fix(jit): keep scalar to_string on tagged path

Author: fffonion

src/vm/jit/recorder.rs

@@ -3274,11 +3274,11 @@ fn select_specialized_builtin_kind(
             });
         }
         BuiltinFunction::ToString => {
-            return Some(if container.known_type == Some(ValueType::String) {
-                SpecializedBuiltinKind::ToStringIdentity
-            } else {
-                SpecializedBuiltinKind::ToString
-            });
+            return match (container.known_type, container.repr) {
+                (Some(ValueType::String), _) => Some(SpecializedBuiltinKind::ToStringIdentity),
+                (_, SsaValueRepr::Tagged) => Some(SpecializedBuiltinKind::ToString),
+                _ => None,
+            };
         }
         BuiltinFunction::StringSplitLiteral => {
             return Some(SpecializedBuiltinKind::StringSplitLiteral);
@@ -5272,6 +5272,38 @@ mod tests {
         );
     }
 
+    #[test]
+    fn scalar_to_string_stays_on_general_call_path() {
+        let program = Program::new(Vec::new(), Vec::new());
+        for info in [
+            ValueInfo::int(None),
+            ValueInfo::float(None),
+            ValueInfo::bool(None),
+        ] {
+            assert_eq!(
+                select_specialized_builtin_kind(
+                    &program,
+                    0,
+                    BuiltinFunction::ToString,
+                    info,
+                    false,
+                ),
+                None,
+                "unboxed scalar to_string needs a tagged Value slot"
+            );
+        }
+        assert_eq!(
+            select_specialized_builtin_kind(
+                &program,
+                0,
+                BuiltinFunction::ToString,
+                ValueInfo::tagged(),
+                false,
+            ),
+            Some(SpecializedBuiltinKind::ToString)
+        );
+    }
+
     #[test]
     fn unboxed_numeric_compare_ignores_stale_operand_type_hint() {
         let program = Program::new(Vec::new(), Vec::new()).with_type_map(crate::TypeMap {

tests/jit/jit_tests.rs

@@ -6100,6 +6100,43 @@ fn trace_jit_specializes_regex_builtins_without_call_boundary() {
     assert!(vm.regex_cache_hit_count() >= 14);
 }
 
+#[test]
+fn trace_jit_scalar_to_string_uses_safe_call_boundary() {
+    if !native_jit_supported() {
+        return;
+    }
+    let source = r#"
+        let mut i = 0;
+        let mut rendered = "";
+        while i < 16 {
+            rendered = i + "";
+            i = i + 1;
+        }
+        rendered;
+    "#;
+    let compiled = compile_source(source).expect("scalar to_string loop should compile");
+    let mut vm = Vm::new(compiled.program.with_local_count(compiled.locals));
+    vm.set_jit_native_direct_links_enabled(false);
+    vm.set_jit_config(JitConfig {
+        enabled: true,
+        hot_loop_threshold: 1,
+        max_trace_len: 512,
+    });
+
+    assert_eq!(
+        vm.run().expect("scalar to_string JIT should run"),
+        VmStatus::Halted
+    );
+    assert_eq!(vm.stack(), &[Value::string("15")]);
+    let snapshot = vm.jit_snapshot();
+    assert!(vm.jit_native_exec_count() > 0, "{}", vm.dump_jit_info());
+    assert!(
+        !snapshot.traces.is_empty(),
+        "scalar to_string should leave surrounding loop traceable:\n{}",
+        vm.dump_jit_info()
+    );
+}
+
 #[test]
 fn trace_jit_executes_hot_loop_inside_script_callable_frame() {
     if !native_jit_supported() {