T Compiler Meeting Agenda 2026 04 23

T-compiler Meeting Agenda 2026-04-23

Announcements

  • Reminder: if you see a PR/issue that seems like there might be legal implications due to copyright/IP/etc, please let us know (or at least message @davidtwco or @Wesley Wiser so we can pass it along).

Other WG meetings

  • None

MCPs/FCPs

Backport nominations

T-compiler beta / T-compiler stable

  • No beta nominations for T-compiler this time.
  • No stable nominations for T-compiler this time.

T-types beta / T-types stable

  • No beta nominations for T-types this time.
  • No stable nominations for T-types this time.

PRs S-waiting-on-t-compiler

T-compiler

Issues of Note

Short Summary

P-critical

T-compiler

  • No P-critical issues for T-compiler this time.

T-types

  • No P-critical issues for T-types this time.

P-high regressions

P-high beta regressions

  • No P-high beta regressions this time.

Unassigned P-high nightly regressions

  • No unassigned P-high nightly regressions this time.

Performance logs

2026-04-21 Triage Log

This week was a bit all over the place, but the largest regressions were either already fixed or they are being investigated. There were also a couple of nice perf. wins.

Triage done by @Kobzol. Revision range: dab8d9d1..9ab01ae5

Summary:

(instructions:u)meanrangecount
Regressions (primary)0.7%[0.2%, 4.6%]39
Regressions (secondary)0.6%[0.2%, 1.4%]31
Improvements (primary)-0.6%[-4.8%, -0.1%]70
Improvements (secondary)-0.7%[-4.1%, -0.0%]93
All (primary)-0.1%[-4.8%, 4.6%]109

3 Regressions, 4 Improvements, 6 Mixed; 4 of them in rollups 41 artifact comparisons made in total

Regressions

Fallback {float} to f32 when f32: From<{float}> and add impl From<f16> for f32 #139087 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)1.1%[0.3%, 2.2%]7
Improvements (primary)--0
Improvements (secondary)--0
All (primary)--0
  • Introduced regressions with the new trait solver.
  • Not marking as triaged yet.

delegation: fix def path hash collision, add per parent disambiguators #153955 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.2%, 0.2%]3
Regressions (secondary)0.3%[0.0%, 0.4%]20
Improvements (primary)--0
Improvements (secondary)--0
All (primary)0.2%[0.2%, 0.2%]3
  • Small regressions primarily in secondary benchmarks.
  • The regressions are being investigated in #155547.
  • Not marking as triaged yet.

Remove HashStable impl for [hir::Attribute]. #154924 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.4%[0.2%, 1.1%]42
Regressions (secondary)0.6%[0.3%, 1.6%]15
Improvements (primary)--0
Improvements (secondary)--0
All (primary)0.4%[0.2%, 1.1%]42
  • Correctness fix, the compiler skipped work it should not have skipped before.
  • Already marked as triaged.

Improvements

Rollup of 10 pull requests #155239 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)1.4%[1.4%, 1.4%]1
Improvements (primary)--0
Improvements (secondary)-0.3%[-0.7%, -0.1%]23
All (primary)--0

Don’t hash DelayedLints #155248 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)0.2%[0.2%, 0.2%]1
Improvements (primary)-0.1%[-0.2%, -0.1%]3
Improvements (secondary)-0.3%[-0.4%, -0.1%]20
All (primary)-0.1%[-0.2%, -0.1%]3

Make region equality emits Eq constraints #155258 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)--0
Improvements (primary)-0.3%[-0.4%, -0.2%]21
Improvements (secondary)-0.5%[-1.2%, -0.1%]26
All (primary)-0.3%[-0.4%, -0.2%]21

Refactor FnDecl and FnSig non-type fields into a new wrapper type #155223 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)--0
Improvements (primary)-0.3%[-0.7%, -0.2%]33
Improvements (secondary)-0.6%[-1.2%, -0.1%]47
All (primary)-0.3%[-0.7%, -0.2%]33

Mixed

Rollup of 18 pull requests #155380 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)0.1%[0.1%, 0.1%]1
Improvements (primary)-0.2%[-0.2%, -0.2%]5
Improvements (secondary)-0.3%[-0.4%, -0.3%]2
All (primary)-0.2%[-0.2%, -0.2%]5
  • More improvements than regressions, but all tiny anyway.
  • Marked as triaged.

Rollup of 5 pull requests #155432 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.2%, 0.2%]2
Regressions (secondary)0.2%[0.0%, 0.2%]9
Improvements (primary)-0.4%[-0.4%, -0.4%]1
Improvements (secondary)--0
All (primary)-0.0%[-0.4%, 0.2%]3
  • Tiny regressions caused by #153178.
  • Marked as triaged.

Remove fewer Storage calls in CopyProp and GVN #142531 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)1.2%[0.2%, 16.9%]35
Regressions (secondary)0.7%[0.2%, 5.5%]22
Improvements (primary)-0.4%[-0.5%, -0.3%]2
Improvements (secondary)-0.4%[-1.5%, -0.2%]24
All (primary)1.1%[-0.5%, 16.9%]37
  • This PR introduced performance regressions caused by wrong CGU estimates.
  • They were later fixed in #155491.
  • Marked as triaged.

Rollup of 11 pull requests #155552 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.4%[0.4%, 0.4%]1
Regressions (secondary)--0
Improvements (primary)--0
Improvements (secondary)-0.4%[-0.8%, -0.0%]9
All (primary)0.4%[0.4%, 0.4%]1
  • One tiny regression in clap_derive, but overall there were more green than red results.
  • Marked as triaged.

Merge BuildReducedGraphVisitor into DefPathVisitor #154945 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)0.6%[0.3%, 1.0%]14
Improvements (primary)-0.4%[-0.5%, -0.2%]6
Improvements (secondary)-0.4%[-1.1%, -0.2%]13
All (primary)-0.4%[-0.5%, -0.2%]6
  • The post-merge results are slightly worse than the pre-merge run, but overall it’s a wash.
  • Marked as triaged.

Fix performance regression introduced in #142531 by excluding Storage{Live,Dead} from CGU size estimation #155491 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)2.1%[0.8%, 4.6%]5
Regressions (secondary)--0
Improvements (primary)-4.2%[-14.1%, -0.2%]9
Improvements (secondary)-3.9%[-9.2%, -1.2%]3
All (primary)-1.9%[-14.1%, 4.6%]14
  • Fixed a previous regression from #142531.
  • Marked as triaged.

Nominated Issues

T-compiler

  • No I-compiler-nominated issues this time.

RFC

  • No I-compiler-nominated RFCs this time.

Oldest PRs waiting for review

T-compiler

  • “Allow forbidden target features to be hard errors” rust#152821 (last review activity: 2 months ago)
    • cc @dingxiangfei2009
  • “[DO NOT MERGE] Remove #[no_link] attribute” rust#152856 (last review activity: 2 months ago)
    • This was waiting on a crater run, here the results
    • @bjorn3 do you need someone to comment the crater run and/or kick off the FCP? Or do you want to work on this further?
  • “Port the without_debuginfo test from backtrace-rs to the testsuite” rust#152860 (last review activity: 2 months ago)
    • Jubilee is at capacity in the last weeks, we might want to reroll (maybe asking before)

Next meeting’s agenda draft: hackmd link