T Compiler Meeting Agenda 2026 08 20

T-compiler Meeting Agenda 2026-08-20

Announcements

  • Today, Rust 1.98 is out, blog post
  • 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

Backport nominations

(all async approved, mentioned here for the record)

T-compiler beta / T-compiler stable

  • :beta: (1.98) revert of “riscv: promote d, e, and f target_features to CfgStableToggleUnstable” rust#161064

    • Authored by beetrees
    • Reverts #156188

  • :beta: (1.100.0) “fix buggy MaybeDangling<&T> validation logic” rust#161125

  • :beta: (1.100.0) “resolving cyclic glob vis-max” rust#161024

    • Authored by calvinrp
    • Voting Zulip topic, all in favor
    • Fixes #160685 a p-high regression found by a crater run (1 crate), now on stable

  • No stable nominations for T-compiler this time.

PRs S-waiting-on-t-compiler

T-compiler

Issues of Note

Short Summary

P-critical

T-compiler

  • No new 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

  • “Rust warns pub glob export is unused” rust#160691
    • Vadim self-assigned
    • A smaller reproduction would be welcome (in case anyone has time to help)

Unassigned P-high nightly regressions

  • No unassigned P-high nightly regressions this time.

Performance logs

2026-08-18 Triage Log

There were almost no regressions this week, while the next trait solver saw several significant performance improvements!

Triage done by @kobzol. Revision range: 771916f9..8fa1c96c

Summary:

(instructions:u)meanrangecount
Regressions (primary)0.4%[0.2%, 0.5%]6
Regressions (secondary)0.6%[0.2%, 1.0%]17
Improvements (primary)-0.5%[-1.7%, -0.2%]166
Improvements (secondary)-2.3%[-16.0%, -0.1%]219
All (primary)-0.5%[-1.7%, 0.5%]172

0 Regressions, 6 Improvements, 7 Mixed; 4 of them in rollups 50 artifact comparisons made in total

Improvements

Mixed

Increase the default stack size to 16 MiB and remove ensure_sufficient_stack #160535 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)0.9%[0.9%, 0.9%]6
Improvements (primary)-0.5%[-1.8%, -0.2%]183
Improvements (secondary)-0.8%[-2.9%, -0.2%]209
All (primary)-0.5%[-1.8%, -0.2%]183
  • One tiny regression on a secondary benchmark, otherwise everything is green.
  • Marked as triaged.

Rollup of 13 pull requests #161014 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.3%[0.1%, 0.8%]15
Regressions (secondary)0.2%[0.2%, 0.2%]2
Improvements (primary)--0
Improvements (secondary)-5.5%[-8.9%, -1.5%]4
All (primary)0.3%[0.1%, 0.8%]15

Rollup of 23 pull requests #161075 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.5%[0.2%, 1.5%]19
Regressions (secondary)0.8%[0.0%, 1.7%]51
Improvements (primary)-0.2%[-0.2%, -0.1%]2
Improvements (secondary)-0.8%[-1.6%, -0.6%]6
All (primary)0.5%[-0.2%, 1.5%]21
  • Regression caused by #160288.
  • Perf is partially recovered in #161200.
  • Already marked as triaged.

stop using fully_perform_locally with the next solver, it’s worse (perf fix for unic-ucd) #160982 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)2.0%[1.4%, 3.1%]3
Improvements (primary)--0
Improvements (secondary)-2.2%[-11.8%, -0.2%]20
All (primary)--0
  • Only next solver changes, more wins than regressions.
  • Marked as triaged.

Avoid allocations when canonicalizing #161077 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.1%[0.1%, 0.1%]2
Regressions (secondary)0.2%[0.2%, 0.2%]3
Improvements (primary)--0
Improvements (secondary)-1.1%[-3.4%, -0.2%]25
All (primary)0.1%[0.1%, 0.1%]2
  • Several nice wins for the next trait solver, and only one tiny regression on libc.
  • Marked as triaged.

Rollup of 5 pull requests #161134 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)1.5%[1.5%, 1.5%]1
Regressions (secondary)0.3%[0.2%, 0.4%]3
Improvements (primary)-0.2%[-0.2%, -0.2%]1
Improvements (secondary)--0
All (primary)0.6%[-0.2%, 1.5%]2
  • The image blip is noise, otherwise essentially no changes.
  • Marked as triaged.

Single-byte ASCII searcher for StrSearcherImpl(pattern.rs) #160408 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.2%, 0.2%]1
Regressions (secondary)0.2%[0.2%, 0.2%]1
Improvements (primary)-1.0%[-1.6%, -0.4%]2
Improvements (secondary)--0
All (primary)-0.6%[-1.6%, 0.2%]3
  • The image blip looks like noise spike returning back, but in general there were almost no significant changes on icounts.
  • 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

  • “Add post-mono MIR optimizations” rust#156858 (last review activity: 2 months ago)
    • PR author is a bit stuck, can anyone with MIR expertise help?
  • “Codegen Overloaded LLVM intrinsics based on their name” rust#157145 (last review activity: 2 months ago)
    • left a ping to LLM folks
  • “Track items behind cfg_select in the same way we do for cfg” rust#157218 (last review activity: 2 months ago)
    • cc @Jana Dönszelmann (IIUC)
  • “Deny partial -Z stack-protector by default in all editions” rust#157941 (last review activity: 2 months ago)
    • Does this need to go through some decisional process? MCP or lang-experiment?- TODO

Next week we will skip the meeting