fix(minifier): make `__proto__` write tracking execution-order independent by Dunqing · Pull Request #24280 · oxc-project/oxc

Dunqing · 2026-07-08T00:46:58Z

What

In propertyWriteSideEffects: false mode, the minifier must keep a property write when the object's __proto__ was written somewhere, because that write may have installed a setter. This tracking ran inside the same pass that removes writes, so a __proto__ write the traversal reaches later — for example inside a hoisted function — was not recorded yet, and the property write was wrongly dropped.

How

The per-pass proto_write_symbols set is replaced by a PROTO_WRITTEN bit on PersistentSymbolFacts (introduced in the parent PR). The Normalize pass seeds it for the whole program before the fixed-point loop starts, so the order the loop visits expressions no longer matters. track_proto_write and the old set are deleted. The scan also catches shapes the old tracking never saw, such as destructuring targets ([o.__proto__] = arr).

Default mode is unaffected: a symbol with a __proto__ write already carries MEMBER_WRITE_HAZARD, which the default path checks separately. tasks/minsize output is byte-identical; the previously commented-out ordering test in remove_unused_expression.rs now runs.

Example

With propertyWriteSideEffects: false:

const obj = {};
f();
obj.a = 1;
function f() {
  g();
  obj.__proto__ = { set a(v) { console.log(v); } };
}

Before: obj.a = 1 was dropped. The setter installed when f() runs never fires, so console.log(1) is lost.

After: obj.a = 1 is kept.

Dunqing · 2026-07-08T00:47:12Z

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codspeed-hq · 2026-07-08T00:51:04Z

Merging this PR will not alter performance

✅ 52 untouched benchmarks
⏩ 19 skipped benchmarks¹

_{Comparing minifier-proto-written-fact (9c1b03b) with minifier-write-only-prop-dce (144288b)}

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. ↩

oxc-guard · 2026-07-08T01:45:37Z

Monitor Oxc

Commit: 9c1b03b099ef

suite	result
Test262	✅
Build	✅
Isolated Declarations	✅
(whitespace)	✅
(mangler)	❌
(transformer)	✅
(compressor)	✅
(dce)	✅
(codegen)	✅
(formatter)	❌
(formatter_dcr)	✅

Dunqing · 2026-07-08T02:27:49Z

Merge activity

Jul 8, 2:27 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
Jul 8, 2:27 AM UTC: Dunqing added this pull request to the Graphite merge queue.
Jul 8, 2:33 AM UTC: Merged by the Graphite merge queue.

…ndent (#24280) Fixes #21248 ### What In `propertyWriteSideEffects: false` mode, the minifier must keep a property write when the object's `__proto__` was written somewhere, because that write may have installed a setter. This tracking ran inside the same pass that removes writes, so a `__proto__` write the traversal reaches later — for example inside a hoisted function — was not recorded yet, and the property write was wrongly dropped. ### How The per-pass `proto_write_symbols` set is replaced by a `PROTO_WRITTEN` bit on `PersistentSymbolFacts` (introduced in the parent PR). The `Normalize` pass seeds it for the whole program before the fixed-point loop starts, so the order the loop visits expressions no longer matters. `track_proto_write` and the old set are deleted. The scan also catches shapes the old tracking never saw, such as destructuring targets (`[o.__proto__] = arr`). Default mode is unaffected: a symbol with a `__proto__` write already carries `MEMBER_WRITE_HAZARD`, which the default path checks separately. `tasks/minsize` output is byte-identical; the previously commented-out ordering test in `remove_unused_expression.rs` now runs. ### Example With `propertyWriteSideEffects: false`: ```js const obj = {}; f(); obj.a = 1; function f() { g(); obj.__proto__ = { set a(v) { console.log(v); } }; } ``` Before: `obj.a = 1` was dropped. The setter installed when `f()` runs never fires, so `console.log(1)` is lost. After: `obj.a = 1` is kept.

### 🚀 Features - 616bfa2 minifier: Remove unreachable code after terminating statements (#24441) (Dunqing) - ddab89a data_structures: Add `likely` and `unlikely` functions (#24368) (overlookmotel) - a3a39f9 react_compiler: Implement enableEmitHookGuards codegen (#24329) (Boshen) - b79eef7 minifier: Apply De Morgan's law to negated comparison chains in jump guards and loop tests (#24279) (Dunqing) - 34ff7b4 minifier: Drop write-only property assignments to unused local bindings by default (#24112) (Dunqing) - 1b829d8 semantic: Record const enums in EnumData (#24268) (Dunqing) - ba0944c semantic: Add `Scoping::set_symbol_span` (#24221) (camc314) ### 🐛 Bug Fixes - 7d33363 minifier: Preserve guaranteed throws from class heritage evaluation (#24349) (Dunqing) - 058a62f semantic: Track ambient contexts in `SemanticBuilder` (#24327) (camc314) - 721eb0b transformer/decorator: Scope accessor class binding (#24330) (camc314) - 1ebdce3 semantic: Allow reserved keywords in ambient declaration types (#24325) (camc314) - 460176a track-memory-allocations: Exclude arena chunks from Sys allocs (#24292) (Dunqing) - af4922b transformer: Clear lowered namespace redeclarations (#24300) (camc314) - ffd2765 semantic: Mark declared computed `MethodDefinition`s as type references (#24296) (camc314) - f17514b isolated-declarations: Emit const readonly fields as types (#24288) (camc314) - 40f769d minifier: Make `__proto__` write tracking execution-order independent (#24280) (Dunqing) - 6371fed transformer: Remove stale enum member bindings (#24272) (camc314) - f05dfab transformer: Correct symbol flags for lowered namespaces (#24271) (Dunqing) - 84eeb55 transformer: Correct symbol flags for lowered enums (#24269) (Dunqing) - c3057da transformer: Preserve generated class binding spans (#24220) (camc314) - 8260096 transformer: Correct span for lowered namespace symbol (#24222) (camc314) - 42d00d3 semantic: Mark declared class heritage as type references (#24237) (camc314) - 588d997 semantic: Mark TS `PropertyDefinition`s computed fields as type references (#24233) (camc314) - 9b95632 semantic: Mark computed method keys in `TSMethodSignature`s as type references (#24232) (camc314) ### ⚡ Performance - 5b26643 transformer_plugins: Dispatch global defines by trailing name (#23666) (Boshen) - dce0f29 react_compiler: Replace all compiled functions in a single AST walk (#24403) (Boshen) - f85f0d8 ast: Delegate inherited enum variants in clone_in and estree derives (#23555) (Boshen) - 3ff0234 allocator: Remove `unwrap` from `ReplaceWith` (#24365) (overlookmotel) - ab22e80 transformer: Fix Rust 1.97 performance regression (#24354) (camc314) - b47585c parser: Use `ReplaceWith` instead of `TakeIn` (#24018) (overlookmotel) - b227a06 minifier: Use `ReplaceWith` instead of `TakeIn` (#24017) (overlookmotel) Co-authored-by: Boshen <[email protected]>

github-actions Bot added the A-minifier Area - Minifier label Jul 8, 2026

Dunqing mentioned this pull request Jul 8, 2026

feat(minifier): drop write-only property assignments to unused local bindings by default #24112

Merged

Dunqing changed the title ~~fix(minifier): make __proto__ write tracking execution-order independent~~ fix(minifier): make __proto__ write tracking execution-order independent Jul 8, 2026

Dunqing added the run-monitor-oxc Add to a PR to dispatch oxc-project/monitor-oxc CI against it label Jul 8, 2026

oxc-guard Bot removed the run-monitor-oxc Add to a PR to dispatch oxc-project/monitor-oxc CI against it label Jul 8, 2026

Dunqing marked this pull request as ready for review July 8, 2026 02:25

Dunqing added the 0-merge Merge with Graphite Merge Queue label Jul 8, 2026

graphite-app Bot force-pushed the minifier-write-only-prop-dce branch from 144288b to 34ff7b4 Compare July 8, 2026 02:28

graphite-app Bot requested a review from overlookmotel as a code owner July 8, 2026 02:28

graphite-app Bot force-pushed the minifier-proto-written-fact branch from 9c1b03b to 40f769d Compare July 8, 2026 02:29

Base automatically changed from minifier-write-only-prop-dce to main July 8, 2026 02:32

graphite-app Bot removed the 0-merge Merge with Graphite Merge Queue label Jul 8, 2026

graphite-app Bot merged commit 40f769d into main Jul 8, 2026
31 checks passed

graphite-app Bot deleted the minifier-proto-written-fact branch July 8, 2026 02:33

oxc-guard Bot mentioned this pull request Jul 13, 2026

release(crates): oxc v0.140.0 #24453

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fix(minifier): make `proto` write tracking execution-order independent#24280

fix(minifier): make `proto` write tracking execution-order independent#24280
graphite-app[bot] merged 1 commit into
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minifier-proto-written-fact

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