Required lifetime bounds for GATs in where clauses get removed

[jam1garner]

Minimal reproduction (playground link, hit 'format' then try building to see it in action):

Before

trait Foo {
    type Arg<'a>;
}

struct Bar<T>(T) where for<'a> T: Foo<Arg<'a> = ()>;

After

trait Foo {
    type Arg<'a>;
}

struct Bar<T>(T)
where
    for<'a> T: Foo<Arg = ()>;
//                             ^^^ note the lack of <'a> bound, which is needed

This will result in the following compiler error:

error[E0107]: missing generics for associated type `Foo::Arg`
 --> src/main.rs:9:20
  |
9 |     for<'a> T: Foo<Arg = ()>;
  |                    ^^^ expected 1 lifetime argument
  |
note: associated type defined here, with 1 lifetime parameter: `'a`
 --> src/main.rs:4:10
  |
4 |     type Arg<'a>;
  |          ^^^ --
help: add missing lifetime argument
  |
9 |     for<'a> T: Foo<Arg<'a> = ()>;
  |                    ~~~~~~~

[raftario]

Also running into this, will try and look into fixing it and open a PR if I can figure it out

[calebcartwright]

Thanks @raftario but this has already been fixed in source, just hasn't been released yet.

[calebcartwright]

we would benefit from getting more test cases added for the various issues that have been reported, can ping you if you're interested in working on that

[raftario]

Oh that's great to hear, thanks for letting me know. Wouldn't mind getting a couple of tests in if that can be of any help

[calebcartwright]

Excellent thanks. Will create a new issue with the details tomorrow and tag you

[calebcartwright]

Closing as the issue was already resolved in source and test case has been added. No ETA yet on exactly when the fix will hit nightly, but it will be in the next release regardless

[chrisbouchard]

Any word when this fix will be available in nightly in light of the push for GAT stabilization?

[calebcartwright]

Any word when this fix will be available in nightly in light of the push for GAT stabilization?

This has already been on nightly for quite some time

[chrisbouchard]

@calebcartwright Yep, you're right. My editor wasn't using nightly rustfmt. Sorry for the noise!

[calebcartwright]

@calebcartwright Yep, you're right. My editor wasn't using nightly rustfmt. Sorry for the noise!

No worries. FWIW I believe it'll land on stable with the 1.58 release too