fix(minifier): avoid merging side effectful expressions to next assignment statement if the side effect may change the left hand side reference

[sapphi-red]

fixes rolldown/rolldown#7093

function wrapper() { let foo; const bar = (() => { foo = {} })(); foo.bar = bar; return foo }

cannot be compressed to

function wrapper() { let foo; return foo.bar = (() => { foo = {} })(), foo }

because foo is read on left hand side.

[sapphi-red]

• fix(minifier): avoid merging side effectful expressions to next assignment statement if the side effect may change the left hand side reference #16165 👈 (View in Graphite)
• main

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[Copilot]

Pull request overview

This PR fixes a minifier bug where side-effectful expressions were incorrectly merged with subsequent assignment statements when those side effects could change the left-hand side reference. The fix prevents invalid optimizations like transforming let foo; const bar = (() => { foo = {} })(); foo.bar = bar into let foo; foo.bar = (() => { foo = {} })(), which would execute in a different order and produce incorrect results.

Key changes:

• Added logic to detect when a replacement expression's side effects could mutate the object being assigned to
• Introduced helper function to check if an expression's reference may be changed by side effects
• Added comprehensive test cases covering various scenarios with mutable references

Reviewed changes

Copilot reviewed 4 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
crates/oxc_minifier/src/peephole/minimize_statements.rs	Core fix: Added check to prevent merging when side effects could change the assignment target's object reference, plus helper function to determine if expressions may be mutated
crates/oxc_minifier/tests/peephole/inline_single_use_variable.rs	Added test cases validating the fix for various scenarios including computed properties, member expressions, and this references
napi/parser/generated/deserialize/ts_range_parent.js	Updated generated code affected by the minifier fix - avoids incorrectly merging assignments where side effects change references
napi/parser/generated/deserialize/ts_parent.js	Updated generated code affected by the minifier fix
napi/parser/generated/deserialize/js_range_parent.js	Updated generated code affected by the minifier fix
napi/parser/generated/deserialize/js_parent.js	Updated generated code affected by the minifier fix
apps/oxlint/src-js/generated/deserialize.js	Updated generated code affected by the minifier fix
tasks/track_memory_allocations/allocs_minifier.snap	Snapshot update showing minimal memory allocation changes from the fix
tasks/minsize/minsize.snap	Snapshot update showing negligible impact on minified output size

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[codspeed-hq]

CodSpeed Performance Report

Merging #16165 will not alter performance

_{Comparing 11-26-fix_minifier_avoid_merging_side_effectful_expressions_to_next_assignment_statement_if_the_side_effect_may_change_the_left_hand_side_reference (25df1e9) with main (48d6ed2)¹}

Summary

✅ 38 untouched
⏩ 7 skipped²

Footnotes

1. No successful run was found on main (e8b6169) during the generation of this report, so 48d6ed2 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 7 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. ↩

[Boshen]

Merge activity

• Nov 27, 7:35 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Nov 27, 7:35 AM UTC: Boshen added this pull request to the Graphite merge queue.
• Nov 27, 7:41 AM UTC: Merged by the Graphite merge queue.