add unreachable_cfgs lint

[folkertdev]

tracking issue: #115585

Split out from #149783. This lint is emitted on branches of a cfg_select! that are statically known to be unreachable. The lint is only emitted when the feature is enabled, so this change specifically does not need an FCP, and the lint will be stabilized alongside the feature (see #149783 (comment)).

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

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[Urgau]

The proposed name for this lint seems quite unfortunate to me. When I first read it I though it had something to do with the #[cfg] attribute, like detecting #[cfg] predicates that are unreachable.

I think it would be better include the word "arm" or "cfg_select" in it, to clearly disambiguate it from the other places where cfgs can appear.

Maybe unreachable_cfg_arms or unreachable_cfg_select_arms?

[folkertdev]

There is some reasoning for then name at #149783 (comment).

cc @traviscross was there any particular reason for you to leave out the select or arm/branch parts?

[traviscross]

The reasoning was:

For match, we call it unreachable-patterns; i.e. we put the focus on the pattern being unreachable rather than the arm. This makes sense; it's more precise. Notably we don't call it unreachable-match-arm-patterns.

Here that would suggest the name unreachable-cfg-predicates, since the constructs on the LHS of each arm are "configuration predicates" and we abbreviate "cfg".

But we have an existing lint, unexpected-cfgs that already uses cfgs to mean "configuration options" (a subset of cfg predicates), so punning a bit and riffing off that system would suggest the name unreachable-cfgs (though as mentioned, unreachable-cfg-predicates is OK too.)

To your point about detecting cfg attributes that are unreachable, I had the same thought when proposing the name, but expected I'd be OK, if we ever did want to lint other kinds of unreachable configuration predicates, using this same name for that.

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Sitting with it now, though, I think unreachable-cfg-select-predicates would be OK.

[bors]

☔ The latest upstream changes (presumably #146348) made this pull request unmergeable. Please resolve the merge conflicts.