Merge pull request #822 from rylev/2021-01-05-update

Update 2021-01-05 with results of investigations into rollup

Author: Mark-Simulacrum

triage/2021-01-05.md

@@ -23,15 +23,14 @@ rustc_serialize: specialize opaque encoding and decoding of some u8 sequences [#
 Rollup of 9 pull requests [#80530](https://github.com/rust-lang/rust/issues/80530)
 - Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=e2267046859c9ceb932abc983561d53a117089f6&end=9775ffef2a4c3a36cadb58b72ea60cefb92c86ae&stat=instructions:u) (up to 2.6% on `full` builds of `ripgrep-opt`)
 - Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=e2267046859c9ceb932abc983561d53a117089f6&end=9775ffef2a4c3a36cadb58b72ea60cefb92c86ae&stat=instructions:u) (up to -1.5% on `incr-patched: println` builds of `cargo-debug`)
-- Unsure which PRs were responsible for the performance impact, but we suspect [#80458](https://github.com/rust-lang/rust/pull/80458). 
+- [#78934](https://github.com/rust-lang/rust/issues/78934) was responsible for the issue. An investigation into what exactly is the cause will happen, but the regression is not severe enough to warrant a revert.
 
 Rollup of 12 pull requests [#80708](https://github.com/rust-lang/rust/issues/80708)
 - Large regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=9919ad6e9ed113557c68c430de2e0f434e4f5b6e&end=f412fb56b8d11c168e7ee49ee74e79c4ab2e5637&stat=instructions:u) (up to 5.9% on `incr-unchanged` builds of `deeply-nested-async-check`)
 - Moderate improvement in [instruction counts](https://perf.rust-lang.org/compare.html?start=9919ad6e9ed113557c68c430de2e0f434e4f5b6e&end=f412fb56b8d11c168e7ee49ee74e79c4ab2e5637&stat=instructions:u) (up to -1.4% on `full` builds of `coercions-debug`)
 - The regression outweighs the improvement
-- Unsure which PR is the cause of the regression, but [#80637](https://github.com/rust-lang/rust/pull/80637) is our best guess.
+- [#80538](https://github.com/rust-lang/rust/issues/80538) was responsible for the issue. An investigation into what exactly is the cause will happen, but the regression is not severe enough to warrant a revert.
 
 #### Nags requiring follow up
 
 - stdarch is still a major contributor to libcore compile times.
-- Investigate the two rollups that had perf impacts [#80530](https://github.com/rust-lang/rust/issues/80530) and [#80708](https://github.com/rust-lang/rust/pull/80708).