fix(minifier): avoid illegal var; when folding unused arguments copy loop#21421
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Dunqing merged 2 commits intoApr 15, 2026
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…y loop When the copy result has no binding pattern, clear the `for` initializer instead of emitting an empty `VariableDeclaration`. Made-with: Cursor
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### 💥 BREAKING CHANGES - 24fb7eb allocator: [**BREAKING**] Rename `Box` and `Vec` methods (#21395) (overlookmotel) ### 🚀 Features - ce5072d parser: Support `turbopack` magic comments (#20803) (Kane Wang) - f5deb55 napi/transform: Expose `optimizeConstEnums` and `optimizeEnums` options (#21388) (Dunqing) - 24b03de data_structures: Introduce `NonNullConst` and `NonNullMut` pointer types (#21425) (overlookmotel) ### 🐛 Bug Fixes - d7a359a ecmascript: Treat update expressions as unconditionally side-effectful (#21456) (Dunqing) - 56af2f4 transformer/async-to-generator: Correct scope of inferred named FE in async-to-generator (#21458) (Dunqing) - b3ed467 minifier: Avoid illegal `var;` when folding unused arguments copy loop (#21421) (fazba) - b0e8f13 minifier: Preserve `var` inside `catch` with same-named parameter (#21366) (Dunqing) - 4fb73a7 transformer/typescript: Preserve execution order for accessor with `useDefineForClassFields: false` (#21369) (Dunqing) ### ⚡ Performance - da3cc16 parser: Refactor out `LexerContext` (#21275) (Ulrich Stark) ### 📚 Documentation - c5b19bb allocator: Reformat comments in `Arena` (#21448) (overlookmotel) - 091e88e lexer: Update doc comment about perf benefit of reading through references (#21423) (overlookmotel) - 922cbee allocator: Remove references to "bump" from comments (#21397) (overlookmotel) Co-authored-by: Dunqing <[email protected]>
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Summary
When peephole-folding the classic
argumentscopy loop, we could end up with an emptyVariableDeclarationinitializer (for (var; …)), which prints as invalidvar;and can confuse downstream folding (e.g.try_fold_forhoisting).Changes
[...arguments]/.slice(offset)array initializer when there is a binding pattern for the copy result (r_id_pat).for_stmt.initinstead of emitting an emptyVariableDeclaration, so the loop becomesfor (; 0;)and dead-code elimination can remove it cleanly.Tests
crates/oxc_minifier/tests/peephole/substitute_alternate_syntax.rscovering unused copy result + consequent (must not become illegalvar;).AI usage
This change was developed with assistance from Cursor; I reviewed the diff and tests locally before opening this PR.