add Str.count_utf8_bytes builtin

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/build/builtin_compiler/main.zig

@@ -140,6 +140,9 @@ fn replaceStrIsEmptyWithLowLevel(env: *ModuleEnv) !std.ArrayList(CIR.Def.Idx) {
     if (env.common.findIdent("Builtin.Str.drop_suffix")) |str_drop_suffix_ident| {
         try low_level_map.put(str_drop_suffix_ident, .str_drop_suffix);
     }
+    if (env.common.findIdent("Builtin.Str.count_utf8_bytes")) |str_count_utf8_bytes_ident| {
+        try low_level_map.put(str_count_utf8_bytes_ident, .str_count_utf8_bytes);
+    }
     if (env.common.findIdent("Builtin.List.len")) |list_len_ident| {
         try low_level_map.put(list_len_ident, .list_len);
     }

src/build/roc/Builtin.roc

@@ -15,6 +15,7 @@ Builtin :: [].{
 		with_prefix : Str, Str -> Str
 		drop_prefix : Str, Str -> Str
 		drop_suffix : Str, Str -> Str
+		count_utf8_bytes : Str -> U64
 	}
 
 	List(_item) :: [ProvidedByCompiler].{

src/canonicalize/Expression.zig

@@ -415,6 +415,7 @@ pub const Expr = union(enum) {
         str_with_prefix,
         str_drop_prefix,
         str_drop_suffix,
+        str_count_utf8_bytes,
 
         // Numeric to_str operations
         u8_to_str,

src/eval/interpreter.zig

@@ -3091,6 +3091,23 @@ pub const Interpreter = struct {
                 out.is_initialized = true;
                 return out;
             },
+            .str_count_utf8_bytes => {
+                // Str.count_utf8_bytes : Str -> U64
+                std.debug.assert(args.len == 1);
+
+                const string_arg = args[0];
+                std.debug.assert(string_arg.ptr != null);
+
+                const string: *const RocStr = @ptrCast(@alignCast(string_arg.ptr.?));
+                const byte_count = builtins.str.countUtf8Bytes(string.*);
+
+                const result_layout = layout.Layout.int(.u64);
+                var out = try self.pushRaw(result_layout, 0);
+                out.is_initialized = false;
+                try out.setInt(@intCast(byte_count));
+                out.is_initialized = true;
+                return out;
+            },
             .list_len => {
                 // List.len : List(a) -> U64
                 // Note: listLen returns usize, but List.len always returns U64.

test/snapshots/repl/str_count_utf8_bytes.md

@@ -0,0 +1,25 @@
+# META
+~~~ini
+description=Str.count_utf8_bytes should return the number of bytes in the string
+type=repl
+~~~
+# SOURCE
+~~~roc
+» Str.count_utf8_bytes("")
+» Str.count_utf8_bytes("hello")
+» Str.count_utf8_bytes("hello world")
+» Str.count_utf8_bytes("é")
+» Str.count_utf8_bytes("🎉")
+~~~
+# OUTPUT
+0
+---
+5
+---
+11
+---
+2
+---
+4
+# PROBLEMS
+NIL