error[E0499]: cannot borrow `x` as mutable more than once at a time
  --> $DIR/issue-11715.rs:5:13
   |
LL |     let y = &mut x;
   |             ------ first mutable borrow occurs here
LL |     let z = &mut x;
   |             ^^^^^^ second mutable borrow occurs here
LL |     z.use_mut();
LL |     y.use_mut();
   |     - first borrow later used here

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0499`.