error[E0596]: cannot borrow `x` as mutable, as it is a captured variable in a `Fn` closure
  --> $DIR/issue-21600.rs:14:20
   |
LL | fn call_it<F>(f: F) where F: Fn() { f(); }
   |                  - change this to accept `FnMut` instead of `Fn`
...
LL |         call_it(|| x.gen_mut());
   |         ------- -- ^ cannot borrow as mutable
   |         |       |
   |         |       in this closure
   |         expects `Fn` instead of `FnMut`

error[E0596]: cannot borrow `x` as mutable, as it is a captured variable in a `Fn` closure
  --> $DIR/issue-21600.rs:14:17
   |
LL | fn call_it<F>(f: F) where F: Fn() { f(); }
   |                  - change this to accept `FnMut` instead of `Fn`
...
LL |     call_it(|| {
   |     ------- -- in this closure
   |     |
   |     expects `Fn` instead of `FnMut`
LL |         call_it(|| x.gen());
LL |         call_it(|| x.gen_mut());
   |                 ^^ - mutable borrow occurs due to use of `x` in closure
   |                 |
   |                 cannot borrow as mutable

error: aborting due to 2 previous errors

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