False positive on non_exhaustive_omitted_patterns_lint with closures

[Nemo157]

I tried this code (playground):

#![warn(non_exhaustive_omitted_patterns)]

// in other library:
// #[non_exhaustive] pub enum Foo { A, B }
use lib::Foo;

fn foo(_: impl Fn(Foo)) {}

fn main() {
    foo(|_: Foo| { todo!() });
}

I expected to see this happen: no warnings.

Instead, this happened:

warning: some variants are not matched explicitly
  --> src/main.rs:10:14
   |
10 |         foo(|_: Foo| { todo!() });
   |              ^ patterns `A` and `B` not covered
   |

Meta

1.64.0-nightly (2022-07-26 4d6d601c8a83284d6b23)

cc #89554

[Nemo157]

(Note: it's not to do with the _ pattern, in the real code I found this in the argument was used).

[Mark-Simulacrum]

I think this is a good thing to fix, but I don't think this would block stabilization since:

• It's unlikely you're enabling the lint across an entire crate -- most of the time, it'll be limited to a few tests or specific matches, probably.
• The lint can be fixed in the future (we should definitely do that though!)