Cannot assign to captured outer variable in an `FnMut` closure diagnostic

[Mark-Simulacrum]

fn foo(mut f: Box<FnMut()>) {
    f();
}

fn main() {
    let y = true;
    foo(Box::new(move || y = false) as Box<_>);
}

produces

error: cannot assign to captured outer variable in an `FnMut` closure
 --> test.rs:7:26
  |
7 |     foo(Box::new(move || y = false) as Box<_>);
  |                          ^^^^^^^^^

error: aborting due to previous error

but should produce

error: cannot assign to captured outer variable in an `FnMut` closure
 --> test.rs:7:26
  |
6 |     let y = true;
  |         - use `mut y` here to make mutable
7 |     foo(Box::new(move || y = false) as Box<_>);
  |                          ^^^^^^^^^

error: aborting due to previous error

cc @estebank, you implemented a similar change for moves: #41523.

[sgrif]

This came up on Reddit today. https://www.reddit.com/r/rust/comments/75170p/why_cant_i_assign_to_captured_outer_variable_in/