T Compiler Meeting Agenda 2025 12 18

T-compiler Meeting Agenda 2025-12-18

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

MCPs/FCPs

Backport nominations

T-compiler beta / T-compiler stable

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

2025-12-16 Triage Log

This week we saw several regressions, partly from the compiler doing more work. The remaining regressions are being investigated.

Triage done by @kobzol. Revision range: 55495234..21ff67df

Summary:

(instructions:u)meanrangecount
Regressions (primary)0.5%[0.1%, 5.1%]40
Regressions (secondary)0.8%[0.0%, 3.0%]63
Improvements (primary)-0.7%[-1.5%, -0.1%]35
Improvements (secondary)-1.0%[-7.4%, -0.0%]73
All (primary)-0.1%[-1.5%, 5.1%]75

3 Regressions, 2 Improvements, 5 Mixed; 2 of them in rollups 36 artifact comparisons made in total

Regressions

resolve: Preserve ambiguous glob reexports in crate metadata #147984 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.3%[0.1%, 0.5%]24
Regressions (secondary)0.3%[0.2%, 0.7%]13
Improvements (primary)--0
Improvements (secondary)-0.1%[-0.1%, -0.1%]1
All (primary)0.3%[0.1%, 0.5%]24
  • The compiler is doing more work after this change, and this work should unblock parallelizing name resolution and macro expansion in the future.
  • Asked on the PR if something can be done about the regression.

Rollup of 9 pull requests #149891 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)1.9%[0.5%, 7.6%]10
Regressions (secondary)1.2%[0.2%, 2.2%]2
Improvements (primary)--0
Improvements (secondary)--0
All (primary)1.9%[0.5%, 7.6%]10
  • The regression in doc builds was caused by #142380.
  • Continuing discussion on the PR.

Externally implementable items #146348 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.6%[0.6%, 0.7%]2
Regressions (secondary)0.3%[0.1%, 0.8%]35
Improvements (primary)--0
Improvements (secondary)-0.0%[-0.1%, -0.0%]2
All (primary)0.6%[0.6%, 0.7%]2
  • The regressions are only on the tiniest crates, and is quite small.
  • Marked as triaged.

Improvements

Remove unwraps from metadata decoding: introduce BlobDecoder #149455 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)--0
Improvements (primary)-0.2%[-0.4%, -0.1%]8
Improvements (secondary)-0.4%[-1.4%, -0.1%]28
All (primary)-0.2%[-0.4%, -0.1%]8

Don’t leak sysroot crates through dependencies #149273 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)--0
Improvements (primary)--0
Improvements (secondary)-2.9%[-6.7%, -0.1%]11
All (primary)--0

Mixed

std: Use more unix.rs code on WASI targets #147572 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.2%, 0.2%]3
Regressions (secondary)0.3%[0.1%, 0.5%]2
Improvements (primary)--0
Improvements (secondary)-0.0%[-0.0%, -0.0%]1
All (primary)0.2%[0.2%, 0.2%]3
  • Except for the tiny helloworld doc regression, this seems to be noise.
  • Marked as triaged.

Revert “early return on duplicate span lowerings” #149757 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)0.1%[0.1%, 0.1%]1
Improvements (primary)-0.3%[-0.7%, -0.1%]94
Improvements (secondary)-0.4%[-0.8%, -0.0%]107
All (primary)-0.3%[-0.7%, -0.1%]94
  • This revert fixed a previous performance regression from #149060.
  • Marked as triaged.

Rollup of 5 pull requests #149818 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.1%, 0.3%]18
Regressions (secondary)0.3%[0.2%, 0.5%]13
Improvements (primary)--0
Improvements (secondary)-0.4%[-0.6%, -0.3%]3
All (primary)0.2%[0.1%, 0.3%]18
  • Small regression caused by #149664, continuing discussion on that PR.
  • Not marking as triaged yet.

Port doc attributes to new attribute API #149645 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.2%, 0.2%]1
Regressions (secondary)1.4%[0.1%, 3.1%]18
Improvements (primary)-0.6%[-1.3%, -0.2%]36
Improvements (secondary)-0.6%[-1.2%, -0.1%]32
All (primary)-0.6%[-1.3%, 0.2%]37
  • More improvements than regressions.
  • Already marked as triaged.

Overhaul filename handling for cross-compiler consistency #149709 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.7%[0.2%, 1.1%]6
Regressions (secondary)0.6%[0.0%, 2.2%]31
Improvements (primary)-0.6%[-1.5%, -0.1%]12
Improvements (secondary)-0.3%[-0.6%, -0.2%]4
All (primary)-0.2%[-1.5%, 1.1%]18
  • The results are a bit all over the place, but overall it’s a wash. Some tiny improvements are expected to be gained from a follow-up in #149989.
  • 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

  • “RISC-V: Implement (Zkne or Zknd) intrinsics correctly” rust#146798 (last review activity: 2 months ago)
    • cc: @Amanieu (I wonder if this comment is relevant)
  • “Do not use dlltool to create DLL Import Libraries for Windows” rust#147274 (last review activity: 2 months ago)
    • cc: @Wesley Wiser
  • “cg_llvm: More preparation for moving FFI bindings to rustc_llvm” rust#147430 (last review activity: 2 months ago)
    • cc: (autoassigned) @Sparrow Li
  • “Uplift module inspect from clippy” rust#147599 (last review activity: 2 months ago)
    • cc: (autoassigned) @Esteban Küber (fixes a P-low issue …)

Next meeting(s) when?

Next two Thursdays fall exactly on Dec, 25th and Jan, 1st :-)