Nightly borrowck regression

[arcnmx]

The following code compiles on stable and beta but not on the current nightly:

#[derive(Clone)]
struct S;

impl S {
    fn borrow(&self) -> Vec<&Self> { vec![self] }

    fn borrow_mut(&mut self, _: &S) { }
}

fn main() {
    let mut s = S;
    for x in s.borrow().into_iter().cloned().collect::<Vec<_>>() {
        s.borrow_mut(&x)
    }
}

[alexcrichton]

triage: I-nominated

[nikomatsakis]

triage: P-high

[nikomatsakis]

Not sure what's going on here just yet.

[arielb1]

This fails on stable (1.3) too if you explicitly deref s.borrow() (either via *, .deref, or Deref::deref) rather than autderefing.

[arielb1]

Minified:

#[derive(Clone)]
struct S;

fn borrow<'a>(s: &'a S) -> Vec<&'a S> { vec![s] }

fn main() {
    let s = S;
    match <Vec<&S> as IntoIterator>::into_iter(borrow(&s)) {
        _ => drop(s)
    };
}

[nikomatsakis]

/me actively investigating now

[nikomatsakis]

I believe this will be fixed by @pnkfelix's PR here: #29186

[nikomatsakis]

@pnkfelix can you confirm?

[pnkfelix]

yes it looks to me like #29186 fixes this.

I also think the test that #29186 adds should suffice to cover this bug; does anyone disagree with that claim?

If no one disagrees then I'll close this as a dupe of #29166

[nikomatsakis]

I agree. Closing as dup of #29166.