T Compiler Meeting Agenda 2025 03 13

T-compiler Meeting Agenda 2025-03-13

Announcements

  • Tomorrow time:2025-03-14T09:00:00-06:00: Design meeting: “Introducing the feature target for the Intel avx512 instruction set” compiler-team#827
    • cc @sayantn
  • 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

  • New MCPs (take a look, see if you like them!)
  • Old MCPs (stale MCP might be closed as per MCP procedure)
    • None at this time
  • Old MCPs (not seconded, take a look)
    • “Add hygiene attributes to compile expanded source code” compiler-team#692 (Zulip) (last review activity: 5 months ago)
    • “Add Hotpatch flag” compiler-team#745 (Zulip) (last review activity: 5 months ago)
    • “Policy change around adding new unstable flags” compiler-team#787 (Zulip) (last review activity: 4 months ago)
    • “Rename “dylib” crate type to “rdylib” (keep old name but deprecate it), and maybe do the same for “staticlib” → “cstaticlib”” compiler-team#825 (Zulip) (last review activity: 2 months ago)
    • “rustc_target for rust-analyzer” compiler-team#839 (Zulip) (last review activity: about 19 days ago)
    • “Compile UI tests as libraries by default” compiler-team#842 (Zulip) (last review activity: about 6 days ago)
    • “Policy: Require MCP and relevant team nominations for adding (ecosystem, custom codegen backend) testing jobs that would block PR/Merge CI and require documenting failure protocol” compiler-team#845 (Zulip) (last review activity: about 6 days ago)
  • Pending FCP requests (check your boxes!)
    • “sanitizers: Stabilize AddressSanitizer and LeakSanitizer for the Tier 1 targets” rust#123617
    • “Warn about C-style octal literals” rust#131309
    • “Stabilize -Zdwarf-version as -Cdwarf-version” rust#136926
  • Things in FCP (make sure you’re good with it)
  • Accepted MCPs
  • MCPs blocked on unresolved concerns
  • Finalized FCPs (disposition merge)
    • “Add --print host-tuple to print host target tuple” rust#125579
    • “make unsupported_calling_conventions a hard error” rust#129935
    • “Fix ICE when passing DefId-creating args to legacy_const_generics.” rust#130443
    • “Stabilize WebAssembly multivalue, reference-types, and tail-call target features” rust#131080
    • “Lint on combining #[no_mangle] and #[export_name]” rust#131558
  • Other teams finalized FCPs
    • “Add lint against function pointer comparisons” rust#118833
    • “Fixup Windows verbatim paths when used with the include! macro” rust#125205
    • “Uplift clippy::double_neg lint as double_negations” rust#126604
    • “Allow dropping dyn Trait principal” rust#126660
    • “atomics: allow atomic and non-atomic reads to race” rust#128778
    • “Lint against getting pointers from immediately dropped temporaries” rust#128985
    • “Do not consider match/let/ref of place that evaluates to ! to diverge, disallow coercions from them too” rust#129392
    • “Make deprecated_cfg_attr_crate_type_name a hard error” rust#129670
    • “Stabilize expr_2021 fragment specifier in all editions” rust#129972
    • “Check elaborated projections from dyn don’t mention unconstrained late bound lifetimes” rust#130367
    • “Finish stabilization of result_ffi_guarantees” rust#130628
    • “Stabilize const ptr::write* and mem::replace” rust#130954
    • “Stabilize s390x inline assembly” rust#131258
    • “Stabilize Arm64EC inline assembly” rust#131781
    • “Always display first line of impl blocks even when collapsed” rust#132155
    • “rework winnowing to sensibly handle global where-bounds” rust#132325
    • “mark is_val_statically_known intrinsic as stably const-callable” rust#132449
    • “Fix ICE when multiple supertrait substitutions need assoc but only one is provided” rust#133392
    • “[rustdoc] Add sans-serif font setting” rust#133636
    • “disallow repr() on invalid items” rust#133925
    • “Make the wasm_c_abi future compat warning a hard error” rust#133951
    • “fully de-stabilize all custom inner attributes” rust#134276
    • “remove long-deprecated no-op attributes no_start and crate_id” rust#134300
    • “Stabilize feature(trait_upcasting)” rust#134367
    • “Fix parsing of ranges after unary operators” rust#134900
    • “Future incompatibility warning unsupported_fn_ptr_calling_conventions: Also warn in dependencies” rust#135767
    • “Reject ?Trait bounds in various places where we unconditionally warned since 1.0” rust#135841
    • “Make cenum_impl_drop_cast a hard error” rust#135964
    • “Allow *const W<dyn A> -> *const dyn A ptr cast” rust#136127
    • “Do not deduplicate list of associated types provided by dyn principal” rust#136458
    • “Do not allow attributes on struct field rest patterns” rust#136490
    • “Make ptr_cast_add_auto_to_object lint into hard error” rust#136764
    • “Turn order dependent trait objects future incompat warning into a hard error” rust#136968
    • “add a “future” edition” rust#137606

Backport nominations

T-compiler beta / T-compiler stable

  • :beta: “rustdoc: when merging target features, keep the highest stability” rust#137632
    • Authored by RalfJung
    • T-rustdoc approves beta backport on Zulip (so probably we can either)
    • Fixes a stable regression but nobody expressed interest in a stable backport
  • :beta: “Revert wf sized check on beta” rust#138122
    • Authored by compiler-errors
    • Fixes a P-medium ICE #137186
    • This patch is a second attempt at fixing it, following a revert of the original patch (#137298)
  • 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-team

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

  • “regression: ICE in rustc_codegen_ssa/src/mir/rvalue.rs:104:55:” rust#137892
    • Fixed by #137894 (beta backport accepted)
  • “regression: literal out of range” rust#137893
    • I think under @oli scrutiny but unsure about its progress
  • “iOS binaries crash with latest nightly” rust#138212

Unassigned P-high nightly regressions

  • No unassigned P-high nightly regressions this time.

Performance logs

triage logs for 2025-03-13

This week we had to merge a lot of large rollups due to many problems with our CI infrastructure, which made analysis harder. Even though the aggregated stats look like there were a lot of regressions, it is skewed by two large regressions happening on an uncommon optimized incremental build and a documentation build of a single crate. The documentation regression is being tracked, and fixes to some other regressions are already in progress.

Triage done by @kobzol. Revision range: daf59857..9fb94b32

Summary:

(instructions:u)meanrangecount
Regressions (primary)1.2%[0.2%, 58.8%]149
Regressions (secondary)4.2%[0.2%, 165.8%]127
Improvements (primary)-1.1%[-14.0%, -0.3%]31
Improvements (secondary)-2.9%[-38.4%, -0.1%]43
All (primary)0.8%[-14.0%, 58.8%]180

2 Regressions, 2 Improvements, 5 Mixed; 4 of them in rollups 37 artifact comparisons made in total

Regressions

Rollup of 6 pull requests #138021 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.2%, 0.2%]1
Regressions (secondary)0.2%[0.2%, 0.2%]9
Improvements (primary)--0
Improvements (secondary)--0
All (primary)0.2%[0.2%, 0.2%]1
  • Tiny doc regressions on the smallest benchmarks, probably caused by #137722. I don’t think we need to investigate more here.
  • Marked as triaged.

Rollup of 12 pull requests #138267 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.7%[0.1%, 1.6%]105
Regressions (secondary)3.5%[0.1%, 12.5%]94
Improvements (primary)--0
Improvements (secondary)--0
All (primary)0.7%[0.1%, 1.6%]105
  • The regression was caused by #136127.
  • A fix is in progress in #138292.
  • Not marking as triaged yet.

Improvements

Change TaskDeps to start preallocated with 128 capacity #137563 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)--0
Improvements (primary)-0.6%[-0.8%, -0.5%]6
Improvements (secondary)--0
All (primary)-0.6%[-0.8%, -0.5%]6

Split the Edges iterator. #137655 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)--0
Improvements (primary)--0
Improvements (secondary)-5.1%[-5.6%, -4.8%]6
All (primary)--0

Mixed

Rollup of 20 pull requests #138058 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.6%[0.2%, 1.6%]29
Regressions (secondary)0.4%[0.1%, 1.0%]16
Improvements (primary)-1.7%[-13.7%, -0.1%]11
Improvements (secondary)-7.8%[-38.3%, -0.3%]10
All (primary)-0.0%[-13.7%, 1.6%]40
  • Most of the perf. changes were likely caused by #137298. The wins and losses kind of balanced themselves.
  • Marked as triaged.

Inline FnOnce/FnMut/Fn shims once again #137907 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)13.6%[0.4%, 59.2%]5
Regressions (secondary)0.6%[0.4%, 1.9%]7
Improvements (primary)-0.4%[-0.9%, -0.1%]25
Improvements (secondary)-0.7%[-3.4%, -0.1%]41
All (primary)2.0%[-0.9%, 59.2%]30
  • Overall there are more wins than regressions, and this change should help optimizations. The single large regression is unlucky CGU scheduling on an optimized incremental build, which is not very common.
  • Marked as triaged.

Rollup of 25 pull requests #138114 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)42.1%[0.2%, 167.8%]4
Improvements (primary)--0
Improvements (secondary)-0.4%[-0.5%, -0.3%]8
All (primary)--0
  • The large regression was caused by #137534. The regression hasn’t been fixed yet, it is being tracked in #138168.
  • Not marking as triaged yet.

Use trunc nuw+br for 0/1 branches even in optimized builds #137500 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.3%[0.2%, 0.8%]37
Regressions (secondary)0.4%[0.2%, 1.2%]61
Improvements (primary)-0.5%[-0.9%, -0.3%]5
Improvements (secondary)-1.0%[-1.4%, -0.4%]4
All (primary)0.2%[-0.9%, 0.8%]42
  • The regressions were expected and deemed acceptable, this should help optimizations.
  • Marked as triaged.

Always inline query_get_at. #137695 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.2%[0.2%, 0.3%]2
Regressions (secondary)0.9%[0.2%, 1.5%]9
Improvements (primary)-0.5%[-1.4%, -0.2%]61
Improvements (secondary)-0.7%[-1.5%, -0.2%]56
All (primary)-0.5%[-1.4%, 0.3%]63
  • Many more wins than regressions.
  • 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 diagnostics to “while loop” and “for loop” that note that it is always determined that it might iterate zero times.” rust#126510 (last review activity: 6 months ago)
    • cc @Esteban Küber
  • “[Coverage][MCDC] Do not initialize mcdc parameters for instances containing no mcdc statements” rust#129989
    • cc: @Zalathar
  • “Handle rustc_query_system cases of rustc::potential_query_instability lint” rust#131200 (last review activity: 4 months ago)
    • cc @cjgillot
  • “JumpThreading: fix bitwise not on non-booleans” rust#131203 (last review activity: 4 months ago)
    • cc @cjgillot
  • “Do not suggest borrow that is already there in fully-qualified call” rust#132469 (last review activity: 4 months ago)
    • cc @Nadrieril
  • “only use generic info when ty var belong it in orphan check” rust#132904 (last review activity: 4 months ago)
    • cc @León Orell Liehr (fmease)

Next meetings’ agenda draft: hackmd link