Remove #[const_trait]

[fmease]

Remove #[const_trait] since we now have const trait. Update all structured diagnostics that still suggested the attribute.

r? @rust-lang/project-const-traits

[rustbot]

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann

Some changes occurred to constck

cc @fee1-dead

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

cc @jdonszelmann

[fmease]

The nyn was incorrect in that it didn't follow the pattern that's explicitly established at the top of the file (where 0 |-> const_trait_impl enabled?, 1 |-> Foo const?, 2 |-> Bar const?) since the last n incorrectly claimed that under this revision Bar was not const despite literally enabling #[const_trait] when active.

Replaced with nny, hence some of the error annotation changes.

[fmease]

This pattern occurs in three more places (tho you can't really abstract over it). It yields a syntactically correct suggestion with any(*) combination of trait modifiers/qualifiers present and can deal with inherited visibility, pub , pub(…) and pub(…) (i.e., no trailing space).

(*): It does not account for specialization's default which theoretically comes before const (like visibility), however that's actually syntactically invalid but the parser does recover from it. Still, it's really not worth accounting for.