T Compiler Meeting Agenda 2025 03 06

T-compiler Meeting Agenda 2025-03-06

Announcements

  • Tomorrow time:2025-03-07T09:00:00-06:00: backlog bonanza cleanup compiler-team#484
  • RustWeek in the Netherland May, 13-17th: join the conversation and suggest topics for talks! There are short-to-medium slots (Zulip discussion)
  • 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!)
    • “Compile UI tests as libraries by default” compiler-team#842 (Zulip)
    • “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)
  • 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)
      • @topic: can we second this? Any concern?
    • “Add Hotpatch flag” compiler-team#745 (Zulip) (last review activity: 4 months ago)
      • @topic: Can anyone with some context have a look at it? thanks
    • “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: about 55 days ago)
    • “rustc_target for rust-analyzer” compiler-team#839 (Zulip) (last review activity: about 12 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

Backport nominations

T-compiler beta / T-compiler stable

  • :beta: “Check signature WF when lowering MIR body” rust#137298
    • Authored by compiler-errors, nominated by @_apiraino as a potential “nice to have” but doesn’t seem urgent
    • Fixes #137186, a P-medium ICE
  • :beta: “rustdoc: when merging target features, keep the highest stability” rust#137632
    • Authored by RalfJung
    • Fixes #137366 (possibly p-high), where rust docs generation failed on some ARM32 targets
  • :beta: “Don’t infer attributes of virtual calls based on the function body” rust#137669
    • Authored by DianQK
    • Fixes #137646, p-critical miscompile, turns out was on stable after all
  • :beta: Revert “store ScalarPair via memset when one side is undef and the other side can be memset” rust#137894
    • Authored by @Michael Goulet (compiler-errors)
    • Fixes #137892, reverts the patch that regressed (#135335)
  • 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

  • “Miscompilation caused by incorrectly-deduced readonly on virtual call” rust#137646

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
    • Taken care by @oli in #137894 beta-nominated (reverted #135335 that caused the regression)
  • “regression: literal out of range” rust#137893
    • It’s on the radar (I think @oli and/or @_Michael Goulet (compiler-errors) )

Unassigned P-high nightly regressions

  • No unassigned P-high nightly regressions this time.

Performance logs

triage logs 2025-03-03

A pretty quiet week, with minimal changes in performance (positive or negative).

Triage done by @simulacrum. Revision range: f5729cfe..daf59857

Summary:

(instructions:u)meanrangecount
Regressions (primary)0.4%[0.2%, 0.6%]16
Regressions (secondary)0.3%[0.1%, 0.6%]27
Improvements (primary)-0.4%[-1.1%, -0.2%]27
Improvements (secondary)-1.0%[-4.1%, -0.1%]63
All (primary)-0.1%[-1.1%, 0.6%]43

1 Regressions, 4 Improvements, 1 Mixed; 2 of them in rollups 29 artifact comparisons made in total

Regressions

Remove NtPat, NtMeta, and NtPath #137517 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.3%[0.2%, 0.4%]16
Regressions (secondary)0.4%[0.3%, 0.6%]9
Improvements (primary)--0
Improvements (secondary)--0
All (primary)0.3%[0.2%, 0.4%]16

The perf regressions are expected and unavoidable, alas.

https://github.com/rust-lang/rust/pull/137517#issuecomment-2692915586

Improvements

Change interners to start preallocated with an increased capacity #137354 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.3%[0.3%, 0.4%]2
Regressions (secondary)0.2%[0.2%, 0.2%]2
Improvements (primary)-0.3%[-0.6%, -0.2%]39
Improvements (secondary)-0.4%[-1.1%, -0.1%]63
All (primary)-0.3%[-0.6%, 0.4%]41

Rollup of 8 pull requests #137710 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)--0
Improvements (primary)--0
Improvements (secondary)-2.0%[-2.8%, -1.3%]9
All (primary)--0

librustdoc: return impl fmt::Display in more places instead of writing to strings #137425 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)--0
Improvements (primary)-0.6%[-1.2%, -0.2%]4
Improvements (secondary)--0
All (primary)-0.6%[-1.2%, -0.2%]4

Optimize empty provenance range checks. #137704 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)--0
Regressions (secondary)--0
Improvements (primary)-0.3%[-0.3%, -0.3%]3
Improvements (secondary)-1.9%[-3.4%, -0.2%]13
All (primary)-0.3%[-0.3%, -0.3%]3

Mixed

Rollup of 10 pull requests #137796 (Comparison Link)

(instructions:u)meanrangecount
Regressions (primary)0.3%[0.1%, 0.4%]9
Regressions (secondary)0.3%[0.2%, 0.5%]8
Improvements (primary)-0.6%[-0.9%, -0.2%]2
Improvements (secondary)--0
All (primary)0.1%[-0.9%, 0.4%]11

Hoping for recovery in subsequent PRs (see comment: https://github.com/rust-lang/rust/pull/137796#issuecomment-2693307115).

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

  • “Do not eagerly reject inference vars when trying to resolve method calls.” rust#126316
    • cc @lcnr
  • “Share the naked asm impl between cg_ssa and cg_clif” rust#134232
    • cc @Esteban Küber
  • “Show pending obligations as unsatisfied constraints in report_similar_impl_candidates” rust#134348 (last review activity: 2 months ago)
    • cc @Michael Goulet (compiler-errors) or @Wesley Wiser can you have a second look if the minimization here is now adequate?
  • “Make Default const and add some const Default impls” rust#134628 (last review activity: 2 months ago)
    • cc @oli
  • “Invalidate all dereferences when encountering non-local assignments” rust#132527 (last review activity: 2 months ago)
    • cc @cjgillot

Next meetings’ agenda draft: hackmd link