fix(es/minifier): Keep function and class declarations whose self-reference is dropped

[Baltasar Blanco (baltasarblanco)]

Description:

The tree shaker's Analyzer deliberately does not count a function's or class's references to itself (VarInfo docs: "This does not include self-references in a function"). The Dropper, which subtracts usages when a node is removed, had no such exclusion. So when an unused local initialized with the enclosing function's name was dropped, the function lost a usage it never gained, hit zero, and was dropped on the next pass while still referenced from outside:

(function () {
  function handler() { var self = handler; }
  setTimeout(handler, 0);
})();
// ReferenceError: handler is not defined

This makes the Dropper track the innermost enclosing named function/class the same way the Analyzer does, and skip self-references when subtracting. The same mechanism applied to class declarations, so both are covered.

Tests:

Two exec tests in tests/exec.rs (function declaration and class declaration variants). Both fail on main with ReferenceError and pass with this change. ./scripts/exec.sh and the full swc_ecma_transforms_optimization and swc_ecma_minifier suites pass with no fixture output changes.

Related issue (if exists):

Closes #12400

[changeset-bot]

🦋 Changeset detected

Latest commit: 3644472

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[codspeed]

Merging this PR will not alter performance

✅ 200 untouched benchmarks
⏩ 67 skipped benchmarks¹

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_{Comparing baltasarblanco:fix/12400-dce-self-ref-usage (3644472) with main (42e4f7e)}

Footnotes

1. 67 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. ↩

[Donny/강동윤 (kdy1)]

Thanks!