Support cfg(true) and cfg(false)

[Jules-Bertholet]

meta does not match true/false, so switch to tt.

[tgross35]

Couple small test requests. The impl itself looks good to me, but @nnethercote would you mind giving this a second set of eyes? Considering you've been looking at these patterns recently and they can be a bit tricky.

[nnethercote]

Seems ok. My only question is how is the behaviour changed if you do something erroneous like if #[cfg(99)]? Probably a different error message?

[Jules-Bertholet]

Probably a different error message?

Yes, from “no rules expected `<token>` in macro call” to “cfg predicate key must be an identifier”. It’s an improvement, I think

[nnethercote]

Sounds good to me.