Implement a naming convention for lint/diagnostic-only `rustc_` attrs

[WaffleLapkin]

Proposal

Make a naming convention such that all all rustc_ attributes that are used only for lint/diagnostic purposes are clearly distinguished from all other rustc_ attributes hich might have semantic implications.

This will make it clear that the lint/diagnostic-only attributes do not affect semantics and are not (as) important.

Personal anecdote

I was just reading sources of UnsafeCell and saw #[rustc_as_ptr] and #[rustc_never_returns_null_ptr] and had a minor heart attack thinking #[rustc_never_returns_null_ptr] changes the LLVM ir to exclude 0 from the valid range of the return (similarly to something I just removed some months ago: rust-lang/rust#154327).

However, it looks like both of these attributes are only used for lints.

Alternative

Add a #[rustc_diagnostic_item_group(...)] similarly to #[rustc_diagnostic_item], but allowing multiple items to have the same "diagnostic group", such that we have a unified way for lints to special-case certain item groups.

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[rustbot]

Important

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cc @rust-lang/compiler

[mejrs]

@rustbot second

[rustbot]

The final comment period is now complete, this major change is now accepted. 🎉

As the automated representative, I want to express gratitude to the author for their work and everyone else who contributed to this major change proposal.

If you think this major change shouldn't have been accepted, feel free to remove the major-change-accepted label and reopen this issue.

Note

Further progress is now tracked over at rust-lang/rust#160808.