From b9dd8cf338247d4b62d612fc8acf5f96a221edca Mon Sep 17 00:00:00 2001
From: Luke Boswell <lukewilliamboswell@gmail.com>
Date: Mon, 1 Dec 2025 21:26:39 +1100
Subject: [PATCH 1/2] fix semantics for Str.split_on

---
 src/canonicalize/Expression.zig               |  2 +-
 test/fx/multiline_split_leak.roc              | 19 +++++++++++++++++++
 .../repl/multiline_string_split_7_lines.md    | 16 ++++++++++++++++
 3 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 test/fx/multiline_split_leak.roc
 create mode 100644 test/snapshots/repl/multiline_string_split_7_lines.md

diff --git a/src/canonicalize/Expression.zig b/src/canonicalize/Expression.zig
index c94f8b50ae..1e20c92ef8 100644
--- a/src/canonicalize/Expression.zig
+++ b/src/canonicalize/Expression.zig
@@ -812,9 +812,9 @@ pub const Expr = union(enum) {
                 .str_reserve => &.{ .consume, .borrow },
                 .str_release_excess_capacity => &.{.consume},
                 .str_join_with => &.{ .consume, .borrow }, // list consumed, separator borrowed
-                .str_split_on => &.{ .consume, .borrow },
 
                 // String operations - borrowing with seamless slice result (incref internally)
+                .str_split_on => &.{ .borrow, .borrow },
                 .str_to_utf8 => &.{.borrow},
                 .str_drop_prefix, .str_drop_suffix => &.{ .borrow, .borrow },
 
diff --git a/test/fx/multiline_split_leak.roc b/test/fx/multiline_split_leak.roc
new file mode 100644
index 0000000000..4fe2841a2b
--- /dev/null
+++ b/test/fx/multiline_split_leak.roc
@@ -0,0 +1,19 @@
+app [main!] { pf: platform "./platform/main.roc" }
+
+import pf.Stdout
+
+input =
+    \\This is a longer line number one
+    \\This is a longer line number two
+	\\L68
+	\\L30
+	\\R48
+	\\L5
+	\\R60
+	\\L55
+	\\L1
+
+main! = || {
+    for line in input.split_on("\n")
+    Stdout.line!(line)
+}
diff --git a/test/snapshots/repl/multiline_string_split_7_lines.md b/test/snapshots/repl/multiline_string_split_7_lines.md
new file mode 100644
index 0000000000..d772e80b76
--- /dev/null
+++ b/test/snapshots/repl/multiline_string_split_7_lines.md
@@ -0,0 +1,16 @@
+# META
+~~~ini
+description=Multiline string with 7 lines split - memory leak test
+type=repl
+~~~
+# SOURCE
+~~~roc
+» input = "L68\nL30\nR48\nL5\nR60\nL55\nL1"
+» input.split_on("\n")
+~~~
+# OUTPUT
+assigned `input`
+---
+["L68", "L30", "R48", "L5", "R60", "L55", "L1"]
+# PROBLEMS
+NIL

From 25e2e0a403fd7030a9577b62140f22e6900fa7cd Mon Sep 17 00:00:00 2001
From: Luke Boswell <lukewilliamboswell@gmail.com>
Date: Tue, 2 Dec 2025 07:16:29 +1100
Subject: [PATCH 2/2] add test for fx split leak

---
 src/cli/test/fx_platform_test.zig | 42 +++++++++++++++++++++++++++++++
 test/fx/multiline_split_leak.roc  |  1 +
 2 files changed, 43 insertions(+)

diff --git a/src/cli/test/fx_platform_test.zig b/src/cli/test/fx_platform_test.zig
index 1fdf2acd18..43179f691d 100644
--- a/src/cli/test/fx_platform_test.zig
+++ b/src/cli/test/fx_platform_test.zig
@@ -745,3 +745,45 @@ test "string literal pattern matching" {
     try testing.expect(has_alice);
     try testing.expect(has_bob);
 }
+
+test "multiline string split_on" {
+    // Tests splitting a multiline string and iterating over the lines.
+    // This is a regression test to ensure split_on works correctly with
+    // multiline strings and doesn't cause memory issues.
+    const allocator = testing.allocator;
+
+    try ensureRocBinary(allocator);
+
+    const run_result = try std.process.Child.run(.{
+        .allocator = allocator,
+        .argv = &[_][]const u8{
+            "./zig-out/bin/roc",
+            "test/fx/multiline_split_leak.roc",
+        },
+    });
+    defer allocator.free(run_result.stdout);
+    defer allocator.free(run_result.stderr);
+
+    switch (run_result.term) {
+        .Exited => |code| {
+            if (code != 0) {
+                std.debug.print("Run failed with exit code {}\n", .{code});
+                std.debug.print("STDOUT: {s}\n", .{run_result.stdout});
+                std.debug.print("STDERR: {s}\n", .{run_result.stderr});
+                return error.RunFailed;
+            }
+        },
+        else => {
+            std.debug.print("Run terminated abnormally: {}\n", .{run_result.term});
+            std.debug.print("STDOUT: {s}\n", .{run_result.stdout});
+            std.debug.print("STDERR: {s}\n", .{run_result.stderr});
+            return error.RunFailed;
+        },
+    }
+
+    // Verify the output contains lines from the multiline string
+    try testing.expect(std.mem.indexOf(u8, run_result.stdout, "This is a longer line number one") != null);
+    try testing.expect(std.mem.indexOf(u8, run_result.stdout, "This is a longer line number two") != null);
+    try testing.expect(std.mem.indexOf(u8, run_result.stdout, "L68") != null);
+    try testing.expect(std.mem.indexOf(u8, run_result.stdout, "The last line is here") != null);
+}
diff --git a/test/fx/multiline_split_leak.roc b/test/fx/multiline_split_leak.roc
index 4fe2841a2b..1b94734d37 100644
--- a/test/fx/multiline_split_leak.roc
+++ b/test/fx/multiline_split_leak.roc
@@ -12,6 +12,7 @@ input =
 	\\R60
 	\\L55
 	\\L1
+	\\The last line is here
 
 main! = || {
     for line in input.split_on("\n")