fix(semantic): hoist Annex B block-scoped function declarations to var scope

[Dunqing]

close: #20705

This PR replaces the mangler-level fix in #20705 with a semantic-level fix that is simpler and allows better name reuse.

Summary

In sloppy mode JavaScript, function declarations inside blocks (if, try, switch, {}) have Annex B.3.2.1 semantics: they create an implicit var-like binding in the enclosing function scope at block exit. The semantic analysis was not modeling this hoisting, which caused the mangler to assign conflicting names to block-scoped functions and outer var bindings — changing program behavior at runtime.

How the fix works

Hoist plain (non-async, non-generator) function declarations from block scopes to the enclosing var scope during binding, using the same pattern as var hoisting (binder.rs lines 46-98):

1. move_binding(block_scope, var_scope, name) — moves the binding
2. set_symbol_scope_id(symbol_id, var_scope) — updates the symbol's scope
3. hoisting_variables.insert(block_scope, name, symbol_id) — keeps a hoisting entry in the block scope for redeclaration checks

This makes the mangler handle Annex B functions naturally alongside other var-scope bindings, with zero mangler changes needed. Name reuse from sibling scopes is preserved (unlike the mangler-level fix which was overly conservative).

Guards

• is_declaration — function expressions are bound in their own (var) scope
• !self.r#async && !self.generator — Annex B only applies to plain functions
• !builder.source_type.is_typescript() — Annex B is JavaScript-only
• !scope_flags.is_var() — already in a var scope, no hoisting needed
• !scope_flags.is_strict_mode() — no Annex B in strict mode / modules

Fixes #20610. Fixes #14316.

Downstream: vitejs/vite#22009, rolldown/rolldown#8791.

Test plan

• 13 sloppy-mode mangler snapshot test cases (all pass without mangler changes)
• 975 linter tests pass
• 40 semantic unit tests pass
• Parser conformance: improved (babel negative 1687→1689, test262 negative stays 100%)
• Semantic conformance: ~10 new mismatches in semantic_test262 — these are transformer rebuild differences where the transformed JS output correctly gets Annex B hoisting that the original TS parse didn't have

[Dunqing]

• fix(semantic): hoist Annex B block-scoped function declarations to var scope #20728 👈 (View in Graphite)
• main

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[codspeed-hq]

Merging this PR will not alter performance

✅ 44 untouched benchmarks
⏩ 12 skipped benchmarks¹

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_{Comparing fix/semantic-annex-b-block-scoped-function (412a86a) with main (0da7c3b)}

Footnotes

1. 12 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. ↩

[Dunqing]

@sapphi-red, could you take a look at the handling in Semantic, which looks clean and follows the spec.

[sapphi-red]

Implementation looks good to me.
I think we should add a test that ensures

console.log(`typeof foo is ${typeof foo}`);
if (true) {
  function foo() {
    return 1;
  }
}

is minified to

console.log(typeof foo); // should not be replaced with 'function' nor 'undefined'
if (true) {
  function foo() {
    return 1;
  }
}

just in case.

[Dunqing]

Implementation looks good to me. I think we should add a test that ensures

console.log(`typeof foo is ${typeof foo}`);
if (true) {
  function foo() {
    return 1;
  }
}

is minified to

console.log(typeof foo); // should not be replaced with 'function' nor 'undefined'
if (true) {
  function foo() {
    return 1;
  }
}

just in case.

Added

[Dunqing]

I will take a look at those failed semantic tests later.

[Dunqing]

Merge activity

• Mar 26, 1:20 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Mar 26, 1:21 AM UTC: Dunqing added this pull request to the Graphite merge queue.
• Mar 26, 1:25 AM UTC: Merged by the Graphite merge queue.