Stack overflow with `impl Trait` and multiple lifetimes... and a closure.

[steffahn]

Seems to stack overflow (I think it's one... on Linux it does give "segfault", technically, and on Windows it produces "thread 'rustc' has overflowed its stack").

(Current nightly behaves like current stable.)

trait Fn1<T>: Fn(T) -> <Self as Fn1<T>>::Output {
    type Output;
}
impl<F: ?Sized, T, O> Fn1<T> for F where F: Fn(T) -> O {type Output = O;}

trait Trait<'a1, 'a2> {}
impl<S: ?Sized> Trait<'_, '_> for S {}

fn foo<'a>(request: &'a str) -> impl for<'b> Fn1<&'b (), Output = impl Trait<'a, 'b>> {
    move |_: &()| ()
}

(playground)

Haven't tried much to minimize further yet, perhaps this can also be done, somehow, without using a closure? Nevermind, I removed the need for closures now

trait Fn1<T> {
    type Output;
}
impl<T, S: ?Sized> Fn1<T> for S {
    type Output = ();
}

trait Trait<'a> {}
impl<'a, S: ?Sized> Trait<'a> for S {}

fn foo() -> impl for<'b> Fn1<&'b (), Output = impl Trait<'b>> {
    ()
}

The same code used to give various ICEs in older Rust versions, stack overflow seems to be since 1.57. I don't have time to investigate further today.

@rustbot label I-crash, A-impl-trait, A-lifetimes, A-closures, T-compiler
@rustbot label -A-closures

---

Labeling question: is ICE -> crash a regression?

[rustbot]

Error: Label ~~A-closures can only be set by Rust team members

Please let @rust-lang/release know if you're having trouble with this bot.

[QuineDot]

More minimal:

trait Trait<'a1, 'a2> {}
trait WithLifetimeAndAssoc<'x> {
    type Assoc;
}

fn foo<'a>(request: &'a str) -> impl for<'b> WithLifetimeAndAssoc<'b, Assoc = impl Trait<'a, 'b>> {
    todo!()
}

Almost surely a dupe of #88236 based on the 1.56 error, and that issue also overflows now.

[steffahn]

Yes, that's a dupe

[steffahn]

I had only searched for I-crash issues :-)