Add test for expansion inside of cfg_attr

[dtolnay]

Test for #148. Requires serde-deprecated/syntex#71 + Syntex release.

[erickt]

r=me once serde-deprecated/syntex#71 lands.

[dtolnay]

I marked this WIP until we get a Syntex release to depend on. As far as I know nobody is waiting urgently for this so we can just wait for the next libsyntax break.

[erickt]

@homu: r+ 675caa8

[erickt]

@dtolnay: syntex is updated, but this is failing on stable and beta with things like:

/Users/erickt/projects/rust/serde/serde/serde_tests/target/debug/build/serde_tests-0dc644810d6621ba/out/test.rs:16260:9: 16260:22 error: the trait bound `test_macros::NamedUnit: std::cmp::PartialEq` is not satisfied [E0277]
/Users/erickt/projects/rust/serde/serde/serde_tests/target/debug/build/serde_tests-0dc644810d6621ba/out/test.rs:16260         assert_tokens(&NamedUnit, vec!(Token :: UnitStruct ( "NamedUnit" )));

Do you know what's up? Maybe the bug hasn't been fixed in syntex yet.

[dtolnay]

That's so strange. It works on the nightly compiler using Syntex, but not on the beta compiler using Syntex. What would be the difference between those?

The generated code looks correct in both cases:

#[cfg_attr(all(), derive(PartialEq, Debug))]
struct NamedUnit;
#[cfg(all())]
const _IMPL_DESERIALIZE_FOR_NamedUnit: () = { /* ... */ };
#[cfg(all())]
const _IMPL_SERIALIZE_FOR_NamedUnit: () = { /* ... */ };

[dtolnay]

Interesting, cargo expand on beta shows #[derive(PartialEq, Debug)] but on nightly it shows them expanded. That must have something to do with it. Now to figure out why...

[dtolnay]

#[cfg_attr(all(), cfg_attr(all(), derive(Debug)))]
struct S;

fn main() {
    println!("{:?}", S);
}

This works on nightly but not stable/beta. Playground link

I am going to write this off as a bug that needed to be fixed in both Syntex and rustc. We can add the test later when we support only versions that contain the rustc side of the fix.