Closure parameter inference and lifetime elision mayhem

[frankmcsherry]

If you throw the following code into playpen, in the fourth case (by my reading of the error message) Rust determines an incompatible number of lifetime parameters for<'r> core::ops::Fn<(&'r u64, &'r u64)> vs. for<'r,'r> core::ops::Fn<(&'r u64, &'r u64)>).

fn test<F: Fn(&u64, &u64)>(f: F) {}

fn main() {
    test(|x,      y     | {});   // works  
    test(|x:&u64, y:&u64| {});   // works
    test(|x:&u64, y     | {});   // works
    test(|x,      y:&u64| {});   // no works
}

I won't try to explain how mysterious this was, manifesting as a "sorry, you don't implement that trait" error (because the closure was not matching the type) without explaining what was awry.

[brson]

Needs a P- tag.

[nikomatsakis]

triage: P-medium

[nikomatsakis]

oh, sorry :)

[arielb1]

Looks like it was caused by the same "unexpected capture" issue as #38714 - it works on nightly.

[arielb1]

This still looks like a distinct "family of examples", so I'll at least like it to be added to the test for that issue.