`impl Trait` should be able to capture long-lived associated types even if the trait contains a lifetime

[RalfJung]

Basically, this should compile:

trait Trait<'foo> { type Assoc: 'static; }

fn test<'foo, T: Trait<'foo>>(x: T::Assoc) -> impl FnOnce() {
    move || { let _x = x; }
}

Thanks to @eddyb I can work around this, writing

trait Trait<'foo> { type Assoc: 'static; }

fn test<'foo, A: 'static, T: Trait<'foo, Assoc=A>>(x: T::Assoc) -> impl FnOnce() {
    move || { let _x = x; }
}

but that is almost impossible to discover.

[eddyb]

It doesn't compile right now because it contains <T as Trait<'foo>>::Assoc, which, despite being 'static, contains the lifetime 'foo, so for soundness reasons, the impl FnOnce() opaque type must also be parametrized over 'foo (to recover the concrete return type).

We could try some erasure scheme, but the "exporting" of the concrete type is already pretty complicated, and there's a huge risk here, of opening up a soundness hole.

cc @nikomatsakis @cramertj @oli-obk

[Arnavion]

(Unexpected lifetime capture by impl-trait is #42940 )

[RalfJung]

@Mark-Simulacrum AFAIK this bug here is specifically about associated types, unlike #42940. Why did you close?

[Mark-Simulacrum]

I got the impression that the underlying bug is the same. But perhaps not. Reopening.

[RalfJung]

This actually compiles with the 2024 edition. :)

@rust-lang/types I assume that means it can be closed?

[lcnr]

the underlying cause still exists:

#![feature(precise_capturing)]
trait Trait<'foo> { type Assoc: 'static; }
fn test<'foo, T: Trait<'foo>>(x: T::Assoc) -> impl use<T> FnOnce() {
    move || { let _x = x; }
}