error[E0387]: cannot borrow data mutably in a captured outer variable in an `Fn` closure
  --> $DIR/issue-21600.rs:24:17
   |
LL |         call_it(|| x.gen_mut()); //~ ERROR cannot borrow data mutably in a captured outer
   |                 ^^
   |
help: consider changing this to accept closures that implement `FnMut`
  --> $DIR/issue-21600.rs:22:13
   |
LL |       call_it(|| {
   |  _____________^
LL | |         call_it(|| x.gen());
LL | |         call_it(|| x.gen_mut()); //~ ERROR cannot borrow data mutably in a captured outer
LL | |         //~^ ERROR cannot borrow data mutably in a captured outer
LL | |     });
   | |_____^

error[E0387]: cannot borrow data mutably in a captured outer variable in an `Fn` closure
  --> $DIR/issue-21600.rs:24:20
   |
LL |         call_it(|| x.gen_mut()); //~ ERROR cannot borrow data mutably in a captured outer
   |                    ^
   |
help: consider changing this closure to take self by mutable reference
  --> $DIR/issue-21600.rs:24:17
   |
LL |         call_it(|| x.gen_mut()); //~ ERROR cannot borrow data mutably in a captured outer
   |                 ^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0387`.