error[E0596]: cannot borrow `x` as mutable, as it is a captured variable in a `Fn` closure
  --> $DIR/issue-21600.rs:14:20
   |
LL |         call_it(|| x.gen_mut());
   |                    ^ cannot borrow as mutable
   |
help: consider changing this to accept closures that implement `FnMut`
  --> $DIR/issue-21600.rs:14:17
   |
LL |         call_it(|| x.gen_mut());
   |                 ^^^^^^^^^^^^^^

error[E0596]: cannot borrow `x` as mutable, as it is a captured variable in a `Fn` closure
  --> $DIR/issue-21600.rs:14:17
   |
LL |         call_it(|| x.gen_mut());
   |                 ^^ - mutable borrow occurs due to use of `x` in closure
   |                 |
   |                 cannot borrow as mutable
   |
help: consider changing this to accept closures that implement `FnMut`
  --> $DIR/issue-21600.rs:12:13
   |
LL |       call_it(|| {
   |  _____________^
LL | |         call_it(|| x.gen());
LL | |         call_it(|| x.gen_mut());
LL | |
LL | |
LL | |     });
   | |_____^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0596`.