Update mgca to use `type const` syntax instead of the `#[type_const]` attribute. by Keith-Cancel · Pull Request #152324 · rust-lang/rust

Keith-Cancel · 2026-02-08T08:27:35Z

This PR changes the #[type_const] attribute to the type const syntax for #132980.

This will fixes #151273 and similar issues, since we need to check type const of items before expansion. The move to add a syntax was mentioned here: #151289 (comment)

The first part of this PR adds support by allowing type const <IDENT>: <TYPE> { = <EXPR> }; syntax in rustc_parse/src/parser/item.rs.

The next part since the AST item does not contain enough information to determine if we have a type const was rework ConstItemRhs into ConstItemRhsKind to store the information since we no longer have the attribute acting as a source of extra data/metadata.

The hir node ConstItemRhsKind current shape mostly works, except in the case of TraitItemKind where it is an option. I initially went about giving hir::ConstItemRhsKind a similar form the AST, but it touches a lot more lines of code and files so because of that, the less invasive option was to add a simple boolean flag to TraitItemKind.

The forth part of this PR includes adding a query I called is_rhs_type_const so that we can handle both local and foreign def_ids.

The fifth aspect of the PR is adding a mgca_type_const_syntax feature gate that is checked before expansion. The standard mgca feature gate is ran after expansion. This feature gate allows for conditional compilation (e.g #[cfg(..)]) of the type const syntax in nightly without min_generic_const_args being enabled.

The last bit is updating all the the tests that used the #[type_const] attribute to use the new syntax that failed because of the changes. This is the bulk of touched/edited files in the PR.

r? @BoxyUwU
@rustbot label +F-associated_const_equality +F-min_generic_const_args

rustbot · 2026-02-08T08:27:39Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

Some changes occurred in tests/ui/sanitizer

cc @rcvalle

BoxyUwU

Would you mind going through and fully removing the type_const attribute? It looks like it shouldn't be too involved. It's mostly just in comments which can be trivially changed to say type const instead of #[type_const] or would just need minor re-flowing. You should be able to find the dozen or so locations by searching for type_const] in the compiler.

Removing the actual parsing/whatnot of #[type_const] should be fairly simple too I think. It's:

delete TypeConstParser
remove type_const from rustc_span/src/symbol.rs
remove it from the list of builtin attrs, rustc_feature/src/builtin_attrs.rs

I realise this PR is already fairly big but I don't think this should make it that much larger, but will pretty much "finish" everything off for this change and that feels quite valuable to me.

View changes since this review

compiler/rustc_hir/src/hir.rs

compiler/rustc_ast_passes/src/feature_gate.rs

compiler/rustc_ast/src/ast.rs

src/tools/rustfmt/src/items.rs

compiler/rustc_ast_passes/src/feature_gate.rs

rustbot · 2026-02-09T04:31:28Z

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

HIR ty lowering was modified

cc @fmease

Some changes occurred to constck

cc @fee1-dead

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

rustbot · 2026-02-09T06:13:51Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

compiler/rustc_ast_passes/src/feature_gate.rs

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

tests/ui/attributes/malformed-attrs.rs

tests/ui/const-generics/type-relative-path-144547.rs

BoxyUwU · 2026-02-09T16:58:04Z

idk if that's spurious or not so lets just do CI again...

BoxyUwU · 2026-02-09T19:49:48Z

@bors r+ rollup=never p=1 (probably bitrots given its size)

rust-bors · 2026-02-09T19:49:51Z

📌 Commit 73a991f has been approved by BoxyUwU

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 1000. This pull request will be tested once the tree is reopened.

rust-bors · 2026-02-10T02:24:48Z

☀️ Test successful - CI
Approved by: BoxyUwU
Duration: 3h 46m 55s
Pushing 381e9ef to main...

github-actions · 2026-02-10T02:28:03Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 18d13b5 (parent) -> 381e9ef (this PR)

Test differences

Show 29 test diffs

Stage 1

[ui] tests/ui/feature-gates/feature-gate-mgca-type-const-syntax.rs: [missing] -> pass (J1)

Stage 2

[ui] tests/ui/feature-gates/feature-gate-mgca-type-const-syntax.rs: [missing] -> pass (J0)

Additionally, 27 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 381e9ef09effbed5f1378a24dc6e461fea35a0e1 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

dist-aarch64-apple: 1h 51m -> 2h 47m (+50.2%)
pr-check-1: 24m 27s -> 31m 23s (+28.4%)
x86_64-gnu-miri: 1h 9m -> 1h 23m (+20.3%)
aarch64-apple: 3h 7m -> 3h 40m (+17.8%)
x86_64-gnu-tools: 54m 14s -> 1h 3m (+17.4%)
pr-check-2: 38m 9s -> 44m 13s (+15.9%)
x86_64-rust-for-linux: 45m 16s -> 51m 52s (+14.6%)
x86_64-mingw-1: 2h 35m -> 2h 57m (+14.3%)
i686-gnu-2: 1h 29m -> 1h 41m (+13.8%)
aarch64-gnu-debug: 1h 8m -> 1h 17m (+13.4%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-02-10T03:08:02Z

Finished benchmarking commit (381e9ef): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.1%, 0.7%]	18
Regressions ❌ (secondary)	0.5%	[0.0%, 1.7%]	13
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.0%]	4
All ❌✅ (primary)	0.4%	[0.1%, 0.7%]	18

Max RSS (memory usage)

Results (primary 2.3%, secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.0%, 2.4%]	3
Regressions ❌ (secondary)	3.7%	[2.2%, 5.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-5.1%, -2.4%]	2
All ❌✅ (primary)	2.3%	[2.0%, 2.4%]	3

Cycles

Results (primary 1.9%, secondary -4.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.9%, 1.9%]	1
Regressions ❌ (secondary)	2.8%	[2.8%, 2.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.4%	[-7.0%, -4.2%]	5
All ❌✅ (primary)	1.9%	[1.9%, 1.9%]	1

Binary size

Results (primary 0.6%, secondary 0.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.0%, 1.2%]	78
Regressions ❌ (secondary)	0.5%	[0.0%, 1.6%]	32
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.0%, 1.2%]	78

Bootstrap: 474.211s -> 509.869s (7.52%)
Artifact size: 397.99 MiB -> 398.00 MiB (0.00%)

Kobzol · 2026-02-17T08:38:00Z

Small regressions on libc and bitmaps, @BoxyUwU do you think there might be something to improve here, or let's accept it due to the size of the PR?

rustbot assigned BoxyUwU Feb 8, 2026

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Keith-Cancel force-pushed the mgca4 branch from 502c224 to 34e95b1 Compare February 9, 2026 02:52

rustbot added the A-attributes Area: Attributes (`#[…]`, `#![…]`) label Feb 9, 2026

Keith-Cancel force-pushed the mgca4 branch from 9398edd to b1a4cfd Compare February 9, 2026 06:13

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Keith-Cancel marked this pull request as draft February 9, 2026 11:45

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 9, 2026

Keith-Cancel force-pushed the mgca4 branch from b1a4cfd to cc0bbf4 Compare February 9, 2026 11:54

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mGCA: Add associated const type check #152146

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rust-bors bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 9, 2026

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fmease mentioned this pull request Feb 10, 2026

Parse mGCA's type-level const items fmease/rasur#10

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rust-bors bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Feb 10, 2026

rust-bors bot merged commit 381e9ef into rust-lang:main Feb 10, 2026
18 of 26 checks passed

rustbot added this to the 1.95.0 milestone Feb 10, 2026

rustbot added the perf-regression Performance regression. label Feb 10, 2026

reddevilmidzy mentioned this pull request Feb 10, 2026

Add regression test for type_const with unit struct ctor under mGCA #152407

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matthiaskrgr mentioned this pull request Feb 14, 2026

[ICE]: Failed to normalize Alias(Free #152633

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lapla-cogito mentioned this pull request Feb 15, 2026

Fix const normalization for generic const items with trait assoc consts #152278

Merged

reddevilmidzy mentioned this pull request Feb 16, 2026

ICE type_of called on const argument's anon const before the const argument was lowered #148730

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Test differences

Stage 1

Stage 2

Job duration changes

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rust-timer commented Feb 10, 2026

Overall result: ❌✅ regressions and improvements - please read the text below

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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