error[E0507]: cannot move out of `bar`, a captured variable in an `FnMut` closure
  --> $DIR/borrowck-move-by-capture.rs:9:29
   |
LL |     let bar: Box<_> = box 3;
   |         --- captured outer variable
LL |     let _g = to_fn_mut(|| {
LL |         let _h = to_fn_once(move || -> isize { *bar });
   |                             ^^^^^^^^^^^^^^^^    ---
   |                             |                   |
   |                             |                   move occurs because `bar` has type `Box<isize>`, which does not implement the `Copy` trait
   |                             |                   move occurs due to use in closure
   |                             move out of `bar` occurs here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0507`.