Add a diagnostic attribute for special casing const bound errors for non-const impls

[oli-obk]

Somewhat of a follow-up to #144194

My plan is to resolve

https://github.com/rust-lang/rust/blob/f4e19c68786211f3c3cf2593442629599678800a/compiler/rustc_hir_typeck/src/callee.rs#L907-913

but doing so without being able to mark impls the way I do in this PR wrould cause all nice diagnostics about for loops and pointer comparisons to just be a *const u32 does not implement [const] PartialEq errors.

[rustbot]

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

cc @jdonszelmann

[rustbot]

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Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1279:29

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[estebank]

Should we also check that the impl is not const? Otherwise the annotation is fully inert in that case, right?

[estebank]

Is that strictly necessary? I feel like at least for std, if we annotate with diagnostic::on_const we'd want that message to appear even in stable.

[oli-obk]

std has the feature enabled. but in this case the aux crate doesn't have it. So if a user crate doesn't have the feature gate, their uses of diagnostic::on_const get ignored

[estebank]

Ah! I see. So the feature must be enabled where the annotation is. Gotcha.