Prohibit multiple predicates

[ghost]

I noticed that cfg-if uses $($meta:meta),* to match cfg predicates:
https://github.com/alexcrichton/cfg-if/blob/f71bf60f212312faddee7da525fcf47daac66499/src/lib.rs#L37
https://github.com/alexcrichton/cfg-if/blob/f71bf60f212312faddee7da525fcf47daac66499/src/lib.rs#L51
https://github.com/alexcrichton/cfg-if/blob/f71bf60f212312faddee7da525fcf47daac66499/src/lib.rs#L53

That is redundant: multiple cfg predicates is not allowed by rustc, and specifying multiple predicates results in #[cfg]s that do not make sense in else parts. Also, mistakes can be easily made when there're many parentheses and commas in the cfg predicate.

[ghost]

Sorry about the typo, but I can't rename the branch.

[alexcrichton]

Oops I think I was a bit too zealous when I first wrote this! Can you gist what the error message for multiple predicates looks like before and after?

[ghost]

Before the patch, specifying multiple predicates compiles, but for cfg_if! { if #[cfg(A, B)] {} else {} }, the if part is guarded by #[cfg(all(A, B))], the else part is guarded by #[cfg(not(any(A, B)))] (instead of #[cfg(not(all(A, B)))]).

After the patch, the compilation fails with error: no rules expected the token `,`.

Note: The "native" #[cfg(A, B)] fails with error: multiple `cfg` predicates are specified.

[alexcrichton]

Ah ok so it does compile but it's actually quite confusing how it works. Definitely unintended, thanks for catching this!