error[E0501]: cannot borrow `x` as immutable because previous closure requires unique access
  --> $DIR/yield-while-ref-reborrowed.rs:45:20
   |
LL |       let mut b = || {
   |  _________________-
LL | |         let a = &mut *x;
LL | |         yield();
LL | |         println!("{}", a);
LL | |     };
   | |_____- closure construction occurs here
LL |       println!("{}", x); //~ ERROR
   |                      ^ borrow occurs here
LL |       b.resume();
   |       - borrow later used here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0501`.