Lint for unused unstable feature gates

[est31]

I've seen two codebases now that used to have unused feature gates:

• diem/diem@2f44be5 (this commit was in july while duration_float stabilized in August)
• diem/diem@e263fbd
• Remove some redundant/unused feature gates nushell/nushell#367

It would be cool to lint for feature gates that are still unstable but are not "used" by the codebase anywhere so they can be removed. That way, it's easy to know which nightly features a codebase actually still needs.

[joshtriplett]

I'd like to see this as well.

[Centril]

This seems reasonably added to the unused lint group as unused_feature.

It shouldn't be too complicated but it will require making sure that all the locations where feature gate checking is done is accounted for. Note that this is not just in libsyntax/feature_gate/check.rs but is also done in an ad-hoc fashion in later stages of compilation. To do this, you'll need to keep a set of used gates and then add features when gates were useful. This is a good opportunity to make the overall gating mechanisms less ad-hoc.

I'd be interested in hearing what @petrochenkov thinks about this from a technical POV though.

[varkor]

This used to exist — the tracking issue is #44232.

[est31]

Oh I see. Closing as a dupe of #44232 then. I hope it gets fixed. @Centril could you put the nominated label onto that issue instead?