Closures cause unnecessary borrows

[mpdn]

Simple example:

use std::cmp::Ordering;

fn something<T,F>(slice: &mut [T], mut compare: F) where
    F: FnMut(&T,&T) -> Ordering
{
    // This compiles fine:
    if compare(&slice[0], &slice[1]) == Ordering::Less {
        slice.swap(0, 1);
    }

    // But this does not:
    let mut cmp = |a,b| compare(a,b) == Ordering::Less;

    if cmp(&slice[0], &slice[1]) {
        slice.swap(0, 1);
    }
}

Playpen: http://is.gd/niaLAa

It seems like it marks the closure return value as a borrowed value from slice, even though it is just a bool.

[steveklabnik]

Triage. This stuff still fails to compile today.

[memoryruins]

Triage: now compiles on 2018.
2015 errors:

error[E0502]: cannot borrow `*slice` as mutable because `slice[..]` is also borrowed as immutable
  --> src/main.rs:15:9
   |
14 |     if cmp(&slice[0], &slice[1]) {
   |             -------- immutable borrow occurs here
15 |         slice.swap(0, 1);
   |         ^^^^^ mutable borrow occurs here
16 |     }
17 | }
   | - immutable borrow ends here

rustc: 1.32.0