Use more principled check for generics in const ops #150519

camelid · 2025-12-30T19:02:02Z

In the future, we likely want to make the check less likely to be missed by reducing the number of external entry points to HIR type lowering.

Note: If this causes pass->error regressions (not truly regressions because those cases were mistakenly accepted), they can easily be avoided for the time being by only running the check if is_self_alias is true or mgca is enabled.

Fix parsing of mgca const blocks in array repeat exprs
Use more principled check for generics in const ops

rustbot · 2025-12-30T19:02:05Z

HIR ty lowering was modified

cc @fmease

rustbot · 2025-12-30T19:02:07Z

r? @mati865

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compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

There was an inconsistency where in the case of array repeat expressions, we forgot to eat the `const` keyword of mgca const blocks. Thus, we ended up parsing them as an anon const containing an inline const, instead of simply an anon const as they should be. This commit fixes that issue and also makes the parsing for mgca const blocks more consistent and simpler in general. For example, we avoid doing a lookahead check to ensure there's a curly brace coming. There should always be one after a `const` keyword in an expression context, and that's handled by the mgca const block parsing function.

tests/ui/const-generics/min_const_generics/self-ty-in-const-2.stderr

tests/ui/const-generics/mgca/explicit_anon_consts-7.rs

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

rustbot · 2026-01-01T23:21:05Z

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

Instead of using a visitor in typeck, we just check, whenever lowering a use of a param, whether the parent item is an MCG anon const during hir ty lowering (and instantiate_value_path). If so, we report an error since MCG anon consts should never be able to use generics. All other kinds of anon consts are at least syntactically allowed to use generic params. We use a `TypeFolder` to accomplish this; this way, we look at the fully explicit semantic representation of the type/const/whatever and don't miss subtle cases like `Self` type aliases.

Now that we avoid a fatal error, they can all be one test.

camelid · 2026-01-02T02:13:40Z

@bors r=BoxyUwU

fixed broken doc link (cc #133661)

bors · 2026-01-02T02:13:43Z

📌 Commit f71e938 has been approved by BoxyUwU

It is now in the queue for this repository.

bors · 2026-01-02T02:23:11Z

⌛ Testing commit f71e938 with merge a47f4df...

bors · 2026-01-02T05:36:38Z

☀️ Test successful - checks-actions
Approved by: BoxyUwU
Pushing a47f4df to main...

github-actions · 2026-01-02T05:39:44Z

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 056908d (parent) -> a47f4df (this PR)

Test differences

Show 59 test diffs

Stage 1

[crashes] tests/crashes/140891.rs: pass -> [missing] (J0)
[ui] tests/ui/const-generics/mgca/early-bound-param-lt-bad.rs: [missing] -> pass (J0)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts-2.rs: pass -> [missing] (J0)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts-3.rs: pass -> [missing] (J0)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts-4.rs: pass -> [missing] (J0)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts-5.rs: pass -> [missing] (J0)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts-6.rs: pass -> [missing] (J0)
[ui] tests/ui/const-generics/mgca/higher-ranked-lts-bad.rs: [missing] -> pass (J0)
[ui] tests/ui/const-generics/mgca/higher-ranked-lts-good.rs: [missing] -> pass (J0)
[ui] tests/ui/const-generics/mgca/late-bound-param-lt-bad.rs: [missing] -> pass (J0)
[ui] tests/ui/const-generics/mgca/selftyalias-containing-param.rs: [missing] -> pass (J0)
[ui] tests/ui/const-generics/mgca/selftyparam.rs: [missing] -> pass (J0)
[ui] tests/ui/const-generics/type-relative-path-144547.rs#mgca: [missing] -> pass (J0)
[ui] tests/ui/const-generics/type-relative-path-144547.rs#min: [missing] -> pass (J0)

Stage 2

[ui] tests/ui/const-generics/mgca/early-bound-param-lt-bad.rs: [missing] -> pass (J1)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts-2.rs: pass -> [missing] (J1)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts-3.rs: pass -> [missing] (J1)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts-4.rs: pass -> [missing] (J1)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts-5.rs: pass -> [missing] (J1)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts-6.rs: pass -> [missing] (J1)
[ui] tests/ui/const-generics/mgca/higher-ranked-lts-bad.rs: [missing] -> pass (J1)
[ui] tests/ui/const-generics/mgca/higher-ranked-lts-good.rs: [missing] -> pass (J1)
[ui] tests/ui/const-generics/mgca/late-bound-param-lt-bad.rs: [missing] -> pass (J1)
[ui] tests/ui/const-generics/mgca/selftyalias-containing-param.rs: [missing] -> pass (J1)
[ui] tests/ui/const-generics/mgca/selftyparam.rs: [missing] -> pass (J1)
[ui] tests/ui/const-generics/type-relative-path-144547.rs#mgca: [missing] -> pass (J1)
[ui] tests/ui/const-generics/type-relative-path-144547.rs#min: [missing] -> pass (J1)
[crashes] tests/crashes/140891.rs: pass -> [missing] (J2)

Additionally, 31 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 a47f4dfd7f496ed51a383c540497899f9e3f91b4 --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

x86_64-gnu-aux: 6993.1s -> 7880.1s (+12.7%)
i686-gnu-1: 7253.7s -> 8162.1s (+12.5%)
x86_64-gnu-gcc: 3039.0s -> 3416.3s (+12.4%)
dist-aarch64-msvc: 5650.7s -> 6290.9s (+11.3%)
aarch64-apple: 9042.1s -> 10044.3s (+11.1%)
x86_64-gnu-llvm-20: 4159.7s -> 4574.0s (+10.0%)
x86_64-gnu-debug: 6605.1s -> 7215.8s (+9.2%)
x86_64-gnu-llvm-21-1: 4200.9s -> 4562.4s (+8.6%)
pr-check-1: 1781.7s -> 1934.7s (+8.6%)
aarch64-msvc-2: 6123.8s -> 6602.7s (+7.8%)

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 · 2026-01-02T06:18:14Z

Finished benchmarking commit (a47f4df): comparison URL.

Overall result: ❌ regressions - 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)	0.2%	[0.1%, 0.3%]	6
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.1%, 0.3%]	6

Max RSS (memory usage)

Results (primary -1.9%, secondary -2.1%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.9%	[-1.9%, -1.9%]	1
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	-1.9%	[-1.9%, -1.9%]	1

Cycles

Results (secondary 7.3%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	7.3%	[7.3%, 7.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-	-	0

Bootstrap: 474.6s -> 473.624s (-0.21%)
Artifact size: 390.83 MiB -> 390.78 MiB (-0.01%)

BoxyUwU · 2026-01-02T07:29:00Z

Perf regression is unfortunate. I'm not sure what we can do about it 🤔 Doing more work here feels somewhat unavoidable and also necessary to fix the ICEs and generally correctly handle forbidding uses of generic parameters introduced after name resolution ☹️

Following rust-lang#150519, the `forbid_generic` field in `Res::SelfTyAlias` is no longer needed. The check for generic `Self` types in anonymous constants is now handled by `check_param_uses_if_mcg` in HIR type lowering, making this field redundant. This removes: - The `forbid_generic` field from `Res::SelfTyAlias` - The hack in `rustc_resolve` that set `forbid_generic: true` when encountering `Self` in constant items - Related pattern matching and field propagation code

…neric-field, r=camelid,BoxyUwU Remove unneeded `forbid_generic` field from `Res::SelfTyAlias` Following rust-lang#150519, the `forbid_generic` field in `Res::SelfTyAlias` is no longer needed and can be removed. - Remove the `forbid_generic: bool` field from `Res::SelfTyAlias` - Simplify the ConstantItem rib handling in `rustc_resolve` - no longer need to mutate res to set `forbid_generic: true` - Update all pattern matches and constructors of `SelfTyAlias` Closes rust-lang#150579 r? `@camelid`

…neric-field, r=camelid,BoxyUwU Remove unneeded `forbid_generic` field from `Res::SelfTyAlias` Following rust-lang#150519, the `forbid_generic` field in `Res::SelfTyAlias` is no longer needed and can be removed. - Remove the `forbid_generic: bool` field from `Res::SelfTyAlias` - Simplify the ConstantItem rib handling in `rustc_resolve` - no longer need to mutate res to set `forbid_generic: true` - Update all pattern matches and constructors of `SelfTyAlias` Closes rust-lang#150579 r? ``@camelid``

Rollup merge of #150589 - AprilNEA:remove-unneeded-forbid-generic-field, r=camelid,BoxyUwU Remove unneeded `forbid_generic` field from `Res::SelfTyAlias` Following #150519, the `forbid_generic` field in `Res::SelfTyAlias` is no longer needed and can be removed. - Remove the `forbid_generic: bool` field from `Res::SelfTyAlias` - Simplify the ConstantItem rib handling in `rustc_resolve` - no longer need to mutate res to set `forbid_generic: true` - Update all pattern matches and constructors of `SelfTyAlias` Closes #150579 r? ``@camelid``

Kobzol · 2026-01-06T08:01:50Z

It's a tiny regression on diesel, I think that we can eat this one.

@rustbot label: +perf-regression-triaged

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Dec 30, 2025

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camelid assigned BoxyUwU and unassigned mati865 Dec 30, 2025

camelid added the A-const-generics Area: const generics (parameters and arguments) label Dec 30, 2025

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camelid added 3 commits January 1, 2026 18:12

mgca: Remove resolved FIXME

bad0c32

mgca: Re-merge split out tests

f71e938

Now that we avoid a fatal error, they can all be one test.

camelid force-pushed the mgca-forbid-params-properly branch from 94e79a8 to f71e938 Compare January 2, 2026 02:12

camelid mentioned this pull request Jan 2, 2026

Remove unneeded forbid_generic field from Res::SelfTyAlias #150579

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bors added the merged-by-bors This PR was explicitly merged by bors. label Jan 2, 2026

bors merged commit a47f4df into rust-lang:main Jan 2, 2026
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rustbot added this to the 1.94.0 milestone Jan 2, 2026

bors mentioned this pull request Jan 2, 2026

Implement consecutive type-relative paths #126651

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rustbot added the perf-regression Performance regression. label Jan 2, 2026

camelid deleted the mgca-forbid-params-properly branch January 2, 2026 07:27

AprilNEA mentioned this pull request Jan 2, 2026

Remove unneeded forbid_generic field from Res::SelfTyAlias #150589

Merged

This was referenced Jan 2, 2026

Tracking Issue for Generic Constant Arguments MVP #132980

Open

Validate HIR ty lowering's produced generic parameters #150620

Open

rustbot added the perf-regression-triaged The performance regression has been triaged. label Jan 6, 2026

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Use more principled check for generics in const ops #150519

Use more principled check for generics in const ops #150519

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Test differences

Stage 1

Stage 2

Job duration changes

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rust-timer commented Jan 2, 2026

Overall result: ❌ regressions - please read the text below

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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