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Merge pull request #2370 from Mark-Simulacrum/triage
Add triage for this week
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‎triage/2026/2026-01-12.md‎

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# 2026-01-12 Triage Log
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Fairly quiet week, most changes due to new features which naturally carry some
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overhead for existing programs. Overall though an improvement.
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Triage done by **@simulacrum**.
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Revision range: [7c04f5d2..840245e9](https://perf.rust-lang.org/?start=7c04f5d216b5dcfff0a55fc20327a1c519004699&end=840245e91b90f22adf9f26d0a0cd98c2416cdef3&absolute=false&stat=instructions%3Au)
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**Summary**:
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| (instructions:u) | mean | range | count |
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|:----------------------------------:|:-----:|:--------------:|:-----:|
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| Regressions ❌ <br /> (primary) | 0.5% | [0.2%, 2.6%] | 20 |
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| Regressions ❌ <br /> (secondary) | 0.4% | [0.1%, 2.5%] | 58 |
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| Improvements ✅ <br /> (primary) | -0.8% | [-6.6%, -0.2%] | 98 |
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| Improvements ✅ <br /> (secondary) | -0.5% | [-1.8%, -0.1%] | 46 |
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| All ❌✅ (primary) | -0.6% | [-6.6%, 2.6%] | 118 |
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3 Regressions, 1 Improvement, 4 Mixed; 2 of them in rollups
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31 artifact comparisons made in total
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#### Regressions
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Rollup of 10 pull requests [#150726](https://github.com/rust-lang/rust/pull/150726) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=74fd7516dac6e7be0e561dddb9bee4e860598f07&end=4d73a008eaf6ba3ca067a3221e5c00602e33a5f2&stat=instructions:u)
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| (instructions:u) | mean | range | count |
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|:----------------------------------:|:----:|:------------:|:-----:|
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| Regressions ❌ <br /> (primary) | 0.4% | [0.2%, 0.8%] | 13 |
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| Regressions ❌ <br /> (secondary) | 0.2% | [0.1%, 0.3%] | 6 |
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| Improvements ✅ <br /> (primary) | - | - | 0 |
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| Improvements ✅ <br /> (secondary) | - | - | 0 |
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| All ❌✅ (primary) | 0.4% | [0.2%, 0.8%] | 13 |
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This is due to "Impls and impl items inherit dead_code lint level of the
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corresponding traits and trait items"
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[#144113](https://github.com/rust-lang/rust/pull/144113) which adds a new
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'feature' to the lint system and so is expected to carry some cost. The costs
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look reasonable enough (small, only ~1 primary benchmark affected), so marking
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as triaged.
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Add allocator parameter to HashMap [#148545](https://github.com/rust-lang/rust/pull/148545) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=4586feb998521343b91c554adbb11ba30547d67a&end=31cd367b9ca1ce359268e7adf4ea540408c0ad85&stat=instructions:u)
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| (instructions:u) | mean | range | count |
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|:----------------------------------:|:----:|:------------:|:-----:|
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| Regressions ❌ <br /> (primary) | 0.3% | [0.2%, 0.5%] | 3 |
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| Regressions ❌ <br /> (secondary) | 0.5% | [0.5%, 0.5%] | 1 |
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| Improvements ✅ <br /> (primary) | - | - | 0 |
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| Improvements ✅ <br /> (secondary) | - | - | 0 |
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| All ❌✅ (primary) | 0.3% | [0.2%, 0.5%] | 3 |
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Expected to cause regressions (actually surprised it's so few). Marked as triaged.
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Rollup of 14 pull requests [#150957](https://github.com/rust-lang/rust/pull/150957) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=08f833aa179952037d5d0c5d949d4c3ec18ec212&end=1279939b38db460bd3844a6ca44e94961a48333d&stat=instructions:u)
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| (instructions:u) | mean | range | count |
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|:----------------------------------:|:----:|:------------:|:-----:|
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| Regressions ❌ <br /> (primary) | 0.1% | [0.1%, 0.2%] | 4 |
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| Regressions ❌ <br /> (secondary) | 0.4% | [0.1%, 1.0%] | 20 |
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| Improvements ✅ <br /> (primary) | - | - | 0 |
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| Improvements ✅ <br /> (secondary) | - | - | 0 |
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| All ❌✅ (primary) | 0.1% | [0.1%, 0.2%] | 4 |
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Kicked off [perf runs](https://github.com/rust-lang/rust/pull/150957#issuecomment-3738810175) on
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component PRs to investigate root cause. If it is related to attribute parsing
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it's likely these regressions should just be accepted though.
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#### Improvements
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resolve: Use `Macros20NormalizedIdent` in more interfaces [#150741](https://github.com/rust-lang/rust/pull/150741) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=7c41ee66db9d08e3989858acaefa1c72a6a88518&end=b68e16cfc0e9f8b29c9ec41bfa6ab0a637eff19c&stat=instructions:u)
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| (instructions:u) | mean | range | count |
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|:----------------------------------:|:-----:|:--------------:|:-----:|
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| Regressions ❌ <br /> (primary) | - | - | 0 |
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| Regressions ❌ <br /> (secondary) | 0.6% | [0.6%, 0.6%] | 1 |
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| Improvements ✅ <br /> (primary) | -0.3% | [-0.6%, -0.1%] | 114 |
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| Improvements ✅ <br /> (secondary) | -0.4% | [-1.2%, -0.1%] | 63 |
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| All ❌✅ (primary) | -0.3% | [-0.6%, -0.1%] | 114 |
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#### Mixed
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`Vec::push` in consts MVP [#147893](https://github.com/rust-lang/rust/pull/147893) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=da476f1942868cdf94ed88b01ea31170cfe95047&end=74fd7516dac6e7be0e561dddb9bee4e860598f07&stat=instructions:u)
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| (instructions:u) | mean | range | count |
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|:----------------------------------:|:-----:|:--------------:|:-----:|
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| Regressions ❌ <br /> (primary) | 0.3% | [0.3%, 0.4%] | 3 |
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| Regressions ❌ <br /> (secondary) | 0.2% | [0.1%, 0.4%] | 5 |
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| Improvements ✅ <br /> (primary) | -0.2% | [-0.2%, -0.2%] | 1 |
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| Improvements ✅ <br /> (secondary) | -1.8% | [-1.9%, -1.7%] | 2 |
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| All ❌✅ (primary) | 0.2% | [-0.2%, 0.4%] | 4 |
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Looking at a cranelift profile it loosely looks like we're spending more time
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in LLVM, maybe because of the extra indirection though the `const_eval_select`?
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Though I don't see obvious signs of this in godbolt diffs vs beta.
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In either case, marking as triaged as further investigation doesn't seem
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warranted given magnitude and general desire for this feature.
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Port `#[cfg]` and `#[cfg_attr]` trace attributes to the new attribute parsers [#150310](https://github.com/rust-lang/rust/pull/150310) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=0aced202c24f9356c1640fc0a7f07433b3a7124f&end=d9617c8d9a55773a96b61ba3a4acb107d65615c1&stat=instructions:u)
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| (instructions:u) | mean | range | count |
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|:----------------------------------:|:-----:|:--------------:|:-----:|
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| Regressions ❌ <br /> (primary) | 0.3% | [0.2%, 0.6%] | 5 |
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| Regressions ❌ <br /> (secondary) | 0.1% | [0.1%, 0.1%] | 1 |
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| Improvements ✅ <br /> (primary) | -0.3% | [-0.4%, -0.1%] | 13 |
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| Improvements ✅ <br /> (secondary) | -0.2% | [-0.3%, -0.1%] | 11 |
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| All ❌✅ (primary) | -0.1% | [-0.4%, 0.6%] | 18 |
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Expected regressions from attribute parsing rework. Marking as triaged. This
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also shrunk crate metadata a good bit, e.g., [2% on `syn`](https://perf.rust-lang.org/compare.html?start=0aced202c24f9356c1640fc0a7f07433b3a7124f&end=d9617c8d9a55773a96b61ba3a4acb107d65615c1&stat=size:crate_metadata).
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Stop emitting UbChecks on every Vec→Slice [#150265](https://github.com/rust-lang/rust/pull/150265) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=3fda0e426ca17b0baa0d6e765a0d23f487350573&end=85d0cdfe3489ff1a4b86daeddba6fcf82b47bd65&stat=instructions:u)
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| (instructions:u) | mean | range | count |
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|:----------------------------------:|:-----:|:--------------:|:-----:|
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| Regressions ❌ <br /> (primary) | 1.3% | [0.1%, 2.6%] | 3 |
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| Regressions ❌ <br /> (secondary) | 2.5% | [2.5%, 2.5%] | 1 |
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| Improvements ✅ <br /> (primary) | -1.5% | [-5.8%, -0.1%] | 25 |
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| Improvements ✅ <br /> (secondary) | -0.5% | [-1.4%, -0.1%] | 6 |
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| All ❌✅ (primary) | -1.2% | [-5.8%, 2.6%] | 28 |
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Mixed results but overall an improvement.
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Also hash spans inside the same file as relative (V2) [#150540](https://github.com/rust-lang/rust/pull/150540) [(Comparison Link)](https://perf.rust-lang.org/compare.html?start=9bc8b40bc314b74e1d5e5ab21a0df39c55a34806&end=08f833aa179952037d5d0c5d949d4c3ec18ec212&stat=instructions:u)
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| (instructions:u) | mean | range | count |
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|:----------------------------------:|:-----:|:--------------:|:-----:|
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| Regressions ❌ <br /> (primary) | 0.3% | [0.1%, 0.4%] | 6 |
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| Regressions ❌ <br /> (secondary) | 0.3% | [0.1%, 0.7%] | 27 |
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| Improvements ✅ <br /> (primary) | -1.2% | [-6.5%, -0.2%] | 20 |
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| Improvements ✅ <br /> (secondary) | -0.3% | [-0.5%, -0.2%] | 8 |
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| All ❌✅ (primary) | -0.9% | [-6.5%, 0.4%] | 26 |
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See [author's note here](https://github.com/rust-lang/rust/pull/150540#issuecomment-3734528974):
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> Matches the expected performance, net positive

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