fix(parser): report for-await in non-async functions (#24855)

## Summary

- report invalid `for await` loops inside non-async functions immediately, including when an unambiguous source later resolves to a module
- update the TypeScript parser conformance snapshot with the newly reported diagnostics

## Root cause

The parser deferred every `for await` context error in unambiguous sources as a script-only error. When later `export` syntax resolved the source as a module, it discarded that deferred error even when the loop was nested inside a non-async function, where `await` is never permitted.

crates/oxc_parser/src/js/statement.rs

@@ -355,9 +355,16 @@ impl<'a, C: Config> ParserImpl<'a, C> {
         // [+Await]
         let r#await = if self.at(Kind::Await) {
             if !self.ctx.has_await() {
-                // For `ModuleKind::Unambiguous`, defer the error until we know whether
-                // this is a Module (where for-await is valid at top-level) or Script.
-                self.error_on_script(diagnostics::await_expression(self.cur_token().span()));
+                let error = diagnostics::await_expression(self.cur_token().span());
+                if self.ctx.has_top_level() {
+                    // For `ModuleKind::Unambiguous`, defer the error until we know whether
+                    // this is a Module (where for-await is valid at top-level) or Script.
+                    self.error_on_script(error);
+                } else {
+                    // A module only permits await at top level, so an await loop inside a
+                    // non-async function is always invalid.
+                    self.error(error);
+                }
             }
             self.bump_any();
             true

tasks/coverage/snapshots/parser_typescript.snap

@@ -3117,6 +3117,15 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/types/wit
     ╰────
   help: Either remove this `await` or add the `async` keyword to the enclosing function
 
+  × `await` is only allowed within async functions and at the top levels of modules
+   ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:4:7]
+ 3 │ function normalFunc(p: Promise<number>) {
+ 4 │   for await (const _ of []);
+   ·       ─────
+ 5 │   return await p;
+   ╰────
+  help: Either remove this `await` or add the `async` keyword to the enclosing function
+
   × `await` is only allowed within async functions and at the top levels of modules
    ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:5:10]
  4 │   for await (const _ of []);
@@ -3126,6 +3135,15 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/types/wit
    ╰────
   help: Either remove this `await` or add the `async` keyword to the enclosing function
 
+  × `await` is only allowed within async functions and at the top levels of modules
+    ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:9:7]
+  8 │ export function exportedFunc(p: Promise<number>) {
+  9 │   for await (const _ of []);
+    ·       ─────
+ 10 │   return await p;
+    ╰────
+  help: Either remove this `await` or add the `async` keyword to the enclosing function
+
   × `await` is only allowed within async functions and at the top levels of modules
     ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:10:10]
   9 │   for await (const _ of []);
@@ -3135,6 +3153,15 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/types/wit
     ╰────
   help: Either remove this `await` or add the `async` keyword to the enclosing function
 
+  × `await` is only allowed within async functions and at the top levels of modules
+    ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:14:7]
+ 13 │ const functionExpression = function(p: Promise<number>) {
+ 14 │   for await (const _ of []);
+    ·       ─────
+ 15 │   await p;
+    ╰────
+  help: Either remove this `await` or add the `async` keyword to the enclosing function
+
   × `await` is only allowed within async functions and at the top levels of modules
     ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:15:3]
  14 │   for await (const _ of []);
@@ -3144,6 +3171,15 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/types/wit
     ╰────
   help: Either remove this `await` or add the `async` keyword to the enclosing function
 
+  × `await` is only allowed within async functions and at the top levels of modules
+    ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:19:7]
+ 18 │ const arrowFunc = (p: Promise<number>) => {
+ 19 │   for await (const _ of []);
+    ·       ─────
+ 20 │   return await p;
+    ╰────
+  help: Either remove this `await` or add the `async` keyword to the enclosing function
+
   × `await` is only allowed within async functions and at the top levels of modules
     ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:20:10]
  19 │   for await (const _ of []);
@@ -3153,6 +3189,15 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/types/wit
     ╰────
   help: Either remove this `await` or add the `async` keyword to the enclosing function
 
+  × `await` is only allowed within async functions and at the top levels of modules
+    ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:24:7]
+ 23 │ function* generatorFunc(p: Promise<number>) {
+ 24 │   for await (const _ of []);
+    ·       ─────
+ 25 │   yield await p;
+    ╰────
+  help: Either remove this `await` or add the `async` keyword to the enclosing function
+
   × `await` is only allowed within async functions and at the top levels of modules
     ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:25:9]
  24 │   for await (const _ of []);
@@ -3162,6 +3207,15 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/types/wit
     ╰────
   help: Either remove this `await` or add the `async` keyword to the enclosing function
 
+  × `await` is only allowed within async functions and at the top levels of modules
+    ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:30:9]
+ 29 │   constructor(p: Promise<number>) {
+ 30 │     for await (const _ of []);
+    ·         ─────
+ 31 │     await p;
+    ╰────
+  help: Either remove this `await` or add the `async` keyword to the enclosing function
+
   × `await` is only allowed within async functions and at the top levels of modules
     ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:31:5]
  30 │     for await (const _ of []);
@@ -3171,6 +3225,15 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/types/wit
     ╰────
   help: Either remove this `await` or add the `async` keyword to the enclosing function
 
+  × `await` is only allowed within async functions and at the top levels of modules
+    ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:34:7]
+ 33 │   method(p: Promise<number>) {
+ 34 │   for await (const _ of []);
+    ·       ─────
+ 35 │     await p;
+    ╰────
+  help: Either remove this `await` or add the `async` keyword to the enclosing function
+
   × `await` is only allowed within async functions and at the top levels of modules
     ╭─[typescript/tests/cases/compiler/awaitInNonAsyncFunction.ts:35:5]
  34 │   for await (const _ of []);