Fulfill additional higher-kinded "outlives" predicates

[apasel422]

Types fail to fulfill higher-kinded "outlives" predicates when the higher-kinded region is present on the left-hand side of the predicate. For example:

fn foo<T>() where for<'a> &'a T: 'a {}

fn main() {
    foo::<i32>();
    //~^ ERROR the requirement `for<'a> &'a i32: 'a` is not satisfied [E0280]
}

CC @nikomatsakis.

[apasel422]

This doesn't actually have to do with diagnostics. A more accurate label would be lifetimes.

[apasel422]

@nikomatsakis Any advice for progressing on this?

[apasel422]

Ping.

[nikomatsakis]

@apasel422 heh, sorry. I think that solving this is actually going to be a bit tricky. What we really want to do, I think, is refactor how the trait resolution code works so that we can more easily introduce predicates into the environment and check under those assumptions. But probably there's an easier, shorter-term fix here, even if it covers less ground.

[beamspease]

I ran into this issue, and as I understand it's not possible without some work on Higher Kinded Lifetimes.
https://www.reddit.com/r/rust/comments/3lfl5p/trait_method_that_returns_owned_copy/

Dunno if this is the correct bug to comment on, but I just wanted to note that it seems like it makes it impossible to implement ToOwned for a trait...

[timotree3]

Is there any progress on this issue? Are Polonius or Chalk going to fix this?

I can reproduce it with something as simple as

trait Foo where for<'a> &'a u64: 'a {}

[Spoonbender]

Triage: resolved

Both examples compile on stable rustc 1.59.0 (9d1b2106e 2022-02-23)

I think we can close this one

[oskgo]

@rustbot label +E-needs-test

[estebank]

We already have a test.