (envs_map.get(module_name)) |auto_imported_type| {
+                if (auto_imported_type.statement_idx != null) {
+                    // This is an imported type module (like Stdout)
+                    // Look up the qualified method name (e.g., "Stdout.line!") in the module's exposed items
+                    const module_env = auto_imported_type.env;
+                    const module_name_text = module_env.module_name;
+                    const auto_import_idx = try self.getOrCreateAutoImport(module_name_text);
+
+                    // Build the qualified method name: "TypeName.method_name"
+                    const type_name_text = self.env.getIdent(module_name);
+                    const method_name_text = self.env.getIdent(method_name);
+                    const qualified_method_name = try self.env.insertQualifiedIdent(type_name_text, method_name_text);
+                    const qualified_text = self.env.getIdent(qualified_method_name);
+
+                    // Look up the qualified method in the module's exposed items
+                    if (module_env.common.findIdent(qualified_text)) |method_ident_idx| {
+                        if (module_env.getExposedNodeIndexById(method_ident_idx)) |method_node_idx| {
+                            // Found the method! Create e_lookup_external + e_call
+                            const func_expr_idx = try self.env.addExpr(CIR.Expr{ .e_lookup_external = .{
+                                .module_idx = auto_import_idx,
+                                .target_node_idx = method_node_idx,
+                                .region = region,
+                            } }, region);
+
+                            // Canonicalize the arguments
+                            const scratch_top = self.env.store.scratchExprTop();
+                            for (self.parse_ir.store.exprSlice(apply.args)) |arg_idx| {
+                                if (try self.canonicalizeExpr(arg_idx)) |canonicalized| {
+                                    try self.env.store.addScratchExpr(canonicalized.get_idx());
+                                }
+                            }
+                            const args_span = try self.env.store.exprSpanFrom(scratch_top);
+
+                            // Create the call expression
+                            const call_expr_idx = try self.env.addExpr(CIR.Expr{
+                                .e_call = .{
+                                    .func = func_expr_idx,
+                                    .args = args_span,
+                                    .called_via = CalledVia.apply,
+                                },
+                            }, region);
+                            return call_expr_idx;
+                        }
+                    }
+
+                    // Method not found in module - generate error
+                    return try self.env.pushMalformed(Expr.Idx, Diagnostic{ .nested_value_not_found = .{
+                        .parent_name = module_name,
+                        .nested_name = method_name,
+                        .region = region,
+                    } });
+                }
+            }
+        }
+
+        // Module exists but is not a type module with a statement_idx - it's a regular module
+        // This means it's something like `SomeModule.someFunc(args)` where someFunc is a regular export
+        // We need to look up the function and create a call
+        const field_text = self.env.getIdent(method_name);
+        const target_node_idx_opt: ?u16 = if (self.module_envs) |envs_map| blk: {
+            if (envs_map.get(module_name)) |auto_imported_type| {
+                const module_env = auto_imported_type.env;
+                if (module_env.common.findIdent(field_text)) |target_ident| {
+                    break :blk module_env.getExposedNodeIndexById(target_ident);
+                } else {
+                    break :blk null;
+                }
+            } else {
+                break :blk null;
+            }
+        } else null;
+
+        if (target_node_idx_opt) |target_node_idx| {
+            // Found the function - create a lookup and call it
+            const func_expr_idx = try self.env.addExpr(CIR.Expr{ .e_lookup_external = .{
+                .module_idx = import_idx,
+                .target_node_idx = target_node_idx,
+                .region = region,
+            } }, region);
+
+            // Canonicalize the arguments
+            const scratch_top = self.env.store.scratchExprTop();
+            for (self.parse_ir.store.exprSlice(apply.args)) |arg_idx| {
+                if (try self.canonicalizeExpr(arg_idx)) |canonicalized| {
+                    try self.env.store.addScratchExpr(canonicalized.get_idx());
+                }
+            }
+            const args_span = try self.env.store.exprSpanFrom(scratch_top);
+
+            // Create the call expression
+            const call_expr_idx = try self.env.addExpr(CIR.Expr{
+                .e_call = .{
+                    .func = func_expr_idx,
+                    .args = args_span,
+                    .called_via = CalledVia.apply,
+                },
+            }, region);
+            return call_expr_idx;
+        } else {
+            // Function not found in module
+            return try self.env.pushMalformed(Expr.Idx, Diagnostic{ .qualified_ident_does_not_exist = .{
+                .ident = method_name,
+                .region = region,
+            } });
+        }
+    }
+
+    // Handle simple field access (not a method call)
+    if (right_expr != .ident) {
+        // Module-qualified access with non-ident, non-apply right side - malformed
+        return try self.env.pushMalformed(Expr.Idx, Diagnostic{ .expr_not_canonicalized = .{
+            .region = region,
+        } });
+    }
 
     const right_ident = right_expr.ident;
-    const field_name = self.parse_ir.tokens.resolveIdentifier(right_ident.token) orelse return null;
-
-    const region = self.parse_ir.tokenizedRegionToRegion(field_access.region);
+    const field_name = self.parse_ir.tokens.resolveIdentifier(right_ident.token) orelse {
+        return try self.env.pushMalformed(Expr.Idx, Diagnostic{ .expr_not_canonicalized = .{
+            .region = region,
+        } });
+    };
 
     // Check if this is a tag access on an auto-imported nominal type (e.g., Bool.True)
     if (self.module_envs) |envs_map| {
@@ -10935,8 +11083,13 @@ fn tryModuleQualifiedLookup(self: *Self, field_access: AST.BinOp) std.mem.Alloca
         }
     } else null;
 
-    // If we didn't find a valid node index, return null to fall through to error handling
-    const target_node_idx = target_node_idx_opt orelse return null;
+    // If we didn't find a valid node index, report an error (don't fall back)
+    const target_node_idx = target_node_idx_opt orelse {
+        return try self.env.pushMalformed(Expr.Idx, Diagnostic{ .qualified_ident_does_not_exist = .{
+            .ident = field_name,
+            .region = region,
+        } });
+    };
 
     // Create the e_lookup_external expression with Import.Idx
     const expr_idx = try self.env.addExpr(CIR.Expr{ .e_lookup_external = .{

src/canonicalize/NodeStore.zig

@@ -674,9 +674,11 @@ pub fn getExpr(store: *const NodeStore, expr: CIR.Expr.Idx) CIR.Expr {
         .expr_suffix_single_question,
         .expr_record_builder,
         => {
-            return CIR.Expr{ .e_runtime_error = .{
-                .diagnostic = @enumFromInt(0),
-            } };
+            return CIR.Expr{
+                .e_runtime_error = .{
+                    .diagnostic = undefined, // deserialized runtime errors don't preserve diagnostics
+                },
+            };
         },
         .expr_ellipsis => {
             return CIR.Expr{ .e_ellipsis = .{} };
@@ -1510,7 +1512,7 @@ pub fn addExpr(store: *NodeStore, expr: CIR.Expr, region: base.Region) Allocator
         .data_1 = 0,
         .data_2 = 0,
         .data_3 = 0,
-        .tag = @enumFromInt(0),
+        .tag = undefined, // set below in switch
     };
 
     switch (expr) {
@@ -2139,7 +2141,7 @@ pub fn addPattern(store: *NodeStore, pattern: CIR.Pattern, region: base.Region)
 
 /// Adds a pattern record field to the store.
 pub fn addPatternRecordField(_: *NodeStore, _: CIR.PatternRecordField) Allocator.Error!CIR.PatternRecordField.Idx {
-    return @enumFromInt(0);
+    @panic("TODO: addPatternRecordField not implemented");
 }
 
 /// Adds a type annotation to the store.
@@ -2151,7 +2153,7 @@ pub fn addTypeAnno(store: *NodeStore, typeAnno: CIR.TypeAnno, region: base.Regio
         .data_1 = 0,
         .data_2 = 0,
         .data_3 = 0,
-        .tag = @enumFromInt(0),
+        .tag = undefined, // set below in switch
     };
 
     switch (typeAnno) {
@@ -2856,7 +2858,7 @@ pub fn addDiagnostic(store: *NodeStore, reason: CIR.Diagnostic) Allocator.Error!
         .data_1 = 0,
         .data_2 = 0,
         .data_3 = 0,
-        .tag = @enumFromInt(0),
+        .tag = undefined, // set below in switch
     };
     var region = base.Region.zero();
 
@@ -3689,7 +3691,7 @@ test "NodeStore basic CompactWriter roundtrip" {
         .data_2 = 0,
         .data_3 = 0,
     };
-    _ = try original.nodes.append(gpa, node1);
+    const node1_idx = try original.nodes.append(gpa, node1);
 
     // Add integer value to extra_data (i128 as 4 u32s)
     const value: i128 = 42;
@@ -3704,7 +3706,7 @@ test "NodeStore basic CompactWriter roundtrip" {
         .start = .{ .offset = 0 },
         .end = .{ .offset = 5 },
     };
-    _ = try original.regions.append(gpa, region);
+    const region1_idx = try original.regions.append(gpa, region);
 
     // Create a temp file
     var tmp_dir = testing.tmpDir(.{});
@@ -3737,7 +3739,7 @@ test "NodeStore basic CompactWriter roundtrip" {
 
     // Verify nodes
     try testing.expectEqual(@as(usize, 1), deserialized.nodes.len());
-    const retrieved_node = deserialized.nodes.get(@enumFromInt(0));
+    const retrieved_node = deserialized.nodes.get(node1_idx);
     try testing.expectEqual(Node.Tag.expr_int, retrieved_node.tag);
     try testing.expectEqual(@as(u32, 0), retrieved_node.data_1);
 
@@ -3750,7 +3752,7 @@ test "NodeStore basic CompactWriter roundtrip" {
 
     // Verify regions
     try testing.expectEqual(@as(usize, 1), deserialized.regions.len());
-    const retrieved_region = deserialized.regions.get(@enumFromInt(0));
+    const retrieved_region = deserialized.regions.get(region1_idx);
     try testing.expectEqual(region.start.offset, retrieved_region.start.offset);
     try testing.expectEqual(region.end.offset, retrieved_region.end.offset);
 }
@@ -3770,7 +3772,7 @@ test "NodeStore multiple nodes CompactWriter roundtrip" {
         .data_2 = 0,
         .data_3 = 0,
     };
-    _ = try original.nodes.append(gpa, var_node);
+    const var_node_idx = try original.nodes.append(gpa, var_node);
 
     // Add expression list node
     const list_node = Node{
@@ -3779,7 +3781,7 @@ test "NodeStore multiple nodes CompactWriter roundtrip" {
         .data_2 = 3, // elems len
         .data_3 = 0,
     };
-    _ = try original.nodes.append(gpa, list_node);
+    const list_node_idx = try original.nodes.append(gpa, list_node);
 
     // Add float node with extra data
     const float_node = Node{
@@ -3788,7 +3790,7 @@ test "NodeStore multiple nodes CompactWriter roundtrip" {
         .data_2 = 0,
         .data_3 = 0,
     };
-    _ = try original.nodes.append(gpa, float_node);
+    const float_node_idx = try original.nodes.append(gpa, float_node);
 
     // Add float value to extra_data
     const float_value: f64 = 3.14159;
@@ -3799,14 +3801,12 @@ test "NodeStore multiple nodes CompactWriter roundtrip" {
     }
 
     // Add regions for each node
-    const regions = [_]Region{
-        .{ .start = .{ .offset = 0 }, .end = .{ .offset = 5 } },
-        .{ .start = .{ .offset = 10 }, .end = .{ .offset = 20 } },
-        .{ .start = .{ .offset = 25 }, .end = .{ .offset = 32 } },
-    };
-    for (regions) |region| {
-        _ = try original.regions.append(gpa, region);
-    }
+    const region1 = Region{ .start = .{ .offset = 0 }, .end = .{ .offset = 5 } };
+    const region2 = Region{ .start = .{ .offset = 10 }, .end = .{ .offset = 20 } };
+    const region3 = Region{ .start = .{ .offset = 25 }, .end = .{ .offset = 32 } };
+    const region1_idx = try original.regions.append(gpa, region1);
+    const region2_idx = try original.regions.append(gpa, region2);
+    const region3_idx = try original.regions.append(gpa, region3);
 
     // Create a temp file
     var tmp_dir = testing.tmpDir(.{});
@@ -3840,32 +3840,36 @@ test "NodeStore multiple nodes CompactWriter roundtrip" {
     // Verify nodes
     try testing.expectEqual(@as(usize, 3), deserialized.nodes.len());
 
-    // Verify var node
-    const retrieved_var = deserialized.nodes.get(@enumFromInt(0));
+    // Verify var node using captured index
+    const retrieved_var = deserialized.nodes.get(var_node_idx);
     try testing.expectEqual(Node.Tag.expr_var, retrieved_var.tag);
     try testing.expectEqual(@as(u32, 5), retrieved_var.data_1);
 
-    // Verify list node
-    const retrieved_list = deserialized.nodes.get(@enumFromInt(1));
+    // Verify list node using captured index
+    const retrieved_list = deserialized.nodes.get(list_node_idx);
     try testing.expectEqual(Node.Tag.expr_list, retrieved_list.tag);
     try testing.expectEqual(@as(u32, 10), retrieved_list.data_1);
     try testing.expectEqual(@as(u32, 3), retrieved_list.data_2);
 
-    // Verify float node and extra data
-    const retrieved_float = deserialized.nodes.get(@enumFromInt(2));
+    // Verify float node and extra data using captured index
+    const retrieved_float = deserialized.nodes.get(float_node_idx);
     try testing.expectEqual(Node.Tag.expr_frac_f64, retrieved_float.tag);
     const retrieved_float_u32s = deserialized.extra_data.items.items[0..2];
     const retrieved_float_u64: u64 = @bitCast(retrieved_float_u32s.*);
     const retrieved_float_value: f64 = @bitCast(retrieved_float_u64);
     try testing.expectApproxEqAbs(float_value, retrieved_float_value, 0.0001);
 
-    // Verify regions
+    // Verify regions using captured indices
     try testing.expectEqual(@as(usize, 3), deserialized.regions.len());
-    for (regions, 0..) |expected_region, i| {
-        const retrieved_region = deserialized.regions.get(@enumFromInt(i));
-        try testing.expectEqual(expected_region.start.offset, retrieved_region.start.offset);
-        try testing.expectEqual(expected_region.end.offset, retrieved_region.end.offset);
-    }
+    const retrieved_region1 = deserialized.regions.get(region1_idx);
+    try testing.expectEqual(region1.start.offset, retrieved_region1.start.offset);
+    try testing.expectEqual(region1.end.offset, retrieved_region1.end.offset);
+    const retrieved_region2 = deserialized.regions.get(region2_idx);
+    try testing.expectEqual(region2.start.offset, retrieved_region2.start.offset);
+    try testing.expectEqual(region2.end.offset, retrieved_region2.end.offset);
+    const retrieved_region3 = deserialized.regions.get(region3_idx);
+    try testing.expectEqual(region3.start.offset, retrieved_region3.start.offset);
+    try testing.expectEqual(region3.end.offset, retrieved_region3.end.offset);
 
     // Verify scratch is null (deserialized NodeStores don't allocate scratch)
     try testing.expect(deserialized.scratch == null);

src/canonicalize/Scope.zig

@@ -363,7 +363,7 @@ pub fn lookupTypeVar(scope: *const Scope, name: Ident.Idx) TypeVarLookupResult {
 
 /// Look up a module alias in this scope
 pub fn lookupModuleAlias(scope: *const Scope, name: Ident.Idx) ModuleAliasLookupResult {
-    // Search by comparing text content, not identifier index
+    // Search by comparing .idx values (integer index into string interner)
     var iter = scope.module_aliases.iterator();
     while (iter.next()) |entry| {
         if (name.idx == entry.key_ptr.idx) {

src/canonicalize/TypeAnnotation.zig

@@ -96,7 +96,12 @@ pub const TypeAnno = union(enum) {
         diagnostic: CIR.Diagnostic.Idx, // The error that occurred
     },
 
-    pub const Idx = enum(u32) { _ };
+    pub const Idx = enum(u32) {
+        /// Placeholder value indicating the anno hasn't been set yet.
+        /// Used during forward reference resolution.
+        placeholder = 0,
+        _,
+    };
     pub const Span = extern struct { span: DataSpan };
 
     pub fn pushToSExprTree(self: *const @This(), ir: *const ModuleEnv, tree: *SExprTree, type_anno_idx: TypeAnno.Idx) std.mem.Allocator.Error!void {

src/canonicalize/test/anno_only_test.zig

@@ -17,27 +17,3 @@ test "e_anno_only expression variant exists" {
         else => return error.WrongExprVariant,
     }
 }
-
-test "e_anno_only can be used in statements" {
-    // This test verifies that e_anno_only expressions can be
-    // used as part of s_decl statements, which is how standalone
-    // type annotations are represented after canonicalization.
-
-    const pattern_idx: CIR.Pattern.Idx = @enumFromInt(0);
-    const expr_idx: CIR.Expr.Idx = @enumFromInt(0);
-    const anno_idx: CIR.Annotation.Idx = @enumFromInt(0);
-
-    const stmt = CIR.Statement{ .s_decl = .{
-        .pattern = pattern_idx,
-        .expr = expr_idx,
-        .anno = anno_idx,
-    } };
-
-    // Verify the statement was created correctly
-    switch (stmt) {
-        .s_decl => |decl| {
-            try testing.expect(decl.anno != null);
-        },
-        else => return error.WrongStatementType,
-    }
-}

src/canonicalize/test/import_store_test.zig

@@ -9,15 +9,22 @@ const Import = CIR.Import;
 const StringLiteral = base.StringLiteral;
 const CompactWriter = collections.CompactWriter;
 
+fn storeContainsModule(store: *const Import.Store, string_store: *const StringLiteral.Store, module_name: []const u8) bool {
+    for (store.imports.items.items) |string_idx| {
+        if (std.mem.eql(u8, string_store.get(string_idx), module_name)) {
+            return true;
+        }
+    }
+    return false;
+}
+
 test "Import.Store deduplicates module names" {
     const testing = std.testing;
     const gpa = testing.allocator;
 
-    // Create a string store for interning module names
     var string_store = try StringLiteral.Store.initCapacityBytes(gpa, 1024);
     defer string_store.deinit(gpa);
 
-    // Create import store
     var store = Import.Store.init();
     defer store.deinit(gpa);
 
@@ -25,7 +32,7 @@ test "Import.Store deduplicates module names" {
     const idx1 = try store.getOrPut(gpa, &string_store, "test.Module");
     const idx2 = try store.getOrPut(gpa, &string_store, "test.Module");
 
-    // Should get the same index
+    // Should get the same index back (deduplication)
     try testing.expectEqual(idx1, idx2);
     try testing.expectEqual(@as(usize, 1), store.imports.len());
 
@@ -39,21 +46,17 @@ test "Import.Store deduplicates module names" {
     try testing.expectEqual(idx1, idx4);
     try testing.expectEqual(@as(usize, 2), store.imports.len());
 
-    // Verify we can retrieve the module names through the string store
-    const str_idx1 = store.imports.items.items[@intFromEnum(idx1)];
-    const str_idx3 = store.imports.items.items[@intFromEnum(idx3)];
-    try testing.expectEqualStrings("test.Module", string_store.get(str_idx1));
-    try testing.expectEqualStrings("other.Module", string_store.get(str_idx3));
+    // Verify both module names are present
+    try testing.expect(storeContainsModule(&store, &string_store, "test.Module"));
+    try testing.expect(storeContainsModule(&store, &string_store, "other.Module"));
 }
 
 test "Import.Store empty CompactWriter roundtrip" {
     const testing = std.testing;
     const gpa = testing.allocator;
 
-    // Create an empty Store
     var original = Import.Store.init();
 
-    // Create a temp file
     var tmp_dir = testing.tmpDir(.{});
     defer tmp_dir.cleanup();
 
@@ -66,15 +69,12 @@ test "Import.Store empty CompactWriter roundtrip" {
     const serialized = try writer.appendAlloc(gpa, Import.Store.Serialized);
     try serialized.serialize(&original, gpa, &writer);
 
-    // Write to file
     try writer.writeGather(gpa, file);
 
-    // Read back
     try file.seekTo(0);
     const buffer = try file.readToEndAlloc(gpa, 1024 * 1024);
     defer gpa.free(buffer);
 
-    // Cast to Serialized and deserialize
     const serialized_ptr = @as(*Import.Store.Serialized, @ptrCast(@alignCast(buffer.ptr)));
     const deserialized = try serialized_ptr.deserialize(@as(i64, @intCast(@intFromPtr(buffer.ptr))), gpa);
 
@@ -87,27 +87,18 @@ test "Import.Store basic CompactWriter roundtrip" {
     const testing = std.testing;
     const gpa = testing.allocator;
 
-    // Create a mock module env with string store
     var string_store = try StringLiteral.Store.initCapacityBytes(gpa, 1024);
     defer string_store.deinit(gpa);
 
-    const MockEnv = struct { strings: *StringLiteral.Store };
-    const mock_env = MockEnv{ .strings = &string_store };
-
-    // Create original store and add some imports
     var original = Import.Store.init();
     defer original.deinit(gpa);
 
-    const idx1 = try original.getOrPut(gpa, mock_env.strings, "json.Json");
-    const idx2 = try original.getOrPut(gpa, mock_env.strings, "core.List");
-    const idx3 = try original.getOrPut(gpa, mock_env.strings, "my.Module");
+    _ = try original.getOrPut(gpa, &string_store, "json.Json");
+    _ = try original.getOrPut(gpa, &string_store, "core.List");
+    _ = try original.getOrPut(gpa, &string_store, "my.Module");
 
-    // Verify indices
-    try testing.expectEqual(@as(u32, 0), @intFromEnum(idx1));
-    try testing.expectEqual(@as(u32, 1), @intFromEnum(idx2));
-    try testing.expectEqual(@as(u32, 2), @intFromEnum(idx3));
+    try testing.expectEqual(@as(usize, 3), original.imports.len());
 
-    // Create a temp file
     var tmp_dir = testing.tmpDir(.{});
     defer tmp_dir.cleanup();
 
@@ -120,30 +111,23 @@ test "Import.Store basic CompactWriter roundtrip" {
     const serialized = try writer.appendAlloc(gpa, Import.Store.Serialized);
     try serialized.serialize(&original, gpa, &writer);
 
-    // Write to file
     try writer.writeGather(gpa, file);
 
-    // Read back
     try file.seekTo(0);
     const buffer = try file.readToEndAlloc(gpa, 1024 * 1024);
     defer gpa.free(buffer);
 
-    // Cast to Serialized and deserialize
     const serialized_ptr: *Import.Store.Serialized = @ptrCast(@alignCast(buffer.ptr));
     var deserialized = try serialized_ptr.deserialize(@as(i64, @intCast(@intFromPtr(buffer.ptr))), gpa);
     defer deserialized.map.deinit(gpa);
 
-    // Verify the imports are accessible
+    // Verify the correct number of imports
     try testing.expectEqual(@as(usize, 3), deserialized.imports.len());
 
-    // Verify the interned string IDs are stored correctly
-    const str_idx1 = deserialized.imports.items.items[0];
-    const str_idx2 = deserialized.imports.items.items[1];
-    const str_idx3 = deserialized.imports.items.items[2];
-
-    try testing.expectEqualStrings("json.Json", string_store.get(str_idx1));
-    try testing.expectEqualStrings("core.List", string_store.get(str_idx2));
-    try testing.expectEqualStrings("my.Module", string_store.get(str_idx3));
+    // Verify all expected module names are present by iterating
+    try testing.expect(storeContainsModule(deserialized, &string_store, "json.Json"));
+    try testing.expect(storeContainsModule(deserialized, &string_store, "core.List"));
+    try testing.expect(storeContainsModule(deserialized, &string_store, "my.Module"));
 
     // Verify the map is repopulated correctly
     try testing.expectEqual(@as(usize, 3), deserialized.map.count());
@@ -153,26 +137,20 @@ test "Import.Store duplicate imports CompactWriter roundtrip" {
     const testing = std.testing;
     const gpa = testing.allocator;
 
-    // Create a mock module env with string store
     var string_store = try StringLiteral.Store.initCapacityBytes(gpa, 1024);
     defer string_store.deinit(gpa);
 
-    const MockEnv = struct { strings: *StringLiteral.Store };
-    const mock_env = MockEnv{ .strings = &string_store };
-
-    // Create store with duplicate imports
     var original = Import.Store.init();
     defer original.deinit(gpa);
 
-    const idx1 = try original.getOrPut(gpa, mock_env.strings, "test.Module");
-    const idx2 = try original.getOrPut(gpa, mock_env.strings, "another.Module");
-    const idx3 = try original.getOrPut(gpa, mock_env.strings, "test.Module"); // duplicate
+    const idx1 = try original.getOrPut(gpa, &string_store, "test.Module");
+    _ = try original.getOrPut(gpa, &string_store, "another.Module");
+    const idx3 = try original.getOrPut(gpa, &string_store, "test.Module"); // duplicate
 
     // Verify deduplication worked
     try testing.expectEqual(idx1, idx3);
     try testing.expectEqual(@as(usize, 2), original.imports.len());
 
-    // Create a temp file
     var tmp_dir = testing.tmpDir(.{});
     defer tmp_dir.cleanup();
 
@@ -185,38 +163,23 @@ test "Import.Store duplicate imports CompactWriter roundtrip" {
     const serialized = try writer.appendAlloc(gpa, Import.Store.Serialized);
     try serialized.serialize(&original, gpa, &writer);
 
-    // Write to file
     try writer.writeGather(gpa, file);
 
-    // Read back
     try file.seekTo(0);
     const buffer = try file.readToEndAlloc(gpa, 1024 * 1024);
     defer gpa.free(buffer);
 
-    // Cast to Serialized and deserialize
     const serialized_ptr: *Import.Store.Serialized = @ptrCast(@alignCast(buffer.ptr));
     var deserialized = try serialized_ptr.deserialize(@as(i64, @intCast(@intFromPtr(buffer.ptr))), gpa);
     defer deserialized.map.deinit(gpa);
 
-    // Verify correct number of imports
+    // Verify correct number of imports (duplicates deduplicated)
     try testing.expectEqual(@as(usize, 2), deserialized.imports.len());
 
-    // Get the string IDs and verify the strings
-    const str_idx1 = deserialized.imports.items.items[@intFromEnum(idx1)];
-    const str_idx2 = deserialized.imports.items.items[@intFromEnum(idx2)];
-
-    try testing.expectEqualStrings("test.Module", string_store.get(str_idx1));
-    try testing.expectEqualStrings("another.Module", string_store.get(str_idx2));
+    // Verify expected module names are present
+    try testing.expect(storeContainsModule(deserialized, &string_store, "test.Module"));
+    try testing.expect(storeContainsModule(deserialized, &string_store, "another.Module"));
 
     // Verify the map was repopulated correctly
     try testing.expectEqual(@as(usize, 2), deserialized.map.count());
-
-    // Check that the map has correct entries for the string indices that were deserialized
-    const str_idx_0 = deserialized.imports.items.items[0];
-    const str_idx_1 = deserialized.imports.items.items[1];
-
-    try testing.expect(deserialized.map.contains(str_idx_0));
-    try testing.expect(deserialized.map.contains(str_idx_1));
-    try testing.expectEqual(@as(Import.Idx, @enumFromInt(0)), deserialized.map.get(str_idx_0).?);
-    try testing.expectEqual(@as(Import.Idx, @enumFromInt(1)), deserialized.map.get(str_idx_1).?);
 }

src/canonicalize/test/import_validation_test.zig

@@ -254,7 +254,7 @@ test "import interner - Import.Idx functionality" {
     // Check that we have the correct number of unique imports (duplicates are deduplicated)
     // Expected: List, Dict, Json, Set (4 unique)
     try expectEqual(@as(usize, 4), result.parse_env.imports.imports.len());
-    // Verify each unique module has an Import.Idx
+    // Verify each unique module has an Import.Idx by checking the imports list
     var found_list = false;
     var found_dict = false;
     var found_json_decode = false;
@@ -276,16 +276,6 @@ test "import interner - Import.Idx functionality" {
     try expectEqual(true, found_dict);
     try expectEqual(true, found_json_decode);
     try expectEqual(true, found_set);
-    // Test the lookup functionality
-    // Get the Import.Idx for "List" (should be used twice)
-    var list_import_idx: ?CIR.Import.Idx = null;
-    for (result.parse_env.imports.imports.items.items, 0..) |import_string_idx, idx| {
-        if (std.mem.eql(u8, result.parse_env.getString(import_string_idx), "List")) {
-            list_import_idx = @enumFromInt(idx);
-            break;
-        }
-    }
-    try testing.expect(list_import_idx != null);
 }
 
 test "import interner - comprehensive usage example" {
@@ -325,22 +315,19 @@ test "import interner - comprehensive usage example" {
     // Check that we have the correct number of unique imports
     // Expected: List, Dict, Try (3 unique)
     try expectEqual(@as(usize, 3), result.parse_env.imports.imports.len());
-    // Verify each unique module has an Import.Idx
+    // Verify each unique module was imported
     var found_list = false;
     var found_dict = false;
     var found_result = false;
-    for (result.parse_env.imports.imports.items.items, 0..) |import_string_idx, idx| {
-        if (std.mem.eql(u8, result.parse_env.getString(import_string_idx), "List")) {
+    for (result.parse_env.imports.imports.items.items) |import_string_idx| {
+        const module_name = result.parse_env.getString(import_string_idx);
+        if (std.mem.eql(u8, module_name, "List")) {
             found_list = true;
-            // Note: We can't verify exposed items count here as Import.Store only stores module names
-        } else if (std.mem.eql(u8, result.parse_env.getString(import_string_idx), "Dict")) {
+        } else if (std.mem.eql(u8, module_name, "Dict")) {
             found_dict = true;
-        } else if (std.mem.eql(u8, result.parse_env.getString(import_string_idx), "Try")) {
+        } else if (std.mem.eql(u8, module_name, "Try")) {
             found_result = true;
         }
-        // Verify Import.Idx can be created from the index
-        const import_idx: CIR.Import.Idx = @enumFromInt(idx);
-        _ = import_idx; // Just verify it compiles
     }
     // Verify all expected modules were found
     try expectEqual(true, found_list);
@@ -348,25 +335,6 @@ test "import interner - comprehensive usage example" {
     try expectEqual(true, found_result);
 }
 
-test "Import.Idx is u32" {
-
-    // Verify that Import.Idx is indeed a u32 enum
-    // Import.Idx is defined as: pub const Idx = enum(u32) { _ };
-    // So we know it's backed by u32
-    // Verify we can create Import.Idx values from u32
-    const test_idx: u32 = 42;
-    const import_idx = @as(CIR.Import.Idx, @enumFromInt(test_idx));
-    const back_to_u32 = @intFromEnum(import_idx);
-    try testing.expectEqual(test_idx, back_to_u32);
-    // Test that we can create valid Import.Idx values
-    const idx1: CIR.Import.Idx = @enumFromInt(0);
-    const idx2: CIR.Import.Idx = @enumFromInt(4294967295); // max u32 value
-    // Verify they are distinct
-    try testing.expect(idx1 != idx2);
-    // Verify the size in memory
-    try testing.expectEqual(@sizeOf(u32), @sizeOf(CIR.Import.Idx));
-}
-
 test "module scopes - imports work in module scope" {
     var gpa_state = std.heap.GeneralPurposeAllocator(.{ .safety = true }){};
     defer std.debug.assert(gpa_state.deinit() == .ok);
@@ -436,18 +404,9 @@ test "module-qualified lookups with e_lookup_external" {
         allocator.destroy(result.parse_env);
     }
     _ = try result.can.canonicalizeFile();
-    // Count e_lookup_external expressions
-    var external_lookup_count: u32 = 0;
-    var found_list_map = false;
-    var found_list_len = false;
-    var found_dict_insert = false;
-    var found_dict_empty = false;
-    // For this test, we're checking that module-qualified lookups work
-    // In the new CIR, we'd need to traverse the expression tree from the root
-    // For now, let's verify that the imports were registered correctly
+    // Verify the module names are correct
     const imports_list = result.parse_env.imports.imports;
     try testing.expect(imports_list.len() >= 2); // List and Dict
-    // Verify the module names are correct
     var has_list = false;
     var has_dict = false;
     for (imports_list.items.items) |import_string_idx| {
@@ -457,19 +416,6 @@ test "module-qualified lookups with e_lookup_external" {
     }
     try testing.expect(has_list);
     try testing.expect(has_dict);
-    // TODO: Once we have proper expression traversal, verify the e_lookup_external nodes
-    // For now, we'll skip counting the actual lookup expressions
-    external_lookup_count = 4; // Expected count
-    found_list_map = true;
-    found_list_len = true;
-    found_dict_insert = true;
-    found_dict_empty = true;
-    // Verify we found all expected external lookups
-    try expectEqual(@as(u32, 4), external_lookup_count);
-    try expectEqual(true, found_list_map);
-    try expectEqual(true, found_list_len);
-    try expectEqual(true, found_dict_insert);
-    try expectEqual(true, found_dict_empty);
 }
 
 test "exposed_items - tracking CIR node indices for exposed items" {
@@ -492,7 +438,7 @@ test "exposed_items - tracking CIR node indices for exposed items" {
         math_env.deinit();
         allocator.destroy(math_env);
     }
-    // Add exposed items and set their node indices
+    // Add exposed items
     const Ident = base.Ident;
     const add_idx = try math_env.common.idents.insert(allocator, Ident.for_text("add"));
     try math_env.addExposedById(add_idx);
@@ -500,11 +446,7 @@ test "exposed_items - tracking CIR node indices for exposed items" {
     try math_env.addExposedById(multiply_idx);
     const pi_idx = try math_env.common.idents.insert(allocator, Ident.for_text("PI"));
     try math_env.addExposedById(pi_idx);
-    // Simulate having CIR node indices for these exposed items
-    // In real usage, these would be set during canonicalization of MathUtils
-    try math_env.common.exposed_items.setNodeIndexById(allocator, @bitCast(add_idx), 100);
-    try math_env.common.exposed_items.setNodeIndexById(allocator, @bitCast(multiply_idx), 200);
-    try math_env.common.exposed_items.setNodeIndexById(allocator, @bitCast(pi_idx), 300);
+
     const math_utils_ident = try temp_idents.insert(allocator, Ident.for_text("MathUtils"));
     const math_utils_qualified_ident = try math_env.common.insertIdent(math_env.gpa, Ident.for_text("MathUtils"));
     try module_envs.put(math_utils_ident, .{ .env = math_env, .qualified_type_ident = math_utils_qualified_ident });
@@ -531,12 +473,7 @@ test "exposed_items - tracking CIR node indices for exposed items" {
         allocator.destroy(result.parse_env);
     }
     _ = try result.can.canonicalizeFile();
-    // Verify that e_lookup_external expressions have the correct target_node_idx values
-    var found_add_with_idx_100 = false;
-    var found_multiply_with_idx_200 = false;
-    var found_pi_with_idx_300 = false;
-    // In the new CIR, we'd need to traverse the expression tree properly
-    // For now, let's verify the imports were registered
+    // Verify the MathUtils import was registered
     const imports_list = result.parse_env.imports.imports;
     var has_mathutils = false;
     for (imports_list.items.items) |import_string_idx| {
@@ -547,62 +484,6 @@ test "exposed_items - tracking CIR node indices for exposed items" {
         }
     }
     try testing.expect(has_mathutils);
-    // TODO: Once we have proper expression traversal, verify the target_node_idx values
-    // For now, we'll assume they work correctly
-    found_add_with_idx_100 = true;
-    found_multiply_with_idx_200 = true;
-    found_pi_with_idx_300 = true;
-    // Verify all lookups have the correct target node indices
-    try expectEqual(true, found_add_with_idx_100);
-    try expectEqual(true, found_multiply_with_idx_200);
-    try expectEqual(true, found_pi_with_idx_300);
-    // Test case where node index is not populated (should get 0)
-    const empty_env = try allocator.create(ModuleEnv);
-    empty_env.* = try ModuleEnv.init(allocator, "");
-    defer {
-        empty_env.deinit();
-        allocator.destroy(empty_env);
-    }
-    const undefined_idx = try empty_env.common.idents.insert(allocator, Ident.for_text("undefined"));
-    try empty_env.addExposedById(undefined_idx);
-    // Don't set node index - should default to 0
-    const empty_module_ident = try temp_idents.insert(allocator, Ident.for_text("EmptyModule"));
-    const empty_qualified_ident = try empty_env.common.insertIdent(empty_env.gpa, Ident.for_text("EmptyModule"));
-    try module_envs.put(empty_module_ident, .{ .env = empty_env, .qualified_type_ident = empty_qualified_ident });
-    const source2 =
-        \\module [test]
-        \\
-        \\import EmptyModule exposing [undefined]
-        \\
-        \\test = undefined
-    ;
-    var result2 = try parseAndCanonicalizeSource(allocator, source2, &module_envs);
-    defer {
-        result2.can.deinit();
-        allocator.destroy(result2.can);
-        result2.ast.deinit(allocator);
-        allocator.destroy(result2.ast);
-        result2.parse_env.deinit();
-        allocator.destroy(result2.parse_env);
-    }
-    _ = try result2.can.canonicalizeFile();
-    // Verify that undefined gets target_node_idx = 0 (not found)
-    var found_undefined_with_idx_0 = false;
-    // Verify EmptyModule was imported
-    const imports_list2 = result2.parse_env.imports.imports;
-    var has_empty_module = false;
-    for (imports_list2.items.items) |import_string_idx| {
-        const import_name = result2.parse_env.getString(import_string_idx);
-        if (std.mem.eql(u8, import_name, "EmptyModule")) {
-            has_empty_module = true;
-            break;
-        }
-    }
-    try testing.expect(has_empty_module);
-    // TODO: Once we have proper expression traversal, verify target_node_idx = 0
-    // For now, we'll assume it works correctly
-    found_undefined_with_idx_0 = true;
-    try expectEqual(true, found_undefined_with_idx_0);
 }
 
 test "export count safety - ensures safe u16 casting" {

src/check/Check.zig

@@ -3169,7 +3169,9 @@ fn checkExpr(self: *Self, expr_idx: CIR.Expr.Idx, env: *Env, expected: Expected)
                     // Here, we unwrap the function, following aliases, to get
                     // the actual function we want to check against
                     var var_ = expected_var;
+                    var guard = types_mod.debug.IterationGuard.init("checkExpr.lambda.unwrapExpectedFunc");
                     while (true) {
+                        guard.tick();
                         switch (self.types.resolveVar(var_).desc.content) {
                             .structure => |flat_type| {
                                 switch (flat_type) {
@@ -3364,7 +3366,9 @@ fn checkExpr(self: *Self, expr_idx: CIR.Expr.Idx, env: *Env, expected: Expected)
                             // Here, we unwrap the function, following aliases, to get
                             // the actual function we want to check against
                             var var_ = func_var;
+                            var guard = types_mod.debug.IterationGuard.init("checkExpr.call.unwrapFuncVar");
                             while (true) {
+                                guard.tick();
                                 switch (self.types.resolveVar(var_).desc.content) {
                                     .structure => |flat_type| {
                                         switch (flat_type) {

src/check/snapshot.zig

@@ -323,8 +323,8 @@ pub const Store = struct {
         return SnapshotStaticDispatchConstraint{
             .fn_name = constraint.fn_name,
             .fn_content = try self.deepCopyVarInternal(store, type_writer, constraint.fn_var),
-            // Dispatcher will be set when collecting constraints during write
-            .dispatcher = @enumFromInt(0),
+            // Dispatcher is set when collecting constraints during write
+            .dispatcher = undefined,
         };
     }
 

src/check/test/type_checking_integration.zig

@@ -1353,9 +1353,10 @@ test "check type - expect" {
         \\  x
         \\}
     ;
-    // With no let-generalization for numeric flex vars, the `x == 1` comparison
-    // adds an is_eq constraint to x (since x is not generalized and remains monomorphic)
-    try checkTypesModule(source, .{ .pass = .last_def }, "a where [a.is_eq : a, a -> Bool, a.from_numeral : Numeral -> Try(a, [InvalidNumeral(Str)])]");
+    // Inside lambdas, numeric flex vars ARE generalized (to support polymorphic functions).
+    // Each use of `x` gets a fresh instance, so constraints from `x == 1` don't
+    // propagate to the generalized type. Only `from_numeral` from the def is captured.
+    try checkTypesModule(source, .{ .pass = .last_def }, "a where [a.from_numeral : Numeral -> Try(a, [InvalidNumeral(Str)])]");
 }
 
 test "check type - expect not bool" {

src/check/test/unify_test.zig

@@ -790,8 +790,10 @@ test "partitionFields - same record" {
     var env = try TestEnv.init(gpa);
     defer env.deinit();
 
-    const field_x = try env.mkRecordField("field_x", @enumFromInt(0));
-    const field_y = try env.mkRecordField("field_y", @enumFromInt(1));
+    const var_x = try env.module_env.types.fresh();
+    const var_y = try env.module_env.types.fresh();
+    const field_x = try env.mkRecordField("field_x", var_x);
+    const field_y = try env.mkRecordField("field_y", var_y);
 
     const range = try env.scratch.appendSliceGatheredFields(&[_]RecordField{ field_x, field_y });
 
@@ -813,9 +815,12 @@ test "partitionFields - disjoint fields" {
     var env = try TestEnv.init(gpa);
     defer env.deinit();
 
-    const a1 = try env.mkRecordField("a1", @enumFromInt(0));
-    const a2 = try env.mkRecordField("a2", @enumFromInt(1));
-    const b1 = try env.mkRecordField("b1", @enumFromInt(2));
+    const var_a1 = try env.module_env.types.fresh();
+    const var_a2 = try env.module_env.types.fresh();
+    const var_b1 = try env.module_env.types.fresh();
+    const a1 = try env.mkRecordField("a1", var_a1);
+    const a2 = try env.mkRecordField("a2", var_a2);
+    const b1 = try env.mkRecordField("b1", var_b1);
 
     const a_range = try env.scratch.appendSliceGatheredFields(&[_]RecordField{ a1, a2 });
     const b_range = try env.scratch.appendSliceGatheredFields(&[_]RecordField{b1});
@@ -839,9 +844,12 @@ test "partitionFields - overlapping fields" {
     var env = try TestEnv.init(gpa);
     defer env.deinit();
 
-    const a1 = try env.mkRecordField("a1", @enumFromInt(0));
-    const both = try env.mkRecordField("both", @enumFromInt(1));
-    const b1 = try env.mkRecordField("b1", @enumFromInt(2));
+    const var_a1 = try env.module_env.types.fresh();
+    const var_both = try env.module_env.types.fresh();
+    const var_b1 = try env.module_env.types.fresh();
+    const a1 = try env.mkRecordField("a1", var_a1);
+    const both = try env.mkRecordField("both", var_both);
+    const b1 = try env.mkRecordField("b1", var_b1);
 
     const a_range = try env.scratch.appendSliceGatheredFields(&[_]RecordField{ a1, both });
     const b_range = try env.scratch.appendSliceGatheredFields(&[_]RecordField{ b1, both });
@@ -868,9 +876,12 @@ test "partitionFields - reordering is normalized" {
     var env = try TestEnv.init(gpa);
     defer env.deinit();
 
-    const f1 = try env.mkRecordField("f1", @enumFromInt(0));
-    const f2 = try env.mkRecordField("f2", @enumFromInt(1));
-    const f3 = try env.mkRecordField("f3", @enumFromInt(2));
+    const var_f1 = try env.module_env.types.fresh();
+    const var_f2 = try env.module_env.types.fresh();
+    const var_f3 = try env.module_env.types.fresh();
+    const f1 = try env.mkRecordField("f1", var_f1);
+    const f2 = try env.mkRecordField("f2", var_f2);
+    const f3 = try env.mkRecordField("f3", var_f3);
 
     const a_range = try env.scratch.appendSliceGatheredFields(&[_]RecordField{ f3, f1, f2 });
     const b_range = try env.scratch.appendSliceGatheredFields(&[_]RecordField{ f1, f2, f3 });
@@ -1027,8 +1038,10 @@ test "partitionTags - same tags" {
     var env = try TestEnv.init(gpa);
     defer env.deinit();
 
-    const tag_x = try env.mkTag("X", &[_]Var{@enumFromInt(0)});
-    const tag_y = try env.mkTag("Y", &[_]Var{@enumFromInt(1)});
+    const var_x = try env.module_env.types.fresh();
+    const var_y = try env.module_env.types.fresh();
+    const tag_x = try env.mkTag("X", &[_]Var{var_x});
+    const tag_y = try env.mkTag("Y", &[_]Var{var_y});
 
     const range = try env.scratch.appendSliceGatheredTags(&[_]Tag{ tag_x, tag_y });
 
@@ -1050,9 +1063,12 @@ test "partitionTags - disjoint fields" {
     var env = try TestEnv.init(gpa);
     defer env.deinit();
 
-    const a1 = try env.mkTag("A1", &[_]Var{@enumFromInt(0)});
-    const a2 = try env.mkTag("A2", &[_]Var{@enumFromInt(1)});
-    const b1 = try env.mkTag("B1", &[_]Var{@enumFromInt(2)});
+    const var_a1 = try env.module_env.types.fresh();
+    const var_a2 = try env.module_env.types.fresh();
+    const var_b1 = try env.module_env.types.fresh();
+    const a1 = try env.mkTag("A1", &[_]Var{var_a1});
+    const a2 = try env.mkTag("A2", &[_]Var{var_a2});
+    const b1 = try env.mkTag("B1", &[_]Var{var_b1});
 
     const a_range = try env.scratch.appendSliceGatheredTags(&[_]Tag{ a1, a2 });
     const b_range = try env.scratch.appendSliceGatheredTags(&[_]Tag{b1});
@@ -1076,9 +1092,12 @@ test "partitionTags - overlapping tags" {
     var env = try TestEnv.init(gpa);
     defer env.deinit();
 
-    const a1 = try env.mkTag("A", &[_]Var{@enumFromInt(0)});
-    const both = try env.mkTag("Both", &[_]Var{@enumFromInt(1)});
-    const b1 = try env.mkTag("B", &[_]Var{@enumFromInt(2)});
+    const var_a = try env.module_env.types.fresh();
+    const var_both = try env.module_env.types.fresh();
+    const var_b = try env.module_env.types.fresh();
+    const a1 = try env.mkTag("A", &[_]Var{var_a});
+    const both = try env.mkTag("Both", &[_]Var{var_both});
+    const b1 = try env.mkTag("B", &[_]Var{var_b});
 
     const a_range = try env.scratch.appendSliceGatheredTags(&[_]Tag{ a1, both });
     const b_range = try env.scratch.appendSliceGatheredTags(&[_]Tag{ b1, both });
@@ -1105,9 +1124,12 @@ test "partitionTags - reordering is normalized" {
     var env = try TestEnv.init(gpa);
     defer env.deinit();
 
-    const f1 = try env.mkTag("F1", &[_]Var{@enumFromInt(0)});
-    const f2 = try env.mkTag("F2", &[_]Var{@enumFromInt(1)});
-    const f3 = try env.mkTag("F3", &[_]Var{@enumFromInt(2)});
+    const var_f1 = try env.module_env.types.fresh();
+    const var_f2 = try env.module_env.types.fresh();
+    const var_f3 = try env.module_env.types.fresh();
+    const f1 = try env.mkTag("F1", &[_]Var{var_f1});
+    const f2 = try env.mkTag("F2", &[_]Var{var_f2});
+    const f3 = try env.mkTag("F3", &[_]Var{var_f3});
 
     const a_range = try env.scratch.appendSliceGatheredTags(&[_]Tag{ f3, f1, f2 });
     const b_range = try env.scratch.appendSliceGatheredTags(&[_]Tag{ f1, f2, f3 });
@@ -1487,7 +1509,7 @@ test "unify - flex with constraints vs structure captures deferred check" {
 
     // Check that constraint was captured
     try std.testing.expectEqual(1, env.scratch.deferred_constraints.len());
-    const deferred = env.scratch.deferred_constraints.get(@enumFromInt(0)).*;
+    const deferred = env.scratch.deferred_constraints.items.items[0];
     try std.testing.expectEqual(
         env.module_env.types.resolveVar(structure_var).var_,
         env.module_env.types.resolveVar(deferred.var_).var_,
@@ -1522,7 +1544,7 @@ test "unify - structure vs flex with constraints captures deferred check (revers
 
     // Check that constraint was captured (note: vars might be swapped due to merge order)
     try std.testing.expectEqual(1, env.scratch.deferred_constraints.len());
-    const deferred = env.scratch.deferred_constraints.get(@enumFromInt(0)).*;
+    const deferred = env.scratch.deferred_constraints.items.items[0];
     try std.testing.expectEqual(
         env.module_env.types.resolveVar(flex_var).var_,
         env.module_env.types.resolveVar(deferred.var_).var_,
@@ -1575,7 +1597,7 @@ test "unify - flex vs nominal type captures constraint" {
 
     // Check that constraint was captured
     try std.testing.expectEqual(1, env.scratch.deferred_constraints.len());
-    const deferred = env.scratch.deferred_constraints.get(@enumFromInt(0)).*;
+    const deferred = env.scratch.deferred_constraints.items.items[0];
     try std.testing.expectEqual(
         env.module_env.types.resolveVar(nominal_var).var_,
         env.module_env.types.resolveVar(deferred.var_).var_,

src/check/unify.zig

@@ -1511,7 +1511,9 @@ const Unifier = struct {
 
         // then recursiv
         var ext = record_ext;
+        var guard = types_mod.debug.IterationGuard.init("gatherRecordFields");
         while (true) {
+            guard.tick();
             switch (ext) {
                 .unbound => {
                     return .{ .ext = ext, .range = range };
@@ -1961,7 +1963,9 @@ const Unifier = struct {
 
         // then loop gathering extensible tags
         var ext_var = tag_union.ext;
+        var guard = types_mod.debug.IterationGuard.init("gatherTagUnionTags");
         while (true) {
+            guard.tick();
             switch (self.types_store.resolveVar(ext_var).desc.content) {
                 .flex => {
                     return .{ .ext = ext_var, .range = range };

src/cli/main.zig

@@ -342,7 +342,7 @@ fn createHardlink(allocs: *Allocators, source: []const u8, dest: []const u8) !vo
                 lpFileName: [*:0]const u16,
                 lpExistingFileName: [*:0]const u16,
                 lpSecurityAttributes: ?*anyopaque,
-            ) callconv(std.os.windows.WINAPI) std.os.windows.BOOL;
+            ) callconv(.winapi) std.os.windows.BOOL;
         };
 
         if (kernel32.CreateHardLinkW(dest_w, source_w, null) == 0) {
@@ -387,11 +387,101 @@ fn generateRandomSuffix(allocs: *Allocators) ![]u8 {
     return suffix;
 }
 
+/// Create a unique temporary directory with PID-based naming.
+/// Returns the path to the directory (allocated from arena, no need to free).
+/// Uses system temp directory to avoid race conditions when cache is cleared.
+pub fn createUniqueTempDir(allocs: *Allocators) ![]const u8 {
+    // Use system temp directory (not roc cache) to avoid race conditions
+    const temp_dir = if (comptime is_windows)
+        std.process.getEnvVarOwned(allocs.arena, "TEMP") catch
+            std.process.getEnvVarOwned(allocs.arena, "TMP") catch try allocs.arena.dupe(u8, "C:\\Windows\\Temp")
+    else
+        std.process.getEnvVarOwned(allocs.arena, "TMPDIR") catch try allocs.arena.dupe(u8, "/tmp");
+
+    const normalized_temp_dir = if (comptime is_windows)
+        std.mem.trimRight(u8, temp_dir, "/\\")
+    else
+        std.mem.trimRight(u8, temp_dir, "/");
+
+    // Get the current process ID for uniqueness
+    const pid = if (comptime is_windows)
+        std.os.windows.GetCurrentProcessId()
+    else
+        std.c.getpid();
+
+    // Try PID-based name first, then fall back to random suffix up to 5 times
+    var attempt: u8 = 0;
+    while (attempt < 6) : (attempt += 1) {
+        const dir_path = if (attempt == 0) blk: {
+            // First attempt: use PID only
+            break :blk if (comptime is_windows)
+                try std.fmt.allocPrint(allocs.arena, "{s}\\roc-{d}", .{ normalized_temp_dir, pid })
+            else
+                try std.fmt.allocPrint(allocs.arena, "{s}/roc-{d}", .{ normalized_temp_dir, pid });
+        } else blk: {
+            // Subsequent attempts: use PID + random 8-char suffix
+            const random_suffix = try generateRandomSuffix(allocs);
+            break :blk if (comptime is_windows)
+                try std.fmt.allocPrint(allocs.arena, "{s}\\roc-{d}-{s}", .{ normalized_temp_dir, pid, random_suffix })
+            else
+                try std.fmt.allocPrint(allocs.arena, "{s}/roc-{d}-{s}", .{ normalized_temp_dir, pid, random_suffix });
+        };
+
+        // Try to create the directory
+        std.fs.cwd().makeDir(dir_path) catch |err| switch (err) {
+            error.PathAlreadyExists => {
+                // Directory already exists, try again with a new random suffix
+                continue;
+            },
+            else => {
+                return err;
+            },
+        };
+
+        return dir_path;
+    }
+
+    // Failed after 6 attempts (1 with PID only, 5 with PID + random suffix)
+    return error.FailedToCreateUniqueTempDir;
+}
+
+/// Write shared memory coordination file (.txt) next to the executable.
+/// This is the file that the child process reads to find the shared memory fd.
+pub fn writeFdCoordinationFile(allocs: *Allocators, temp_exe_path: []const u8, shm_handle: SharedMemoryHandle) !void {
+    // The coordination file is at {temp_dir}.txt where temp_dir is the directory containing the exe
+    const temp_dir = std.fs.path.dirname(temp_exe_path) orelse return error.InvalidPath;
+
+    // Ensure we have no trailing slashes
+    var dir_path = temp_dir;
+    while (dir_path.len > 0 and (dir_path[dir_path.len - 1] == '/' or dir_path[dir_path.len - 1] == '\\')) {
+        dir_path = dir_path[0 .. dir_path.len - 1];
+    }
+
+    const fd_file_path = try std.fmt.allocPrint(allocs.arena, "{s}.txt", .{dir_path});
+
+    // Create the file (exclusive - fail if exists to detect collisions)
+    const fd_file = std.fs.cwd().createFile(fd_file_path, .{ .exclusive = true }) catch |err| switch (err) {
+        error.PathAlreadyExists => {
+            // File already exists - this is unexpected since we have unique temp dirs
+            std.log.err("Coordination file already exists at '{s}'", .{fd_file_path});
+            return err;
+        },
+        else => return err,
+    };
+    defer fd_file.close();
+
+    // Write shared memory info to file
+    const fd_str = try std.fmt.allocPrint(allocs.arena, "{}\n{}", .{ shm_handle.fd, shm_handle.size });
+    try fd_file.writeAll(fd_str);
+    try fd_file.sync();
+}
+
 /// Create the temporary directory structure for fd communication.
 /// Returns the path to the executable in the temp directory (allocated from arena, no need to free).
 /// If a cache directory is provided, it will be used for temporary files; otherwise
 /// falls back to the system temp directory.
-pub fn createTempDirStructure(allocs: *Allocators, exe_path: []const u8, shm_handle: SharedMemoryHandle, cache_dir: ?[]const u8) ![]const u8 {
+/// The exe_display_name is the name that will appear in `ps` output (e.g., "app.roc").
+pub fn createTempDirStructure(allocs: *Allocators, exe_path: []const u8, exe_display_name: []const u8, shm_handle: SharedMemoryHandle, cache_dir: ?[]const u8) ![]const u8 {
     // Use provided cache dir or fall back to system temp directory
     const temp_dir = if (cache_dir) |dir|
         try allocs.arena.dupe(u8, dir)
@@ -401,20 +491,34 @@ pub fn createTempDirStructure(allocs: *Allocators, exe_path: []const u8, shm_han
     else
         std.process.getEnvVarOwned(allocs.arena, "TMPDIR") catch try allocs.arena.dupe(u8, "/tmp");
 
-    // Try up to 10 times to create a unique directory
-    var attempt: u8 = 0;
-    while (attempt < 10) : (attempt += 1) {
-        const random_suffix = try generateRandomSuffix(allocs);
+    const normalized_temp_dir = if (comptime is_windows)
+        std.mem.trimRight(u8, temp_dir, "/\\")
+    else
+        std.mem.trimRight(u8, temp_dir, "/");
 
-        // Create the full path with .txt suffix first
-        const normalized_temp_dir = if (comptime is_windows)
-            std.mem.trimRight(u8, temp_dir, "/\\")
-        else
-            std.mem.trimRight(u8, temp_dir, "/");
-        const dir_name_with_txt = if (comptime is_windows)
-            try std.fmt.allocPrint(allocs.arena, "{s}\\roc-tmp-{s}.txt", .{ normalized_temp_dir, random_suffix })
-        else
-            try std.fmt.allocPrint(allocs.arena, "{s}/roc-tmp-{s}.txt", .{ normalized_temp_dir, random_suffix });
+    // Get the current process ID for uniqueness
+    const pid = if (comptime is_windows)
+        std.os.windows.GetCurrentProcessId()
+    else
+        std.c.getpid();
+
+    // Try PID-based name first, then fall back to random suffix up to 5 times
+    var attempt: u8 = 0;
+    while (attempt < 6) : (attempt += 1) {
+        const dir_name_with_txt = if (attempt == 0) blk: {
+            // First attempt: use PID only
+            break :blk if (comptime is_windows)
+                try std.fmt.allocPrint(allocs.arena, "{s}\\roc-{d}.txt", .{ normalized_temp_dir, pid })
+            else
+                try std.fmt.allocPrint(allocs.arena, "{s}/roc-{d}.txt", .{ normalized_temp_dir, pid });
+        } else blk: {
+            // Subsequent attempts: use PID + random 8-char suffix
+            const random_suffix = try generateRandomSuffix(allocs);
+            break :blk if (comptime is_windows)
+                try std.fmt.allocPrint(allocs.arena, "{s}\\roc-{d}-{s}.txt", .{ normalized_temp_dir, pid, random_suffix })
+            else
+                try std.fmt.allocPrint(allocs.arena, "{s}/roc-{d}-{s}.txt", .{ normalized_temp_dir, pid, random_suffix });
+        };
 
         // Get the directory path by slicing off the .txt suffix
         const dir_path_len = dir_name_with_txt.len - 4; // Remove ".txt"
@@ -456,9 +560,8 @@ pub fn createTempDirStructure(allocs: *Allocators, exe_path: []const u8, shm_han
         try fd_file.sync(); // Ensure data is written to disk
         fd_file.close();
 
-        // Create hardlink to executable in temp directory
-        const exe_basename = std.fs.path.basename(exe_path);
-        const temp_exe_path = try std.fs.path.join(allocs.arena, &.{ temp_dir_path, exe_basename });
+        // Create hardlink to executable in temp directory with display name
+        const temp_exe_path = try std.fs.path.join(allocs.arena, &.{ temp_dir_path, exe_display_name });
 
         // Try to create a hardlink first (more efficient than copying)
         createHardlink(allocs, exe_path, temp_exe_path) catch {
@@ -470,7 +573,7 @@ pub fn createTempDirStructure(allocs: *Allocators, exe_path: []const u8, shm_han
         return temp_exe_path;
     }
 
-    // Failed after 10 attempts
+    // Failed after 6 attempts (1 with PID only, 5 with PID + random suffix)
     return error.FailedToCreateUniqueTempDir;
 }
 
@@ -480,6 +583,11 @@ var debug_allocator: std.heap.DebugAllocator(.{}) = .{
 
 /// The CLI entrypoint for the Roc compiler.
 pub fn main() !void {
+    // Install stack overflow handler early, before any significant work.
+    // This gives us a helpful error message instead of a generic segfault
+    // if the compiler blows the stack (e.g., due to infinite recursion in type translation).
+    _ = base.stack_overflow.install();
+
     var gpa_tracy: tracy.TracyAllocator(null) = undefined;
     var gpa, const is_safe = gpa: {
         if (builtin.os.tag == .wasi) break :gpa .{ std.heap.wasm_allocator, false };
@@ -724,26 +832,51 @@ fn rocRun(allocs: *Allocators, args: cli_args.RunArgs) !void {
         },
     };
 
-    // Generate executable name based on the roc file path
-    // TODO use something more interesting like a hash from the platform.main or platform/host.a etc
-    const exe_base_name = std.fmt.allocPrint(allocs.arena, "roc_run_{}", .{std.hash.crc.Crc32.hash(args.path)}) catch |err| {
-        std.log.err("Failed to generate executable name: {}", .{err});
-        return err;
-    };
+    // The final executable name seen in `ps` is the roc filename (e.g., "app.roc")
+    const exe_display_name = std.fs.path.basename(args.path);
 
-    // Add .exe extension on Windows
-    const exe_name = if (builtin.target.os.tag == .windows)
-        std.fmt.allocPrint(allocs.arena, "{s}.exe", .{exe_base_name}) catch |err| {
-            std.log.err("Failed to generate executable name with extension: {}", .{err});
+    // Display name for temp directory (what shows in ps)
+    const exe_display_name_with_ext = if (builtin.target.os.tag == .windows)
+        std.fmt.allocPrint(allocs.arena, "{s}.exe", .{exe_display_name}) catch |err| {
+            std.log.err("Failed to generate display name with extension: {}", .{err});
             return err;
         }
     else
-        allocs.arena.dupe(u8, exe_base_name) catch |err| {
-            std.log.err("Failed to duplicate executable name: {}", .{err});
+        allocs.arena.dupe(u8, exe_display_name) catch |err| {
+            std.log.err("Failed to duplicate display name: {}", .{err});
             return err;
         };
 
-    const exe_path = std.fs.path.join(allocs.arena, &.{ exe_cache_dir, exe_name }) catch |err| {
+    // Cache executable name uses hash of path (no PID - collision is fine since same content)
+    const exe_cache_name = std.fmt.allocPrint(allocs.arena, "roc_{x}", .{std.hash.crc.Crc32.hash(args.path)}) catch |err| {
+        std.log.err("Failed to generate cache executable name: {}", .{err});
+        return err;
+    };
+
+    const exe_cache_name_with_ext = if (builtin.target.os.tag == .windows)
+        std.fmt.allocPrint(allocs.arena, "{s}.exe", .{exe_cache_name}) catch |err| {
+            std.log.err("Failed to generate cache name with extension: {}", .{err});
+            return err;
+        }
+    else
+        allocs.arena.dupe(u8, exe_cache_name) catch |err| {
+            std.log.err("Failed to duplicate cache name: {}", .{err});
+            return err;
+        };
+
+    const exe_cache_path = std.fs.path.join(allocs.arena, &.{ exe_cache_dir, exe_cache_name_with_ext }) catch |err| {
+        std.log.err("Failed to create cache executable path: {}", .{err});
+        return err;
+    };
+
+    // Create unique temp directory for this build (uses PID for uniqueness)
+    const temp_dir_path = createUniqueTempDir(allocs) catch |err| {
+        std.log.err("Failed to create temp directory: {}", .{err});
+        return err;
+    };
+
+    // The executable is built directly in the temp dir with the display name
+    const exe_path = std.fs.path.join(allocs.arena, &.{ temp_dir_path, exe_display_name_with_ext }) catch |err| {
         std.log.err("Failed to create executable path: {}", .{err});
         return err;
     };
@@ -780,42 +913,44 @@ fn rocRun(allocs: *Allocators, args: cli_args.RunArgs) !void {
         return error.NoPlatformSource;
     }
 
-    // Check if the interpreter executable already exists (cached)
-    const exe_exists = if (args.no_cache) false else blk: {
-        std.fs.accessAbsolute(exe_path, .{}) catch {
+    // Check if the interpreter executable already exists in cache
+    const cache_exists = if (args.no_cache) false else blk: {
+        std.fs.accessAbsolute(exe_cache_path, .{}) catch {
             break :blk false;
         };
         break :blk true;
     };
 
-    if (!exe_exists) {
+    if (cache_exists) {
+        // Cached executable exists - hardlink from cache to temp dir
+        std.log.debug("Using cached executable: {s}", .{exe_cache_path});
+        createHardlink(allocs, exe_cache_path, exe_path) catch |err| {
+            // If hardlinking fails, fall back to copying
+            std.log.debug("Hardlink from cache failed, copying: {}", .{err});
+            std.fs.cwd().copyFile(exe_cache_path, std.fs.cwd(), exe_path, .{}) catch |copy_err| {
+                std.log.err("Failed to copy cached executable: {}", .{copy_err});
+                return copy_err;
+            };
+        };
+    } else {
 
-        // Check for cached shim library, extract if not present
+        // Extract shim library to temp dir to avoid race conditions
         const shim_filename = if (builtin.target.os.tag == .windows) "roc_shim.lib" else "libroc_shim.a";
-        const shim_path = std.fs.path.join(allocs.arena, &.{ exe_cache_dir, shim_filename }) catch |err| {
+        const shim_path = std.fs.path.join(allocs.arena, &.{ temp_dir_path, shim_filename }) catch |err| {
             std.log.err("Failed to create shim library path: {}", .{err});
             return err;
         };
 
-        // Extract shim if not cached or if --no-cache is used
-        const shim_exists = if (args.no_cache) false else blk: {
-            std.fs.cwd().access(shim_path, .{}) catch {
-                break :blk false;
-            };
-            break :blk true;
+        // Always extract to temp dir (unique per process, no race condition)
+        extractReadRocFilePathShimLibrary(allocs, shim_path) catch |err| {
+            std.log.err("Failed to extract read roc file path shim library: {}", .{err});
+            return err;
         };
 
-        if (!shim_exists) {
-            // Shim not found in cache or cache disabled, extract it
-            extractReadRocFilePathShimLibrary(allocs, shim_path) catch |err| {
-                std.log.err("Failed to extract read roc file path shim library: {}", .{err});
-                return err;
-            };
-        }
-
         // Generate platform host shim using the detected entrypoints
+        // Use temp dir to avoid race conditions when multiple processes run in parallel
 
-        const platform_shim_path = generatePlatformHostShim(allocs, exe_cache_dir, entrypoints.items, shim_target) catch |err| {
+        const platform_shim_path = generatePlatformHostShim(allocs, temp_dir_path, entrypoints.items, shim_target) catch |err| {
             std.log.err("Failed to generate platform host shim: {}", .{err});
             return err;
         };
@@ -948,6 +1083,22 @@ fn rocRun(allocs: *Allocators, args: cli_args.RunArgs) !void {
                 return err;
             },
         };
+
+        // After building, hardlink to cache for future runs
+        // Force-hardlink (delete existing first) since hash collision means identical content
+        std.log.debug("Caching executable to: {s}", .{exe_cache_path});
+        std.fs.cwd().deleteFile(exe_cache_path) catch |err| switch (err) {
+            error.FileNotFound => {}, // OK, doesn't exist
+            else => std.log.debug("Could not delete existing cache file: {}", .{err}),
+        };
+        createHardlink(allocs, exe_path, exe_cache_path) catch |err| {
+            // If hardlinking fails, fall back to copying
+            std.log.debug("Hardlink to cache failed, copying: {}", .{err});
+            std.fs.cwd().copyFile(exe_path, std.fs.cwd(), exe_cache_path, .{}) catch |copy_err| {
+                // Non-fatal - just means future runs won't be cached
+                std.log.debug("Failed to copy to cache: {}", .{copy_err});
+            };
+        };
     }
 
     // Set up shared memory with ModuleEnv
@@ -986,7 +1137,7 @@ fn rocRun(allocs: *Allocators, args: cli_args.RunArgs) !void {
     } else {
         // POSIX: Use existing file descriptor inheritance approach
         std.log.debug("Using POSIX file descriptor inheritance approach", .{});
-        runWithPosixFdInheritance(allocs, exe_path, shm_handle, &cache_manager, args.app_args) catch |err| {
+        runWithPosixFdInheritance(allocs, exe_path, shm_handle, args.app_args) catch |err| {
             return err;
         };
     }
@@ -1132,40 +1283,32 @@ fn runWithWindowsHandleInheritance(allocs: *Allocators, exe_path: []const u8, sh
 }
 
 /// Run child process using POSIX file descriptor inheritance (existing approach for Unix)
-fn runWithPosixFdInheritance(allocs: *Allocators, exe_path: []const u8, shm_handle: SharedMemoryHandle, cache_manager: *CacheManager, app_args: []const []const u8) !void {
-    // Get cache directory for temporary files
-    const temp_cache_dir = cache_manager.config.getTempDir(allocs.arena) catch |err| {
-        std.log.err("Failed to get temp cache directory: {}", .{err});
+/// The exe_path should already be in a unique temp directory created by createUniqueTempDir.
+fn runWithPosixFdInheritance(allocs: *Allocators, exe_path: []const u8, shm_handle: SharedMemoryHandle, app_args: []const []const u8) !void {
+    // Write the coordination file (.txt) next to the executable
+    // The executable is already in a unique temp directory
+    std.log.debug("Writing fd coordination file for: {s}", .{exe_path});
+    writeFdCoordinationFile(allocs, exe_path, shm_handle) catch |err| {
+        std.log.err("Failed to write fd coordination file: {}", .{err});
         return err;
     };
+    std.log.debug("Coordination file written successfully", .{});
 
-    // Ensure temp cache directory exists
-    std.fs.cwd().makePath(temp_cache_dir) catch |err| switch (err) {
-        error.PathAlreadyExists => {},
-        else => {
-            std.log.err("Failed to create temp cache directory: {}", .{err});
-            return err;
-        },
-    };
-
-    // Create temporary directory structure for fd communication
-    std.log.debug("Creating temporary directory structure for fd communication", .{});
-    const temp_exe_path = createTempDirStructure(allocs, exe_path, shm_handle, temp_cache_dir) catch |err| {
-        std.log.err("Failed to create temp dir structure: {}", .{err});
-        return err;
-    };
-    std.log.debug("Temporary executable created at: {s}", .{temp_exe_path});
-
-    // Configure fd inheritance
-    var flags = posix.fcntl(shm_handle.fd, posix.F_GETFD, 0);
-    if (flags < 0) {
+    // Configure fd inheritance - clear FD_CLOEXEC so child process inherits the fd
+    // NOTE: The doNotOptimizeAway calls are required to prevent the ReleaseFast
+    // optimizer from incorrectly optimizing away or reordering the fcntl calls.
+    const getfd_result = posix.fcntl(shm_handle.fd, posix.F_GETFD, 0);
+    std.mem.doNotOptimizeAway(&getfd_result);
+    if (getfd_result < 0) {
         std.log.err("Failed to get fd flags: {}", .{c._errno().*});
         return error.FdConfigFailed;
     }
 
-    flags &= ~@as(c_int, posix.FD_CLOEXEC);
-
-    if (posix.fcntl(shm_handle.fd, posix.F_SETFD, flags) < 0) {
+    const new_flags = getfd_result & ~@as(c_int, posix.FD_CLOEXEC);
+    std.mem.doNotOptimizeAway(&new_flags);
+    const setfd_result = posix.fcntl(shm_handle.fd, posix.F_SETFD, new_flags);
+    std.mem.doNotOptimizeAway(&setfd_result);
+    if (setfd_result < 0) {
         std.log.err("Failed to set fd flags: {}", .{c._errno().*});
         return error.FdConfigFailed;
     }
@@ -1175,7 +1318,7 @@ fn runWithPosixFdInheritance(allocs: *Allocators, exe_path: []const u8, shm_hand
         std.log.err("Failed to allocate argv: {}", .{err});
         return err;
     };
-    argv[0] = temp_exe_path;
+    argv[0] = exe_path;
     for (app_args, 0..) |arg, i| {
         argv[1 + i] = arg;
     }
@@ -1192,10 +1335,10 @@ fn runWithPosixFdInheritance(allocs: *Allocators, exe_path: []const u8, shm_hand
     child.stderr_behavior = .Inherit;
 
     // Spawn the child process
-    std.log.debug("Spawning child process: {s} with {} app args", .{ temp_exe_path, app_args.len });
+    std.log.debug("Spawning child process: {s} with {} app args", .{ exe_path, app_args.len });
     std.log.debug("Child process working directory: {s}", .{child.cwd.?});
     child.spawn() catch |err| {
-        std.log.err("Failed to spawn {s}: {}", .{ temp_exe_path, err });
+        std.log.err("Failed to spawn {s}: {}", .{ exe_path, err });
         return err;
     };
     std.log.debug("Child process spawned successfully (PID: {})", .{child.id});
@@ -1213,12 +1356,12 @@ fn runWithPosixFdInheritance(allocs: *Allocators, exe_path: []const u8, shm_hand
                 std.log.debug("Child process completed successfully", .{});
             } else {
                 // Propagate the exit code from the child process to our parent
-                std.log.debug("Child process {s} exited with code: {}", .{ temp_exe_path, exit_code });
+                std.log.debug("Child process {s} exited with code: {}", .{ exe_path, exit_code });
                 std.process.exit(exit_code);
             }
         },
         .Signal => |signal| {
-            std.log.err("Child process {s} killed by signal: {}", .{ temp_exe_path, signal });
+            std.log.err("Child process {s} killed by signal: {}", .{ exe_path, signal });
             if (signal == 11) { // SIGSEGV
                 std.log.err("Child process crashed with segmentation fault (SIGSEGV)", .{});
             } else if (signal == 6) { // SIGABRT
@@ -1230,11 +1373,11 @@ fn runWithPosixFdInheritance(allocs: *Allocators, exe_path: []const u8, shm_hand
             std.process.exit(128 +| @as(u8, @truncate(signal)));
         },
         .Stopped => |signal| {
-            std.log.err("Child process {s} stopped by signal: {}", .{ temp_exe_path, signal });
+            std.log.err("Child process {s} stopped by signal: {}", .{ exe_path, signal });
             return error.ProcessStopped;
         },
         .Unknown => |status| {
-            std.log.err("Child process {s} terminated with unknown status: {}", .{ temp_exe_path, status });
+            std.log.err("Child process {s} terminated with unknown status: {}", .{ exe_path, status });
             return error.ProcessUnknownTermination;
         },
     }
@@ -1422,44 +1565,12 @@ pub fn setupSharedMemoryWithModuleEnv(allocs: *Allocators, roc_file_path: []cons
         const module_env_ptr = try compileModuleToSharedMemory(
             allocs,
             module_path,
-            module_filename,
+            module_name, // Use just "Stdout" (not "Stdout.roc") so type-module detection works
             shm_allocator,
             &builtin_modules,
             &.{},
         );
 
-        // Add exposed item aliases with "pf." prefix for import resolution
-        // The canonicalizer builds lookup names like "Stdout.roc.pf.Stdout.line!"
-        // because the import "pf.Stdout" creates an alias Stdout -> pf.Stdout,
-        // and scopeLookupModule returns "pf.Stdout" which becomes part of the qualified name.
-        // We need to add aliases that match this pattern.
-        module_env_ptr.common.exposed_items.ensureSorted(shm_allocator);
-        const exposed_entries = module_env_ptr.common.exposed_items.items.entries.items;
-        for (exposed_entries) |entry| {
-            const key_ident: base.Ident.Idx = @bitCast(entry.key);
-            const key_text = module_env_ptr.common.getIdent(key_ident);
-
-            // Check if this is a qualified name like "Stdout.roc.Stdout.line!"
-            // We want to create an alias "Stdout.roc.pf.Stdout.line!"
-            // The pattern is: "{module}.roc.{Type}.{method}"
-            // We want to create: "{module}.roc.pf.{Type}.{method}"
-            if (std.mem.indexOf(u8, key_text, ".roc.")) |roc_pos| {
-                const prefix = key_text[0 .. roc_pos + 5]; // "Stdout.roc."
-                const suffix = key_text[roc_pos + 5 ..]; // "Stdout.line!"
-
-                // Create the aliased name "Stdout.roc.pf.Stdout.line!"
-                const aliased_name = try std.fmt.allocPrint(shm_allocator, "{s}pf.{s}", .{ prefix, suffix });
-                // Note: We don't defer free because this is allocated in shm_allocator (shared memory)
-
-                // Insert the aliased name into the platform env's ident table
-                const aliased_ident = try module_env_ptr.insertIdent(base.Ident.for_text(aliased_name));
-
-                // First add to exposed items, then set node index
-                try module_env_ptr.common.exposed_items.addExposedById(shm_allocator, @bitCast(aliased_ident));
-                try module_env_ptr.common.exposed_items.setNodeIndexById(shm_allocator, @bitCast(aliased_ident), entry.value);
-            }
-        }
-
         // Store platform modules at indices 0..N-2, app will be at N-1
         module_env_offsets_ptr[i] = @intFromPtr(module_env_ptr) - @intFromPtr(shm.base_ptr);
         platform_env_ptrs[i] = module_env_ptr;
@@ -1605,19 +1716,29 @@ pub fn setupSharedMemoryWithModuleEnv(allocs: *Allocators, roc_file_path: []cons
     // Two keys are needed for each platform module:
     // 1. "pf.Stdout" - used during import validation (import pf.Stdout)
     // 2. "Stdout" - used during expression canonicalization (Stdout.line!)
-    // Also set statement_idx to a non-null value to trigger qualified name lookup,
-    // since associated items are stored as "Stdout.roc.Stdout.line!", not just "line!".
+    // Also set statement_idx to the actual type node index, which is needed for
+    // creating e_nominal_external and e_lookup_external expressions.
     for (exposed_modules.items, 0..) |module_name, i| {
         const platform_env = platform_env_ptrs[i];
-        // For platform modules, the qualified type name is "ModuleName.roc.ModuleName"
-        // This matches how associated items are stored (e.g., "Stdout.roc.Stdout.line!")
+        // For platform modules (type modules), the qualified type name is just the type name.
+        // Type modules like Stdout.roc store associated items as "Stdout.line!" (not "Stdout.roc.Stdout.line!")
+        // because processTypeDeclFirstPass uses parent_name=null for top-level types.
         // Insert into app_env (calling module) since Ident.Idx values are not transferable between stores.
-        const qualified_type_name = try std.fmt.allocPrint(allocs.gpa, "{s}.roc.{s}", .{ module_name, module_name });
-        defer allocs.gpa.free(qualified_type_name);
-        const type_qualified_ident = try app_env.insertIdent(base.Ident.for_text(qualified_type_name));
+        const type_qualified_ident = try app_env.insertIdent(base.Ident.for_text(module_name));
+
+        // Look up the type in the platform module's exposed_items to get the actual node index
+        const type_ident_in_platform = platform_env.common.findIdent(module_name) orelse {
+            std.log.err("Platform module '{s}' does not expose a type named '{s}'", .{ module_name, module_name });
+            return error.MissingTypeInPlatformModule;
+        };
+        const type_node_idx = platform_env.getExposedNodeIndexById(type_ident_in_platform) orelse {
+            std.log.err("Platform module type '{s}' has no node index in exposed_items", .{module_name});
+            return error.MissingNodeIndexForPlatformType;
+        };
+
         const auto_type = Can.AutoImportedType{
             .env = platform_env,
-            .statement_idx = @enumFromInt(0), // Non-null triggers qualified name building
+            .statement_idx = @enumFromInt(type_node_idx), // actual type node index for e_lookup_external
             .qualified_type_ident = type_qualified_ident,
         };
 

src/collections/safe_list.zig

@@ -23,7 +23,7 @@ pub fn SafeRange(comptime Idx: type) type {
 
         /// An empty range
         pub fn empty() Self {
-            return .{ .start = @enumFromInt(0), .count = 0 };
+            return .{ .start = undefined, .count = 0 };
         }
 
         // Drop first elem from the span, if possible
@@ -99,6 +99,8 @@ pub fn SafeList(comptime T: type) type {
 
         /// An index for an item in the list.
         pub const Idx = enum(u32) {
+            /// The first valid index in the list.
+            first = 0,
             _,
 
             /// Get the raw u32 value for storage
@@ -246,6 +248,11 @@ pub fn SafeList(comptime T: type) type {
 
         /// Convert a range to a slice
         pub fn sliceRange(self: *const SafeList(T), range: Range) Slice {
+            // Empty ranges have undefined start, return empty slice directly
+            if (range.count == 0) {
+                return &.{};
+            }
+
             const start: usize = @intFromEnum(range.start);
             const end: usize = start + range.count;
 
@@ -368,7 +375,7 @@ pub fn SafeList(comptime T: type) type {
             return Iterator{
                 .array = self,
                 .len = self.len(),
-                .current = @enumFromInt(0),
+                .current = .first,
             };
         }
     };
@@ -396,7 +403,7 @@ pub fn SafeMultiList(comptime T: type) type {
         items: std.MultiArrayList(T) = .{},
 
         /// Index of an item in the list.
-        pub const Idx = enum(u32) { zero = 0, _ };
+        pub const Idx = enum(u32) { first = 0, _ };
 
         /// A non-type-safe slice of the list.
         pub const Slice = std.MultiArrayList(T).Slice;
@@ -461,7 +468,7 @@ pub fn SafeMultiList(comptime T: type) type {
 
         pub fn appendSlice(self: *SafeMultiList(T), gpa: Allocator, elems: []const T) std.mem.Allocator.Error!Range {
             if (elems.len == 0) {
-                return .{ .start = .zero, .count = 0 };
+                return .{ .start = .first, .count = 0 };
             }
             const start_length = self.len();
             try self.items.ensureUnusedCapacity(gpa, elems.len);
@@ -474,6 +481,17 @@ pub fn SafeMultiList(comptime T: type) type {
 
         /// Convert a range to a slice
         pub fn sliceRange(self: *const SafeMultiList(T), range: Range) Slice {
+            // Empty ranges have undefined start, return empty slice directly
+            if (range.count == 0) {
+                const base = self.items.slice();
+                // Return a zero-length slice based on the existing slice
+                return .{
+                    .ptrs = base.ptrs,
+                    .len = 0,
+                    .capacity = 0,
+                };
+            }
+
             const start: usize = @intFromEnum(range.start);
             const end: usize = start + range.count;
 
@@ -963,7 +981,7 @@ test "SafeList edge cases serialization" {
 
         try testing.expectEqual(@as(usize, 0), deserialized.list_u32.len());
         try testing.expectEqual(@as(usize, 1), deserialized.list_u8.len());
-        try testing.expectEqual(@as(u8, 123), deserialized.list_u8.get(@enumFromInt(0)).*);
+        try testing.expectEqual(@as(u8, 123), deserialized.list_u8.get(.first).*);
     }
 }
 
@@ -1048,11 +1066,12 @@ test "SafeList CompactWriter complete roundtrip example" {
     const deserialized = serialized_ptr.deserialize(@as(i64, @intCast(@intFromPtr(buffer.ptr))));
 
     // Step 8: Verify data is accessible and correct
+    const Idx = SafeList(u32).Idx;
     try testing.expectEqual(@as(usize, 4), deserialized.len());
-    try testing.expectEqual(@as(u32, 100), deserialized.get(@enumFromInt(0)).*);
-    try testing.expectEqual(@as(u32, 200), deserialized.get(@enumFromInt(1)).*);
-    try testing.expectEqual(@as(u32, 300), deserialized.get(@enumFromInt(2)).*);
-    try testing.expectEqual(@as(u32, 400), deserialized.get(@enumFromInt(3)).*);
+    try testing.expectEqual(@as(u32, 100), deserialized.get(.first).*);
+    try testing.expectEqual(@as(u32, 200), deserialized.get(@as(Idx, @enumFromInt(1))).*);
+    try testing.expectEqual(@as(u32, 300), deserialized.get(@as(Idx, @enumFromInt(2))).*);
+    try testing.expectEqual(@as(u32, 400), deserialized.get(@as(Idx, @enumFromInt(3))).*);
 }
 
 test "SafeList CompactWriter multiple lists with different alignments" {
@@ -1155,10 +1174,11 @@ test "SafeList CompactWriter multiple lists with different alignments" {
     offset = std.mem.alignForward(usize, offset, @alignOf(u8));
     offset += 3 * @sizeOf(u8);
 
+    const U8Idx = SafeList(u8).Idx;
     try testing.expectEqual(@as(usize, 3), deser_u8.len());
-    try testing.expectEqual(@as(u8, 10), deser_u8.get(@enumFromInt(0)).*);
-    try testing.expectEqual(@as(u8, 20), deser_u8.get(@enumFromInt(1)).*);
-    try testing.expectEqual(@as(u8, 30), deser_u8.get(@enumFromInt(2)).*);
+    try testing.expectEqual(@as(u8, 10), deser_u8.get(.first).*);
+    try testing.expectEqual(@as(u8, 20), deser_u8.get(@as(U8Idx, @enumFromInt(1))).*);
+    try testing.expectEqual(@as(u8, 30), deser_u8.get(@as(U8Idx, @enumFromInt(2))).*);
 
     // 2. Deserialize u16 list
     offset = std.mem.alignForward(usize, offset, @alignOf(SafeList(u16).Serialized));
@@ -1169,9 +1189,10 @@ test "SafeList CompactWriter multiple lists with different alignments" {
     offset = std.mem.alignForward(usize, offset, @alignOf(u16));
     offset += 2 * @sizeOf(u16);
 
+    const U16Idx = SafeList(u16).Idx;
     try testing.expectEqual(@as(usize, 2), deser_u16.len());
-    try testing.expectEqual(@as(u16, 1000), deser_u16.get(@enumFromInt(0)).*);
-    try testing.expectEqual(@as(u16, 2000), deser_u16.get(@enumFromInt(1)).*);
+    try testing.expectEqual(@as(u16, 1000), deser_u16.get(.first).*);
+    try testing.expectEqual(@as(u16, 2000), deser_u16.get(@as(U16Idx, @enumFromInt(1))).*);
 
     // 3. Deserialize u32 list
     offset = std.mem.alignForward(usize, offset, @alignOf(SafeList(u32).Serialized));
@@ -1182,11 +1203,12 @@ test "SafeList CompactWriter multiple lists with different alignments" {
     offset = std.mem.alignForward(usize, offset, @alignOf(u32));
     offset += 4 * @sizeOf(u32);
 
+    const U32Idx = SafeList(u32).Idx;
     try testing.expectEqual(@as(usize, 4), deser_u32.len());
-    try testing.expectEqual(@as(u32, 100_000), deser_u32.get(@enumFromInt(0)).*);
-    try testing.expectEqual(@as(u32, 200_000), deser_u32.get(@enumFromInt(1)).*);
-    try testing.expectEqual(@as(u32, 300_000), deser_u32.get(@enumFromInt(2)).*);
-    try testing.expectEqual(@as(u32, 400_000), deser_u32.get(@enumFromInt(3)).*);
+    try testing.expectEqual(@as(u32, 100_000), deser_u32.get(.first).*);
+    try testing.expectEqual(@as(u32, 200_000), deser_u32.get(@as(U32Idx, @enumFromInt(1))).*);
+    try testing.expectEqual(@as(u32, 300_000), deser_u32.get(@as(U32Idx, @enumFromInt(2))).*);
+    try testing.expectEqual(@as(u32, 400_000), deser_u32.get(@as(U32Idx, @enumFromInt(3))).*);
 
     // 4. Deserialize u64 list
     offset = std.mem.alignForward(usize, offset, @alignOf(SafeList(u64).Serialized));
@@ -1197,22 +1219,24 @@ test "SafeList CompactWriter multiple lists with different alignments" {
     offset = std.mem.alignForward(usize, offset, @alignOf(u64));
     offset += 2 * @sizeOf(u64);
 
+    const U64Idx = SafeList(u64).Idx;
     try testing.expectEqual(@as(usize, 2), deser_u64.len());
-    try testing.expectEqual(@as(u64, 10_000_000_000), deser_u64.get(@enumFromInt(0)).*);
-    try testing.expectEqual(@as(u64, 20_000_000_000), deser_u64.get(@enumFromInt(1)).*);
+    try testing.expectEqual(@as(u64, 10_000_000_000), deser_u64.get(.first).*);
+    try testing.expectEqual(@as(u64, 20_000_000_000), deser_u64.get(@as(U64Idx, @enumFromInt(1))).*);
 
     // 5. Deserialize struct list
     offset = std.mem.alignForward(usize, offset, @alignOf(SafeList(AlignedStruct).Serialized));
     const s_struct = @as(*SafeList(AlignedStruct).Serialized, @ptrCast(@alignCast(buffer.ptr + offset)));
     const deser_struct = s_struct.deserialize(@as(i64, @intCast(base_addr)));
 
+    const StructIdx = SafeList(AlignedStruct).Idx;
     try testing.expectEqual(@as(usize, 2), deser_struct.len());
-    const item0 = deser_struct.get(@enumFromInt(0));
+    const item0 = deser_struct.get(.first);
     try testing.expectEqual(@as(u32, 42), item0.x);
     try testing.expectEqual(@as(u64, 1337), item0.y);
     try testing.expectEqual(@as(u8, 255), item0.z);
 
-    const item1 = deser_struct.get(@enumFromInt(1));
+    const item1 = deser_struct.get(@as(StructIdx, @enumFromInt(1)));
     try testing.expectEqual(@as(u32, 99), item1.x);
     try testing.expectEqual(@as(u64, 9999), item1.y);
     try testing.expectEqual(@as(u8, 128), item1.z);
@@ -1318,10 +1342,11 @@ test "SafeList CompactWriter interleaved pattern with alignment tracking" {
     offset = std.mem.alignForward(usize, offset, @alignOf(u8));
     offset += 3; // 3 u8 elements
 
+    const D1Idx = SafeList(u8).Idx;
     try testing.expectEqual(@as(usize, 3), d1.len());
-    try testing.expectEqual(@as(u8, 1), d1.get(@enumFromInt(0)).*);
-    try testing.expectEqual(@as(u8, 2), d1.get(@enumFromInt(1)).*);
-    try testing.expectEqual(@as(u8, 3), d1.get(@enumFromInt(2)).*);
+    try testing.expectEqual(@as(u8, 1), d1.get(.first).*);
+    try testing.expectEqual(@as(u8, 2), d1.get(@as(D1Idx, @enumFromInt(1))).*);
+    try testing.expectEqual(@as(u8, 3), d1.get(@as(D1Idx, @enumFromInt(2))).*);
 
     // 2. Second list - u64
     offset = std.mem.alignForward(usize, offset, @alignOf(SafeList(u64).Serialized));
@@ -1331,9 +1356,10 @@ test "SafeList CompactWriter interleaved pattern with alignment tracking" {
     offset = std.mem.alignForward(usize, offset, @alignOf(u64));
     offset += 2 * @sizeOf(u64); // 2 u64 elements
 
+    const D2Idx = SafeList(u64).Idx;
     try testing.expectEqual(@as(usize, 2), d2.len());
-    try testing.expectEqual(@as(u64, 1_000_000), d2.get(@enumFromInt(0)).*);
-    try testing.expectEqual(@as(u64, 2_000_000), d2.get(@enumFromInt(1)).*);
+    try testing.expectEqual(@as(u64, 1_000_000), d2.get(.first).*);
+    try testing.expectEqual(@as(u64, 2_000_000), d2.get(@as(D2Idx, @enumFromInt(1))).*);
 
     // 3. Third list - u16
     offset = std.mem.alignForward(usize, offset, @alignOf(SafeList(u16).Serialized));
@@ -1343,11 +1369,12 @@ test "SafeList CompactWriter interleaved pattern with alignment tracking" {
     offset = std.mem.alignForward(usize, offset, @alignOf(u16));
     offset += 4 * @sizeOf(u16); // 4 u16 elements
 
+    const D3Idx = SafeList(u16).Idx;
     try testing.expectEqual(@as(usize, 4), d3.len());
-    try testing.expectEqual(@as(u16, 100), d3.get(@enumFromInt(0)).*);
-    try testing.expectEqual(@as(u16, 200), d3.get(@enumFromInt(1)).*);
-    try testing.expectEqual(@as(u16, 300), d3.get(@enumFromInt(2)).*);
-    try testing.expectEqual(@as(u16, 400), d3.get(@enumFromInt(3)).*);
+    try testing.expectEqual(@as(u16, 100), d3.get(.first).*);
+    try testing.expectEqual(@as(u16, 200), d3.get(@as(D3Idx, @enumFromInt(1))).*);
+    try testing.expectEqual(@as(u16, 300), d3.get(@as(D3Idx, @enumFromInt(2))).*);
+    try testing.expectEqual(@as(u16, 400), d3.get(@as(D3Idx, @enumFromInt(3))).*);
 
     // 4. Fourth list - u32
     offset = std.mem.alignForward(usize, offset, @alignOf(SafeList(u32).Serialized));
@@ -1355,7 +1382,7 @@ test "SafeList CompactWriter interleaved pattern with alignment tracking" {
     const d4 = s4.deserialize(@as(i64, @intCast(base)));
 
     try testing.expectEqual(@as(usize, 1), d4.len());
-    try testing.expectEqual(@as(u32, 42), d4.get(@enumFromInt(0)).*);
+    try testing.expectEqual(@as(u32, 42), d4.get(.first).*);
 }
 
 test "SafeList CompactWriter brute-force alignment verification" {
@@ -1476,7 +1503,7 @@ test "SafeList CompactWriter brute-force alignment verification" {
             offset += 1; // 1 u8 element
 
             try testing.expectEqual(@as(usize, 1), d_u8.len());
-            try testing.expectEqual(@as(u8, 42), d_u8.get(@enumFromInt(0)).*);
+            try testing.expectEqual(@as(u8, 42), d_u8.get(.first).*);
 
             // Second list
             offset = std.mem.alignForward(usize, offset, @alignOf(SafeList(T).Serialized));
@@ -1551,28 +1578,32 @@ test "SafeMultiList CompactWriter roundtrip with file" {
     const deserialized = serialized_ptr.deserialize(@as(i64, @intCast(@intFromPtr(buffer.ptr))));
 
     // Verify the data
+    const Idx = SafeMultiList(TestStruct).Idx;
     try testing.expectEqual(@as(usize, 4), deserialized.len());
 
     // Verify all the data
-    try testing.expectEqual(@as(u32, 100), deserialized.get(@enumFromInt(0)).id);
-    try testing.expectEqual(@as(u64, 1000), deserialized.get(@enumFromInt(0)).value);
-    try testing.expectEqual(true, deserialized.get(@enumFromInt(0)).flag);
-    try testing.expectEqual(@as(u8, 10), deserialized.get(@enumFromInt(0)).data);
+    try testing.expectEqual(@as(u32, 100), deserialized.get(.first).id);
+    try testing.expectEqual(@as(u64, 1000), deserialized.get(.first).value);
+    try testing.expectEqual(true, deserialized.get(.first).flag);
+    try testing.expectEqual(@as(u8, 10), deserialized.get(.first).data);
 
-    try testing.expectEqual(@as(u32, 200), deserialized.get(@enumFromInt(1)).id);
-    try testing.expectEqual(@as(u64, 2000), deserialized.get(@enumFromInt(1)).value);
-    try testing.expectEqual(false, deserialized.get(@enumFromInt(1)).flag);
-    try testing.expectEqual(@as(u8, 20), deserialized.get(@enumFromInt(1)).data);
+    const second_idx: Idx = @enumFromInt(1);
+    try testing.expectEqual(@as(u32, 200), deserialized.get(second_idx).id);
+    try testing.expectEqual(@as(u64, 2000), deserialized.get(second_idx).value);
+    try testing.expectEqual(false, deserialized.get(second_idx).flag);
+    try testing.expectEqual(@as(u8, 20), deserialized.get(second_idx).data);
 
-    try testing.expectEqual(@as(u32, 300), deserialized.get(@enumFromInt(2)).id);
-    try testing.expectEqual(@as(u64, 3000), deserialized.get(@enumFromInt(2)).value);
-    try testing.expectEqual(true, deserialized.get(@enumFromInt(2)).flag);
-    try testing.expectEqual(@as(u8, 30), deserialized.get(@enumFromInt(2)).data);
+    const third_idx: Idx = @enumFromInt(2);
+    try testing.expectEqual(@as(u32, 300), deserialized.get(third_idx).id);
+    try testing.expectEqual(@as(u64, 3000), deserialized.get(third_idx).value);
+    try testing.expectEqual(true, deserialized.get(third_idx).flag);
+    try testing.expectEqual(@as(u8, 30), deserialized.get(third_idx).data);
 
-    try testing.expectEqual(@as(u32, 400), deserialized.get(@enumFromInt(3)).id);
-    try testing.expectEqual(@as(u64, 4000), deserialized.get(@enumFromInt(3)).value);
-    try testing.expectEqual(false, deserialized.get(@enumFromInt(3)).flag);
-    try testing.expectEqual(@as(u8, 40), deserialized.get(@enumFromInt(3)).data);
+    const fourth_idx: Idx = @enumFromInt(3);
+    try testing.expectEqual(@as(u32, 400), deserialized.get(fourth_idx).id);
+    try testing.expectEqual(@as(u64, 4000), deserialized.get(fourth_idx).value);
+    try testing.expectEqual(false, deserialized.get(fourth_idx).flag);
+    try testing.expectEqual(@as(u8, 40), deserialized.get(fourth_idx).data);
 }
 
 test "SafeMultiList empty list CompactWriter roundtrip" {
@@ -1702,30 +1733,31 @@ test "SafeMultiList CompactWriter multiple lists different alignments" {
     const base = @as(i64, @intCast(@intFromPtr(buffer.ptr)));
 
     // Deserialize list1 (at offset1)
+    const D1Idx = SafeMultiList(Type1).Idx;
     const d1_serialized = @as(*SafeMultiList(Type1).Serialized, @ptrCast(@alignCast(buffer.ptr + offset1)));
     const d1 = d1_serialized.deserialize(base);
     try testing.expectEqual(@as(usize, 3), d1.len());
-    try testing.expectEqual(@as(u8, 10), d1.get(@enumFromInt(0)).a);
-    try testing.expectEqual(@as(u16, 100), d1.get(@enumFromInt(0)).b);
-    try testing.expectEqual(@as(u8, 20), d1.get(@enumFromInt(1)).a);
-    try testing.expectEqual(@as(u16, 200), d1.get(@enumFromInt(1)).b);
-    try testing.expectEqual(@as(u8, 30), d1.get(@enumFromInt(2)).a);
-    try testing.expectEqual(@as(u16, 300), d1.get(@enumFromInt(2)).b);
+    try testing.expectEqual(@as(u8, 10), d1.get(.first).a);
+    try testing.expectEqual(@as(u16, 100), d1.get(.first).b);
+    try testing.expectEqual(@as(u8, 20), d1.get(@as(D1Idx, @enumFromInt(1))).a);
+    try testing.expectEqual(@as(u16, 200), d1.get(@as(D1Idx, @enumFromInt(1))).b);
+    try testing.expectEqual(@as(u8, 30), d1.get(@as(D1Idx, @enumFromInt(2))).a);
+    try testing.expectEqual(@as(u16, 300), d1.get(@as(D1Idx, @enumFromInt(2))).b);
 
     // Deserialize list2 (at offset2)
     const d2_serialized = @as(*SafeMultiList(Type2).Serialized, @ptrCast(@alignCast(buffer.ptr + offset2)));
     const d2 = d2_serialized.deserialize(base);
     try testing.expectEqual(@as(usize, 2), d2.len());
-    try testing.expectEqual(@as(u32, 1000), d2.get(@enumFromInt(0)).x);
-    try testing.expectEqual(@as(u64, 10000), d2.get(@enumFromInt(0)).y);
+    try testing.expectEqual(@as(u32, 1000), d2.get(.first).x);
+    try testing.expectEqual(@as(u64, 10000), d2.get(.first).y);
 
     // Deserialize list3 (at offset3)
     const d3_serialized = @as(*SafeMultiList(Type3).Serialized, @ptrCast(@alignCast(buffer.ptr + offset3)));
     const d3 = d3_serialized.deserialize(base);
     try testing.expectEqual(@as(usize, 2), d3.len());
-    try testing.expectEqual(@as(u64, 999), d3.get(@enumFromInt(0)).id);
-    try testing.expectEqual(@as(u8, 42), d3.get(@enumFromInt(0)).data);
-    try testing.expectEqual(true, d3.get(@enumFromInt(0)).flag);
+    try testing.expectEqual(@as(u64, 999), d3.get(.first).id);
+    try testing.expectEqual(@as(u8, 42), d3.get(.first).data);
+    try testing.expectEqual(true, d3.get(.first).flag);
 }
 
 test "SafeMultiList CompactWriter brute-force alignment verification" {
@@ -1815,10 +1847,11 @@ test "SafeMultiList CompactWriter brute-force alignment verification" {
         const d2_serialized = @as(*SafeMultiList(TestType).Serialized, @ptrCast(@alignCast(buffer.ptr + offset2)));
         const d2 = d2_serialized.deserialize(base);
         if (length > 0) {
+            const d2_first_idx: SafeMultiList(TestType).Idx = .first;
             try testing.expectEqual(@as(usize, 1), d2.len());
-            try testing.expectEqual(@as(u8, 255), d2.get(@enumFromInt(0)).a);
-            try testing.expectEqual(@as(u32, 999999), d2.get(@enumFromInt(0)).b);
-            try testing.expectEqual(@as(u64, 888888888), d2.get(@enumFromInt(0)).c);
+            try testing.expectEqual(@as(u8, 255), d2.get(d2_first_idx).a);
+            try testing.expectEqual(@as(u32, 999999), d2.get(d2_first_idx).b);
+            try testing.expectEqual(@as(u64, 888888888), d2.get(d2_first_idx).c);
         } else {
             try testing.expectEqual(@as(usize, 0), d2.len());
         }
@@ -2286,7 +2319,8 @@ test "SafeMultiList.Serialized roundtrip" {
     try testing.expectEqual(@as(u8, 64), c_values[2]);
 
     // Check get() method
-    const item1 = list.get(@as(SafeMultiList(TestStruct).Idx, @enumFromInt(0)));
+    const first_idx: SafeMultiList(TestStruct).Idx = .first;
+    const item1 = list.get(first_idx);
     try testing.expectEqual(@as(u32, 100), item1.a);
     try testing.expectEqual(@as(f32, 1.5), item1.b);
     try testing.expectEqual(@as(u8, 255), item1.c);

src/compile/test/type_printing_bug_test.zig

@@ -64,7 +64,7 @@ test "canonicalizeAndTypeCheckModule preserves Try types in type printing" {
     defer result.deinit();
 
     // Now get the type of map_result and convert it to a string
-    // Find the map_result definition
+    // Find the map_result definition and get its type var from the expression
     const defs_slice = env.store.sliceDefs(env.all_defs);
     var map_result_var: ?types.Var = null;
     for (defs_slice) |def_idx| {
@@ -74,8 +74,8 @@ test "canonicalizeAndTypeCheckModule preserves Try types in type printing" {
             const ident_idx = pattern.assign.ident;
             const ident_text = env.getIdent(ident_idx);
             if (std.mem.eql(u8, ident_text, "map_result")) {
-                // Get the type variable from the first definition - it's the first in the defs list
-                map_result_var = @enumFromInt(0); // First variable
+                // Get the type variable from the definition's expression
+                map_result_var = ModuleEnv.varFrom(def.expr);
                 break;
             }
         }

src/eval/StackValue.zig

@@ -34,14 +34,203 @@ const Expr = CIR.Expr;
 
 const StackValue = @This();
 
+// ============================================================================
+// Internal helper functions for memory operations that don't need rt_var
+// ============================================================================
+
+/// Increment reference count for a value given its layout and pointer.
+/// Used internally when we don't need full StackValue type information.
+fn increfLayoutPtr(layout: Layout, ptr: ?*anyopaque, layout_cache: *LayoutStore) void {
+    if (layout.tag == .scalar and layout.data.scalar.tag == .str) {
+        if (ptr == null) return;
+        const roc_str = @as(*const RocStr, @ptrCast(@alignCast(ptr.?))).*;
+        roc_str.incref(1);
+        return;
+    }
+    if (layout.tag == .list) {
+        if (ptr == null) return;
+        const list_value = @as(*const RocList, @ptrCast(@alignCast(ptr.?))).*;
+        list_value.incref(1, false);
+        return;
+    }
+    if (layout.tag == .box) {
+        if (ptr == null) return;
+        const slot: *usize = @ptrCast(@alignCast(ptr.?));
+        if (slot.* != 0) {
+            const data_ptr: [*]u8 = @as([*]u8, @ptrFromInt(slot.*));
+            builtins.utils.increfDataPtrC(@as(?[*]u8, data_ptr), 1);
+        }
+        return;
+    }
+    if (layout.tag == .record) {
+        if (ptr == null) return;
+        const record_data = layout_cache.getRecordData(layout.data.record.idx);
+        if (record_data.fields.count == 0) return;
+
+        const field_layouts = layout_cache.record_fields.sliceRange(record_data.getFields());
+        const base_ptr = @as([*]u8, @ptrCast(ptr.?));
+
+        var field_index: usize = 0;
+        while (field_index < field_layouts.len) : (field_index += 1) {
+            const field_info = field_layouts.get(field_index);
+            const field_layout = layout_cache.getLayout(field_info.layout);
+            const field_offset = layout_cache.getRecordFieldOffset(layout.data.record.idx, @intCast(field_index));
+            const field_ptr = @as(*anyopaque, @ptrCast(base_ptr + field_offset));
+            increfLayoutPtr(field_layout, field_ptr, layout_cache);
+        }
+        return;
+    }
+    if (layout.tag == .tuple) {
+        if (ptr == null) return;
+        const tuple_data = layout_cache.getTupleData(layout.data.tuple.idx);
+        if (tuple_data.fields.count == 0) return;
+
+        const element_layouts = layout_cache.tuple_fields.sliceRange(tuple_data.getFields());
+        const base_ptr = @as([*]u8, @ptrCast(ptr.?));
+
+        var elem_index: usize = 0;
+        while (elem_index < element_layouts.len) : (elem_index += 1) {
+            const elem_info = element_layouts.get(elem_index);
+            const elem_layout = layout_cache.getLayout(elem_info.layout);
+            const elem_offset = layout_cache.getTupleElementOffset(layout.data.tuple.idx, @intCast(elem_index));
+            const elem_ptr = @as(*anyopaque, @ptrCast(base_ptr + elem_offset));
+            increfLayoutPtr(elem_layout, elem_ptr, layout_cache);
+        }
+        return;
+    }
+    if (layout.tag == .tag_union) {
+        if (ptr == null) return;
+        // For unions, we need to read the tag and incref the appropriate payload
+        // This is complex - for now just skip (caller should handle specific union types)
+        return;
+    }
+    // Other layout types (scalar ints/floats, zst, etc.) don't need refcounting
+}
+
+/// Decrement reference count for a value given its layout and pointer.
+/// Used internally when we don't need full StackValue type information.
+fn decrefLayoutPtr(layout: Layout, ptr: ?*anyopaque, layout_cache: *LayoutStore, ops: *RocOps) void {
+    if (layout.tag == .scalar and layout.data.scalar.tag == .str) {
+        if (ptr == null) return;
+        const roc_str = @as(*const RocStr, @ptrCast(@alignCast(ptr.?))).*;
+        roc_str.decref(ops);
+        return;
+    }
+    if (layout.tag == .list) {
+        if (ptr == null) return;
+        const list_header: *const RocList = @ptrCast(@alignCast(ptr.?));
+        const list_value = list_header.*;
+        const elem_layout = layout_cache.getLayout(layout.data.list);
+        const alignment_u32: u32 = @intCast(elem_layout.alignment(layout_cache.targetUsize()).toByteUnits());
+        const element_width: usize = @intCast(layout_cache.layoutSize(elem_layout));
+        const elements_refcounted = elem_layout.isRefcounted();
+
+        // Decref elements when unique
+        if (list_value.isUnique()) {
+            if (list_value.getAllocationDataPtr()) |source| {
+                const count = list_value.getAllocationElementCount(elements_refcounted);
+                var idx: usize = 0;
+                while (idx < count) : (idx += 1) {
+                    const elem_ptr = source + idx * element_width;
+                    decrefLayoutPtr(elem_layout, @ptrCast(elem_ptr), layout_cache, ops);
+                }
+            }
+        }
+        list_value.decref(alignment_u32, element_width, elements_refcounted, null, &builtins.list.rcNone, ops);
+        return;
+    }
+    if (layout.tag == .box) {
+        if (ptr == null) return;
+        const slot: *usize = @ptrCast(@alignCast(ptr.?));
+        const raw_ptr = slot.*;
+        if (raw_ptr == 0) return;
+        const data_ptr = @as([*]u8, @ptrFromInt(raw_ptr));
+        const target_usize = layout_cache.targetUsize();
+        const elem_layout = layout_cache.getLayout(layout.data.box);
+        const elem_alignment: u32 = @intCast(elem_layout.alignment(target_usize).toByteUnits());
+
+        const ptr_int = @intFromPtr(data_ptr);
+        const tag_mask: usize = if (@sizeOf(usize) == 8) 0b111 else 0b11;
+        const unmasked_ptr = ptr_int & ~tag_mask;
+        const payload_ptr = @as([*]u8, @ptrFromInt(unmasked_ptr));
+        const refcount_ptr: *isize = @as(*isize, @ptrFromInt(unmasked_ptr - @sizeOf(isize)));
+
+        if (builtins.utils.rcUnique(refcount_ptr.*)) {
+            if (elem_layout.isRefcounted()) {
+                decrefLayoutPtr(elem_layout, @ptrCast(@alignCast(payload_ptr)), layout_cache, ops);
+            }
+        }
+        builtins.utils.decrefDataPtrC(@as(?[*]u8, payload_ptr), elem_alignment, false, ops);
+        slot.* = 0;
+        return;
+    }
+    if (layout.tag == .record) {
+        if (ptr == null) return;
+        const record_data = layout_cache.getRecordData(layout.data.record.idx);
+        if (record_data.fields.count == 0) return;
+
+        const field_layouts = layout_cache.record_fields.sliceRange(record_data.getFields());
+        const base_ptr = @as([*]u8, @ptrCast(ptr.?));
+
+        var field_index: usize = 0;
+        while (field_index < field_layouts.len) : (field_index += 1) {
+            const field_info = field_layouts.get(field_index);
+            const field_layout = layout_cache.getLayout(field_info.layout);
+            const field_offset = layout_cache.getRecordFieldOffset(layout.data.record.idx, @intCast(field_index));
+            const field_ptr = @as(*anyopaque, @ptrCast(base_ptr + field_offset));
+            decrefLayoutPtr(field_layout, field_ptr, layout_cache, ops);
+        }
+        return;
+    }
+    if (layout.tag == .tuple) {
+        if (ptr == null) return;
+        const tuple_data = layout_cache.getTupleData(layout.data.tuple.idx);
+        if (tuple_data.fields.count == 0) return;
+
+        const element_layouts = layout_cache.tuple_fields.sliceRange(tuple_data.getFields());
+        const base_ptr = @as([*]u8, @ptrCast(ptr.?));
+
+        var elem_index: usize = 0;
+        while (elem_index < element_layouts.len) : (elem_index += 1) {
+            const elem_info = element_layouts.get(elem_index);
+            const elem_layout = layout_cache.getLayout(elem_info.layout);
+            const elem_offset = layout_cache.getTupleElementOffset(layout.data.tuple.idx, @intCast(elem_index));
+            const elem_ptr = @as(*anyopaque, @ptrCast(base_ptr + elem_offset));
+            decrefLayoutPtr(elem_layout, elem_ptr, layout_cache, ops);
+        }
+        return;
+    }
+    if (layout.tag == .closure) {
+        if (ptr == null) return;
+        // Get the closure header to find the captures layout
+        const closure_header: *const layout_mod.Closure = @ptrCast(@alignCast(ptr.?));
+        const captures_layout = layout_cache.getLayout(closure_header.captures_layout_idx);
+
+        // Only decref if there are actual captures (record with fields)
+        if (captures_layout.tag == .record) {
+            const record_data = layout_cache.getRecordData(captures_layout.data.record.idx);
+            if (record_data.fields.count > 0) {
+                const header_size = @sizeOf(layout_mod.Closure);
+                const cap_align = captures_layout.alignment(layout_cache.targetUsize());
+                const aligned_off = std.mem.alignForward(usize, header_size, @intCast(cap_align.toByteUnits()));
+                const base_ptr: [*]u8 = @ptrCast(@alignCast(ptr.?));
+                const rec_ptr: *anyopaque = @ptrCast(base_ptr + aligned_off);
+                decrefLayoutPtr(captures_layout, rec_ptr, layout_cache, ops);
+            }
+        }
+        return;
+    }
+    // Other layout types (scalar ints/floats, zst, etc.) don't need refcounting
+}
+
 /// Type and memory layout information for the result value
 layout: Layout,
 /// Ptr to the actual value in stack memory
 ptr: ?*anyopaque,
 /// Flag to track whether the memory has been initialized
 is_initialized: bool = false,
-/// Optional runtime type variable for type information (used in constant folding)
-rt_var: ?types.Var = null,
+/// Runtime type variable for type information (used for method dispatch and constant folding)
+rt_var: types.Var,
 
 /// Copy this stack value to a destination pointer with bounds checking
 pub fn copyToPtr(self: StackValue, layout_cache: *LayoutStore, dest_ptr: *anyopaque) !void {
@@ -226,13 +415,7 @@ pub fn copyToPtr(self: StackValue, layout_cache: *LayoutStore, dest_ptr: *anyopa
             const field_offset = layout_cache.getRecordFieldOffset(self.layout.data.record.idx, @intCast(field_index));
             const field_ptr = @as(*anyopaque, @ptrCast(base_ptr + field_offset));
 
-            const field_value = StackValue{
-                .layout = field_layout,
-                .ptr = field_ptr,
-                .is_initialized = true,
-            };
-
-            field_value.incref(layout_cache);
+            increfLayoutPtr(field_layout, field_ptr, layout_cache);
         }
         return;
     }
@@ -263,13 +446,7 @@ pub fn copyToPtr(self: StackValue, layout_cache: *LayoutStore, dest_ptr: *anyopa
             const elem_offset = layout_cache.getTupleElementOffset(self.layout.data.tuple.idx, @intCast(elem_index));
             const elem_ptr = @as(*anyopaque, @ptrCast(base_ptr + elem_offset));
 
-            const elem_value = StackValue{
-                .layout = elem_layout,
-                .ptr = elem_ptr,
-                .is_initialized = true,
-            };
-
-            elem_value.incref(layout_cache);
+            increfLayoutPtr(elem_layout, elem_ptr, layout_cache);
         }
         return;
     }
@@ -304,29 +481,8 @@ pub fn copyToPtr(self: StackValue, layout_cache: *LayoutStore, dest_ptr: *anyopa
                 const base_ptr: [*]u8 = @ptrCast(@alignCast(self.ptr.?));
                 const rec_ptr: [*]u8 = @ptrCast(base_ptr + aligned_off);
 
-                // Iterate over each field in the captures record and incref all fields.
-                // We call incref on ALL fields (not just isRefcounted()) because:
-                // - For directly refcounted types (str, list, box): increfs them
-                // - For nested records/tuples: recursively handles their contents
-                // - For scalars: incref is a no-op
-                // This is symmetric with decref.
-                const field_layouts = layout_cache.record_fields.sliceRange(record_data.getFields());
-                var field_index: usize = 0;
-                while (field_index < field_layouts.len) : (field_index += 1) {
-                    const field_info = field_layouts.get(field_index);
-                    const field_layout = layout_cache.getLayout(field_info.layout);
-
-                    const field_offset = layout_cache.getRecordFieldOffset(captures_layout.data.record.idx, @intCast(field_index));
-                    const field_ptr = @as(*anyopaque, @ptrCast(rec_ptr + field_offset));
-
-                    const field_value = StackValue{
-                        .layout = field_layout,
-                        .ptr = field_ptr,
-                        .is_initialized = true,
-                    };
-
-                    field_value.incref(layout_cache);
-                }
+                // Incref the entire captures record (which handles all fields recursively)
+                increfLayoutPtr(captures_layout, @ptrCast(rec_ptr), layout_cache);
             }
         }
         return;
@@ -365,13 +521,7 @@ pub fn copyToPtr(self: StackValue, layout_cache: *LayoutStore, dest_ptr: *anyopa
         }
 
         // Incref only the active variant's payload (at offset 0)
-        const payload_value = StackValue{
-            .layout = variant_layout,
-            .ptr = @as(*anyopaque, @ptrCast(base_ptr)),
-            .is_initialized = true,
-        };
-
-        payload_value.incref(layout_cache);
+        increfLayoutPtr(variant_layout, @as(*anyopaque, @ptrCast(base_ptr)), layout_cache);
         return;
     }
 
@@ -722,7 +872,7 @@ pub const TupleAccessor = struct {
     element_layouts: layout_mod.TupleField.SafeMultiList.Slice,
 
     /// Get a StackValue for the element at the given original index (before sorting)
-    pub fn getElement(self: TupleAccessor, original_index: usize) !StackValue {
+    pub fn getElement(self: TupleAccessor, original_index: usize, elem_rt_var: types.Var) !StackValue {
         // Find the sorted index corresponding to this original index
         const sorted_index = self.findElementIndexByOriginal(original_index) orelse return error.TupleIndexOutOfBounds;
 
@@ -748,13 +898,24 @@ pub const TupleAccessor = struct {
             .layout = element_layout,
             .ptr = element_ptr,
             .is_initialized = true, // Elements in existing tuples are initialized
+            .rt_var = elem_rt_var,
         };
     }
 
+    /// Get just the element pointer without needing type information (for internal operations like setElement)
+    pub fn getElementPtr(self: TupleAccessor, original_index: usize) !*anyopaque {
+        const sorted_index = self.findElementIndexByOriginal(original_index) orelse return error.TupleIndexOutOfBounds;
+        std.debug.assert(self.base_value.is_initialized);
+        std.debug.assert(self.base_value.ptr != null);
+        const element_offset = self.layout_cache.getTupleElementOffset(self.tuple_layout.data.tuple.idx, @intCast(sorted_index));
+        const base_ptr = @as([*]u8, @ptrCast(self.base_value.ptr.?));
+        return @as(*anyopaque, @ptrCast(base_ptr + element_offset));
+    }
+
     /// Set an element by copying from a source StackValue
     pub fn setElement(self: TupleAccessor, index: usize, source: StackValue) !void {
-        const dest_element = try self.getElement(index);
-        try source.copyToPtr(self.layout_cache, dest_element.ptr.?);
+        const dest_ptr = try self.getElementPtr(index);
+        try source.copyToPtr(self.layout_cache, dest_ptr);
     }
 
     /// Find the sorted element index corresponding to an original tuple position
@@ -871,11 +1032,11 @@ pub const ListAccessor = struct {
         return self.list.len();
     }
 
-    pub fn getElement(self: ListAccessor, index: usize) !StackValue {
+    pub fn getElement(self: ListAccessor, index: usize, elem_rt_var: types.Var) !StackValue {
         if (index >= self.list.len()) return error.ListIndexOutOfBounds;
 
         if (self.element_size == 0) {
-            return StackValue{ .layout = self.element_layout, .ptr = null, .is_initialized = true };
+            return StackValue{ .layout = self.element_layout, .ptr = null, .is_initialized = true, .rt_var = elem_rt_var };
         }
 
         const base_ptr = self.list.bytes orelse return error.NullStackPointer;
@@ -884,8 +1045,18 @@ pub const ListAccessor = struct {
             .layout = self.element_layout,
             .ptr = @ptrCast(base_ptr + offset),
             .is_initialized = true,
+            .rt_var = elem_rt_var,
         };
     }
+
+    /// Get just the element pointer without needing type information (for internal operations)
+    pub fn getElementPtr(self: ListAccessor, index: usize) !?*anyopaque {
+        if (index >= self.list.len()) return error.ListIndexOutOfBounds;
+        if (self.element_size == 0) return null;
+        const base_ptr = self.list.bytes orelse return error.NullStackPointer;
+        const offset = index * self.element_size;
+        return @ptrCast(base_ptr + offset);
+    }
 };
 
 fn storeListElementCount(list: *RocList, elements_refcounted: bool) void {
@@ -961,7 +1132,7 @@ pub const RecordAccessor = struct {
     field_layouts: layout_mod.RecordField.SafeMultiList.Slice,
 
     /// Get a StackValue for the field at the given index
-    pub fn getFieldByIndex(self: RecordAccessor, index: usize) !StackValue {
+    pub fn getFieldByIndex(self: RecordAccessor, index: usize, field_rt_var: types.Var) !StackValue {
         if (index >= self.field_layouts.len) {
             return error.RecordIndexOutOfBounds;
         }
@@ -988,11 +1159,12 @@ pub const RecordAccessor = struct {
             .layout = field_layout,
             .ptr = field_ptr,
             .is_initialized = true, // Fields in existing records are initialized
+            .rt_var = field_rt_var,
         };
     }
 
     /// Get a StackValue for the field with the given name
-    pub fn getFieldByName(self: RecordAccessor, field_name_idx: Ident.Idx) !?StackValue {
+    pub fn getFieldByName(self: RecordAccessor, field_name_idx: Ident.Idx, field_rt_var: types.Var) !?StackValue {
         const field_offset = self.layout_cache.getRecordFieldOffsetByName(
             self.record_layout.data.record.idx,
             field_name_idx,
@@ -1026,12 +1198,13 @@ pub const RecordAccessor = struct {
             .layout = field_layout.?,
             .ptr = field_ptr,
             .is_initialized = true,
+            .rt_var = field_rt_var,
         };
     }
 
     /// Set a field by copying from a source StackValue
     pub fn setFieldByIndex(self: RecordAccessor, index: usize, source: StackValue) !void {
-        const dest_field = try self.getFieldByIndex(index);
+        const dest_field = try self.getFieldByIndex(index, source.rt_var);
         try source.copyToPtr(self.layout_cache, dest_field.ptr.?);
     }
 
@@ -1168,15 +1341,6 @@ pub fn copyTo(self: StackValue, dest: StackValue, layout_cache: *LayoutStore) vo
     );
 }
 
-/// Create a StackValue view of a memory region (no copy)
-pub fn fromPtr(layout: Layout, ptr: *anyopaque) StackValue {
-    return StackValue{
-        .layout = layout,
-        .ptr = ptr,
-        .is_initialized = true,
-    };
-}
-
 /// Copy value data to another StackValue WITHOUT incrementing refcounts (move semantics)
 pub fn copyWithoutRefcount(self: StackValue, dest: StackValue, layout_cache: *LayoutStore) void {
     std.debug.assert(self.is_initialized);
@@ -1269,56 +1433,12 @@ pub fn incref(self: StackValue, layout_cache: *LayoutStore) void {
     }
     // Handle records by recursively incref'ing each field (symmetric with decref)
     if (self.layout.tag == .record) {
-        if (self.ptr == null) return;
-        const record_data = layout_cache.getRecordData(self.layout.data.record.idx);
-        if (record_data.fields.count == 0) return;
-
-        const field_layouts = layout_cache.record_fields.sliceRange(record_data.getFields());
-        const base_ptr = @as([*]u8, @ptrCast(self.ptr.?));
-
-        var field_index: usize = 0;
-        while (field_index < field_layouts.len) : (field_index += 1) {
-            const field_info = field_layouts.get(field_index);
-            const field_layout = layout_cache.getLayout(field_info.layout);
-
-            const field_offset = layout_cache.getRecordFieldOffset(self.layout.data.record.idx, @intCast(field_index));
-            const field_ptr = @as(*anyopaque, @ptrCast(base_ptr + field_offset));
-
-            const field_value = StackValue{
-                .layout = field_layout,
-                .ptr = field_ptr,
-                .is_initialized = true,
-            };
-
-            field_value.incref(layout_cache);
-        }
+        increfLayoutPtr(self.layout, self.ptr, layout_cache);
         return;
     }
     // Handle tuples by recursively incref'ing each element (symmetric with decref)
     if (self.layout.tag == .tuple) {
-        if (self.ptr == null) return;
-        const tuple_data = layout_cache.getTupleData(self.layout.data.tuple.idx);
-        if (tuple_data.fields.count == 0) return;
-
-        const element_layouts = layout_cache.tuple_fields.sliceRange(tuple_data.getFields());
-        const base_ptr = @as([*]u8, @ptrCast(self.ptr.?));
-
-        var elem_index: usize = 0;
-        while (elem_index < element_layouts.len) : (elem_index += 1) {
-            const elem_info = element_layouts.get(elem_index);
-            const elem_layout = layout_cache.getLayout(elem_info.layout);
-
-            const elem_offset = layout_cache.getTupleElementOffset(self.layout.data.tuple.idx, @intCast(elem_index));
-            const elem_ptr = @as(*anyopaque, @ptrCast(base_ptr + elem_offset));
-
-            const elem_value = StackValue{
-                .layout = elem_layout,
-                .ptr = elem_ptr,
-                .is_initialized = true,
-            };
-
-            elem_value.incref(layout_cache);
-        }
+        increfLayoutPtr(self.layout, self.ptr, layout_cache);
         return;
     }
     // Handle tag unions by reading discriminant and incref'ing only the active variant's payload
@@ -1342,17 +1462,11 @@ pub fn incref(self: StackValue, layout_cache: *LayoutStore) void {
         const variant_layout = layout_cache.getLayout(variants.get(discriminant).payload_layout);
 
         // Incref only the active variant's payload (at offset 0)
-        const payload_value = StackValue{
-            .layout = variant_layout,
-            .ptr = @as(*anyopaque, @ptrCast(base_ptr)),
-            .is_initialized = true,
-        };
-
         if (comptime trace_refcount) {
             traceRefcount("INCREF tag_union disc={} variant_layout.tag={}", .{ discriminant, @intFromEnum(variant_layout.tag) });
         }
 
-        payload_value.incref(layout_cache);
+        increfLayoutPtr(variant_layout, @as(*anyopaque, @ptrCast(base_ptr)), layout_cache);
         return;
     }
 }
@@ -1450,12 +1564,7 @@ pub fn decref(self: StackValue, layout_cache: *LayoutStore, ops: *RocOps) void {
                     var idx: usize = 0;
                     while (idx < count) : (idx += 1) {
                         const elem_ptr = source + idx * element_width;
-                        const elem_value = StackValue{
-                            .layout = elem_layout,
-                            .ptr = @ptrCast(elem_ptr),
-                            .is_initialized = true,
-                        };
-                        elem_value.decref(layout_cache, ops);
+                        decrefLayoutPtr(elem_layout, @ptrCast(elem_ptr), layout_cache, ops);
                     }
                 }
             }
@@ -1498,12 +1607,7 @@ pub fn decref(self: StackValue, layout_cache: *LayoutStore, ops: *RocOps) void {
 
             if (builtins.utils.rcUnique(refcount_ptr.*)) {
                 if (elem_layout.isRefcounted()) {
-                    const payload_value = StackValue{
-                        .layout = elem_layout,
-                        .ptr = @ptrCast(@alignCast(payload_ptr)),
-                        .is_initialized = true,
-                    };
-                    payload_value.decref(layout_cache, ops);
+                    decrefLayoutPtr(elem_layout, @ptrCast(@alignCast(payload_ptr)), layout_cache, ops);
                 }
             }
 
@@ -1523,26 +1627,7 @@ pub fn decref(self: StackValue, layout_cache: *LayoutStore, ops: *RocOps) void {
                 });
             }
 
-            const field_layouts = layout_cache.record_fields.sliceRange(record_data.getFields());
-            const base_ptr = @as([*]u8, @ptrCast(self.ptr.?));
-
-            var field_index: usize = 0;
-            while (field_index < field_layouts.len) : (field_index += 1) {
-                const field_info = field_layouts.get(field_index);
-                const field_layout = layout_cache.getLayout(field_info.layout);
-
-                const field_offset = layout_cache.getRecordFieldOffset(self.layout.data.record.idx, @intCast(field_index));
-                const field_ptr = @as(*anyopaque, @ptrCast(base_ptr + field_offset));
-
-                const field_value = StackValue{
-                    .layout = field_layout,
-                    .ptr = field_ptr,
-                    .is_initialized = true,
-                };
-
-                field_value.decref(layout_cache, ops);
-            }
-
+            decrefLayoutPtr(self.layout, self.ptr, layout_cache, ops);
             return;
         },
         .box_of_zst => {
@@ -1563,61 +1648,11 @@ pub fn decref(self: StackValue, layout_cache: *LayoutStore, ops: *RocOps) void {
                 });
             }
 
-            const element_layouts = layout_cache.tuple_fields.sliceRange(tuple_data.getFields());
-            const base_ptr = @as([*]u8, @ptrCast(self.ptr.?));
-
-            var elem_index: usize = 0;
-            while (elem_index < element_layouts.len) : (elem_index += 1) {
-                const elem_info = element_layouts.get(elem_index);
-                const elem_layout = layout_cache.getLayout(elem_info.layout);
-
-                const elem_offset = layout_cache.getTupleElementOffset(self.layout.data.tuple.idx, @intCast(elem_index));
-                const elem_ptr = @as(*anyopaque, @ptrCast(base_ptr + elem_offset));
-
-                const elem_value = StackValue{
-                    .layout = elem_layout,
-                    .ptr = elem_ptr,
-                    .is_initialized = true,
-                };
-
-                elem_value.decref(layout_cache, ops);
-            }
-
+            decrefLayoutPtr(self.layout, self.ptr, layout_cache, ops);
             return;
         },
         .closure => {
-            if (self.ptr == null) return;
-            // Get the closure header to find the captures layout
-            const closure = self.asClosure();
-            const captures_layout = layout_cache.getLayout(closure.captures_layout_idx);
-
-            // Only decref if there are actual captures (record with fields)
-            if (captures_layout.tag == .record) {
-                const record_data = layout_cache.getRecordData(captures_layout.data.record.idx);
-                if (record_data.fields.count > 0) {
-                    if (comptime trace_refcount) {
-                        traceRefcount("DECREF closure ptr=0x{x} captures={}", .{
-                            @intFromPtr(self.ptr),
-                            record_data.fields.count,
-                        });
-                    }
-
-                    // Calculate the offset to the captures record (after header, with alignment)
-                    const header_size = @sizeOf(layout_mod.Closure);
-                    const cap_align = captures_layout.alignment(layout_cache.targetUsize());
-                    const aligned_off = std.mem.alignForward(usize, header_size, @intCast(cap_align.toByteUnits()));
-                    const base_ptr: [*]u8 = @ptrCast(@alignCast(self.ptr.?));
-                    const rec_ptr: *anyopaque = @ptrCast(base_ptr + aligned_off);
-
-                    // Create a StackValue for the captures record and decref it
-                    const captures_value = StackValue{
-                        .layout = captures_layout,
-                        .ptr = rec_ptr,
-                        .is_initialized = true,
-                    };
-                    captures_value.decref(layout_cache, ops);
-                }
-            }
+            decrefLayoutPtr(self.layout, self.ptr, layout_cache, ops);
             return;
         },
         .tag_union => {
@@ -1649,13 +1684,7 @@ pub fn decref(self: StackValue, layout_cache: *LayoutStore, ops: *RocOps) void {
             }
 
             // Decref only the active variant's payload (at offset 0)
-            const payload_value = StackValue{
-                .layout = variant_layout,
-                .ptr = @as(*anyopaque, @ptrCast(base_ptr)),
-                .is_initialized = true,
-            };
-
-            payload_value.decref(layout_cache, ops);
+            decrefLayoutPtr(variant_layout, @as(*anyopaque, @ptrCast(base_ptr)), layout_cache, ops);
             return;
         },
         else => {},

src/eval/comptime_evaluator.zig

@@ -348,16 +348,8 @@ pub const ComptimeEvaluator = struct {
         // Convert StackValue to CIR expression based on layout
         const layout = stack_value.layout;
 
-        // Get the runtime type variable from the StackValue first, or fall back to expression type
-        const rt_var: types_mod.Var = if (stack_value.rt_var) |sv_rt_var|
-            sv_rt_var
-        else blk: {
-            // Fall back to expression type variable
-            const ct_var = ModuleEnv.varFrom(def.expr);
-            break :blk self.interpreter.translateTypeVar(self.env, ct_var) catch {
-                return error.NotImplemented;
-            };
-        };
+        // Get the runtime type variable from the StackValue
+        const rt_var = stack_value.rt_var;
         const resolved = self.interpreter.runtime_types.resolveVar(rt_var);
 
         // Check if it's a tag union type
@@ -471,7 +463,8 @@ pub const ComptimeEvaluator = struct {
 
         // Get variant_var and ext_var
         const variant_var: types_mod.Var = bool_rt_var;
-        var ext_var: types_mod.Var = @enumFromInt(0);
+        // ext_var will be set if this is a tag_union type
+        var ext_var: types_mod.Var = undefined;
 
         if (resolved.desc.content == .structure) {
             if (resolved.desc.content.structure == .tag_union) {
@@ -492,33 +485,33 @@ pub const ComptimeEvaluator = struct {
     /// Fold a tag union (represented as scalar, like Bool) to an e_zero_argument_tag expression
     fn foldTagUnionScalar(self: *ComptimeEvaluator, def_idx: CIR.Def.Idx, expr_idx: CIR.Expr.Idx, stack_value: eval_mod.StackValue) !void {
         _ = def_idx; // unused now that we get rt_var from stack_value
-        // The value is the tag index directly (scalar integer)
+        // The value is the tag index directly (scalar integer).
+        // The caller already verified layout.tag == .scalar, and scalar tag unions are always ints.
+        std.debug.assert(stack_value.layout.tag == .scalar and stack_value.layout.data.scalar.tag == .int);
         const tag_index: usize = @intCast(stack_value.asI128());
 
-        // Get the runtime type variable from the StackValue (already validated in tryFoldConstant)
-        const rt_var = stack_value.rt_var orelse return error.NotImplemented;
+        // Get the runtime type variable from the StackValue
+        const rt_var = stack_value.rt_var;
 
         // Get the list of tags for this union type
         var tag_list = std.array_list.AlignedManaged(types_mod.Tag, null).init(self.allocator);
         defer tag_list.deinit();
         try self.interpreter.appendUnionTags(rt_var, &tag_list);
 
-        if (tag_index >= tag_list.items.len) {
-            return error.NotImplemented;
-        }
+        // Tag index from the value must be valid
+        std.debug.assert(tag_index < tag_list.items.len);
 
         const tag_info = tag_list.items[tag_index];
         const arg_vars = self.interpreter.runtime_types.sliceVars(tag_info.args);
 
-        // Only fold zero-argument tags (like True, False)
-        if (arg_vars.len != 0) {
-            return error.NotImplemented;
-        }
+        // Scalar tag unions don't have payloads, so arg_vars must be empty
+        std.debug.assert(arg_vars.len == 0);
 
         // Get variant_var and ext_var from type information
         const resolved = self.interpreter.runtime_types.resolveVar(rt_var);
         const variant_var: types_mod.Var = rt_var;
-        var ext_var: types_mod.Var = @enumFromInt(0);
+        // ext_var will be set if this is a tag_union type
+        var ext_var: types_mod.Var = undefined;
 
         if (resolved.desc.content == .structure) {
             if (resolved.desc.content.structure == .tag_union) {
@@ -543,17 +536,18 @@ pub const ComptimeEvaluator = struct {
         var acc = try stack_value.asTuple(&self.interpreter.runtime_layout_store);
 
         // Element 1 is the tag discriminant - getElement takes original index directly
-        const tag_field = try acc.getElement(1);
+        const tag_elem_rt_var = try self.interpreter.runtime_types.fresh();
+        const tag_field = try acc.getElement(1, tag_elem_rt_var);
 
         // Extract tag index
         if (tag_field.layout.tag != .scalar or tag_field.layout.data.scalar.tag != .int) {
             return error.NotImplemented;
         }
-        const tmp_sv = eval_mod.StackValue{ .layout = tag_field.layout, .ptr = tag_field.ptr, .is_initialized = true };
+        const tmp_sv = eval_mod.StackValue{ .layout = tag_field.layout, .ptr = tag_field.ptr, .is_initialized = true, .rt_var = tag_elem_rt_var };
         const tag_index: usize = @intCast(tmp_sv.asI128());
 
-        // Get the runtime type variable from the StackValue (already validated in tryFoldConstant)
-        const rt_var = stack_value.rt_var orelse return error.NotImplemented;
+        // Get the runtime type variable from the StackValue
+        const rt_var = stack_value.rt_var;
 
         // Get the list of tags for this union type
         var tag_list = std.array_list.AlignedManaged(types_mod.Tag, null).init(self.allocator);
@@ -575,7 +569,8 @@ pub const ComptimeEvaluator = struct {
         // Get variant_var and ext_var from type information
         const resolved = self.interpreter.runtime_types.resolveVar(rt_var);
         const variant_var: types_mod.Var = rt_var;
-        var ext_var: types_mod.Var = @enumFromInt(0);
+        // ext_var will be set if this is a tag_union type
+        var ext_var: types_mod.Var = undefined;
 
         if (resolved.desc.content == .structure) {
             if (resolved.desc.content.structure == .tag_union) {
@@ -996,7 +991,8 @@ pub const ComptimeEvaluator = struct {
         }
 
         // Build is_negative Bool
-        const is_neg_value = try self.interpreter.pushRaw(layout_mod.Layout.int(.u8), 0);
+        const bool_rt_var = try self.interpreter.getCanonicalBoolRuntimeVar();
+        const is_neg_value = try self.interpreter.pushRaw(layout_mod.Layout.int(.u8), 0, bool_rt_var);
         if (is_neg_value.ptr) |ptr| {
             @as(*u8, @ptrCast(@alignCast(ptr))).* = @intFromBool(num_lit_info.is_negative);
         }
@@ -1132,7 +1128,7 @@ pub const ComptimeEvaluator = struct {
             try self.interpreter.bindings.append(.{
                 .pattern_idx = params[0],
                 .value = num_literal_record,
-                .expr_idx = @enumFromInt(0),
+                .expr_idx = null, // No source expression for synthetic binding
                 .source_env = origin_env,
             });
             defer _ = self.interpreter.bindings.pop();
@@ -1192,7 +1188,8 @@ pub const ComptimeEvaluator = struct {
         const list_layout_idx = try self.interpreter.runtime_layout_store.insertList(layout_mod.Idx.u8);
         const list_layout = self.interpreter.runtime_layout_store.getLayout(list_layout_idx);
 
-        const dest = try self.interpreter.pushRaw(list_layout, 0);
+        // rt_var not needed for List(U8) construction - only layout matters
+        const dest = try self.interpreter.pushRaw(list_layout, 0, undefined);
         if (dest.ptr == null) return dest;
 
         const header: *builtins.list.RocList = @ptrCast(@alignCast(dest.ptr.?));
@@ -1242,7 +1239,8 @@ pub const ComptimeEvaluator = struct {
         const record_layout_idx = try self.interpreter.runtime_layout_store.putRecord(self.env, &field_layouts, &field_names);
         const record_layout = self.interpreter.runtime_layout_store.getLayout(record_layout_idx);
 
-        var dest = try self.interpreter.pushRaw(record_layout, 0);
+        // rt_var not needed for Numeral record construction - only layout matters
+        var dest = try self.interpreter.pushRaw(record_layout, 0, undefined);
         var accessor = try dest.asRecord(&self.interpreter.runtime_layout_store);
 
         // Use self.env for field lookups since the record was built with self.env's idents
@@ -1315,7 +1313,8 @@ pub const ComptimeEvaluator = struct {
             // Use layout store's env for field lookups since records use that env's idents
             const layout_env = self.interpreter.runtime_layout_store.env;
             const tag_idx = accessor.findFieldIndex(layout_env.idents.tag) orelse return true;
-            const tag_field = accessor.getFieldByIndex(tag_idx) catch return true;
+            const tag_rt_var = self.interpreter.runtime_types.fresh() catch return true;
+            const tag_field = accessor.getFieldByIndex(tag_idx, tag_rt_var) catch return true;
 
             if (tag_field.layout.tag == .scalar and tag_field.layout.data.scalar.tag == .int) {
                 const tag_value = tag_field.asI128();
@@ -1343,7 +1342,8 @@ pub const ComptimeEvaluator = struct {
             var accessor = result.asTuple(&self.interpreter.runtime_layout_store) catch return true;
 
             // Element 1 is tag discriminant - getElement takes original index directly
-            const tag_field = accessor.getElement(1) catch return true;
+            const tag_elem_rt_var = self.interpreter.runtime_types.fresh() catch return true;
+            const tag_field = accessor.getElement(1, tag_elem_rt_var) catch return true;
 
             if (tag_field.layout.tag == .scalar and tag_field.layout.data.scalar.tag == .int) {
                 const tag_value = tag_field.asI128();
@@ -1396,7 +1396,10 @@ pub const ComptimeEvaluator = struct {
             // This should never happen - Try type must have a payload field
             return try std.fmt.allocPrint(self.allocator, "Internal error: from_numeral returned malformed Try value (missing payload field)", .{});
         };
-        const payload_field = try_accessor.getFieldByIndex(payload_idx) catch {
+        const payload_rt_var = self.interpreter.runtime_types.fresh() catch {
+            return try std.fmt.allocPrint(self.allocator, "Internal error: from_numeral returned malformed Try value (could not create rt_var)", .{});
+        };
+        const payload_field = try_accessor.getFieldByIndex(payload_idx, payload_rt_var) catch {
             return try std.fmt.allocPrint(self.allocator, "Internal error: from_numeral returned malformed Try value (could not access payload)", .{});
         };
 
@@ -1411,7 +1414,10 @@ pub const ComptimeEvaluator = struct {
             // Check if this has a payload field (for the Str)
             // Single-tag unions might not have a "tag" field, so we look for payload first
             if (err_accessor.findFieldIndex(layout_env.idents.payload)) |err_payload_idx| {
-                const err_payload = err_accessor.getFieldByIndex(err_payload_idx) catch {
+                const err_payload_rt_var = self.interpreter.runtime_types.fresh() catch {
+                    return try std.fmt.allocPrint(self.allocator, "Internal error: could not create rt_var for InvalidNumeral payload", .{});
+                };
+                const err_payload = err_accessor.getFieldByIndex(err_payload_idx, err_payload_rt_var) catch {
                     return try std.fmt.allocPrint(self.allocator, "Internal error: could not access InvalidNumeral payload", .{});
                 };
                 return try self.extractStrFromValue(err_payload);
@@ -1421,7 +1427,8 @@ pub const ComptimeEvaluator = struct {
             // Iterate through fields looking for a Str
             var field_idx: usize = 0;
             while (true) : (field_idx += 1) {
-                const field = err_accessor.getFieldByIndex(field_idx) catch break;
+                const iter_field_rt_var = self.interpreter.runtime_types.fresh() catch break;
+                const field = err_accessor.getFieldByIndex(field_idx, iter_field_rt_var) catch break;
                 if (field.layout.tag == .scalar and field.layout.data.scalar.tag == .str) {
                     return try self.extractStrFromValue(field);
                 }
@@ -1507,8 +1514,12 @@ pub const ComptimeEvaluator = struct {
                         try self.reportProblem(expect_info.message, expect_info.region, .expect_failed);
                     },
                     .error_eval => |error_info| {
-                        const error_name = @errorName(error_info.err);
-                        try self.reportProblem(error_name, error_info.region, .error_eval);
+                        // Provide user-friendly messages for specific errors
+                        const error_message = switch (error_info.err) {
+                            error.DivisionByZero => "Division by zero",
+                            else => @errorName(error_info.err),
+                        };
+                        try self.reportProblem(error_message, error_info.region, .error_eval);
                     },
                 }
             }

src/eval/render_helpers.zig

@@ -130,7 +130,8 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                 const count = tup_acc.getElementCount();
                 if (count > 0) {
                     // Get tag index from the last element
-                    const tag_elem = try tup_acc.getElement(count - 1);
+                    // rt_var not needed for tag discriminant access (it's always an integer)
+                    const tag_elem = try tup_acc.getElement(count - 1, undefined);
                     if (tag_elem.layout.tag == .scalar and tag_elem.layout.data.scalar.tag == .int) {
                         if (std.math.cast(usize, tag_elem.asI128())) |tag_idx| {
                             tag_index = tag_idx;
@@ -150,26 +151,28 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                         if (arg_vars.len == 1) {
                             // Single payload: first element
                             // Get the correct layout from the type variable, not the payload union layout
-                            const payload_elem = try tup_acc.getElement(0);
                             const arg_var = arg_vars[0];
+                            const payload_elem = try tup_acc.getElement(0, arg_var);
                             const layout_idx = try ctx.layout_store.addTypeVar(arg_var, ctx.type_scope);
                             const arg_layout = ctx.layout_store.getLayout(layout_idx);
                             const payload_value = StackValue{
                                 .layout = arg_layout,
                                 .ptr = payload_elem.ptr,
                                 .is_initialized = payload_elem.is_initialized,
+                                .rt_var = arg_var,
                             };
                             const rendered = try renderValueRocWithType(ctx, payload_value, arg_var);
                             defer gpa.free(rendered);
                             try out.appendSlice(rendered);
                         } else {
                             // Multiple payloads: first element is a nested tuple containing all payload args
-                            const payload_elem = try tup_acc.getElement(0);
+                            // rt_var undefined for tuple access (we have the individual element types)
+                            const payload_elem = try tup_acc.getElement(0, undefined);
                             if (payload_elem.layout.tag == .tuple) {
                                 var payload_tup = try payload_elem.asTuple(ctx.layout_store);
                                 var j: usize = 0;
                                 while (j < arg_vars.len) : (j += 1) {
-                                    const elem_value = try payload_tup.getElement(j);
+                                    const elem_value = try payload_tup.getElement(j, arg_vars[j]);
                                     const rendered = try renderValueRocWithType(ctx, elem_value, arg_vars[j]);
                                     defer gpa.free(rendered);
                                     try out.appendSlice(rendered);
@@ -189,9 +192,10 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
             } else if (value.layout.tag == .record) {
                 var acc = try value.asRecord(ctx.layout_store);
                 if (acc.findFieldIndex(ctx.env.idents.tag)) |idx| {
-                    const tag_field = try acc.getFieldByIndex(idx);
+                    const field_rt = try ctx.runtime_types.fresh();
+                    const tag_field = try acc.getFieldByIndex(idx, field_rt);
                     if (tag_field.layout.tag == .scalar and tag_field.layout.data.scalar.tag == .int) {
-                        const tmp_sv = StackValue{ .layout = tag_field.layout, .ptr = tag_field.ptr, .is_initialized = true };
+                        const tmp_sv = StackValue{ .layout = tag_field.layout, .ptr = tag_field.ptr, .is_initialized = true, .rt_var = undefined };
                         // Only treat as tag if value fits in usize (valid tag discriminants are small)
                         if (std.math.cast(usize, tmp_sv.asI128())) |tag_idx| {
                             tag_index = tag_idx;
@@ -205,7 +209,8 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                     errdefer out.deinit();
                     try out.appendSlice(tag_name);
                     if (acc.findFieldIndex(ctx.env.idents.payload)) |pidx| {
-                        const payload = try acc.getFieldByIndex(pidx);
+                        const field_rt = try ctx.runtime_types.fresh();
+                        const payload = try acc.getFieldByIndex(pidx, field_rt);
                         const args_range = tags.items(.args)[tag_index];
                         const arg_vars = ctx.runtime_types.sliceVars(toVarRange(args_range));
                         if (arg_vars.len > 0) {
@@ -218,6 +223,7 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                                     .layout = arg_layout,
                                     .ptr = payload.ptr,
                                     .is_initialized = payload.is_initialized,
+                                    .rt_var = arg_var,
                                 };
                                 const rendered = try renderValueRocWithType(ctx, payload_value, arg_var);
                                 defer gpa.free(rendered);
@@ -237,6 +243,7 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                                     .layout = tuple_layout,
                                     .ptr = payload.ptr,
                                     .is_initialized = payload.is_initialized,
+                                    .rt_var = undefined, // not needed - type known from layout
                                 };
                                 if (tuple_size == 0 or payload.ptr == null) {
                                     var j: usize = 0;
@@ -247,6 +254,7 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                                                 .layout = elem_layouts[j],
                                                 .ptr = null,
                                                 .is_initialized = true,
+                                                .rt_var = arg_vars[j],
                                             },
                                             arg_vars[j],
                                         );
@@ -259,7 +267,7 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                                     var j: usize = 0;
                                     while (j < arg_vars.len) : (j += 1) {
                                         const sorted_idx = tup_acc.findElementIndexByOriginal(j) orelse return error.TypeMismatch;
-                                        const elem_value = try tup_acc.getElement(sorted_idx);
+                                        const elem_value = try tup_acc.getElement(sorted_idx, arg_vars[j]);
                                         const rendered = try renderValueRocWithType(ctx, elem_value, arg_vars[j]);
                                         defer gpa.free(rendered);
                                         try out.appendSlice(rendered);
@@ -308,6 +316,7 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                                 .layout = arg_layout,
                                 .ptr = payload_ptr,
                                 .is_initialized = true,
+                                .rt_var = arg_var,
                             };
                             const rendered = try renderValueRocWithType(ctx, payload_value, arg_var);
                             defer gpa.free(rendered);
@@ -333,6 +342,7 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                                             .layout = elem_layouts[j],
                                             .ptr = null,
                                             .is_initialized = true,
+                                            .rt_var = arg_vars[j],
                                         },
                                         arg_vars[j],
                                     );
@@ -345,12 +355,13 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                                     .layout = tuple_layout,
                                     .ptr = payload_ptr,
                                     .is_initialized = true,
+                                    .rt_var = undefined, // not needed - type known from layout
                                 };
                                 var tup_acc = try tuple_value.asTuple(ctx.layout_store);
                                 var j: usize = 0;
                                 while (j < arg_vars.len) : (j += 1) {
                                     const sorted_idx = tup_acc.findElementIndexByOriginal(j) orelse return error.TypeMismatch;
-                                    const elem_value = try tup_acc.getElement(sorted_idx);
+                                    const elem_value = try tup_acc.getElement(sorted_idx, arg_vars[j]);
                                     const rendered = try renderValueRocWithType(ctx, elem_value, arg_vars[j]);
                                     defer gpa.free(rendered);
                                     try out.appendSlice(rendered);
@@ -383,6 +394,7 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                     .layout = payload_layout,
                     .ptr = null,
                     .is_initialized = true,
+                    .rt_var = payload_var,
                 };
 
                 switch (value.layout.tag) {
@@ -464,7 +476,8 @@ pub fn renderValueRocWithType(ctx: *RenderCtx, value: StackValue, rt_var: types.
                     const idx = acc.findFieldIndex(f.name) orelse {
                         std.debug.panic("Record field not found in layout: type says field '{s}' exists but layout doesn't have it", .{name_text});
                     };
-                    const field_val = try acc.getFieldByIndex(idx);
+                    const field_rt = try ctx.runtime_types.fresh();
+                    const field_val = try acc.getFieldByIndex(idx, field_rt);
                     const rendered = try renderValueRocWithType(ctx, field_val, f.var_);
                     defer gpa.free(rendered);
                     try out.appendSlice(rendered);
@@ -537,7 +550,8 @@ pub fn renderValueRoc(ctx: *RenderCtx, value: StackValue) ![]u8 {
         const count = acc.getElementCount();
         var i: usize = 0;
         while (i < count) : (i += 1) {
-            const elem = try acc.getElement(i);
+            // rt_var undefined (no type info available in this context)
+            const elem = try acc.getElement(i, undefined);
             const rendered = try renderValueRoc(ctx, elem);
             defer gpa.free(rendered);
             try out.appendSlice(rendered);
@@ -560,7 +574,7 @@ pub fn renderValueRoc(ctx: *RenderCtx, value: StackValue) ![]u8 {
             while (i < len) : (i += 1) {
                 if (roc_list.bytes) |bytes| {
                     const elem_ptr: *anyopaque = @ptrCast(bytes + i * elem_size);
-                    const elem_val = StackValue{ .layout = elem_layout, .ptr = elem_ptr, .is_initialized = true };
+                    const elem_val = StackValue{ .layout = elem_layout, .ptr = elem_ptr, .is_initialized = true, .rt_var = undefined };
                     const rendered = try renderValueRoc(ctx, elem_val);
                     defer gpa.free(rendered);
                     try out.appendSlice(rendered);
@@ -601,7 +615,7 @@ pub fn renderValueRoc(ctx: *RenderCtx, value: StackValue) ![]u8 {
             const field_layout = ctx.layout_store.getLayout(fld.layout);
             const base_ptr: [*]u8 = @ptrCast(@alignCast(value.ptr.?));
             const field_ptr: *anyopaque = @ptrCast(base_ptr + offset);
-            const field_val = StackValue{ .layout = field_layout, .ptr = field_ptr, .is_initialized = true };
+            const field_val = StackValue{ .layout = field_layout, .ptr = field_ptr, .is_initialized = true, .rt_var = undefined };
             const rendered = try renderValueRoc(ctx, field_val);
             defer gpa.free(rendered);
             try out.appendSlice(rendered);

src/eval/test/comptime_eval_test.zig

@@ -1763,3 +1763,66 @@ test "comptime eval - to_str on unbound number literal" {
     // Flex var defaults to Dec; Dec.to_str is provided by builtins
     try testing.expectEqual(@as(usize, 0), result.problems.len());
 }
+
+// --- Division by zero tests ---
+
+test "comptime eval - division by zero produces error" {
+    const src =
+        \\x = 5 // 0
+    ;
+
+    var result = try parseCheckAndEvalModule(src);
+    defer cleanupEvalModule(&result);
+
+    const summary = try result.evaluator.evalAll();
+
+    // Should evaluate 1 declaration with no crashes (it's an error, not a crash)
+    try testing.expectEqual(@as(u32, 1), summary.evaluated);
+    try testing.expectEqual(@as(u32, 0), summary.crashed);
+
+    // Should have 1 problem reported (division by zero)
+    try testing.expect(result.problems.len() >= 1);
+    try testing.expect(errorContains(result.problems, "Division by zero"));
+}
+
+test "comptime eval - division by zero in expression" {
+    const src =
+        \\a = 10
+        \\b = 0
+        \\c = a // b
+    ;
+
+    var result = try parseCheckAndEvalModule(src);
+    defer cleanupEvalModule(&result);
+
+    const summary = try result.evaluator.evalAll();
+
+    // Should evaluate 3 declarations, c will cause an error
+    try testing.expectEqual(@as(u32, 3), summary.evaluated);
+
+    // Should have 1 problem reported (division by zero)
+    try testing.expect(result.problems.len() >= 1);
+    try testing.expect(errorContains(result.problems, "Division by zero"));
+}
+
+test "comptime eval - modulo by zero produces error" {
+    const src =
+        \\x = 10 % 0
+    ;
+
+    var result = try parseCheckAndEvalModule(src);
+    defer cleanupEvalModule(&result);
+
+    const summary = try result.evaluator.evalAll();
+
+    // Should evaluate 1 declaration
+    try testing.expectEqual(@as(u32, 1), summary.evaluated);
+
+    // Should have 1 problem reported (division by zero for modulo)
+    try testing.expect(result.problems.len() >= 1);
+    try testing.expect(errorContains(result.problems, "Division by zero"));
+}
+
+// Note: "division by zero does not halt other defs" test is skipped because
+// the interpreter state after an eval error may not allow continuing evaluation
+// of subsequent definitions that share the same evaluation context.

src/eval/test/eval_test.zig

@@ -1387,3 +1387,25 @@ test "if block with local bindings - regression" {
         \\else 99
     , 0, .no_trace);
 }
+
+test "List.len returns proper U64 nominal type for method calls - regression" {
+    // Regression test for InvalidMethodReceiver when calling methods on List.len result
+    // Bug report: `n = List.len([]); _str = n.to_str()` crashed with InvalidMethodReceiver
+    // The issue was that List.len created a fresh runtime type variable instead of using
+    // the return_rt_var parameter, which prevented method resolution from finding the
+    // U64 nominal type information needed to look up .to_str()
+    try runExpectStr(
+        \\{
+        \\    n = List.len([])
+        \\    n.to_str()
+        \\}
+    , "0", .no_trace);
+
+    // Also test with non-empty list
+    try runExpectStr(
+        \\{
+        \\    n = List.len([1, 2, 3])
+        \\    n.to_str()
+        \\}
+    , "3", .no_trace);
+}

src/eval/test/helpers.zig

@@ -331,7 +331,8 @@ pub fn runExpectTuple(src: []const u8, expected_elements: []const ExpectedElemen
 
     for (expected_elements) |expected_element| {
         // Get the element at the specified index
-        const element = try tuple_accessor.getElement(@intCast(expected_element.index));
+        // Use the result's rt_var since we're accessing elements of the evaluated expression
+        const element = try tuple_accessor.getElement(@intCast(expected_element.index), result.rt_var);
 
         // Check if this is an integer or Dec
         try std.testing.expect(element.layout.tag == .scalar);
@@ -397,6 +398,7 @@ pub fn runExpectRecord(src: []const u8, expected_fields: []const ExpectedField,
                     .layout = field_layout,
                     .ptr = field_ptr,
                     .is_initialized = true,
+                    .rt_var = result.rt_var, // use result's rt_var for field access
                 };
                 // Check if this is an integer or Dec
                 const int_val = if (field_layout.data.scalar.tag == .int) blk: {
@@ -453,7 +455,8 @@ pub fn runExpectListI64(src: []const u8, expected_elements: []const i64, should_
     try std.testing.expectEqual(expected_elements.len, list_accessor.len());
 
     for (expected_elements, 0..) |expected_val, i| {
-        const element = try list_accessor.getElement(i);
+        // Use the result's rt_var since we're accessing elements of the evaluated expression
+        const element = try list_accessor.getElement(i, result.rt_var);
 
         // Check if this is an integer
         try std.testing.expect(element.layout.tag == .scalar);

src/eval/test/stack_test.zig

@@ -17,10 +17,10 @@ test "Stack.alloca basic allocation" {
     var stack = try Stack.initCapacity(std.testing.allocator, 1024);
     defer stack.deinit();
 
-    const ptr1 = try stack.alloca(10, @enumFromInt(0));
+    const ptr1 = try stack.alloca(10, .@"1");
     try std.testing.expectEqual(@as(u32, 10), stack.used);
 
-    const ptr2 = try stack.alloca(20, @enumFromInt(0));
+    const ptr2 = try stack.alloca(20, .@"1");
     try std.testing.expectEqual(@as(u32, 30), stack.used);
 
     // The pointers should be different
@@ -42,7 +42,7 @@ test "Stack.alloca with alignment" {
 
         // Create initial misalignment
         if (misalign > 0) {
-            _ = try stack.alloca(@intCast(misalign), @enumFromInt(0));
+            _ = try stack.alloca(@intCast(misalign), .@"1");
         }
 
         // Test each alignment with the current misalignment
@@ -70,7 +70,7 @@ test "Stack.alloca with alignment" {
     stack.used = 0;
     for (alignments) |alignment| {
         // Create some misalignment
-        _ = try stack.alloca(3, @enumFromInt(0));
+        _ = try stack.alloca(3, .@"1");
 
         const before_used = stack.used;
         const ptr = try stack.alloca(alignment * 2, @enumFromInt(std.math.log2_int(u32, alignment)));
@@ -88,10 +88,10 @@ test "Stack.alloca overflow" {
     defer stack.deinit();
 
     // This should succeed
-    _ = try stack.alloca(50, @enumFromInt(0));
+    _ = try stack.alloca(50, .@"1");
 
     // This should fail (would total 150 bytes)
-    try std.testing.expectError(StackOverflow.StackOverflow, stack.alloca(100, @enumFromInt(0)));
+    try std.testing.expectError(StackOverflow.StackOverflow, stack.alloca(100, .@"1"));
 
     // Stack should still be in valid state
     try std.testing.expectEqual(@as(u32, 50), stack.used);
@@ -102,14 +102,14 @@ test "Stack.restore" {
     defer stack.deinit();
 
     const checkpoint = stack.next();
-    _ = try stack.alloca(100, @enumFromInt(0));
+    _ = try stack.alloca(100, .@"1");
     try std.testing.expectEqual(@as(u32, 100), stack.used);
 
     stack.restore(checkpoint);
     try std.testing.expectEqual(@as(u32, 0), stack.used);
 
     // Allocate again after restore
-    const ptr1 = try stack.alloca(50, @enumFromInt(0));
+    const ptr1 = try stack.alloca(50, .@"1");
     try std.testing.expectEqual(@intFromPtr(checkpoint), @intFromPtr(ptr1));
 }
 
@@ -120,7 +120,7 @@ test "Stack.isEmpty" {
     try std.testing.expect(stack.isEmpty());
     try std.testing.expectEqual(@as(u32, 100), stack.available());
 
-    _ = try stack.alloca(30, @enumFromInt(0));
+    _ = try stack.alloca(30, .@"1");
     try std.testing.expect(!stack.isEmpty());
     try std.testing.expectEqual(@as(u32, 70), stack.available());
 }
@@ -129,8 +129,8 @@ test "Stack zero-size allocation" {
     var stack = try Stack.initCapacity(std.testing.allocator, 100);
     defer stack.deinit();
 
-    const ptr1 = try stack.alloca(0, @enumFromInt(0));
-    const ptr2 = try stack.alloca(0, @enumFromInt(0));
+    const ptr1 = try stack.alloca(0, .@"1");
+    const ptr2 = try stack.alloca(0, .@"1");
 
     // Zero-size allocations should return the same pointer
     try std.testing.expectEqual(@intFromPtr(ptr1), @intFromPtr(ptr2));
@@ -147,8 +147,8 @@ test "Stack memory is aligned to max_roc_alignment" {
     try std.testing.expectEqual(@as(usize, 0), start_addr % max_alignment_value);
 
     // Also verify after some allocations
-    _ = try stack.alloca(100, @enumFromInt(0));
-    _ = try stack.alloca(200, @enumFromInt(0));
+    _ = try stack.alloca(100, .@"1");
+    _ = try stack.alloca(200, .@"1");
 
     // The start pointer should still be aligned
     try std.testing.expectEqual(@as(usize, 0), start_addr % max_alignment_value);

src/flake.nix

@@ -34,7 +34,7 @@
           testcmd() {
             zig build snapshot && zig build test
           }
-          export -f testscmd
+          export -f testcmd
 
           fmtcmd() {
             zig build fmt

src/fmt/fmt.zig

@@ -1353,6 +1353,11 @@ const Formatter = struct {
                 region = i.region;
                 try fmt.formatIdent(i.ident_tok, null);
             },
+            .var_ident => |i| {
+                region = i.region;
+                try fmt.pushAll("var ");
+                try fmt.formatIdent(i.ident_tok, null);
+            },
             .tag => |t| {
                 region = t.region;
 

src/ipc/coordination.zig

@@ -100,9 +100,9 @@ fn readFdInfoFromFile(allocator: std.mem.Allocator) CoordinationError!FdInfo {
     };
     const dir_basename = std.fs.path.basename(exe_dir);
 
-    // Verify it has the expected prefix
-    if (!std.mem.startsWith(u8, dir_basename, "roc-tmp-")) {
-        std.log.err("Unexpected directory name: expected 'roc-tmp-*', got '{s}'", .{dir_basename});
+    // Verify it has the expected prefix (roc-{pid} or roc-{pid}-{suffix})
+    if (!std.mem.startsWith(u8, dir_basename, "roc-")) {
+        std.log.err("Unexpected directory name: expected 'roc-*', got '{s}'", .{dir_basename});
         return error.FdInfoReadFailed;
     }
 

src/ipc/platform.zig

@@ -310,11 +310,21 @@ pub fn mapMemory(handle: Handle, size: usize, base_addr: ?*anyopaque) SharedMemo
                 posix.MAP_SHARED,
                 handle,
                 0,
-            ) orelse {
-                std.log.err("POSIX: Failed to map shared memory (size: {})", .{size});
+            );
+            // mmap returns MAP_FAILED (which is (void *)-1) on error, not null
+            // Need to check for both null and MAP_FAILED
+            if (ptr == null) {
+                std.log.err("POSIX: Failed to map shared memory - null returned (size: {})", .{size});
                 return error.MmapFailed;
-            };
-            return ptr;
+            }
+            const ptr_value = @intFromPtr(ptr.?);
+            if (ptr_value == std.math.maxInt(usize)) {
+                // This is MAP_FAILED (-1 cast to pointer)
+                const errno = std.c._errno().*;
+                std.log.err("POSIX: Failed to map shared memory - MAP_FAILED (size: {}, fd: {}, errno: {})", .{ size, handle, errno });
+                return error.MmapFailed;
+            }
+            return ptr.?;
         },
         else => return error.UnsupportedPlatform,
     }

src/parse/AST.zig

@@ -1242,6 +1242,11 @@ pub const Pattern = union(enum) {
         ident_tok: Token.Idx,
         region: TokenizedRegion,
     },
+    /// A mutable variable binding in a pattern, e.g., `var $x` in `|var $x, y|`
+    var_ident: struct {
+        ident_tok: Token.Idx,
+        region: TokenizedRegion,
+    },
     tag: struct {
         tag_tok: Token.Idx,
         args: Pattern.Span,
@@ -1305,6 +1310,7 @@ pub const Pattern = union(enum) {
     pub fn to_tokenized_region(self: @This()) TokenizedRegion {
         return switch (self) {
             .ident => |p| p.region,
+            .var_ident => |p| p.region,
             .tag => |p| p.region,
             .int => |p| p.region,
             .frac => |p| p.region,
@@ -1339,6 +1345,21 @@ pub const Pattern = union(enum) {
 
                 try tree.endNode(begin, attrs);
             },
+            .var_ident => |ident| {
+                const begin = tree.beginNode();
+                try tree.pushStaticAtom("p-var-ident");
+                try ast.appendRegionInfoToSexprTree(env, tree, ident.region);
+
+                // Add raw attribute
+                const raw_begin = tree.beginNode();
+                try tree.pushStaticAtom("raw");
+                try tree.pushString(ast.resolve(ident.ident_tok));
+                const attrs2 = tree.beginNode();
+                try tree.endNode(raw_begin, attrs2);
+                const attrs = tree.beginNode();
+
+                try tree.endNode(begin, attrs);
+            },
             .tag => |tag| {
                 const begin = tree.beginNode();
                 try tree.pushStaticAtom("p-tag");

src/parse/Node.zig

@@ -250,6 +250,10 @@ pub const Tag = enum {
     /// * lhs - LHS DESCRIPTION
     /// * rhs - RHS DESCRIPTION
     ident_patt,
+    /// Mutable variable binding in pattern
+    /// Example: `var $x` in `|var $x, y|`
+    /// * main_token - the identifier token
+    var_ident_patt,
     /// DESCRIPTION
     /// Example: EXAMPLE
     /// * lhs - LHS DESCRIPTION

src/parse/NodeStore.zig

@@ -21,6 +21,9 @@ const sexpr = base.sexpr;
 /// packing optional data into u32 fields where 0 would otherwise be ambiguous.
 const OPTIONAL_VALUE_OFFSET: u32 = 1;
 
+/// The root node is always stored at index 0 in the node list.
+pub const root_node_idx: Node.List.Idx = .first;
+
 const NodeStore = @This();
 
 gpa: std.mem.Allocator,
@@ -46,7 +49,7 @@ pub const AST_HEADER_NODE_COUNT = 6;
 /// Count of the statement nodes in the AST
 pub const AST_STATEMENT_NODE_COUNT = 13;
 /// Count of the pattern nodes in the AST
-pub const AST_PATTERN_NODE_COUNT = 14;
+pub const AST_PATTERN_NODE_COUNT = 15;
 /// Count of the type annotation nodes in the AST
 pub const AST_TYPE_ANNO_NODE_COUNT = 10;
 /// Count of the expression nodes in the AST
@@ -166,7 +169,7 @@ pub fn addMalformed(store: *NodeStore, comptime T: type, reason: Diagnostic.Tag,
 /// Adds a file node to the store.
 pub fn addFile(store: *NodeStore, file: AST.File) std.mem.Allocator.Error!void {
     try store.extra_data.append(store.gpa, @intFromEnum(file.header));
-    store.nodes.set(@enumFromInt(0), .{
+    store.nodes.set(root_node_idx, .{
         .tag = .root,
         .main_token = 0,
         .data = .{ .lhs = file.statements.span.start, .rhs = file.statements.span.len },
@@ -478,6 +481,11 @@ pub fn addPattern(store: *NodeStore, pattern: AST.Pattern) std.mem.Allocator.Err
             node.region = i.region;
             node.main_token = i.ident_tok;
         },
+        .var_ident => |i| {
+            node.tag = .var_ident_patt;
+            node.region = i.region;
+            node.main_token = i.ident_tok;
+        },
         .tag => |t| {
             const data_start = @as(u32, @intCast(store.extra_data.items.len));
             try store.extra_data.append(store.gpa, t.args.span.len);
@@ -1014,7 +1022,7 @@ pub fn addTypeAnno(store: *NodeStore, anno: AST.TypeAnno) std.mem.Allocator.Erro
 
 /// TODO
 pub fn getFile(store: *const NodeStore) AST.File {
-    const node = store.nodes.get(@enumFromInt(0));
+    const node = store.nodes.get(root_node_idx);
     const header_ed_idx = @as(usize, @intCast(node.data.lhs + node.data.rhs));
     const header = store.extra_data.items[header_ed_idx];
     return .{
@@ -1387,6 +1395,12 @@ pub fn getPattern(store: *const NodeStore, pattern_idx: AST.Pattern.Idx) AST.Pat
                 .region = node.region,
             } };
         },
+        .var_ident_patt => {
+            return .{ .var_ident = .{
+                .ident_tok = node.main_token,
+                .region = node.region,
+            } };
+        },
         .tag_patt => {
             const args_start = node.data.lhs;
 

src/parse/Parser.zig

@@ -197,7 +197,7 @@ pub fn parseFile(self: *Parser) Error!void {
 
     self.store.emptyScratch();
     try self.store.addFile(.{
-        .header = @as(AST.Header.Idx, @enumFromInt(0)),
+        .header = undefined, // overwritten below after parseHeader()
         .statements = AST.Statement.Span{ .span = base.DataSpan.empty() },
         .region = AST.TokenizedRegion.empty(),
     });
@@ -1452,6 +1452,19 @@ pub fn parsePattern(self: *Parser, alternatives: Alternatives) Error!AST.Pattern
                     .region = .{ .start = start, .end = self.pos },
                 } });
             },
+            .KwVar => {
+                // Mutable variable binding in pattern, e.g., `var $x`
+                self.advance();
+                if (self.peek() != .LowerIdent) {
+                    return try self.pushMalformed(AST.Pattern.Idx, .var_must_have_ident, self.pos);
+                }
+                const ident_tok = self.pos;
+                self.advance();
+                pattern = try self.store.addPattern(.{ .var_ident = .{
+                    .ident_tok = ident_tok,
+                    .region = .{ .start = start, .end = self.pos },
+                } });
+            },
             .NamedUnderscore => {
                 self.advance();
                 pattern = try self.store.addPattern(.{ .ident = .{
@@ -2069,9 +2082,6 @@ pub fn parseExprWithBp(self: *Parser, min_bp: u8) Error!AST.Expr.Idx {
                             },
                         }
                         lookahead_pos += 1;
-
-                        // Limit lookahead to prevent infinite loops
-                        if (lookahead_pos > saved_pos + 100) break;
                     }
                 }
 

src/parse/test/ast_node_store_test.zig

@@ -281,6 +281,12 @@ test "NodeStore round trip - Pattern" {
             .region = rand_region(),
         },
     });
+    try patterns.append(gpa, AST.Pattern{
+        .var_ident = .{
+            .ident_tok = rand_token_idx(),
+            .region = rand_region(),
+        },
+    });
     try patterns.append(gpa, AST.Pattern{
         .tag = .{
             .args = AST.Pattern.Span{ .span = rand_span() },

src/repl/eval.zig

@@ -855,16 +855,7 @@ pub const Repl = struct {
             try self.generateAndStoreDebugHtml(module_env, final_expr_idx);
         }
 
-        const output = blk: {
-            if (result.rt_var) |rt_var| {
-                break :blk try interpreter.renderValueRocWithType(result, rt_var, self.roc_ops);
-            }
-            const expr_ct_var = can.ModuleEnv.varFrom(final_expr_idx);
-            const expr_rt_var = interpreter.translateTypeVar(module_env, expr_ct_var) catch {
-                break :blk try interpreter.renderValueRoc(result);
-            };
-            break :blk try interpreter.renderValueRocWithType(result, expr_rt_var, self.roc_ops);
-        };
+        const output = try interpreter.renderValueRocWithType(result, result.rt_var, self.roc_ops);
 
         result.decref(&interpreter.runtime_layout_store, self.roc_ops);
         return .{ .expression = output };

src/types/TypeWriter.zig

@@ -9,6 +9,7 @@ const std = @import("std");
 const base = @import("base");
 const types_mod = @import("types.zig");
 const import_mapping_mod = @import("import_mapping.zig");
+const debug = @import("debug.zig");
 
 const TypesStore = @import("store.zig").Store;
 const Allocator = std.mem.Allocator;
@@ -610,7 +611,9 @@ fn gatherRecordFields(self: *TypeWriter, fields: RecordField.SafeMultiList.Range
     }
 
     var ext = initial_ext;
+    var guard = debug.IterationGuard.init("TypeWriter.gatherRecordFields");
     while (true) {
+        guard.tick();
         const resolved = self.types.resolveVar(ext);
         switch (resolved.desc.content) {
             .flex => |flex| {

src/types/debug.zig

@@ -0,0 +1,70 @@
+//! Debug utilities for type checking
+//!
+//! These utilities are only active in debug builds and help catch infinite loops
+//! in type-checking code by limiting the number of iterations.
+
+const std = @import("std");
+const builtin = @import("builtin");
+
+/// Maximum number of iterations before panicking in debug builds.
+/// This is set high enough to handle legitimate complex types but low enough
+/// to catch infinite loops quickly during development.
+pub const MAX_ITERATIONS: u32 = 100_000;
+
+/// A debug-only iteration guard that panics if a loop exceeds MAX_ITERATIONS.
+/// In release builds, this is a no-op.
+///
+/// Usage:
+/// ```
+/// var guard = IterationGuard.init("myFunction");
+/// while (condition) {
+///     guard.tick();
+///     // ... loop body
+/// }
+/// ```
+pub const IterationGuard = struct {
+    count: u32,
+    location: []const u8,
+
+    const Self = @This();
+
+    pub fn init(location: []const u8) Self {
+        return .{
+            .count = 0,
+            .location = location,
+        };
+    }
+
+    /// Call this at the start of each loop iteration.
+    /// In debug builds, panics if MAX_ITERATIONS is exceeded.
+    /// In release builds, this is a no-op that should be optimized away.
+    pub inline fn tick(self: *Self) void {
+        if (builtin.mode == .Debug) {
+            self.count += 1;
+            if (self.count > MAX_ITERATIONS) {
+                std.debug.panic(
+                    "Infinite loop detected in type-checking at '{s}' after {d} iterations. " ++
+                        "This usually indicates a cyclic type or bug in the type checker.",
+                    .{ self.location, self.count },
+                );
+            }
+        }
+    }
+
+    /// Returns the current iteration count (useful for debugging).
+    pub fn getCount(self: *const Self) u32 {
+        return self.count;
+    }
+};
+
+test "IterationGuard does not panic for normal iteration counts" {
+    var guard = IterationGuard.init("test");
+    var i: u32 = 0;
+    while (i < 1000) : (i += 1) {
+        guard.tick();
+    }
+    // In release builds, tick() is a no-op so count stays at 0.
+    // In debug builds, count should be 1000.
+    const expected: u32 = if (builtin.mode == .Debug) 1000 else 0;
+    try std.testing.expectEqual(expected, guard.getCount());
+}

src/types/generalize.zig

@@ -205,12 +205,18 @@ pub const Generalizer = struct {
                 if (@intFromEnum(resolved.desc.rank) < rank_to_generalize_int) {
                     // Rank was lowered during adjustment - variable escaped
                     try var_pool.addVarToRank(resolved.var_, resolved.desc.rank);
-                } else if (self.hasNumeralConstraint(resolved.desc.content)) {
-                    // Flex var with numeric constraint - don't generalize.
+                } else if (rank_to_generalize_int == @intFromEnum(Rank.top_level) and self.hasNumeralConstraint(resolved.desc.content)) {
+                    // Flex var with numeric constraint at TOP LEVEL - don't generalize.
                     // This ensures numeric literals like `x = 15` stay monomorphic so that
                     // later usage like `I64.to_str(x)` can constrain x to I64.
                     // Without this, let-generalization would create a fresh copy at each use,
                     // leaving the original as an unconstrained flex var that defaults to Dec.
+                    //
+                    // However, at rank > top_level (inside lambdas OR inside nested blocks),
+                    // we DO generalize numeric literals. This allows:
+                    // - Polymorphic functions like `|a| a + 1` to work correctly
+                    // - Numeric literals in blocks like `{ n = 42; use_as_i64(n); use_as_dec(n) }`
+                    //   to be used polymorphically within that block's scope.
                     try var_pool.addVarToRank(resolved.var_, resolved.desc.rank);
                 } else {
                     // Rank unchanged - safe to generalize

src/types/mod.zig

@@ -12,6 +12,7 @@ pub const store = @import("store.zig");
 pub const instantiate = @import("instantiate.zig");
 pub const generalize = @import("generalize.zig");
 pub const import_mapping = @import("import_mapping.zig");
+pub const debug = @import("debug.zig");
 
 pub const TypeWriter = @import("TypeWriter.zig");
 

src/types/store.zig

@@ -7,6 +7,7 @@ const collections = @import("collections");
 const serialization = @import("serialization");
 
 const types = @import("types.zig");
+const debug = @import("debug.zig");
 
 const Allocator = std.mem.Allocator;
 const Desc = types.Descriptor;
@@ -588,7 +589,9 @@ pub const Store = struct {
         if (initial_var != redirected_root_var) {
             var compressed_slot_idx = Self.varToSlotIdx(initial_var);
             var compressed_slot: Slot = self.slots.get(compressed_slot_idx);
+            var guard = debug.IterationGuard.init("resolveVarAndCompressPath");
             while (true) {
+                guard.tick();
                 switch (compressed_slot) {
                     .redirect => |next_redirect_var| {
                         self.slots.set(compressed_slot_idx, Slot{ .redirect = redirected_root_var });
@@ -610,8 +613,10 @@ pub const Store = struct {
         var redirected_slot: Slot = self.slots.get(redirected_slot_idx);
 
         var is_root = true;
+        var guard = debug.IterationGuard.init("resolveVar");
 
         while (true) {
+            guard.tick();
             switch (redirected_slot) {
                 .redirect => |next_redirect_var| {
                     redirected_slot_idx = Self.varToSlotIdx(next_redirect_var);
@@ -1006,7 +1011,10 @@ const SlotStore = struct {
     }
 
     /// A type-safe index into the store
-    const Idx = enum(u32) { _ };
+    const Idx = enum(u32) {
+        first = 0,
+        _,
+    };
 };
 
 /// Represents a store of descriptors
@@ -1109,7 +1117,10 @@ const DescStore = struct {
 
     /// A type-safe index into the store
     /// This type is made public below
-    const Idx = enum(u32) { _ };
+    const Idx = enum(u32) {
+        first = 0,
+        _,
+    };
 };
 
 /// An index into the desc store
@@ -1385,13 +1396,27 @@ test "SlotStore.Serialized roundtrip" {
     const gpa = std.testing.allocator;
     const CompactWriter = collections.CompactWriter;
 
+    // Use a real Store to get real Var and DescStore.Idx values
+    var store = try Store.init(gpa);
+    defer store.deinit();
+
+    // Create real type variables - fresh() creates a flex var with a root slot
+    const var_a = try store.fresh();
+    const var_b = try store.fresh();
+    const var_c = try store.fresh();
+
+    // Get the DescStore.Idx from the root slots
+    const desc_idx_a = store.getSlot(var_a).root;
+    const desc_idx_c = store.getSlot(var_c).root;
+
+    // Create a separate SlotStore for serialization testing
     var slot_store = try SlotStore.init(gpa, 4);
     defer slot_store.deinit(gpa);
 
-    // Add some slots
-    _ = try slot_store.insert(gpa, .{ .root = @enumFromInt(100) });
-    _ = try slot_store.insert(gpa, .{ .redirect = @enumFromInt(0) });
-    _ = try slot_store.insert(gpa, .{ .root = @enumFromInt(200) });
+    // Add slots and capture returned indices
+    const slot_a = try slot_store.insert(gpa, .{ .root = desc_idx_a });
+    const slot_b = try slot_store.insert(gpa, .{ .redirect = var_b });
+    const slot_c = try slot_store.insert(gpa, .{ .root = desc_idx_c });
 
     // Create temp file
     var tmp_dir = std.testing.tmpDir(.{});
@@ -1424,11 +1449,11 @@ test "SlotStore.Serialized roundtrip" {
     const deser_ptr = @as(*SlotStore.Serialized, @ptrCast(@alignCast(buffer.ptr)));
     const deserialized = deser_ptr.deserialize(@as(i64, @intCast(@intFromPtr(buffer.ptr))));
 
-    // Verify
+    // Verify using captured indices
     try std.testing.expectEqual(@as(u64, 3), deserialized.backing.len());
-    try std.testing.expectEqual(Slot{ .root = @enumFromInt(100) }, deserialized.get(@enumFromInt(0)));
-    try std.testing.expectEqual(Slot{ .redirect = @enumFromInt(0) }, deserialized.get(@enumFromInt(1)));
-    try std.testing.expectEqual(Slot{ .root = @enumFromInt(200) }, deserialized.get(@enumFromInt(2)));
+    try std.testing.expectEqual(Slot{ .root = desc_idx_a }, deserialized.get(slot_a));
+    try std.testing.expectEqual(Slot{ .redirect = var_b }, deserialized.get(slot_b));
+    try std.testing.expectEqual(Slot{ .root = desc_idx_c }, deserialized.get(slot_c));
 }
 
 test "DescStore.Serialized roundtrip" {
@@ -1438,7 +1463,7 @@ test "DescStore.Serialized roundtrip" {
     var desc_store = try DescStore.init(gpa, 4);
     defer desc_store.deinit(gpa);
 
-    // Add some descriptors
+    // Add some descriptors and capture returned indices
     const desc1 = Descriptor{
         .content = Content{ .flex = Flex.init() },
         .rank = Rank.generalized,
@@ -1450,8 +1475,8 @@ test "DescStore.Serialized roundtrip" {
         .mark = Mark.visited,
     };
 
-    _ = try desc_store.insert(gpa, desc1);
-    _ = try desc_store.insert(gpa, desc2);
+    const desc_idx_1 = try desc_store.insert(gpa, desc1);
+    const desc_idx_2 = try desc_store.insert(gpa, desc2);
 
     // Create temp file
     var tmp_dir = std.testing.tmpDir(.{});
@@ -1489,10 +1514,10 @@ test "DescStore.Serialized roundtrip" {
     const deserialized = deser_ptr.deserialize(@as(i64, @intCast(@intFromPtr(buffer.ptr))));
     // Note: deserialize already handles relocation, don't call relocate again
 
-    // Verify
+    // Verify using captured indices
     try std.testing.expectEqual(@as(usize, 2), deserialized.backing.items.len);
-    try std.testing.expectEqual(desc1, deserialized.get(@enumFromInt(0)));
-    try std.testing.expectEqual(desc2, deserialized.get(@enumFromInt(1)));
+    try std.testing.expectEqual(desc1, deserialized.get(desc_idx_1));
+    try std.testing.expectEqual(desc2, deserialized.get(desc_idx_2));
 }
 
 test "Store.Serialized roundtrip" {

test/fx-open/app_with_custom_error.roc

@@ -5,9 +5,9 @@ import pf.Stdout
 # Test: both Exit and CustomError in different branches
 # This triggers the type error
 main! = |args| {
-    if List.is_empty(args) {
-        Err(Exit(42))
-    } else {
-        Err(CustomError)
-    }
+	if List.is_empty(args) {
+		Err(Exit(42))
+	} else {
+		Err(CustomError)
+	}
 }

test/fx-open/platform/main.roc

@@ -13,5 +13,8 @@ main_for_host! = |args|
     match main!(args) {
         Ok({}) => 0
         Err(Exit(code)) => code
-        _ => 1
+        Err(other) => {
+            Stderr.line!("exited with other error: ${inspect other})
+            1
+        }
     }

test/fx/division_by_zero.roc

@@ -0,0 +1,14 @@
+app [main!] { pf: platform "./platform/main.roc" }
+
+import pf.Stdout
+
+# Use a mutable variable to prevent compile-time evaluation
+main! = || {
+    # The var keyword creates a runtime variable that can't be constant-folded
+    var $divisor = 0
+
+    # This will trigger a division by zero error at runtime
+    result = 42 / $divisor
+
+    Stdout.line!("Result: ${U64.to_str(result)}")
+}

test/fx/platform/host.zig

@@ -1,13 +1,233 @@
-///! Platform host that tests effectful functions writing to stdout and stderr.
+//! Platform host for testing effectful Roc applications.
+//!
+//! This host provides stdin/stdout/stderr effects and includes a test mode for
+//! verifying IO behavior without performing actual syscalls.
+//!
+//! ## Test Mode
+//!
+//! Run with `--test <spec>` to simulate IO and verify behavior:
+//! ```
+//! ./zig-out/bin/roc app.roc -- --test "1>Hello, world!"
+//! ```
+//!
+//! Spec format uses pipe-separated operations:
+//! - `0<input` - provide "input" as stdin
+//! - `1>output` - expect "output" on stdout
+//! - `2>output` - expect "output" on stderr
+//!
+//! Example with multiple operations:
+//! ```
+//! --test "0<user input|1>Before stdin|1>After stdin"
+//! ```
+//!
+//! Use `--test-verbose <spec>` for detailed output during test execution.
+//!
+//! Exit codes:
+//! - 0: All expectations matched in order
+//! - 1: Test failed (mismatch, missing output, extra output, or invalid spec)
 const std = @import("std");
+const builtin = @import("builtin");
 const builtins = @import("builtins");
 const build_options = @import("build_options");
+const posix = if (builtin.os.tag != .windows and builtin.os.tag != .wasi) std.posix else undefined;
 
 const trace_refcount = build_options.trace_refcount;
 
+/// Error message to display on stack overflow in a Roc program
+const STACK_OVERFLOW_MESSAGE = "\nThis Roc application overflowed its stack memory and crashed.\n\n";
+
+/// Callback for stack overflow in a Roc program
+fn handleRocStackOverflow() noreturn {
+    if (comptime builtin.os.tag == .windows) {
+        const DWORD = u32;
+        const HANDLE = ?*anyopaque;
+        const STD_ERROR_HANDLE: DWORD = @bitCast(@as(i32, -12));
+
+        const kernel32 = struct {
+            extern "kernel32" fn GetStdHandle(nStdHandle: DWORD) callconv(.winapi) HANDLE;
+            extern "kernel32" fn WriteFile(hFile: HANDLE, lpBuffer: [*]const u8, nNumberOfBytesToWrite: DWORD, lpNumberOfBytesWritten: ?*DWORD, lpOverlapped: ?*anyopaque) callconv(.winapi) i32;
+            extern "kernel32" fn ExitProcess(uExitCode: c_uint) callconv(.winapi) noreturn;
+        };
+
+        const stderr_handle = kernel32.GetStdHandle(STD_ERROR_HANDLE);
+        var bytes_written: DWORD = 0;
+        _ = kernel32.WriteFile(stderr_handle, STACK_OVERFLOW_MESSAGE.ptr, STACK_OVERFLOW_MESSAGE.len, &bytes_written, null);
+        kernel32.ExitProcess(134);
+    } else if (comptime builtin.os.tag != .wasi) {
+        _ = posix.write(posix.STDERR_FILENO, STACK_OVERFLOW_MESSAGE) catch {};
+        posix.exit(134);
+    } else {
+        std.process.exit(134);
+    }
+}
+
+/// Callback for access violation in a Roc program
+fn handleRocAccessViolation(fault_addr: usize) noreturn {
+    if (comptime builtin.os.tag == .windows) {
+        const DWORD = u32;
+        const HANDLE = ?*anyopaque;
+        const STD_ERROR_HANDLE: DWORD = @bitCast(@as(i32, -12));
+
+        const kernel32 = struct {
+            extern "kernel32" fn GetStdHandle(nStdHandle: DWORD) callconv(.winapi) HANDLE;
+            extern "kernel32" fn WriteFile(hFile: HANDLE, lpBuffer: [*]const u8, nNumberOfBytesToWrite: DWORD, lpNumberOfBytesWritten: ?*DWORD, lpOverlapped: ?*anyopaque) callconv(.winapi) i32;
+            extern "kernel32" fn ExitProcess(uExitCode: c_uint) callconv(.winapi) noreturn;
+        };
+
+        var addr_buf: [18]u8 = undefined;
+        const addr_str = builtins.handlers.formatHex(fault_addr, &addr_buf);
+
+        const msg1 = "\nSegmentation fault (SIGSEGV) in this Roc program.\nFault address: ";
+        const msg2 = "\n\n";
+        const stderr_handle = kernel32.GetStdHandle(STD_ERROR_HANDLE);
+        var bytes_written: DWORD = 0;
+        _ = kernel32.WriteFile(stderr_handle, msg1.ptr, msg1.len, &bytes_written, null);
+        _ = kernel32.WriteFile(stderr_handle, addr_str.ptr, @intCast(addr_str.len), &bytes_written, null);
+        _ = kernel32.WriteFile(stderr_handle, msg2.ptr, msg2.len, &bytes_written, null);
+        kernel32.ExitProcess(139);
+    } else {
+        // POSIX (and WASI fallback)
+        const msg = "\nSegmentation fault (SIGSEGV) in this Roc program.\nFault address: ";
+        _ = posix.write(posix.STDERR_FILENO, msg) catch {};
+
+        var addr_buf: [18]u8 = undefined;
+        const addr_str = builtins.handlers.formatHex(fault_addr, &addr_buf);
+        _ = posix.write(posix.STDERR_FILENO, addr_str) catch {};
+        _ = posix.write(posix.STDERR_FILENO, "\n\n") catch {};
+        posix.exit(139);
+    }
+}
+
+/// Error message to display on division by zero in a Roc program
+const DIVISION_BY_ZERO_MESSAGE = "\nThis Roc application divided by zero and crashed.\n\n";
+
+/// Callback for arithmetic errors (division by zero) in a Roc program
+fn handleRocArithmeticError() noreturn {
+    if (comptime builtin.os.tag == .windows) {
+        const DWORD = u32;
+        const HANDLE = ?*anyopaque;
+        const STD_ERROR_HANDLE: DWORD = @bitCast(@as(i32, -12));
+
+        const kernel32 = struct {
+            extern "kernel32" fn GetStdHandle(nStdHandle: DWORD) callconv(.winapi) HANDLE;
+            extern "kernel32" fn WriteFile(hFile: HANDLE, lpBuffer: [*]const u8, nNumberOfBytesToWrite: DWORD, lpNumberOfBytesWritten: ?*DWORD, lpOverlapped: ?*anyopaque) callconv(.winapi) i32;
+            extern "kernel32" fn ExitProcess(uExitCode: c_uint) callconv(.winapi) noreturn;
+        };
+
+        const stderr_handle = kernel32.GetStdHandle(STD_ERROR_HANDLE);
+        var bytes_written: DWORD = 0;
+        _ = kernel32.WriteFile(stderr_handle, DIVISION_BY_ZERO_MESSAGE.ptr, DIVISION_BY_ZERO_MESSAGE.len, &bytes_written, null);
+        kernel32.ExitProcess(136);
+    } else if (comptime builtin.os.tag != .wasi) {
+        _ = posix.write(posix.STDERR_FILENO, DIVISION_BY_ZERO_MESSAGE) catch {};
+        posix.exit(136); // 128 + 8 (SIGFPE)
+    } else {
+        std.process.exit(136);
+    }
+}
+
+/// Type of IO operation in test spec
+const EffectType = enum(u8) {
+    stdin_input, // 0<
+    stdout_expect, // 1>
+    stderr_expect, // 2>
+};
+
+/// A single entry in the test spec
+const SpecEntry = struct {
+    effect_type: EffectType,
+    value: []const u8,
+    spec_line: usize, // For error reporting
+};
+
+/// Test state for simulated IO mode
+const TestState = struct {
+    enabled: bool,
+    verbose: bool,
+    entries: []const SpecEntry,
+    current_index: usize,
+    failed: bool,
+    failure_info: ?FailureInfo,
+
+    const FailureInfo = struct {
+        expected_type: EffectType,
+        expected_value: []const u8,
+        actual_type: EffectType,
+        actual_value: []const u8,
+        spec_line: usize,
+    };
+
+    fn init() TestState {
+        return .{
+            .enabled = false,
+            .verbose = false,
+            .entries = &.{},
+            .current_index = 0,
+            .failed = false,
+            .failure_info = null,
+        };
+    }
+};
+
+/// Parse error for invalid spec format
+const ParseError = error{
+    InvalidSpecFormat,
+    OutOfMemory,
+};
+
+/// Parse test spec string into array of SpecEntry
+/// Format: "0<input|1>output|2>error" (pipe-separated)
+/// Returns error if any segment doesn't start with a valid pattern (0<, 1>, 2>)
+fn parseTestSpec(allocator: std.mem.Allocator, spec: []const u8) ParseError![]SpecEntry {
+    var entries = std.ArrayList(SpecEntry).initCapacity(allocator, 8) catch return ParseError.OutOfMemory;
+    errdefer entries.deinit(allocator);
+
+    var line_num: usize = 1;
+    // Split on pipe character
+    var iter = std.mem.splitScalar(u8, spec, '|');
+
+    while (iter.next()) |segment| {
+        defer line_num += 1;
+
+        // Skip empty segments (e.g., trailing pipe)
+        if (segment.len == 0) continue;
+
+        // Check for valid pattern prefix
+        if (segment.len < 2) {
+            const stderr_file: std.fs.File = .stderr();
+            stderr_file.writeAll("Error: Invalid spec segment '") catch {};
+            stderr_file.writeAll(segment) catch {};
+            stderr_file.writeAll("' - must start with 0<, 1>, or 2>\n") catch {};
+            return ParseError.InvalidSpecFormat;
+        }
+
+        const effect_type: EffectType = blk: {
+            if (segment[0] == '0' and segment[1] == '<') break :blk .stdin_input;
+            if (segment[0] == '1' and segment[1] == '>') break :blk .stdout_expect;
+            if (segment[0] == '2' and segment[1] == '>') break :blk .stderr_expect;
+            // Invalid pattern - report error
+            const stderr_file: std.fs.File = .stderr();
+            stderr_file.writeAll("Error: Invalid spec segment '") catch {};
+            stderr_file.writeAll(segment) catch {};
+            stderr_file.writeAll("' - must start with 0<, 1>, or 2>\n") catch {};
+            return ParseError.InvalidSpecFormat;
+        };
+
+        entries.append(allocator, .{
+            .effect_type = effect_type,
+            .value = segment[2..],
+            .spec_line = line_num,
+        }) catch return ParseError.OutOfMemory;
+    }
+
+    return entries.toOwnedSlice(allocator) catch ParseError.OutOfMemory;
+}
+
+
 /// Host environment - contains GeneralPurposeAllocator for leak detection
 const HostEnv = struct {
     gpa: std.heap.GeneralPurposeAllocator(.{}),
+    test_state: TestState,
 };
 
 /// Roc allocation function with size-tracking metadata
@@ -163,16 +383,47 @@ fn __main() callconv(.c) void {}
 
 // C compatible main for runtime
 fn main(argc: c_int, argv: [*][*:0]u8) callconv(.c) c_int {
-    _ = argc;
-    _ = argv;
-    platform_main() catch |err| {
-        const stderr: std.fs.File = .stderr();
-        stderr.writeAll("HOST ERROR: ") catch {};
-        stderr.writeAll(@errorName(err)) catch {};
-        stderr.writeAll("\n") catch {};
+    // Parse --test or --test-verbose argument
+    var test_spec: ?[]const u8 = null;
+    var test_verbose: bool = false;
+    var i: usize = 1;
+    const arg_count: usize = @intCast(argc);
+    const stderr_file: std.fs.File = .stderr();
+    while (i < arg_count) : (i += 1) {
+        const arg = std.mem.span(argv[i]);
+        if (std.mem.eql(u8, arg, "--test-verbose")) {
+            if (i + 1 < arg_count) {
+                i += 1;
+                test_spec = std.mem.span(argv[i]);
+                test_verbose = true;
+            } else {
+                stderr_file.writeAll("Error: --test-verbose requires a spec argument\n") catch {};
+                return 1;
+            }
+        } else if (std.mem.eql(u8, arg, "--test")) {
+            if (i + 1 < arg_count) {
+                i += 1;
+                test_spec = std.mem.span(argv[i]);
+            } else {
+                stderr_file.writeAll("Error: --test requires a spec argument\n") catch {};
+                return 1;
+            }
+        } else if (arg.len >= 2 and arg[0] == '-' and arg[1] == '-') {
+            stderr_file.writeAll("Error: unknown flag '") catch {};
+            stderr_file.writeAll(arg) catch {};
+            stderr_file.writeAll("'\n") catch {};
+            stderr_file.writeAll("Usage: <app> [--test <spec>] [--test-verbose <spec>]\n") catch {};
+            return 1;
+        }
+    }
+
+    const exit_code = platform_main(test_spec, test_verbose) catch |err| {
+        stderr_file.writeAll("HOST ERROR: ") catch {};
+        stderr_file.writeAll(@errorName(err)) catch {};
+        stderr_file.writeAll("\n") catch {};
         return 1;
     };
-    return 0;
+    return exit_code;
 }
 
 // Use the actual RocStr from builtins instead of defining our own
@@ -182,14 +433,69 @@ const RocStr = builtins.str.RocStr;
 /// Follows RocCall ABI: (ops, ret_ptr, args_ptr)
 /// Returns {} and takes Str as argument
 fn hostedStderrLine(ops: *builtins.host_abi.RocOps, ret_ptr: *anyopaque, args_ptr: *anyopaque) callconv(.c) void {
-    _ = ops;
     _ = ret_ptr; // Return value is {} which is zero-sized
 
     // Arguments struct for single Str parameter
     const Args = extern struct { str: RocStr };
     const args: *Args = @ptrCast(@alignCast(args_ptr));
-
     const message = args.str.asSlice();
+
+    const host: *HostEnv = @ptrCast(@alignCast(ops.env));
+
+    // Test mode: verify output matches expected
+    if (host.test_state.enabled) {
+        const stderr_file: std.fs.File = .stderr();
+        if (host.test_state.current_index < host.test_state.entries.len) {
+            const entry = host.test_state.entries[host.test_state.current_index];
+            if (entry.effect_type == .stderr_expect and std.mem.eql(u8, entry.value, message)) {
+                host.test_state.current_index += 1;
+                if (host.test_state.verbose) {
+                    stderr_file.writeAll("[OK] stderr: \"") catch {};
+                    stderr_file.writeAll(message) catch {};
+                    stderr_file.writeAll("\"\n") catch {};
+                }
+                return; // Match!
+            }
+            // Mismatch - must allocate a copy of the message since the RocStr may be freed
+            const actual_copy = host.gpa.allocator().dupe(u8, message) catch "";
+            host.test_state.failed = true;
+            host.test_state.failure_info = .{
+                .expected_type = entry.effect_type,
+                .expected_value = entry.value,
+                .actual_type = .stderr_expect,
+                .actual_value = actual_copy,
+                .spec_line = entry.spec_line,
+            };
+            if (host.test_state.verbose) {
+                stderr_file.writeAll("[FAIL] stderr: \"") catch {};
+                stderr_file.writeAll(message) catch {};
+                stderr_file.writeAll("\" (expected ") catch {};
+                stderr_file.writeAll(effectTypeName(entry.effect_type)) catch {};
+                stderr_file.writeAll(": \"") catch {};
+                stderr_file.writeAll(entry.value) catch {};
+                stderr_file.writeAll("\")\n") catch {};
+            }
+        } else {
+            // Extra output not in spec - must allocate a copy of the message
+            const actual_copy = host.gpa.allocator().dupe(u8, message) catch "";
+            host.test_state.failed = true;
+            host.test_state.failure_info = .{
+                .expected_type = .stderr_expect, // We expected nothing
+                .expected_value = "",
+                .actual_type = .stderr_expect,
+                .actual_value = actual_copy,
+                .spec_line = 0,
+            };
+            if (host.test_state.verbose) {
+                stderr_file.writeAll("[FAIL] stderr: \"") catch {};
+                stderr_file.writeAll(message) catch {};
+                stderr_file.writeAll("\" (unexpected - no more expected operations)\n") catch {};
+            }
+        }
+        return;
+    }
+
+    // Normal mode: write to stderr
     const stderr: std.fs.File = .stderr();
     stderr.writeAll(message) catch {};
     stderr.writeAll("\n") catch {};
@@ -201,19 +507,69 @@ fn hostedStderrLine(ops: *builtins.host_abi.RocOps, ret_ptr: *anyopaque, args_pt
 fn hostedStdinLine(ops: *builtins.host_abi.RocOps, ret_ptr: *anyopaque, args_ptr: *anyopaque) callconv(.c) void {
     _ = args_ptr; // Argument is {} which is zero-sized
 
-    // Read a line from stdin
+    const host: *HostEnv = @ptrCast(@alignCast(ops.env));
+    const result: *RocStr = @ptrCast(@alignCast(ret_ptr));
+
+    // Test mode: consume next stdin_input entry from spec
+    if (host.test_state.enabled) {
+        const stderr_file: std.fs.File = .stderr();
+        if (host.test_state.current_index < host.test_state.entries.len) {
+            const entry = host.test_state.entries[host.test_state.current_index];
+            if (entry.effect_type == .stdin_input) {
+                host.test_state.current_index += 1;
+                result.* = RocStr.fromSlice(entry.value, ops);
+                if (host.test_state.verbose) {
+                    stderr_file.writeAll("[OK] stdin: \"") catch {};
+                    stderr_file.writeAll(entry.value) catch {};
+                    stderr_file.writeAll("\"\n") catch {};
+                }
+                return;
+            }
+            // Wrong type - expected stdin but spec has output
+            host.test_state.failed = true;
+            host.test_state.failure_info = .{
+                .expected_type = entry.effect_type,
+                .expected_value = entry.value,
+                .actual_type = .stdin_input,
+                .actual_value = "(stdin read)",
+                .spec_line = entry.spec_line,
+            };
+            if (host.test_state.verbose) {
+                stderr_file.writeAll("[FAIL] stdin read (expected ") catch {};
+                stderr_file.writeAll(effectTypeName(entry.effect_type)) catch {};
+                stderr_file.writeAll(": \"") catch {};
+                stderr_file.writeAll(entry.value) catch {};
+                stderr_file.writeAll("\")\n") catch {};
+            }
+        } else {
+            // Ran out of entries - app tried to read more stdin than provided
+            host.test_state.failed = true;
+            host.test_state.failure_info = .{
+                .expected_type = .stdin_input,
+                .expected_value = "",
+                .actual_type = .stdin_input,
+                .actual_value = "(stdin read)",
+                .spec_line = 0,
+            };
+            if (host.test_state.verbose) {
+                stderr_file.writeAll("[FAIL] stdin read (unexpected - no more expected operations)\n") catch {};
+            }
+        }
+        result.* = RocStr.empty();
+        return;
+    }
+
+    // Normal mode: Read a line from stdin
     var buffer: [4096]u8 = undefined;
     const stdin_file: std.fs.File = .stdin();
     const bytes_read = stdin_file.read(&buffer) catch {
         // Return empty string on error
-        const result: *RocStr = @ptrCast(@alignCast(ret_ptr));
         result.* = RocStr.empty();
         return;
     };
 
     // Handle EOF (no bytes read)
     if (bytes_read == 0) {
-        const result: *RocStr = @ptrCast(@alignCast(ret_ptr));
         result.* = RocStr.empty();
         return;
     }
@@ -233,7 +589,6 @@ fn hostedStdinLine(ops: *builtins.host_abi.RocOps, ret_ptr: *anyopaque, args_ptr
     // Create RocStr from the read line and return it
     // RocStr.fromSlice handles allocation internally (either inline for small strings
     // or via roc_alloc for big strings with proper refcount tracking)
-    const result: *RocStr = @ptrCast(@alignCast(ret_ptr));
     result.* = RocStr.fromSlice(line, ops);
 }
 
@@ -241,14 +596,69 @@ fn hostedStdinLine(ops: *builtins.host_abi.RocOps, ret_ptr: *anyopaque, args_ptr
 /// Follows RocCall ABI: (ops, ret_ptr, args_ptr)
 /// Returns {} and takes Str as argument
 fn hostedStdoutLine(ops: *builtins.host_abi.RocOps, ret_ptr: *anyopaque, args_ptr: *anyopaque) callconv(.c) void {
-    _ = ops;
     _ = ret_ptr; // Return value is {} which is zero-sized
 
     // Arguments struct for single Str parameter
     const Args = extern struct { str: RocStr };
     const args: *Args = @ptrCast(@alignCast(args_ptr));
-
     const message = args.str.asSlice();
+
+    const host: *HostEnv = @ptrCast(@alignCast(ops.env));
+
+    // Test mode: verify output matches expected
+    if (host.test_state.enabled) {
+        const stderr_file: std.fs.File = .stderr();
+        if (host.test_state.current_index < host.test_state.entries.len) {
+            const entry = host.test_state.entries[host.test_state.current_index];
+            if (entry.effect_type == .stdout_expect and std.mem.eql(u8, entry.value, message)) {
+                host.test_state.current_index += 1;
+                if (host.test_state.verbose) {
+                    stderr_file.writeAll("[OK] stdout: \"") catch {};
+                    stderr_file.writeAll(message) catch {};
+                    stderr_file.writeAll("\"\n") catch {};
+                }
+                return; // Match!
+            }
+            // Mismatch - must allocate a copy of the message since the RocStr may be freed
+            const actual_copy = host.gpa.allocator().dupe(u8, message) catch "";
+            host.test_state.failed = true;
+            host.test_state.failure_info = .{
+                .expected_type = entry.effect_type,
+                .expected_value = entry.value,
+                .actual_type = .stdout_expect,
+                .actual_value = actual_copy,
+                .spec_line = entry.spec_line,
+            };
+            if (host.test_state.verbose) {
+                stderr_file.writeAll("[FAIL] stdout: \"") catch {};
+                stderr_file.writeAll(message) catch {};
+                stderr_file.writeAll("\" (expected ") catch {};
+                stderr_file.writeAll(effectTypeName(entry.effect_type)) catch {};
+                stderr_file.writeAll(": \"") catch {};
+                stderr_file.writeAll(entry.value) catch {};
+                stderr_file.writeAll("\")\n") catch {};
+            }
+        } else {
+            // Extra output not in spec - must allocate a copy of the message
+            const actual_copy = host.gpa.allocator().dupe(u8, message) catch "";
+            host.test_state.failed = true;
+            host.test_state.failure_info = .{
+                .expected_type = .stdout_expect, // We expected nothing
+                .expected_value = "",
+                .actual_type = .stdout_expect,
+                .actual_value = actual_copy,
+                .spec_line = 0,
+            };
+            if (host.test_state.verbose) {
+                stderr_file.writeAll("[FAIL] stdout: \"") catch {};
+                stderr_file.writeAll(message) catch {};
+                stderr_file.writeAll("\" (unexpected - no more expected operations)\n") catch {};
+            }
+        }
+        return;
+    }
+
+    // Normal mode: write to stdout
     const stdout: std.fs.File = .stdout();
     stdout.writeAll(message) catch {};
     stdout.writeAll("\n") catch {};
@@ -263,11 +673,36 @@ const hosted_function_ptrs = [_]builtins.host_abi.HostedFn{
 };
 
 /// Platform host entrypoint
-fn platform_main() !void {
+fn platform_main(test_spec: ?[]const u8, test_verbose: bool) !c_int {
+    // Install signal handlers for stack overflow, access violations, and division by zero
+    // This allows us to display helpful error messages instead of crashing
+    _ = builtins.handlers.install(handleRocStackOverflow, handleRocAccessViolation, handleRocArithmeticError);
+
     var host_env = HostEnv{
         .gpa = std.heap.GeneralPurposeAllocator(.{}){},
+        .test_state = TestState.init(),
     };
+
+    // Parse test spec if provided
+    if (test_spec) |spec| {
+        host_env.test_state.entries = try parseTestSpec(host_env.gpa.allocator(), spec);
+        host_env.test_state.enabled = true;
+        host_env.test_state.verbose = test_verbose;
+    }
+
     defer {
+        // Free duplicated actual_value if allocated (on test failure)
+        if (host_env.test_state.failure_info) |info| {
+            if (info.actual_value.len > 0) {
+                host_env.gpa.allocator().free(info.actual_value);
+            }
+        }
+
+        // Free test entries if allocated
+        if (host_env.test_state.entries.len > 0) {
+            host_env.gpa.allocator().free(host_env.test_state.entries);
+        }
+
         const leaked = host_env.gpa.deinit();
         if (leaked == .leak) {
             std.log.err("\x1b[33mMemory leak detected!\x1b[0m", .{});
@@ -298,4 +733,70 @@ fn platform_main() !void {
     // causing a segfault if you pass null. This should be changed! Dereferencing
     // garbage memory is obviously pointless, and there's no reason we should do it.
     roc__main(&roc_ops, @as(*anyopaque, @ptrCast(&ret)), @as(*anyopaque, @ptrCast(&args)));
+
+    // Check test results if in test mode
+    if (host_env.test_state.enabled) {
+        // Check if test failed or not all entries were consumed
+        if (host_env.test_state.failed or host_env.test_state.current_index != host_env.test_state.entries.len) {
+            const stderr_file: std.fs.File = .stderr();
+
+            // Print failure info
+            if (host_env.test_state.failure_info) |info| {
+                if (info.spec_line == 0) {
+                    // Extra/unexpected output
+                    stderr_file.writeAll("TEST FAILED: Unexpected ") catch {};
+                    stderr_file.writeAll(effectTypeName(info.actual_type)) catch {};
+                    stderr_file.writeAll(" output: \"") catch {};
+                    stderr_file.writeAll(info.actual_value) catch {};
+                    stderr_file.writeAll("\"\n") catch {};
+                } else {
+                    var buf: [512]u8 = undefined;
+                    const msg = std.fmt.bufPrint(&buf, "TEST FAILED at spec line {d}:\n  Expected: {s} \"{s}\"\n  Got:      {s} \"{s}\"\n", .{
+                        info.spec_line,
+                        effectTypeName(info.expected_type),
+                        info.expected_value,
+                        effectTypeName(info.actual_type),
+                        info.actual_value,
+                    }) catch "TEST FAILED\n";
+                    stderr_file.writeAll(msg) catch {};
+                }
+            } else if (host_env.test_state.current_index < host_env.test_state.entries.len) {
+                // Not all entries were consumed - list what's remaining
+                const remaining = host_env.test_state.entries.len - host_env.test_state.current_index;
+                var buf: [256]u8 = undefined;
+                const msg = std.fmt.bufPrint(&buf, "TEST FAILED: {d} expected IO operation(s) not performed:\n", .{remaining}) catch "TEST FAILED: expected IO operations not performed\n";
+                stderr_file.writeAll(msg) catch {};
+
+                // List up to 5 unconsumed entries
+                const max_to_show: usize = 5;
+                var shown: usize = 0;
+                for (host_env.test_state.entries[host_env.test_state.current_index..]) |entry| {
+                    if (shown >= max_to_show) {
+                        stderr_file.writeAll("  ...\n") catch {};
+                        break;
+                    }
+                    stderr_file.writeAll("  - ") catch {};
+                    stderr_file.writeAll(effectTypeName(entry.effect_type)) catch {};
+                    stderr_file.writeAll(": \"") catch {};
+                    stderr_file.writeAll(entry.value) catch {};
+                    stderr_file.writeAll("\"\n") catch {};
+                    shown += 1;
+                }
+            } else {
+                stderr_file.writeAll("TEST FAILED\n") catch {};
+            }
+
+            return 1;
+        }
+    }
+
+    return 0;
+}
+
+fn effectTypeName(effect_type: EffectType) []const u8 {
+    return switch (effect_type) {
+        .stdin_input => "stdin",
+        .stdout_expect => "stdout",
+        .stderr_expect => "stderr",
+    };
 }

test/fx/stack_overflow_runtime.roc

@@ -0,0 +1,16 @@
+app [main!] { pf: platform "./platform/main.roc" }
+
+import pf.Stdout
+
+# This function causes infinite recursion, leading to stack overflow at runtime.
+# It cannot be tail-call optimized because there's work after the recursive call.
+overflow : I64 -> I64
+overflow = |n|
+    # Prevent tail-call optimization by adding to the result after recursion
+    overflow(n + 1) + 1
+
+main! = || {
+    # This will overflow the stack at runtime
+    result = overflow(0)
+    Stdout.line!("Result: ${I64.to_str(result)}")
+}

test/fx/stdin_test.roc

@@ -1,13 +1,10 @@
 app [main!] { pf: platform "./platform/main.roc" }
 
-import pf.Stdout
 import pf.Stdin
-
-str : Str -> Str
-str = |s| s
+import pf.Stdout
 
 main! = || {
-    Stdout.line!(str("Before stdin"))
-    temp = Stdin.line!()
-    Stdout.line!(str("After stdin"))
+    Stdout.line!("Before stdin")
+    _line = Stdin.line!()
+    Stdout.line!("After stdin")
 }

test/fx/sublist_method_segfault.roc

@@ -0,0 +1,15 @@
+app [main!] { pf: platform "./platform/main.roc" }
+
+# Regression test: Calling .sublist() method on a List(U8) from "".to_utf8()
+# causes a segfault when the variable doesn't have an explicit type annotation.
+# Error was: "Roc crashed: Error evaluating from shared memory: InvalidMethodReceiver"
+# The bug was that translateTypeVar was using the wrong module (closure's source module)
+# instead of the caller's module when translating the return type.
+main! = || {
+    # Test case 1: Method call without type annotation (original bug)
+    s = "".to_utf8()
+    _slice = s.sublist({ start: 0, len: 0 })
+
+    # Test case 2: Comparing empty list with method result
+    _ignore = "".to_utf8() == []
+}

test/fx/var_interp_segfault.roc

@@ -0,0 +1,29 @@
+app [main!] { pf: platform "./platform/main.roc" }
+
+import pf.Stdout
+
+print! : Str => {}
+print! = |msg| msg.split_on("\n").for_each!(Stdout.line!)
+
+fnA! : Str => Try(I64, _)
+fnA! = |_input| {
+    var $x = 1
+    Ok($x)
+}
+
+fnB! : Str => Try(I64, _)
+fnB! = |_input| {
+    var $y = 2
+    Ok($y)
+}
+
+run! = || {
+    print!("A1: ${fnA!("test")?.to_str()}")
+    print!("A2: ${fnA!("test")?.to_str()}")
+    print!("A3: ${fnA!("test")?.to_str()}")
+    Ok({})
+}
+
+main! = || {
+    _ignore = run!()
+}

test/snapshots/numeric_let_generalize_in_block.md

@@ -0,0 +1,91 @@
+# META
+~~~ini
+description=Numeric let-generalization inside nested block (rank > top_level)
+type=expr
+~~~
+# SOURCE
+~~~roc
+{
+    n = 42
+    a = I64.to_str(n)
+    b = Dec.to_str(n)
+    Str.concat(a, b)
+}
+~~~
+# EXPECTED
+NIL
+# PROBLEMS
+NIL
+# TOKENS
+~~~zig
+OpenCurly,
+LowerIdent,OpAssign,Int,
+LowerIdent,OpAssign,UpperIdent,NoSpaceDotLowerIdent,NoSpaceOpenRound,LowerIdent,CloseRound,
+LowerIdent,OpAssign,UpperIdent,NoSpaceDotLowerIdent,NoSpaceOpenRound,LowerIdent,CloseRound,
+UpperIdent,NoSpaceDotLowerIdent,NoSpaceOpenRound,LowerIdent,Comma,LowerIdent,CloseRound,
+CloseCurly,
+EndOfFile,
+~~~
+# PARSE
+~~~clojure
+(e-block
+	(statements
+		(s-decl
+			(p-ident (raw "n"))
+			(e-int (raw "42")))
+		(s-decl
+			(p-ident (raw "a"))
+			(e-apply
+				(e-ident (raw "I64.to_str"))
+				(e-ident (raw "n"))))
+		(s-decl
+			(p-ident (raw "b"))
+			(e-apply
+				(e-ident (raw "Dec.to_str"))
+				(e-ident (raw "n"))))
+		(e-apply
+			(e-ident (raw "Str.concat"))
+			(e-ident (raw "a"))
+			(e-ident (raw "b")))))
+~~~
+# FORMATTED
+~~~roc
+{
+	n = 42
+	a = I64.to_str(n)
+	b = Dec.to_str(n)
+	Str.concat(a, b)
+}
+~~~
+# CANONICALIZE
+~~~clojure
+(e-block
+	(s-let
+		(p-assign (ident "n"))
+		(e-num (value "42")))
+	(s-let
+		(p-assign (ident "a"))
+		(e-call
+			(e-lookup-external
+				(builtin))
+			(e-lookup-local
+				(p-assign (ident "n")))))
+	(s-let
+		(p-assign (ident "b"))
+		(e-call
+			(e-lookup-external
+				(builtin))
+			(e-lookup-local
+				(p-assign (ident "n")))))
+	(e-call
+		(e-lookup-external
+			(builtin))
+		(e-lookup-local
+			(p-assign (ident "a")))
+		(e-lookup-local
+			(p-assign (ident "b")))))
+~~~
+# TYPES
+~~~clojure
+(expr (type "Str"))
+~~~

test/snapshots/repl/list_count_if.md

@@ -0,0 +1,13 @@
+# META
+~~~ini
+description=List.count_if counts elements where predicate returns true
+type=repl
+~~~
+# SOURCE
+~~~roc
+» List.count_if([1, 2, 3, 4, 5], |x| x > 2)
+~~~
+# OUTPUT
+3
+# PROBLEMS
+NIL

test/snapshots/repl/list_count_if_all_match.md

@@ -0,0 +1,13 @@
+# META
+~~~ini
+description=List.count_if returns list length when all elements match
+type=repl
+~~~
+# SOURCE
+~~~roc
+» List.count_if([1, 2, 3, 4, 5], |x| x > 0)
+~~~
+# OUTPUT
+5
+# PROBLEMS
+NIL

test/snapshots/repl/list_count_if_empty.md

@@ -0,0 +1,13 @@
+# META
+~~~ini
+description=List.count_if on empty list returns 0
+type=repl
+~~~
+# SOURCE
+~~~roc
+» List.count_if([], |x| x > 2)
+~~~
+# OUTPUT
+0
+# PROBLEMS
+NIL

test/snapshots/repl/list_count_if_none_match.md

@@ -0,0 +1,13 @@
+# META
+~~~ini
+description=List.count_if returns 0 when no elements match
+type=repl
+~~~
+# SOURCE
+~~~roc
+» List.count_if([1, 2, 3], |x| x > 10)
+~~~
+# OUTPUT
+0
+# PROBLEMS
+NIL

test/snapshots/repl/numeric_multiple_diff_types.md

@@ -1,6 +1,6 @@
 # META
 ~~~ini
-description=Numeric without annotation, multiple uses with different types (produces type error)
+description=Numeric without annotation, multiple uses with different types (each use gets fresh type)
 type=repl
 ~~~
 # SOURCE
@@ -17,6 +17,6 @@ assigned `a`
 ---
 assigned `b`
 ---
-TYPE MISMATCH
+"4242.0"
 # PROBLEMS
 NIL

test/snapshots/repl/numeric_sum_to_str.md

@@ -0,0 +1,22 @@
+# META
+~~~ini
+description=Numeric sum then convert to I16 string
+type=repl
+~~~
+# SOURCE
+~~~roc
+» a = 4
+» b = 5
+» sum = a + b
+» I16.to_str(sum)
+~~~
+# OUTPUT
+assigned `a`
+---
+assigned `b`
+---
+assigned `sum`
+---
+"9"
+# PROBLEMS
+NIL

test/snapshots/repl/u8_range_to.md

@@ -0,0 +1,19 @@
+# META
+~~~ini
+description=U8.to - creates a list of integers from start to end (inclusive)
+type=repl
+~~~
+# SOURCE
+~~~roc
+» 1u8.to(5u8)
+» 0u8.to(0u8)
+» 5u8.to(3u8)
+~~~
+# OUTPUT
+[1, 2, 3, 4, 5]
+---
+[0]
+---
+[]
+# PROBLEMS
+NIL

test/snapshots/repl/u8_range_until.md

@@ -0,0 +1,19 @@
+# META
+~~~ini
+description=U8.until - creates a list of integers from start to end (exclusive)
+type=repl
+~~~
+# SOURCE
+~~~roc
+» 0u8.until(3u8)
+» 1u8.until(1u8)
+» 5u8.until(3u8)
+~~~
+# OUTPUT
+[0, 1, 2]
+---
+[]
+---
+[]
+# PROBLEMS
+NIL

test/snapshots/repl/var_in_lambda_param.md

@@ -0,0 +1,17 @@
+# META
+~~~ini
+description=Test var in lambda parameters
+type=repl
+~~~
+# SOURCE
+~~~roc
+» f = |var $x, y| { $x = $x + y
+    $x }
+» f(1, 2)
+~~~
+# OUTPUT
+assigned `f`
+---
+3
+# PROBLEMS
+NIL

typos.toml

@@ -22,3 +22,4 @@ HSA = "HSA"
 typ = "typ"
 ba = "ba"
 Trys = "Trys"
+sigfault = "sigfault"