Feature gate custom attributes

[Manishearth]

fixes#22203

r? @nikomatsakis

This breaks code that might be using attributes randomly, so it's technically a

[breaking-change]

[Manishearth]

cc @nick29581

[Manishearth]

This doesn't completely implement the backwards compatibility we need for attributes. There's also the case of attributes like #[must_use] being used places where they shouldn't be (eg mods).

But the requirements for each of the builtin attributes are different so I don't see a way of doing this here. Adding some sort of individual check for each sounds doable though.

[nikomatsakis]

The interaction between #22336 and this is interesting. I think I'd like a way to include something in the table of custom attributes but still declare that it requires a (perhaps generic) feature-gate. This would be the case for things like rustc_move_fragments and rustc_variance. Or we could add them to the list above, but it seems a bit redundant. (It might also be nice to be able to just write something like FeatureGate("tag") and get a generic message like "The attribute {} has unstable semantics and requires the feature-gate {}".

[Manishearth]

Yeah, Gated("tag") (and perhaps even Deprecated("reason")) sound like useful additions to the list.

[Manishearth]

(fixed)

[nikomatsakis]

@Manishearth ok r+ with the addition of Gated("tag"), which should be used for rustc_* (and perhaps Deprecated("reason"))

[eddyb]

@bors r=nikomatsakis 0112 aka cute little fibonacci

[nikomatsakis]

@bors r+ 1fffdaf

[nikomatsakis]

@bors r- until rustc_variance and friends are feature-gated

[nikomatsakis]

@bors r+ 917fce2

[nikomatsakis]

@bors r+ f64d71b