fix(transformer): hoist lowered async declarations (#22770)

Fixes semantic scope mismatches when async function declarations are lowered inside sloppy block scopes.

Before this change, `async_to_generator` kept the original async function binding, and the generated helper binding, in the current block scope. After transform those declarations are no longer async:

```js
with (obj) {
  async function ref() {}
}
```

becomes roughly:

```js
with (obj) {
  function ref() {
    return _ref.apply(this, arguments);
  }
  function _ref() {
    _ref = babelHelpers.asyncToGenerator(function* () {});
    return _ref.apply(this, arguments);
  }
}
```

A fresh semantic rebuild treats those lowered plain function declarations with Annex B hoisting rules, so in sloppy block scopes their bindings can belong to the enclosing var scope instead of the block scope. The transformed semantic tree was still using the pre-lowered block placement, which caused rebuild mismatches in Test262 coverage.

This PR makes `async_to_generator` choose the same binding scope that a rebuilt semantic tree would use for the emitted plain function declarations, while preserving the existing behavior for TypeScript, strict mode, var scopes, and cases where the hoist scope already has a binding.

crates/oxc_transformer/src/es2017/async_to_generator.rs

@@ -451,9 +451,17 @@ impl<'a> AsyncGeneratorExecutor<'a> {
             Self::create_placeholder_params(&wrapper_function.params, wrapper_scope_id, ctx);
         let params = mem::replace(&mut wrapper_function.params, params);
 
+        let function_binding_scope_id =
+            Self::function_declaration_binding_scope_id(wrapper_function.id.as_ref(), ctx);
+        if function_binding_scope_id != ctx.current_scope_id()
+            && let Some(id) = &wrapper_function.id
+        {
+            Self::move_binding_identifier_to_target_scope(function_binding_scope_id, id, ctx);
+        }
+
         let bound_ident = Self::create_bound_identifier(
             wrapper_function.id.as_ref(),
-            ctx.current_scope_id(),
+            Self::function_declaration_binding_scope_id(None, ctx),
             SymbolFlags::Function,
             ctx,
         );
@@ -510,6 +518,31 @@ impl<'a> AsyncGeneratorExecutor<'a> {
         }
     }
 
+    /// Returns the binding scope a rebuilt semantic tree would use for a plain function
+    /// declaration emitted at the current position.
+    fn function_declaration_binding_scope_id(
+        id: Option<&BindingIdentifier<'a>>,
+        ctx: &TraverseCtx<'a>,
+    ) -> ScopeId {
+        if ctx.state.source_type.is_typescript() {
+            return ctx.current_scope_id();
+        }
+
+        let scope_flags = ctx.current_scope_flags();
+        if scope_flags.is_var() || scope_flags.is_strict_mode() {
+            return ctx.current_scope_id();
+        }
+
+        let hoist_scope_id = ctx.current_hoist_scope_id();
+        if let Some(id) = id
+            && ctx.scoping().scope_has_binding(hoist_scope_id, id.name)
+        {
+            ctx.current_scope_id()
+        } else {
+            hoist_scope_id
+        }
+    }
+
     /// Transforms async arrow functions into generator functions wrapped in the asyncToGenerator helper.
     pub(self) fn transform_arrow_function(
         &self,

tasks/coverage/snapshots/semantic_test262.snap

@@ -2,7 +2,7 @@ commit: de8e621c
 
 semantic_test262 Summary:
 AST Parsed     : 47240/47240 (100.00%)
-Positive Passed: 47020/47240 (99.53%)
+Positive Passed: 47028/47240 (99.55%)
 semantic Error: tasks/coverage/test262/test/language/eval-code/direct/async-func-decl-a-following-parameter-is-named-arguments-declare-arguments-and-assign.js
 Scope flags mismatch:
 after transform: ScopeId(4): ScopeFlags(Function)
@@ -760,67 +760,11 @@ Scope flags mismatch:
 after transform: ScopeId(6): ScopeFlags(Function)
 rebuilt        : ScopeId(2): ScopeFlags(Function | DirectEval)
 
-semantic Error: tasks/coverage/test262/test/language/statements/async-function/unscopables-with-in-nested-fn.js
-Bindings mismatch:
-after transform: ScopeId(0): ["_asyncToGenerator", "callCount", "count", "v"]
-rebuilt        : ScopeId(0): ["_asyncToGenerator", "_ref", "callCount", "count", "ref", "v"]
-Bindings mismatch:
-after transform: ScopeId(1): ["_ref", "ref"]
-rebuilt        : ScopeId(1): []
-Symbol scope ID mismatch for "ref":
-after transform: SymbolId(3): ScopeId(1)
-rebuilt        : SymbolId(4): ScopeId(0)
-Symbol scope ID mismatch for "_ref":
-after transform: SymbolId(7): ScopeId(1)
-rebuilt        : SymbolId(6): ScopeId(0)
-
-semantic Error: tasks/coverage/test262/test/language/statements/async-function/unscopables-with.js
-Bindings mismatch:
-after transform: ScopeId(0): ["_asyncToGenerator", "callCount", "count", "v"]
-rebuilt        : ScopeId(0): ["_asyncToGenerator", "_ref", "callCount", "count", "ref", "v"]
-Bindings mismatch:
-after transform: ScopeId(1): ["_ref", "ref"]
-rebuilt        : ScopeId(1): []
-Symbol scope ID mismatch for "ref":
-after transform: SymbolId(3): ScopeId(1)
-rebuilt        : SymbolId(4): ScopeId(0)
-Symbol scope ID mismatch for "_ref":
-after transform: SymbolId(7): ScopeId(1)
-rebuilt        : SymbolId(6): ScopeId(0)
-
 semantic Error: tasks/coverage/test262/test/language/statements/async-generator/eval-var-scope-syntax-err.js
 Scope flags mismatch:
 after transform: ScopeId(4): ScopeFlags(Function)
 rebuilt        : ScopeId(2): ScopeFlags(Function | DirectEval)
 
-semantic Error: tasks/coverage/test262/test/language/statements/async-generator/unscopables-with-in-nested-fn.js
-Bindings mismatch:
-after transform: ScopeId(0): ["_wrapAsyncGenerator", "callCount", "count", "v"]
-rebuilt        : ScopeId(0): ["_ref", "_wrapAsyncGenerator", "callCount", "count", "ref", "v"]
-Bindings mismatch:
-after transform: ScopeId(1): ["_ref", "ref"]
-rebuilt        : ScopeId(1): []
-Symbol scope ID mismatch for "ref":
-after transform: SymbolId(3): ScopeId(1)
-rebuilt        : SymbolId(4): ScopeId(0)
-Symbol scope ID mismatch for "_ref":
-after transform: SymbolId(7): ScopeId(1)
-rebuilt        : SymbolId(6): ScopeId(0)
-
-semantic Error: tasks/coverage/test262/test/language/statements/async-generator/unscopables-with.js
-Bindings mismatch:
-after transform: ScopeId(0): ["_wrapAsyncGenerator", "callCount", "count", "v"]
-rebuilt        : ScopeId(0): ["_ref", "_wrapAsyncGenerator", "callCount", "count", "ref", "v"]
-Bindings mismatch:
-after transform: ScopeId(1): ["_ref", "ref"]
-rebuilt        : ScopeId(1): []
-Symbol scope ID mismatch for "ref":
-after transform: SymbolId(3): ScopeId(1)
-rebuilt        : SymbolId(4): ScopeId(0)
-Symbol scope ID mismatch for "_ref":
-after transform: SymbolId(7): ScopeId(1)
-rebuilt        : SymbolId(6): ScopeId(0)
-
 semantic Error: tasks/coverage/test262/test/language/statements/class/cpn-class-decl-fields-computed-property-name-from-arrow-function-expression.js
 Scope flags mismatch:
 after transform: ScopeId(2): ScopeFlags(StrictMode | Function | Arrow)
@@ -1209,34 +1153,6 @@ Scope flags mismatch:
 after transform: ScopeId(1): ScopeFlags(StrictMode | DirectEval)
 rebuilt        : ScopeId(1): ScopeFlags(StrictMode)
 
-semantic Error: tasks/coverage/test262/test/language/statements/for-await-of/async-func-decl-dstr-obj-id-put-unresolvable-no-strict.js
-Bindings mismatch:
-after transform: ScopeId(0): ["_asyncIterator", "_asyncToGenerator"]
-rebuilt        : ScopeId(0): ["_asyncIterator", "_asyncToGenerator", "_fn", "fn"]
-Bindings mismatch:
-after transform: ScopeId(1): ["_fn", "fn", "iterCount", "promise"]
-rebuilt        : ScopeId(1): ["iterCount", "promise"]
-Symbol scope ID mismatch for "fn":
-after transform: SymbolId(1): ScopeId(1)
-rebuilt        : SymbolId(3): ScopeId(0)
-Symbol scope ID mismatch for "_fn":
-after transform: SymbolId(10): ScopeId(1)
-rebuilt        : SymbolId(4): ScopeId(0)
-
-semantic Error: tasks/coverage/test262/test/language/statements/for-await-of/async-func-decl-dstr-obj-prop-put-unresolvable-no-strict.js
-Bindings mismatch:
-after transform: ScopeId(0): ["_asyncIterator", "_asyncToGenerator"]
-rebuilt        : ScopeId(0): ["_asyncIterator", "_asyncToGenerator", "_fn", "fn"]
-Bindings mismatch:
-after transform: ScopeId(1): ["_fn", "fn", "iterCount", "promise"]
-rebuilt        : ScopeId(1): ["iterCount", "promise"]
-Symbol scope ID mismatch for "fn":
-after transform: SymbolId(1): ScopeId(1)
-rebuilt        : SymbolId(3): ScopeId(0)
-Symbol scope ID mismatch for "_fn":
-after transform: SymbolId(10): ScopeId(1)
-rebuilt        : SymbolId(4): ScopeId(0)
-
 semantic Error: tasks/coverage/test262/test/language/statements/for-await-of/async-func-decl-dstr-obj-rest-getter.js
 Scope parent mismatch:
 after transform: ScopeId(3): Some(ScopeId(2))
@@ -1257,34 +1173,6 @@ Scope parent mismatch:
 after transform: ScopeId(3): Some(ScopeId(2))
 rebuilt        : ScopeId(6): Some(ScopeId(5))
 
-semantic Error: tasks/coverage/test262/test/language/statements/for-await-of/async-gen-decl-dstr-obj-id-put-unresolvable-no-strict.js
-Bindings mismatch:
-after transform: ScopeId(0): ["_asyncIterator", "_awaitAsyncGenerator", "_wrapAsyncGenerator"]
-rebuilt        : ScopeId(0): ["_asyncIterator", "_awaitAsyncGenerator", "_fn", "_wrapAsyncGenerator", "fn"]
-Bindings mismatch:
-after transform: ScopeId(1): ["_fn", "fn", "iterCount", "promise"]
-rebuilt        : ScopeId(1): ["iterCount", "promise"]
-Symbol scope ID mismatch for "fn":
-after transform: SymbolId(1): ScopeId(1)
-rebuilt        : SymbolId(4): ScopeId(0)
-Symbol scope ID mismatch for "_fn":
-after transform: SymbolId(11): ScopeId(1)
-rebuilt        : SymbolId(5): ScopeId(0)
-
-semantic Error: tasks/coverage/test262/test/language/statements/for-await-of/async-gen-decl-dstr-obj-prop-put-unresolvable-no-strict.js
-Bindings mismatch:
-after transform: ScopeId(0): ["_asyncIterator", "_awaitAsyncGenerator", "_wrapAsyncGenerator"]
-rebuilt        : ScopeId(0): ["_asyncIterator", "_awaitAsyncGenerator", "_fn", "_wrapAsyncGenerator", "fn"]
-Bindings mismatch:
-after transform: ScopeId(1): ["_fn", "fn", "iterCount", "promise"]
-rebuilt        : ScopeId(1): ["iterCount", "promise"]
-Symbol scope ID mismatch for "fn":
-after transform: SymbolId(1): ScopeId(1)
-rebuilt        : SymbolId(4): ScopeId(0)
-Symbol scope ID mismatch for "_fn":
-after transform: SymbolId(11): ScopeId(1)
-rebuilt        : SymbolId(5): ScopeId(0)
-
 semantic Error: tasks/coverage/test262/test/language/statements/for-await-of/async-gen-decl-dstr-obj-rest-getter.js
 Scope parent mismatch:
 after transform: ScopeId(3): Some(ScopeId(2))
@@ -1328,21 +1216,9 @@ after transform: SymbolId(1) "x"
 rebuilt        : SymbolId(1) "x"
 
 semantic Error: tasks/coverage/test262/test/language/statements/switch/scope-lex-async-function.js
-Bindings mismatch:
-after transform: ScopeId(0): ["_asyncToGenerator"]
-rebuilt        : ScopeId(0): ["_asyncToGenerator", "_x", "x"]
-Bindings mismatch:
-after transform: ScopeId(1): ["_x", "x"]
-rebuilt        : ScopeId(1): []
-Symbol scope ID mismatch for "x":
-after transform: SymbolId(0): ScopeId(1)
-rebuilt        : SymbolId(1): ScopeId(0)
 Symbol reference IDs mismatch for "x":
 after transform: SymbolId(0): []
 rebuilt        : SymbolId(1): [ReferenceId(7)]
-Symbol scope ID mismatch for "_x":
-after transform: SymbolId(1): ScopeId(1)
-rebuilt        : SymbolId(2): ScopeId(0)
 Reference symbol mismatch for "x":
 after transform: <None>
 rebuilt        : SymbolId(1) "x"
@@ -1351,21 +1227,9 @@ after transform: ["arguments", "require", "x"]
 rebuilt        : ["arguments", "require"]
 
 semantic Error: tasks/coverage/test262/test/language/statements/switch/scope-lex-async-generator.js
-Bindings mismatch:
-after transform: ScopeId(0): ["_wrapAsyncGenerator"]
-rebuilt        : ScopeId(0): ["_wrapAsyncGenerator", "_x", "x"]
-Bindings mismatch:
-after transform: ScopeId(1): ["_x", "x"]
-rebuilt        : ScopeId(1): []
-Symbol scope ID mismatch for "x":
-after transform: SymbolId(0): ScopeId(1)
-rebuilt        : SymbolId(1): ScopeId(0)
 Symbol reference IDs mismatch for "x":
 after transform: SymbolId(0): []
 rebuilt        : SymbolId(1): [ReferenceId(7)]
-Symbol scope ID mismatch for "_x":
-after transform: SymbolId(1): ScopeId(1)
-rebuilt        : SymbolId(2): ScopeId(0)
 Reference symbol mismatch for "x":
 after transform: <None>
 rebuilt        : SymbolId(1) "x"