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Lifetime generics on Generic Const Items should not affect if const is checked for evaluatability #136204

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compiler-errors opened this issue Jan 28, 2025 · 0 comments
Open

Lifetime generics on Generic Const Items should not affect if const is checked for evaluatability #136204

compiler-errors opened this issue Jan 28, 2025 · 0 comments
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@fmease
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C-bug Category: This is a bug. F-generic_const_items `#![feature(generic_const_items)]` T-types Relevant to the types team, which will review and decide on the PR/issue.

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@compiler-errors
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There's a difference between const _: () = panic!(); and const _<'a>: () = panic!();: The former is a pre-mono error, the latter is a post-mono error.

This seems inconsistent, and pretty bad. But luckily something we can change before stabilizing generic consts :)

I think we should probably change the code introduced in #121387 to instead call requires_monomorphization instead of is_empty on the generics.

It should also preferably check for impossible predicates in the same way we do for -Clink-dead-code/-Zcollect-mono-items=eager, since we must avoid monomorphizing consts with impossible (possibly trivial) preds. You could probably turn that into an ICE today.

Originally posted by @compiler-errors in #136168 (review)
@rustbot rustbot added the needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. label Jan 28, 2025
@compiler-errors compiler-errors added T-types Relevant to the types team, which will review and decide on the PR/issue. F-generic_const_items `#![feature(generic_const_items)]` C-bug Category: This is a bug. and removed needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. labels Jan 28, 2025
@fmease fmease mentioned this issue Feb 2, 2025
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bors added a commit to rust-lang-ci/rust that referenced this issue Feb 2, 2025
@bors
Auto merge of rust-lang#136429 - fmease:gci-fix-def-site-checks, r=<try>
ea1cdda 
GCI: At their def site, actually wfcheck the where-clause & always eval free lifetime-generic constants

1st commit: Partially addresses rust-lang#136204 by turning const eval errors from post to pre-mono for free lifetime-generic constants.
Re. 2nd commit: Oof, that's embarrassing! How could I miss that in the initial impl?

This doesn't fully address rust-lang#136204 because I still haven't figured out how & where to properly & best suppress const eval of free constants whose predicates don't hold at the def site. The motivating example is `#![feature(trivial_bounds)] const _UNUSED: () = () where String: Copy;` which can also be found over at the tracking issue rust-lang#113521.

r? compiler-errors or reassign
bors added a commit to rust-lang-ci/rust that referenced this issue Feb 3, 2025
@bors
Auto merge of rust-lang#136429 - fmease:gci-fix-def-site-checks, r=<try>
6f11489 
GCI: At their def site, actually wfcheck the where-clause & always eval free lifetime-generic constants

1st commit: Partially addresses rust-lang#136204 by turning const eval errors from post to pre-mono for free lifetime-generic constants.
Re. 2nd commit: Oof, yeah, I missed that in the initial impl!

This doesn't fully address rust-lang#136204 because I still haven't figured out how & where to properly & best suppress const eval of free constants whose predicates don't hold at the def site. The motivating example is `#![feature(trivial_bounds)] const _UNUSED: () = () where String: Copy;` which can also be found over at the tracking issue rust-lang#113521.

r? compiler-errors or reassign
@fmease fmease moved this to In Progress in Generic Const Items (GCI) Feb 11, 2025
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C-bug Category: This is a bug. F-generic_const_items `#![feature(generic_const_items)]` T-types Relevant to the types team, which will review and decide on the PR/issue.
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Generic Const Items (GCI)
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