fix(minifier): disallow merging assignments to let declarations when TDZ error would be introduced

[sapphi-red]

let x;
x = (() => x?.a)();

should not be transformed to

let x = (() => x?.a)();

, as it would introduce a TDZ error.

To fix that, I limited the right hand side of the assignment to be a literal value for let declarations.

The precise check would be to check whether the right hand side includes a reference to the variable declared on the left hand side, but I didn't do that for now as it is complicated. If we implement that, we can improve this condition too.

oxc/crates/oxc_minifier/src/peephole/minimize_statements.rs

Lines 544 to 548 in 5d597f7

	// should not move assignment above other variables for let
	// this could cause TDZ errors (e.g. `let a, b; b = a;`)
	if decl.kind == VariableDeclarationKind::Let {
	break;
	}

fixes #13615
refs #13270

[sapphi-red]

• fix(minifier): disallow merging assignments to let declarations when TDZ error would be introduced #13635 👈 (View in Graphite)
• main

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #13635 will not alter performance

_{Comparing 09-10-fix_minifier_disallow_merging_assignments_to_let_declarations_when_tdz_error_would_be_introduced (f9fd65b) with main (476729b)¹}

Summary

✅ 37 untouched benchmarks

Footnotes

1. No successful run was found on main (f9fd65b) during the generation of this report, so 476729b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[Copilot]

Pull Request Overview

This PR fixes a bug in the minifier where merging assignments to let declarations could introduce Temporal Dead Zone (TDZ) errors. The fix prevents merging when the assignment's right-hand side is not a literal value for let declarations, ensuring that expressions like let x; x = (() => x?.a)() are not incorrectly transformed to let x = (() => x?.a)().

• Added safety check to prevent TDZ errors when merging assignments to let declarations
• Restricted merging to literal values only for let declarations while keeping existing behavior for var
• Added comprehensive test cases covering various TDZ scenarios

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/oxc_minifier/src/peephole/minimize_statements.rs	Core logic fix adding TDZ safety check for let declarations
crates/oxc_minifier/tests/peephole/merge_assignments_to_declarations.rs	Test cases covering TDZ error scenarios
tasks/track_memory_allocations/allocs_minifier.snap	Memory allocation tracking update reflecting the code changes

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[Boshen]

Merge activity

• Sep 10, 4:10 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Sep 10, 4:10 AM UTC: Boshen added this pull request to the Graphite merge queue.
• Sep 10, 4:38 AM UTC: Merged by the Graphite merge queue.