T Compiler Meeting Agenda 2026 06 04

T-compiler Meeting Agenda 2026-06-04

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 @Boxy so we can pass it along).

Other WG meetings

MCPs/FCPs

Interesting MCPs/FCPs

Note: let’s review them one by one (maybe send pings?)

Backport nominations

T-compiler beta / T-compiler stable

  • 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

:warning: Note: a new beta (1.96) crater-run has just completed and we have a few regressions (will look into them)

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

2026-06-01 Triage Log

This week we saw nice wins across the board thanks to merging several compiler queries together (#155678), and also substantial improvements in doc performance thanks to doing less work when sorting trait impls (#157179).

Triage done by @Kobzol. Revision range: 783eb8c8..4804ad7e

Summary:

(instructions:u)meanrangecount
Regressions (primary)0.3%[0.1%, 0.7%]14
Regressions (secondary)0.4%[0.1%, 0.9%]39
Improvements (primary)-0.9%[-6.8%, -0.2%]111
Improvements (secondary)-1.1%[-2.9%, -0.1%]53
All (primary)-0.8%[-6.8%, 0.7%]125

3 Regressions, 1 Improvement, 2 Mixed; 4 of them in rollups 35 artifact comparisons made in total

Regressions

Rollup of 10 pull requests #157031 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.2%, 0.2%]3
Regressions (secondary)0.3%[0.2%, 0.6%]7
Improvements (primary)--0
Improvements (secondary)--0
All (primary)0.2%[0.2%, 0.2%]3
  • Noise that reverted back in follow-up PRs.
  • Already marked as triaged.

Rollup of 8 pull requests #157161 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.1%, 0.6%]15
Regressions (secondary)0.5%[0.1%, 2.4%]18
Improvements (primary)--0
Improvements (secondary)-0.0%[-0.0%, -0.0%]1
All (primary)0.2%[0.1%, 0.6%]15
  • Regression caused by #156875, continuing discussion on that PR.
  • Marked as triaged.

Rollup of 2 pull requests #157177 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.1%[0.1%, 0.1%]3
Regressions (secondary)--0
Improvements (primary)--0
Improvements (secondary)--0
All (primary)0.1%[0.1%, 0.1%]3
  • Either noise or #157165, but the regression is tiny and only on a single benchmark, not worth digging deeper.
  • Marked as triaged.

Improvements

rustdoc: optimize impl sorting #157179 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)--0
Improvements (primary)-1.5%[-6.7%, -0.3%]13
Improvements (secondary)-1.1%[-1.9%, -0.4%]2
All (primary)-1.5%[-6.7%, -0.3%]13

Mixed

Merge several HIR-level queries into one #155678 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)0.4%[0.0%, 1.1%]11
Improvements (primary)-0.7%[-1.7%, -0.1%]109
Improvements (secondary)-0.9%[-2.8%, -0.1%]62
All (primary)-0.7%[-1.7%, -0.1%]109
  • Wins greatly outweigh the regressions.
  • Marked as triaged.

Rollup of 16 pull requests #157121 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)0.6%[0.3%, 1.0%]9
Improvements (primary)--0
Improvements (secondary)-0.1%[-0.2%, -0.1%]2
All (primary)--0
  • Regression caused by #157076.
  • The regression should be fixed by #157182.
  • Already marked as triaged.

Nominated Issues

T-compiler

RFC

  • “MIR move elimination” rfcs#3943
    • Asked on Zulip (thread)
    • Work in progress implementation of RFC#3943 in #156046, shows promising perf. gains
    • Nominated by @_Amanieu d’Antras (needs some eyeballs from T-compiler or just kick off an FCP)

Oldest PRs waiting for review

T-compiler

  • “lint ImproperCTypes: refactor linting architecture (part 3)” rust#146275 (last review activity: 7 months ago)
    • maybe cc @_Jana (why T-bootstrap, btw?)
  • “Dont bail in error predicate unless self ty is error in new solver” rust#150453 (last review activity: 4 months ago)
    • cc @_lcnr

Next meetings’ agenda draft: hackmd link