Add warn-by-default lint for visibility on const _ declarations

[Jules-Bertholet]

Add a warn-by-default unused_visibilities lint for visibility qualifiers on const _ declarations—e.g. pub const _: () = ();. Such qualifiers have no effect.

A Sourcegraph search suggests that this pattern is relatively rare, and mostly found in tests (with only 3 exceptions). So perhaps this could become an FCW/hard error in the future.

@rustbot label T-lang A-lints A-visibility

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy
^^ Reviewed Clippy part by @blyxyas

[rustbot]

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[jdonszelmann]

cc: @rust-lang/lang

[jdonszelmann]

seems reasonable from me (compiler) then, gotta wait for lang approval

[scottmcm]

I think this seems fine to warn on from the perspective of "don't write things that you don't need", though probably with the usual "well if it came from a macro it's fine" kinds of limits because if the vis or ident was passed-in then complaining about it might not be worth it.

@rfcbot fcp merge

---

Personally I don't know that I'd want to FCW this. For me it's not the "we don't want to define a meaning for this" case where turning things into hard errors is most valuable; it's more the "well, you don't need to do this but what it does is clear (even if what it does is functionally nothing)".

But that's not something we need to decide in this issue and that decision isn't part of the FCP.

[rust-rfcbot]

Team member @scottmcm has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[Jules-Bertholet]

Personally, I would support an FCW, for the same reasons we disallow let mut _. But that discussion can happen later

[traviscross]

Agree with @scottmcm here, including the analysis of whether an FCW is warranted (which we're not deciding here).

@rfcbot reviewed

[scottmcm]

for the same reasons we disallow let mut _

Note that mut is a binding modifier, not an item modifier, so it's not quite analogous. let mut _ doesn't have a binding to mut, but pub const _ does arguably have a constant to pub.

(aka mut is part of the same grammar thing as the binding itself, whereas the visibility it part of the item grammar that isn't split out by whether it's a _ or not.)

But we don't need to resolve this here either way. The FCP is compatible with both paths in the future.