[ty] Use a SmallVec for seen type aliases#26187
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Typing conformance resultsNo changes detected ✅Current numbersThe percentage of diagnostics emitted that were expected errors held steady at 94.37%. The percentage of expected errors that received a diagnostic held steady at 89.00%. The number of fully passing files held steady at 94/134. |
Memory usage reportMemory usage unchanged ✅ |
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AlexWaygood
approved these changes
Jun 21, 2026
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Summary
Use a
SmallVecfor type aliases visited while checking self-referential type-variable defaults.Running ty over the entire ecosystem corpus shows this distribution
A
SmallVecwith an inline capacity avoids allocations in 99%, and the linear scan is a non-brainer for up to 6 elements. There's a risk that largerseenresult inO(n)scans at every level, which would justify a hash map. I'd argue that this isn't an issue in practice and we also acceptO(n)for our recursive Type visitor.I'd say we use a
SmallVec, we can always use aSmallSetif this ever shows up on a real world project.Performance
Consistent 2-3% improvement across all projects
Test Plan
Testing: Ran focused mdtests, repository hooks, and the 162-project ecosystem corpus.