Parse error for traits without parameter names

[Manishearth]

Discovered in mozilla/cbindgen#665, breaks on https://github.com/japaric/cast.rs. Minimal example:

trait Foo {
    fn bar(i32) -> i32;
}

error: Syn unable to parse file
 --> test.rs:2:14
  |
2 |     fn bar(i32) -> i32;
  |               ^ expected `:`

[Manishearth]

This is basically because TraitItem::Method just reuses Signature. I'm not sure what the best route forward is here, we can either create a separate Signature, or treat Signature to parse this as _: ty

[mystor]

Support for this deprecated trait syntax was actually intentionally (IIRC) removed a while back pre-1.0, in 9b98fce. I can't find the associated issue, though.

I think it was to simplify types before the 1.0 release, but I'm sure @dtolnay remembers more context.

EDIT: I think this is an interesting example of how syn is expected to be able to parse both rust 2018 and rust 2015 syntax accurately without knowing which edition the crate in question is using. This may become more of an issue, and we may need to add flags for e.g. specifying which edition to parse with in the future. This will also probably require some support from rustc itself to tell us what edition specific tokens originate from.

[Manishearth]

Hmm. I have wished before that there be a context object proc macros can get this information from :/

This should at least be a future incompat lint on the older edition. Oh well.

[Manishearth]

Also opened rust-lang/rust#82918 so crates get nudged to newer editions

[dtolnay]

Yeah this is intentionally omitted from the 1.0 series.