Add triage for this week

triage/2021-10-05.md

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+# 2021-10-05 Triage Log
+
+A fairly busy week, with a relatively high percentage of PRs landing with
+regressions and improvements. The overall trajectory is fairly neutral for this
+week though.
+
+Triage done by **@simulacrum**.
+Revision range: [83f147b3baf21acfc367a6da1045d212cd3957e4..25ec8273855fde2d72ae877b397e054de5300e10](https://perf.rust-lang.org/?start=83f147b3baf21acfc367a6da1045d212cd3957e4&end=25ec8273855fde2d72ae877b397e054de5300e10&absolute=false&stat=instructions%3Au)
+
+5 Regressions, 5 Improvements, 5 Mixed; 1 of them in rollups
+
+43 comparisons made in total
+
+#### Regressions
+
+Don't anonymize bound region names during typeck [#89250](https://github.com/rust-lang/rust/issues/89250)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=30acf6def32a340b4bfce6e1e2638f1b05ac2cd9&end=69c1c6a173dcae20c245348f6c7d19074b6109b7&stat=instructions:u) (up to 0.6% on `full` builds of `coercions`)
+- Regression seems to be real, though no regressions in cycle counts are
+  reported, so this is likely an acceptable hit.
+
+Constify ?-operator for Result and Option [#86853](https://github.com/rust-lang/rust/issues/86853)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=69c1c6a173dcae20c245348f6c7d19074b6109b7&end=c6007fdc7059c677a6c089e8d2915b264c0d1326&stat=instructions:u) (up to 0.5% on `incr-full` builds of `stm32f4`)
+- This is a small regression on the stm32f4 crate, due to a number of extra
+  obligations that are created, presumably due to the checks for whether ?
+  operators in this crate can be const. It is likely an acceptable change for
+  the time being, particularly as the const-impls functionality is still in
+  nascent stages and optimizations may come down the line.
+- The PR ended up temporarily reverted by
+  [#89450](https://github.com/rust-lang/rust/issues/89450), though not due to
+  performance reasons.
+
+Fix read_to_end to not grow an exact size buffer [#89165](https://github.com/rust-lang/rust/issues/89165)
+- Small regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=e737694a4d66b01308b73d4559a35b43e414faf9&end=d25de31a0eeb14ab0c8c4613496fe2d3d9a085dd&stat=instructions:u) (up to 0.7% on `full` builds of `piston-image`)
+- This regression is unclear in cause, and investigation on the PR did not lead
+  to a conclusion. However, the regression is fairly small and the change is
+  expected to have impacts on more I/O heavy benchmarks than rustc is.
+  Syscall counts were shown to improve on the PR, including for rustc, so the
+  optimization seems to work in general.
+
+Fix unsound optimization with explicit variant discriminants [#89489](https://github.com/rust-lang/rust/issues/89489)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=44593aeb1387b1be355aeaf0040d5927bd80f060&end=a4797664ba9c7d71e586122853858eeb6c153bb9&stat=instructions:u) (up to 2.3% on `full` builds of `style-servo`)
+- This is an expected regression, but is clearly justified by the fix for an
+  unsound optimization. We expect to reland it with appropriate changes in the
+  future.
+
+Fix clippy lints [#89405](https://github.com/rust-lang/rust/issues/89405)
+- Small regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=a8387aef8c378a771686878062e544af4d5e2245&end=b27661eb33c74cb514dba059b47d86b6582ac1c2&stat=instructions:u) (up to 0.4% on `incr-unchanged` builds of `helloworld`)
+- Likely to be painful to investigate -- changes are widely scattered across the
+  codebase and in theory "no-ops".
+
+
+#### Improvements
+
+Restructure std::rt [#89011](https://github.com/rust-lang/rust/issues/89011)
+- Large improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=50f9f7810c975234ce4730488d32661a76a00428&end=11491938f80988c7261a1179cf71a25c379c8783&stat=instructions:u) (up to -4.0% on `incr-patched: b9b3e592dd cherry picked` builds of `style-servo`)
+- Significant improvement for this particular incremental benchmark (due to
+  avoiding inlining from particular modules and as such recompilation of some
+  CGUs) and -- as noted on the PR -- also a code size improvement, particularly
+  relevant for small binaries.
+
+Remove visible path calculation from allowed deprecation lint [#89395](https://github.com/rust-lang/rust/issues/89395)
+- Very large improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=598d89bf142823b5d84e2eb0f0f9e418ee966a4b&end=4e4942dfa66667c0addfff8e0882a59b035d45ca&stat=instructions:u) (up to -10.9% on `incr-patched: minor change` builds of `tokio-webpush-simple`)
+- Excellent improvement in multiple benchmarks, though unlikely to be terribly
+  representative. Most of these benchmarks have -A deprecated passed by the
+  perf.rlo runtime currently (since we do not meaningfully update benchmarks
+  over time, we build up deprecation warnings).
+
+
+Avoid nondeterminism in trimmed_def_paths [#89408](https://github.com/rust-lang/rust/issues/89408)
+- Large improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=b27661eb33c74cb514dba059b47d86b6582ac1c2&end=edebf77e0090195bf80c0d8cda821e1bf9d03053&stat=instructions:u) (up to -2.0% on `incr-unchanged` builds of `webrender-wrench`)
+- Fixes a regression introduced in
+  [#83343](https://github.com/rust-lang/rust/pull/83343), not a standalone
+  improvement.
+
+
+Rollup of 14 pull requests [#89512](https://github.com/rust-lang/rust/issues/89512)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=d25de31a0eeb14ab0c8c4613496fe2d3d9a085dd&end=44593aeb1387b1be355aeaf0040d5927bd80f060&stat=instructions:u) (up to -1.4% on `incr-patched: static str 6144` builds of `issue-46449`)
+- Unclear cause, but unlikely to receive much investigation as it's an
+  improvement in a rollup -- we prioritize regressions.
+
+Deriving: Include bound generic params in type parameters for where clause [#89341](https://github.com/rust-lang/rust/issues/89341)
+- Small improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=d14731cb3ced8318d7fc83cbe838f0e7f2fb3b40&end=f03eb6bef8ced8a243858b819e013b9caf83d757&stat=instructions:u) (up to -0.4% on `incr-unchanged` builds of `helloworld`)
+
+
+#### Mixed
+
+Add more tracing instrumentation [#89048](https://github.com/rust-lang/rust/issues/89048)
+- Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=1d71ba862309d59df710078a845c8772ffb22aba&end=8f8092cc32ec171becef8ceacec7dbb06c5d7d7e&stat=instructions:u) (up to -1.6% on `full` builds of `keccak`)
+- Very large regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=1d71ba862309d59df710078a845c8772ffb22aba&end=8f8092cc32ec171becef8ceacec7dbb06c5d7d7e&stat=instructions:u) (up to 12.0% on `full` builds of `ctfe-stress-4`)
+- Regression is mitigated by
+  [#89363](https://github.com/rust-lang/rust/pull/89363), which just barely
+  missed being included in this report.
+
+
+Add an intermediate representation to exhaustiveness checking [#88950](https://github.com/rust-lang/rust/issues/88950)
+- Large improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=8f8092cc32ec171becef8ceacec7dbb06c5d7d7e&end=6df1d82869d06b88ff413e63a1e8efbb311e3b5c&stat=instructions:u) (up to -2.6% on `full` builds of `unicode_normalization`)
+- Large regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=8f8092cc32ec171becef8ceacec7dbb06c5d7d7e&end=6df1d82869d06b88ff413e63a1e8efbb311e3b5c&stat=instructions:u) (up to 2.9% on `full` builds of `match-stress-enum`)
+- Code quality improvements mitigate the regressions, which are largely limited
+  to a stress test.
+
+
+Rollup of 6 pull requests [#89435](https://github.com/rust-lang/rust/issues/89435)
+- Small improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=ed937594d3912ced11f6f35a90bb8bf591909d2a&end=b6057bf7b7ee7c58e6a39ead02eaa13b75f908c2&stat=instructions:u) (up to -0.3% on `incr-unchanged` builds of `helloworld`)
+- Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=ed937594d3912ced11f6f35a90bb8bf591909d2a&end=b6057bf7b7ee7c58e6a39ead02eaa13b75f908c2&stat=instructions:u) (up to 0.8% on `incr-unchanged` builds of `externs`)
+- Unclear cause for the regression. Hasn't been investigated yet.
+
+
+Rework HIR API to make invocations of the hir_crate query harder. [#88880](https://github.com/rust-lang/rust/issues/88880)
+- Small improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=b6057bf7b7ee7c58e6a39ead02eaa13b75f908c2&end=c02371c442f811878ab3a0f5a813402b6dfd45d2&stat=instructions:u) (up to -0.4% on `incr-unchanged` builds of `helloworld`)
+- Small regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=b6057bf7b7ee7c58e6a39ead02eaa13b75f908c2&end=c02371c442f811878ab3a0f5a813402b6dfd45d2&stat=instructions:u) (up to 0.2% on `full` builds of `many-assoc-items`)
+- Regressions are smaller than improvements. Original perf run didn't return
+  regressions at all. Tagged for investigation.
+
+resolve: Cache module loading for all foreign modules [#89239](https://github.com/rust-lang/rust/issues/89239)
+- Small regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=edebf77e0090195bf80c0d8cda821e1bf9d03053&end=d14731cb3ced8318d7fc83cbe838f0e7f2fb3b40&stat=instructions:u) (up to 0.8% on `incr-unchanged` builds of `tokio-webpush-simple`)
+- From the PR: This corrects behaviour for macros 2.0 and avoids an ICE, so a
+  0.8% regression seems acceptable.
+
+#### Untriaged Pull Requests
+
+Investigation work continues, and we're down to 22 from 28 untriaged PRs week
+over week.
+
+- [#89435 Rollup of 6 pull requests](https://github.com/rust-lang/rust/pull/89435)
+- [#89363 Fix performance regression with #[instrument]](https://github.com/rust-lang/rust/pull/89363)
+- [#89263 Suggest both of immutable and mutable trait implementations](https://github.com/rust-lang/rust/pull/89263)
+- [#89165 Fix read_to_end to not grow an exact size buffer](https://github.com/rust-lang/rust/pull/89165)
+- [#89125 Don't use projection cache or candidate cache in intercrate mode](https://github.com/rust-lang/rust/pull/89125)
+- [#89103 Migrate in-tree crates to 2021](https://github.com/rust-lang/rust/pull/89103)
+- [#89047 Rollup of 10 pull requests](https://github.com/rust-lang/rust/pull/89047)
+- [#89030 Introduce `Rvalue::ShallowInitBox`](https://github.com/rust-lang/rust/pull/89030)
+- [#88945 Remove concept of 'completion' from the projection cache](https://github.com/rust-lang/rust/pull/88945)
+- [#88824 Rollup of 15 pull requests](https://github.com/rust-lang/rust/pull/88824)
+- [#88804 Revise never type fallback algorithm](https://github.com/rust-lang/rust/pull/88804)
+- [#88719 Point at argument instead of call for their obligations](https://github.com/rust-lang/rust/pull/88719)
+- [#88703 Gather module items after lowering.](https://github.com/rust-lang/rust/pull/88703)
+- [#88627 Do not preallocate HirIds](https://github.com/rust-lang/rust/pull/88627)
+- [#88575 Querify `FnAbi::of_{fn_ptr,instance}` as `fn_abi_of_{fn_ptr,instance}`.](https://github.com/rust-lang/rust/pull/88575)
+- [#88308 Morph `layout_raw` query into `layout_of`.](https://github.com/rust-lang/rust/pull/88308)
+- [#87781 Remove box syntax from compiler and tools](https://github.com/rust-lang/rust/pull/87781)
+- [#87688 Introduce `let...else`](https://github.com/rust-lang/rust/pull/87688)
+- [#87064 Support `#[track_caller]` on closures and generators](https://github.com/rust-lang/rust/pull/87064)
+- [#83698 Use undef for uninitialized bytes in constants](https://github.com/rust-lang/rust/pull/83698)
+- [#83302 Get piece unchecked in `write`](https://github.com/rust-lang/rust/pull/83302)
+- [#80522 Split rustc_mir](https://github.com/rust-lang/rust/pull/80522)