T Compiler Meeting Agenda 2026 01 08

T-compiler Meeting Agenda 2026-01-08

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

  • :beta: “Perf regression fix” rust#150056
    • Authored by Kivooeo
    • Follow-up to another regression fix (#149664)
    • Voting Zulip topic mostly in favor; one comment points out it fixes a tiny perf. regression so maybe not worth per se
  • :beta: “Don’t try to evaluate const blocks during constant promotion” rust#150557
    • Authored by dianne
    • (Review in progress)
    • Voting Zulip topic
  • 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

2026-01-06 Triage Log

Not many PRs were merged, as it was still mostly a holiday week. #149681 caused small regressions across the board, this is pending investigation.

Triage done by @kobzol. Revision range: 112a2742..7c04f5d2

Summary:

(instructions:u)meanrangecount
Regressions (primary)0.5%[0.1%, 1.4%]146
Regressions (secondary)0.6%[0.0%, 3.5%]91
Improvements (primary)-3.1%[-4.7%, -1.5%]2
Improvements (secondary)-0.7%[-6.4%, -0.1%]15
All (primary)0.4%[-4.7%, 1.4%]148

2 Regressions, 0 Improvements, 7 Mixed; 4 of them in rollups 51 artifact comparisons made in total

Regressions

Use more principled check for generics in const ops #150519 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.1%, 0.3%]6
Regressions (secondary)0.2%[0.2%, 0.2%]1
Improvements (primary)--0
Improvements (secondary)--0
All (primary)0.2%[0.1%, 0.3%]6
  • Tiny regression on the diesel benchmark.
  • The compiler does more work here, and this fixes an ICE, so it seems warranted.
  • Marked as triaged.

MGCA: Support for tuple constructors #150603 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)0.1%[0.1%, 0.2%]11
Improvements (primary)--0
Improvements (secondary)--0
All (primary)--0
  • The regressions are only on a few secondary stress tests and they are tiny.
  • Marked as triaged.

Mixed

Remove the explicit branch hint from const_panic #150386 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.6%[0.5%, 0.7%]2
Regressions (secondary)--0
Improvements (primary)-0.4%[-0.5%, -0.4%]2
Improvements (secondary)-0.1%[-0.1%, -0.0%]3
All (primary)0.1%[-0.5%, 0.7%]4
  • Over all benchmarks it’s a wash.
  • Marked as triaged.

Make inliner cycle detection a fallible process #147361 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.6%[0.3%, 1.0%]4
Regressions (secondary)0.8%[0.1%, 1.3%]6
Improvements (primary)-1.0%[-4.9%, -0.1%]8
Improvements (secondary)-0.6%[-6.5%, -0.1%]18
All (primary)-0.5%[-4.9%, 1.0%]12
  • The nalgebra win looks real.
  • This changes inlining behavior, so that always perturbs compile times.
  • Marked as triaged.

Rollup of 3 pull requests #150582 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.2%, 0.2%]1
Regressions (secondary)0.1%[0.1%, 0.2%]2
Improvements (primary)-0.1%[-0.2%, -0.1%]4
Improvements (secondary)-0.7%[-1.8%, -0.1%]3
All (primary)-0.1%[-0.2%, 0.2%]5
  • Tiny changes, overall it’s a wash.
  • Marked as triaged.

Rollup of 7 pull requests #150669 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.7%[0.7%, 0.7%]1
Regressions (secondary)0.1%[0.1%, 0.1%]1
Improvements (primary)--0
Improvements (secondary)-0.5%[-0.6%, -0.2%]4
All (primary)0.7%[0.7%, 0.7%]1
  • The regressed cargo benchmark seems a bit noisy.
  • Over all benchmarks it’s a wash, I don’t think we need to investigate further.
  • Marked as triaged.

Rollup of 3 pull requests #150682 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.5%[0.1%, 1.5%]154
Regressions (secondary)0.7%[0.0%, 3.5%]87
Improvements (primary)--0
Improvements (secondary)-0.1%[-0.1%, -0.1%]4
All (primary)0.5%[0.1%, 1.5%]154
  • The regression was caused by #149681, continuing discussion there.
  • Marked as triaged.

Rename the gcc component to gcc-dev #150535 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)0.6%[0.5%, 0.6%]3
Improvements (primary)--0
Improvements (secondary)-0.1%[-0.2%, -0.1%]9
All (primary)--0
  • Noise, this only changed a CI component.
  • Marked as triaged.

Rollup of 3 pull requests #150700 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)0.0%[0.0%, 0.1%]2
Improvements (primary)-0.1%[-0.1%, -0.1%]1
Improvements (secondary)-0.4%[-0.8%, -0.0%]4
All (primary)-0.1%[-0.1%, -0.1%]1
  • Noise.
  • Marked as triaged.

Nominated Issues

T-compiler

  • “Volatile reads and writes on aarch64 sometimes generate instructions not suitable for MMIO in protected VMs” rust#131894
  • “Diesel stops building with nightly-2025-12-10” rust#149845
    • not nominated, just a FIY
    • regression (still in nightly) after #147984, fixed downstream by Vadim (thanks! see comment)
    • Diesel author asked if we could hold this change for a release cycle or two, given that he has very limited resources to maintain Diesel and is afraid his userbase might not have had enough time to install the fixed version of Diesel before we land this change in 1.93 (in 2 weeks)
    • Author of # 147984 (Vadim) prefers landing this change either way

RFC

  • No I-compiler-nominated RFCs this time.

Oldest PRs waiting for review

T-compiler

  • TODO

Next meeting’s agenda draft: hackmd link