NLL: Implied bounds computation misses some relationships

[lqd]

Found while triaging the NLL-enabled crater run, and extracted from specs-0.10.

#![feature(nll)]
#![allow(unused_variables)]

struct Drain<'a, T: 'a> {
    _marker: ::std::marker::PhantomData<&'a T>,
}

trait Join {
    type Value;
    fn get(value: &mut Self::Value);
}

impl<'a, T> Join for Drain<'a, T> {
    type Value = &'a mut Option<T>;
    
    fn get<'b>(value: &'b mut Self::Value) {
    }
}

fn main() {
}

Playground

Note: the added 'b is to make the error message clearer, about a the missed relationship between 'a and 'b from the argument type (maybe because it is going through the associated type).

Otherwise fn get(value: &mut Self::Value) yields this more obscure free region "'a" does not outlive free region "".

[nikomatsakis]

Turns out to be fixed by #51959