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::new(3);
   |         ---  ------ move occurs because `bar` has type `Box<isize>`, which does not implement the `Copy` trait
   |         |
   |         captured outer variable
LL |     let _g = to_fn_mut(|| {
   |                        -- captured by this `FnMut` closure
LL |         let _h = to_fn_once(move || -> isize { *bar });
   |                             ^^^^^^^^^^^^^^^^   ---- variable moved due to use in closure
   |                             |
   |                             `bar` is moved here
   |
help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but `FnOnce` closures may consume them only once
  --> $DIR/borrowck-move-by-capture.rs:3:37
   |
LL | fn to_fn_mut<A:std::marker::Tuple,F:FnMut<A>>(f: F) -> F { f }
   |                                     ^^^^^^^^
help: consider cloning the value before moving it into the closure
   |
LL ~         let value = bar.clone();
LL ~         let _h = to_fn_once(move || -> isize { value });
   |

error: aborting due to 1 previous error

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