T Compiler Meeting Agenda 2025 04 10

T-compiler Meeting Agenda 2025-04-10

Announcements

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: 6 months ago)
    • “Add Hotpatch flag” compiler-team#745 (Zulip) (last review activity: 6 months ago)
    • “Policy change around adding new unstable flags” compiler-team#787 (Zulip) (last review activity: 5 months ago)
    • “setup typos check in CI (for rust repo)” compiler-team#817 (Zulip) (last review activity: about 27 days 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: 3 months ago)
    • “rustc_target for rust-analyzer” compiler-team#839 (Zulip) (last review activity: about 47 days ago)
    • “Compile UI tests as libraries by default” compiler-team#842 (Zulip) (last review activity: about 33 days ago)
    • “Basic target naming rules” compiler-team#850 (Zulip) (last review activity: about 20 days ago)
    • “Add one more “late metadata”/“digest” file to rlib archives” compiler-team#853 (Zulip) (last review activity: about 20 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
  • Things in FCP (make sure you’re good with it)
  • Accepted MCPs
    • “Retire the mailing list and make all decisions on zulip” compiler-team#649 (Zulip)
    • “Enable warn(unused_crate_dependencies) on rustc_* crates” compiler-team#844 (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)
    • “Add --print=llvm-target-tuple” compiler-team#846 (Zulip)
    • “Split run-make test suite into slower-building test suite with suitably-staged cargo and faster-building test suite without cargo” compiler-team#847 (Zulip)
    • “Renaming Zulip T-compiler WGs streams” compiler-team#848 (Zulip)
    • “Add -Zsplit-metadata flag to reduce disk usage” compiler-team#851 (Zulip)
    • “Allow boolean literals in --check-cfg” compiler-team#854 (Zulip)
    • “Review rotation policy: remove reviewer from active review rotation if there’s prolonged lack of feedback on randomly-rolled PRs / indication that unavailability is temporary” compiler-team#856 (Zulip)
  • 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
    • “Remove backticks from ShouldPanic::YesWithMessage’s TrFailedMsg” rust#136160
    • “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
    • “Stabilize #![feature(precise_capturing_in_traits)]” rust#138128
    • “Prefer built-in sized impls (and only sized impls) for rigid types always” rust#138176
    • “Stabilize repr128” rust#138285
    • “Stabilize cfg_boolean_literals” rust#138632

Backport nominations

T-compiler beta / T-compiler stable

  • :beta: “Ignore zero-sized types in wasm future-compat warning” rust#139498
    • Authored by alexcrichton
    • From comment:

      This commit fixes a false positive of the warning triggered for #138762 (“Tracking issue for wasm32-unknown-unknown’s future-incompat warning for C ABI changes”) and the fix is to codify that zero-sized types are “safe” in both the old and new ABIs.

    • nominated because the false positive warning progressed to beta
  • 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

  • No P-high beta regressions this time.

Unassigned P-high nightly regressions

  • “Performance regression in nightly with target-cpu=native on znver4” rust#139370

Performance logs

triage logs for 2025-04-08

A busy week with lots of performance improvements. The largest performance improvement was from a revert of a previous week’s regression just in time for the beta release. Another large improvement came to small tweaks in the query system showing that there still are opportunities for small, targeted performance improvements in the compiler.

Triage done by @rylev. Revision range: 2ea33b59..e643f59f

Summary:

(instructions:u)meanrangecount
Regressions ❌ (primary)0.8%[0.2%, 1.9%]11
Regressions ❌ (secondary)8.4%[0.2%, 38.5%]16
Improvements ✅ (primary)-1.0%[-35.1%, -0.2%]206
Improvements ✅ (secondary)-1.8%[-8.6%, -0.1%]155
All ❌✅ (primary)-0.9%[-35.1%, 1.9%]217

2 Regressions, 9 Improvements, 5 Mixed; 4 of them in rollups 48 artifact comparisons made in total

Regressions

Initial support for auto traits with default bounds #120706 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)0.8%[0.2%, 1.3%]7
Regressions ❌ (secondary)0.4%[0.2%, 1.1%]21
Improvements ✅ (primary)--0
Improvements ✅ (secondary)--0
All ❌✅ (primary)0.8%[0.2%, 1.3%]7

Do not visit whole crate to compute lints_that_dont_need_to_run. #133781 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)0.3%[0.2%, 0.4%]7
Regressions ❌ (secondary)0.3%[0.1%, 0.6%]25
Improvements ✅ (primary)--0
Improvements ✅ (secondary)--0
All ❌✅ (primary)0.3%[0.2%, 0.4%]7

Improvements

Revert “Rollup merge of #136127 - WaffleLapkin:dyn_ptr_unwrap_cast, r=compiler-errors” #138892 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)--0
Regressions ❌ (secondary)0.1%[0.0%, 0.2%]2
Improvements ✅ (primary)-0.7%[-1.5%, -0.2%]99
Improvements ✅ (secondary)-3.0%[-11.3%, -0.1%]105
All ❌✅ (primary)-0.7%[-1.5%, -0.2%]99

Move ast::Item::ident into ast::ItemKind #138740 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)--0
Regressions ❌ (secondary)--0
Improvements ✅ (primary)-0.2%[-0.3%, -0.1%]3
Improvements ✅ (secondary)-0.5%[-0.8%, -0.3%]2
All ❌✅ (primary)-0.2%[-0.3%, -0.1%]3

Misc query tweaks #139234 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)--0
Regressions ❌ (secondary)--0
Improvements ✅ (primary)-0.6%[-1.4%, -0.2%]71
Improvements ✅ (secondary)-1.3%[-2.2%, -0.6%]9
All ❌✅ (primary)-0.6%[-1.4%, -0.2%]71

Rollup of 7 pull requests #139301 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)--0
Regressions ❌ (secondary)--0
Improvements ✅ (primary)-0.3%[-0.5%, -0.2%]8
Improvements ✅ (secondary)-0.3%[-0.5%, -0.1%]35
All ❌✅ (primary)-0.3%[-0.5%, -0.2%]8

Rollup of 9 pull requests #139336 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)--0
Regressions ❌ (secondary)--0
Improvements ✅ (primary)-0.3%[-0.3%, -0.2%]6
Improvements ✅ (secondary)-0.3%[-0.6%, -0.1%]21
All ❌✅ (primary)-0.3%[-0.3%, -0.2%]6

Folder experiment: Monomorphize region resolver #139287 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)--0
Regressions ❌ (secondary)--0
Improvements ✅ (primary)-0.3%[-0.7%, -0.2%]34
Improvements ✅ (secondary)-0.9%[-1.6%, -0.2%]19
All ❌✅ (primary)-0.3%[-0.7%, -0.2%]34

hygiene: Avoid recursion in syntax context decoding #139281 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)--0
Regressions ❌ (secondary)--0
Improvements ✅ (primary)-0.5%[-0.7%, -0.4%]3
Improvements ✅ (secondary)-0.6%[-0.7%, -0.3%]14
All ❌✅ (primary)-0.5%[-0.7%, -0.4%]3

Folder experiment: Micro-optimize RegionEraserVisitor #139292 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)--0
Regressions ❌ (secondary)--0
Improvements ✅ (primary)-4.7%[-34.9%, -0.4%]9
Improvements ✅ (secondary)--0
All ❌✅ (primary)-4.7%[-34.9%, -0.4%]9

Rollup of 9 pull requests #139482 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)--0
Regressions ❌ (secondary)--0
Improvements ✅ (primary)-0.3%[-0.4%, -0.2%]8
Improvements ✅ (secondary)-0.3%[-0.7%, -0.2%]22
All ❌✅ (primary)-0.3%[-0.4%, -0.2%]8

Mixed

Remove NtExpr and NtLiteral #138478 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)0.5%[0.1%, 2.6%]80
Regressions ❌ (secondary)6.8%[0.2%, 38.2%]41
Improvements ✅ (primary)--0
Improvements ✅ (secondary)-0.2%[-0.3%, -0.2%]9
All ❌✅ (primary)0.5%[0.1%, 2.6%]80
  • Performance impact was expected by the author.

Update dependencies #138848 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)0.4%[0.4%, 0.4%]1
Regressions ❌ (secondary)0.3%[0.1%, 0.5%]5
Improvements ✅ (primary)-0.2%[-0.4%, -0.1%]14
Improvements ✅ (secondary)-0.2%[-0.3%, -0.1%]15
All ❌✅ (primary)-0.2%[-0.4%, 0.4%]15
  • Improvements outweigh the regressions and the number of regressions is small.

Various local trait item iteration cleanups #139018 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)0.2%[0.2%, 0.2%]2
Regressions ❌ (secondary)--0
Improvements ✅ (primary)-0.2%[-0.3%, -0.2%]6
Improvements ✅ (secondary)-0.2%[-0.6%, -0.1%]40
All ❌✅ (primary)-0.1%[-0.3%, 0.2%]8
  • Perf regressions outweighed by the perf improvements.

Rollup of 5 pull requests #139257 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)0.3%[0.2%, 0.4%]13
Regressions ❌ (secondary)0.2%[0.1%, 0.6%]32
Improvements ✅ (primary)-0.7%[-0.7%, -0.6%]4
Improvements ✅ (secondary)-1.0%[-1.0%, -1.0%]1
All ❌✅ (primary)0.1%[-0.7%, 0.4%]17

gvn: Invalid dereferences for all non-local mutations #132527 (Comparison Link)

(instructions:u)meanrangecount
Regressions ❌ (primary)0.3%[0.1%, 0.9%]37
Regressions ❌ (secondary)0.5%[0.1%, 1.3%]29
Improvements ✅ (primary)-0.9%[-1.7%, -0.6%]6
Improvements ✅ (secondary)-1.7%[-1.8%, -1.5%]6
All ❌✅ (primary)0.2%[-1.7%, 0.9%]43
  • This PR seems to have brought about a decent number of perf regressions.
  • Asked the author and reviewer

Nominated Issues

T-compiler

  • “Add target_env = "macabi" and target_env = "sim"” rust#139451
    • This is a user-facing change so @Jubilee asks is it needs an FCP (comment)

RFC

  • None

Oldest PRs waiting for review

T-compiler

  • “Show pending obligations as unsatisfied constraints in report_similar_impl_candidates” rust#134348 (last review activity: 3 months ago)
    • cc @Michael Goulet (compiler-errors) @Wesley Wiser
  • “Look at proc-macro attributes when encountering unknown attribute” rust#134841 (last review activity: 3 months ago)
    • cc @Wesley Wiser , @Esteban Küber for a rebase when you have a chance
  • “Implement asymmetrical precedence for closures and jumps” rust#134847 (last review activity: 3 months ago)
    • cc @fmease
  • “Add a new lint that warns for pointers to stack memory” rust#134218 (last review activity: 3 months ago)
    • @Esteban Küber @Michael Goulet (compiler-errors)

Next meetings’ agenda draft: hackmd link