MIR borrowck: spurious cannot borrow immutable item error with a closure

[arielb1]

e.g. this variant of the test run-pass/issue-16671.rs

fn foo<F: FnOnce()>(_f: F) { }

fn main() {
    let mut var = 0;
    foo(move|| {
        &mut var;
    });
}

Causes this error:

error[E0596]: cannot borrow immutable item `var` as mutable (Mir)
  --> ../src/test/run-pass/issue-16671.rs:18:9
   |
18 |         &mut var;
   |         ^^^^^^^^ cannot borrow as mutable
   |
   = note: Value not mutable causing this error: `_`

error: aborting due to previous error

[davidtwco]

Can be closed now, @arielb1 with #46236 merged?

[nikomatsakis]

Looks like it.