NLL ICE: "unresolved inference variable in outlives"

[nikomatsakis]

The crater run turned up a number of crates encountering an ICE with a message like "unresolved inference variable in outlives" (see list in #52054). I'm not sure what is the problem yet but I'm filing this bug to track the issue.

A good representative is combine-language-2.0.0-beta4, which failed with this log.

[matthewjasper]

Self contained example. Related to #51649?

// Delayed ICE:
// error: internal compiler error: unresolved inference variable in outlives: _#0t
#![feature(nll)]

pub trait Parser {
    type Input;

    #[inline(always)]
    fn parse_first(input: &mut Self::Input);
}

impl<'a, I, P: ?Sized> Parser for &'a mut P
where
    P: Parser<Input = I>,
{
    type Input = I;

    fn parse_first(_: &mut Self::Input) {}
}

fn main() {}

[pnkfelix]

triage re-assigning to self

[nikomatsakis]

I think this may have been fixed by #51959. At least @matthewjasper's example no longer reproduces. I'll have to try the original crate.

[nikomatsakis]

OK wait. @matthewjasper's example now fails to build, whereas it builds without nll. It no longer ICEs though.