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fix(transformer): hoist lowered async declarations#22770

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codex/hoist-lowered-async-declarations
Jul 2, 2026
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fix(transformer): hoist lowered async declarations#22770
graphite-app[bot] merged 1 commit into
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codex/hoist-lowered-async-declarations

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@camc314 camc314 commented May 27, 2026

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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:

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

becomes roughly:

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.

@github-actions github-actions Bot added the A-transformer Area - Transformer / Transpiler label May 27, 2026
@camc314
camc314 marked this pull request as ready for review May 27, 2026 13:46
Copilot AI review requested due to automatic review settings May 27, 2026 13:46
@codspeed-hq

codspeed-hq Bot commented May 27, 2026

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Merging this PR will not alter performance

✅ 52 untouched benchmarks
⏩ 19 skipped benchmarks1


Comparing codex/hoist-lowered-async-declarations (b984403) with main (851ee43)

Open in CodSpeed

Footnotes

  1. 19 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports.

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Pull request overview

This PR fixes semantic scope mismatches in async-to-generator lowering by registering emitted plain function declarations in the same scope a rebuilt semantic tree would use under sloppy-mode Annex B hoisting.

Changes:

  • Adds scope selection logic for lowered async function declarations.
  • Moves original lowered function bindings to the appropriate hoist scope when needed.
  • Updates the Test262 semantic snapshot to reflect newly passing cases.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File Description
crates/oxc_transformer/src/es2017/async_to_generator.rs Adds helper logic to choose binding scopes for lowered function declarations and generated helper declarations.
tasks/coverage/snapshots/semantic_test262.snap Updates semantic coverage results after resolving related binding mismatches.

@camc314
camc314 force-pushed the codex/hoist-lowered-async-declarations branch from d6ebfa3 to b984403 Compare July 2, 2026 19:35
@camc314 camc314 added the 0-merge Merge with Graphite Merge Queue label Jul 2, 2026

camc314 commented Jul 2, 2026

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Merge activity

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.
@graphite-app
graphite-app Bot force-pushed the codex/hoist-lowered-async-declarations branch from b984403 to 018c0e5 Compare July 2, 2026 19:47
@graphite-app
graphite-app Bot merged commit 018c0e5 into main Jul 2, 2026
29 checks passed
@graphite-app graphite-app Bot removed the 0-merge Merge with Graphite Merge Queue label Jul 2, 2026
@graphite-app
graphite-app Bot deleted the codex/hoist-lowered-async-declarations branch July 2, 2026 19:52
camc314 added a commit that referenced this pull request Jul 3, 2026
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.
camc314 added a commit that referenced this pull request Jul 3, 2026
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.
Boshen added a commit that referenced this pull request Jul 6, 2026
### 🚀 Features

- 260425f semantic/examples: Include unresolved references (#24214)
(camc314)
- 2d9b0b3 minifier: Fold boolean-literal ternary branches in value
contexts (#24110) (Dunqing)
- 61fbf10 ast: Implement `ReplaceWith` on all AST types (#24013)
(overlookmotel)
- 7db7a29 allocator: Add `ReplaceWith` trait (#24012) (overlookmotel)
- 4eb074e mangler: Add `reserved` option for names that must not be
mangled (#24041) (Dunqing)
- 2e62012 data_structures: Add `StringExt` trait (#24006)
(overlookmotel)
- 60e7160 minifier: Drop side-effect-free IIFEs whose result is unused
(#23967) (Dunqing)
- 26dd9e2 ast: Add method to widen inherited enum ref to parent ref
(#23961) (overlookmotel)

### 🐛 Bug Fixes

- e8b50ee transformer: Clean up semantics for stripped TypeScript syntax
(#24180) (camc314)
- d966d0b react_compiler: Remove clippy allows (#24168) (Boshen)
- 854ef8d react_compiler: Compile generic functions instead of
over-bailing on type-param hoisting (#24158) (Boshen)
- 093586c react_compiler: Align memoization cache-slot allocation with
Babel (#24157) (Boshen)
- 09c8f59 react_compiler: Normalize snapshot fixture paths (#24142)
(camc314)
- f13df97 react_compiler: Drop stray empty statement from catch bindings
(#24133) (Boshen)
- cb2a505 react_compiler: Codegen destructuring reassignment targets
(#24131) (Boshen)
- b82c394 react_compiler: Propagate codegen invariants instead of
emitting empty bodies (#24128) (Boshen)
- 5771982 react_compiler: Render unchanged programs as source in fixture
snapshots (#24129) (Boshen)
- 4b16e1a transformer/async-to-generator: Preserve direct eval scope
flags (#24136) (camc314)
- 4e9194f react_compiler: Lower `delete obj.prop` to
Property/ComputedDelete (#24123) (Boshen)
- 0b25582 ast: Type binding node `typeAnnotation` as `TSTypeAnnotation |
null` (#23113) (Boshen)
- 018c0e5 transformer: Hoist lowered async declarations (#22770)
(camc314)
- 652fbaf mangler: Keep names of destructured exported bindings (#24036)
(Dunqing)
- e274415 minifier: Don't drop global calls that throw despite pure
arguments (#23917) (Dunqing)
- 59abb30 minifier: Only merge string literals in `try_fold_add` when
the inner operator is `+` (#23622) (Jerry Zhao)

### ⚡ Performance

- c5ca77b transformer: Avoid cloning refresh options (#24191) (camc314)
- bf1a151 react_compiler: Compile out debug printers (#24184) (Boshen)
- abb44a0 transformer: Build fixed object-rest arguments (#24190)
(camc314)
- a4db731 isolated_declarations: Use `ReplaceWith` instead of `TakeIn`
(#24016) (overlookmotel)
- ff10855 transformer: Use `ReplaceWith` instead of `TakeIn` (#24015)
(overlookmotel)
- bd49aff ecmascript: Avoid heap-allocating Math.min/max/imul operands
(#23941) (Lawrence Lin)
- e4b708b react_compiler: Skip compiled files before prefilters (#24171)
(Boshen)
- c59f2fe rust: Return impl ExactSizeIterator from slice-backed
accessors (#24144) (Boshen)
- 5d6d04a codegen: SWAR-skip boring byte runs in sourcemap line/column
scan (#24023) (Boshen)
- a55e0be traverse: Reduce string operations in `get_var_name_from_node`
(#24007) (overlookmotel)
- e6d48e1 transformer/nullish_coalescing: Move cold path into separate
function (#23989) (overlookmotel)
- c4e35b5 transformer/object_rest_spread: Pre-allocate capacity in `Vec`
(#23988) (overlookmotel)
- 527b8e5 transformer/decorators: Narrow type earlier (#23987)
(overlookmotel)

### 📚 Documentation

- 30d17f5 allocator: Clarify docs for `TakeIn::take_in_box` (#24093)
(overlookmotel)
- 675e6a8 ast: Correct doc comment for `PrivateFieldExpression` (#24008)
(overlookmotel)
- e4c30e6 minifier: Explain what `dce` mode means (#23994) (Dunqing)
- 37cbf88 ast_macros: Document fields of `StructDetails` (#23959)
(overlookmotel)
- 4de3e54 ast: Correct doc comment (#23948) (overlookmotel)

Co-authored-by: Boshen <[email protected]>
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