error[E0594]: cannot assign to captured outer variable in an `FnMut` closure
  --> $DIR/closure-immutable-outer-variable.rs:19:26
   |
18 |     let y = true;
   |         - help: consider making `y` mutable: `mut y`
19 |     foo(Box::new(move || y = false) as Box<_>); //~ ERROR cannot assign to captured outer variable
   |                          ^^^^^^^^^

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0594"