Async struct method, <_'> lifetime shadowing

[fominok]

Hello, I ran into a problem using <'_> lifetime with async struct methods, here is the example:

struct Foo<'a> { swag: &'a i32 }

impl Foo<'_> {
    async fn bar(&self) -> i32 {
        1337
    }
}

The error is:
error[E0496]: lifetime name '_ shadows a lifetime name that is already in scope

Everything is fine if I use explicit lifetime like 'a.

Version: 1.39 Nighly (2019-09-19 9b9d2af on playground)

[cramertj]

This is happening because the desugaring of async fn uses the '_ lifetime. This is sort of unfortunate, but seems backwards compatible to fix, so I don't think we should block on this.

[nikomatsakis]

We decided to focus on this one in the WG meeting -- @matthewjasper you still interested in working on it?

[matthewjasper]

Yes, I'm still interested in working on this. I don't think that this should be too hard to fix though if someone else wants it.