Build broken by type alias enforcing outlives relations

[MingweiSamuel]

rust-lang/rust#95519

[MingweiSamuel]

rust-lang/rust#95922

[MingweiSamuel]

#![feature(generic_associated_types)]
#![feature(type_alias_impl_trait)]

trait Gats {
    type Build<'a, 'b>: Fn() -> String;
    fn build<'a, 'b>(a: &'a str, b: &'b str) -> Self::Build<'a, 'b>;
}

type AlphaBuild<'a, 'b> = impl Fn() -> String;

struct Alpha;
impl Gats for Alpha {
    type Build<'a, 'b> = AlphaBuild<'a, 'b>;
    fn build<'a, 'b>(a: &'a str, b: &'b str) -> Self::Build<'a, 'b> {
        || format!("{}{}", a, b)
    }
}

Playground Link

[MingweiSamuel]

doesn't quite reproduce

#![feature(generic_associated_types)]
#![feature(type_alias_impl_trait)]

use std::fmt::Display;

trait GatsBase {
    type Build<'a, 'b>: Fn() -> String;
}
trait Gats: GatsBase {
    fn build<'a, 'b>(&'a self, b: &'b str) -> Self::Build<'a, 'b>;
}

type AlphaBuild<'a, 'b, T> = impl 'a + 'b + Fn() -> String;

struct Alpha<T: Display>(T);

impl<T: Display> GatsBase for Alpha<T> {
    type Build<'a, 'b> = AlphaBuild<'a, 'b, T>;
}
impl<T: Display> Gats for Alpha<T> {
    fn build<'a, 'b>(&'a self, b: &'b str) -> Self::Build<'a, 'b> {
        || format!("{}{}", self.0, b)
    }
}

[aliemjay]

I know nothing about hydroflow but noticed that adding move to the closure makes it pass.

Was it accpeted previously? That would be surprising!

[MingweiSamuel]

updated playground

Thanks! Yeah, that does fix this example, so I guess this wasn't a reproduction of the issue we're having in hf itself. Still working on it.

more complicated playground also works

[aliemjay]

reproduced on playground

Fixed by adding where T: 'a to the GAT and to the type alias. playgound

[MingweiSamuel]

Oh I didn't know the where Self: 'a clause let you bound other type parameters as 'a, I think that will work!

[MingweiSamuel]

Although I guess writing where Self: 'a (instead of where T: 'a) also works? Since T is a parameter of Self, I guess.