fix(es/minifier): Avoid generating mangled property names that collide with existing properties by baltasarblanco · Pull Request #11839 · swc-project/swc

baltasarblanco · 2026-05-02T13:06:23Z

What's the problem this PR addresses?

When jsc.minify.mangle.props is enabled, the property mangler can rename a property in a subclass to a name that already exists as a property of a base class (or of any other object in the program), producing observable shadowing at runtime.

Minimal repro from the issue:

class Class1 { e = 1; }
class Class2 extends Class1 { someField = 2; }
console.log(new Class2().e); // expected: 1

Before the fix, someField was mangled to e, so new Class2().e returned 2 instead of the inherited 1.

How did you fix it?

Root cause is in crates/swc_ecma_minifier/src/pass/mangle_props.rs. The state already builds an unmangleable set containing both reserved DOM/JS property names (loaded from domprops.json / jsprops.json) and all property names seen in the program. But gen_name called Base54Chars::encode directly, which only skips JavaScript reserved words — it never consults unmangleable.

In the repro, e is present in domprops.json, so Class1.e is correctly preserved by is_reserved. The encoder then walks a, b, c, ... and may emit e for someField, colliding with the inherited property.

The fix advances the encoder in a loop until it produces a candidate not in unmangleable. The set already aggregates exactly what we need to avoid, so no extra bookkeeping is needed. In practice this is 0-2 extra encode calls per generated name.

Testing

Added issue_11027_inherited_class_property_collision in crates/swc_ecma_minifier/tests/mangle.rs using the existing assert_mangled harness with props mangling enabled (the fixture-based suite at tests/mangle/** does not enable property mangling, so an inline test is the right vehicle here, in line with the other props-related tests in this file).
cargo test -p swc_ecma_minifier --test mangle passes (2217 tests).
./scripts/exec.sh passes (2129/2130, 1 ignored, unchanged).
cargo fmt --all and cargo clippy -p swc_ecma_minifier --all-targets -- -D warnings clean.

…e with existing properties The property mangler's encoder only skipped JavaScript reserved words when emitting new names, but not the names already in use as properties in the program. The state already aggregated those names in the `unmangleable` set (reserved DOM/JS props plus all property atoms seen in the module), but `gen_name` never consulted it. As a result, for programs containing a one-letter property like `e` (also present in `domprops.json`), the encoder could emit `e` again as the mangled name for an unrelated property in a subclass, producing observable runtime shadowing. Make `gen_name` advance the encoder until a candidate that is not in `unmangleable` is produced. In practice this costs 0-2 extra encode calls per generated name. Closes swc-project#11027

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codspeed-hq · 2026-05-02T13:13:23Z

Merging this PR will not alter performance

✅ 219 untouched benchmarks
⏩ 31 skipped benchmarks¹

_{Comparing baltasarblanco:fix/11027-mangle-props-name-collision (5efb4e2) with main (9300ede)}

31 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. ↩

github-actions · 2026-05-02T13:21:44Z

Binary Sizes

File	Size
`swc.linux-x64-gnu.node`	27M (27759176 bytes)

Commit: e290c52

baltasarblanco · 2026-05-02T15:24:54Z

Regarding the CodSpeed regression on es/large/codegen/es2016 (-2.23%, 37.1ms → 37.9ms): this benchmark exercises the codegen path, while this PR only modifies swc_ecma_minifier::pass::mangle_props — the change is gated behind mangle.props and never executes in a pure codegen run. CodSpeed also flagged "Different runtime environments detected" and notes that no successful main run was available, so the comparison fell back to 16a56d0 which may contain unrelated changes. The variance is consistent with runner noise (~0.8ms on a 37ms bench). Acknowledging on CodSpeed.

kdy1

Can you add some complex test cases?

@kdy1

Cover three additional scenarios that exercise the property mangler's collision avoidance: - Multi-level inheritance chain (`A -> B -> C`) with multiple fields per level, ensuring no mangled name in a deeper subclass collides with a property inherited from any ancestor. - Object literal sharing the namespace with a class, including a reserved DOM/JS property (`valueOf`), confirming the mangler treats both kinds of property declarations consistently. - Mixed shapes (getter on the base, plain methods on both base and derived, instance fields on the derived), confirming the fix holds regardless of how the inherited member is declared. Requested by @kdy1 on swc-project#11839.

baltasarblanco · 2026-05-04T17:10:08Z

Added three additional test cases covering deeper inheritance chains, object-literal-and-class namespace sharing, and mixed shapes (getters / methods / fields). Marking as ready for review.

kdy1 · 2026-05-04T18:00:02Z

Sorry, I should have been more specific.

Please add test cases for lots of properties to mangle

@kdy1

Per @kdy1's request on swc-project#11839, add a test case with a large number of properties to be mangled. The base class declares six fields whose names (`a` through `f`) are all present in `domprops.json`, and the derived class declares 25 fresh fields. Since roughly half of the single-character base54 alphabet collides with reserved DOM property names, generating 25 mangled names exercises the collision-avoidance loop in `gen_name` repeatedly, confirming the fix scales correctly when many candidates must be skipped.

baltasarblanco · 2026-05-04T20:07:47Z

Added issue_11027_many_properties_stress_test: a base class with six fields whose names (a through f) are all in domprops.json, plus a derived class declaring 25 fresh fields. The mangled output confirms none of the 25 generated names collide with the reserved-or-inherited names — the encoder skipped the unavailable single-character candidates and continued onward (i, l, o, t, m, s, g, ...), exercising the collision-avoidance loop repeatedly within a single mangle pass.

baltasarblanco · 2026-05-05T03:30:59Z

Thanks for refactoring the stress test to compare runtime stdout via run_mangle_props_exec_test — that's a much better way to validate semantic preservation than matching the mangled output textually. I'll keep this pattern in mind for future contributions.

kdy1 · 2026-05-05T03:35:08Z

Thanks!

baltasarblanco requested review from a team as code owners May 2, 2026 13:06

Merge branch 'main' into fix/11027-mangle-props-name-collision

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kdy1 requested changes May 2, 2026

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kdy1 marked this pull request as draft May 4, 2026 02:05

baltasarblanco added 2 commits May 4, 2026 13:54

Merge branch 'main' into fix/11027-mangle-props-name-collision

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baltasarblanco marked this pull request as ready for review May 4, 2026 17:10

test(es/minifier): Cover mangle props runtime collisions

5efb4e2

kdy1 approved these changes May 5, 2026

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kdy1 merged commit 9b4fab5 into swc-project:main May 5, 2026
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github-actions Bot added this to the Planned milestone May 5, 2026

swc-project locked as resolved and limited conversation to collaborators Jun 4, 2026

kdy1 modified the milestones: Planned, v1.15.41, v1.15.40 Jun 10, 2026

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fix(es/minifier): Avoid generating mangled property names that collide with existing properties#11839

fix(es/minifier): Avoid generating mangled property names that collide with existing properties#11839
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baltasarblanco:fix/11027-mangle-props-name-collision

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baltasarblanco commented May 2, 2026

What's the problem this PR addresses?

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