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Deny const auto traits #149307

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@lapla-cogito lapla-cogito commented Nov 25, 2025 •
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close #149285

The AST validation now detects and rejects const auto traits. Additionally, I updated an existing test that was using const unsafe auto trait.

r? fmease

@rustbot 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 Nov 25, 2025
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rustbot commented Nov 25, 2025

fmease is currently at their maximum review capacity.
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fmease approved these changes Nov 25, 2025
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tests/ui/traits/const-traits/const-auto-trait.stderr
LL | const auto trait Marker {}
| ^^^^^
|
= help: remove the `const` keyword
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We could provide a structured suggestion but yeah, it's not really worth the effort.

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I initially thought about that, but decided not to, mainly because not every suggestion can be fixed automatically with rustfix, for example the code sample in the original issue that triggers a panic in next-gen solvers, and because we don't really have anything meaningful to add beyond what the current message already communicates.

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Ah, the next-gen solver still crashes despite this change? Right, of course.

I guess we should replace the unreachable!(…) in consider_auto_trait_candidate of HostEffectPredicate with a cx.delay_bug. Sorry, I forgot about that.

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Ah, that’s a bit different from what I was aiming to accomplish in this PR, so I had planned to handle it in a follow-up PR instead (though I should have mentioned that). But since integrating the tests is straightforward, I’ve decided to include it in this PR as well: c80ab42

@fmease fmease 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 Nov 25, 2025
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fmease commented Nov 25, 2025

Thanks! Two small nitpicks

@lapla-cogito lapla-cogito force-pushed the deny_const_auto_trait branch 2 times, most recently from 44f4a9b to 0f63688 Compare November 25, 2025 12:06
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lapla-cogito commented Nov 25, 2025

@rustbot ready

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Nov 25, 2025
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fmease commented Nov 25, 2025

@bors r+ rollup

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bors commented Nov 25, 2025

📌 Commit 3326fbd has been approved by fmease

It is now in the queue for this repository.

@bors bors 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 Nov 25, 2025
@fmease fmease added the PG-const-traits Project group: Const traits label Nov 25, 2025
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fmease commented Nov 25, 2025 •
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Ah, we also need to replace the unreachable!() with a cx.delay_bug() in consider_auto_trait_candidate, so it no longer crashes under -Znext-solver. #149307 (comment).

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@bors bors 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-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Nov 25, 2025
@rustbot rustbot added the WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) label Nov 25, 2025
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lapla-cogito commented Nov 25, 2025

@rustbot ready

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Nov 25, 2025
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fmease commented Nov 25, 2025

Thanks again @bors r+

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bors commented Nov 25, 2025

📌 Commit ae699c8 has been approved by fmease

It is now in the queue for this repository.

@bors bors 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 Nov 25, 2025
jhpratt added a commit to jhpratt/rust that referenced this pull request Nov 26, 2025
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Rollup merge of rust-lang#149307 - lapla-cogito:deny_const_auto_trait…
b435ee4 
…, r=fmease

Deny const auto traits

close rust-lang#149285

The AST validation now detects and rejects const auto traits. Additionally, I updated an existing test that was using `const unsafe auto trait`.

r? fmease
@jhpratt jhpratt mentioned this pull request Nov 26, 2025
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PG-const-traits Project group: Const traits S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver)

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We don't reject const auto traits

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