implied bounds from projections in impl header can be unsound

[aliemjay]

A variant of #98543 that has a different failure path and thus is not fixed by #99217. cc @lcnr

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=da2da1d93c1e2c99c640000a78e9a80f

trait Trait {
    type Type;
}

impl<T> Trait for T {
    type Type = ();
}

trait Extend<'a, 'b> {
    fn extend(self, s: &'a str) -> &'b str;
}

impl<'a, 'b> Extend<'a, 'b> for <&'b &'a () as Trait>::Type
where
    for<'what, 'ever> &'what &'ever (): Trait,
{
    fn extend(self, s: &'a str) -> &'b str {
        s
    }
}

fn main() {
    let y = <() as Extend<'_, '_>>::extend((), &String::from("Hello World"));
    println!("{}", y);
}

@rustbot label C-bug I-unsound T-types A-implied-bounds A-associated-items

[lcnr]

ah, that's fun 👍

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high

[lcnr]

@rustbot claim