Error with nested async fn and anonymous lifetimes

[udoprog]

The following breaks on nightly (see playground)

#![feature(async_await)]

struct Foo<'a>(&'a ());

impl Foo<'_> {
    fn test() {
        async fn test() {}
    }
}

With:

error[E0261]: use of undeclared lifetime name `'_`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0261`.
error: Could not compile `playground`.

To learn more, run the command again with --verbose.

CC: dtolnay/async-trait#18

[Centril]

Reduced:

#![feature(async_await)]

struct Foo<'a>(&'a ());

impl<'a> Foo<'a> {
    fn test() {
        async fn test() {}
    }
}

[udoprog]

Forgot to mention in case this helps, giving the inner fn a matching lifetime avoids this issue for some reason:

#![feature(async_await)]

struct Foo<'a>(&'a ());

impl<'a> Foo<'a> {
    fn test() {
        async fn test<'a>() {}
    }
}

[cramertj]

The reason this is happening is that the desugared version of async fn applies all in-scope lifetimes to the returned existential type, which in this case is creating -> Fut<'_>, which is invalid. I'm not sure exactly what the best fix is for this, though, since elided lifetimes in the impl signature are intentionally unnameable. The closest idea I have would be to create some special new lifetime that is '_ but can refer to lifetimes originating from the impl header. cc @nikomatsakis who may have opinions.

[nikomatsakis]

Hmm @cramertj wouldn't you expect the nested async fn to not inherit anything from its enclosing scopes? Seems like a bug to me that it does? Am I confused about something?

[cramertj]

@nikomatsakis The function in the middle is actually a bit of a source of confusion here, although it does indicate that there's a second bug since we shouldn't be inheriting generics since the nested async fn isn't part of the impl.

The bug I was focusing on is better illustrated by this example:

#![feature(async_await)]

struct Foo<'a>(&'a u8);

impl Foo<'_> {
    async fn bar() {}
}

which produces this error:

error[E0261]: use of undeclared lifetime name `'_`

error: aborting due to previous error

This is because the code is transformed into

impl Foo<'_> {
    fn bar() -> OpaqueTy<'_> {}
}

and in that case '_ does not refer back to the usage from the impl header.

[nikomatsakis]

@cramertj I see. Indeed, I would expect that, internally, we'd have some way to refer back to the '_ from the impl header. I believe this should be possible, no? In particular, the '_ in impls expands (internally) to LifetimeName::Param(ParamName::Fresh(N)) for some integer N, which I believe is basically treated like any other named lifetime.