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GCI: At their def site, actually wfcheck the where-clause & always eval free lifetime-generic constants * 1st commit: Partially addresses [#136204](https://github.com/rust-lang/rust/issues/136204) by turning const eval errors from post to pre-mono for free lifetime-generic constants. * As the linked issue/comment states, on master there's a difference between `const _: () = panic!();` (pre-mono error) and `const _<'a>: () = panic!();` (post-mono error) which feels wrong. * With this PR, both become pre-mono ones! * 2nd commit: Oof, yeah, I missed that in the initial impl! This doesn't fully address #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 `const _UNUSED: () = () where for<'_delay> String: Copy;` which can also be found over at the tracking issue #113521. r? compiler-errors or reassign
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.