error[E0499]: cannot borrow `i` as mutable more than once at a time
  --> $DIR/E0499.rs:14:22
   |
13 |     let mut x = &mut i;
   |                      - first mutable borrow occurs here
14 |     let mut a = &mut i; //~ ERROR E0499
   |                      ^ second mutable borrow occurs here
15 | }
   | - first borrow ends here

error: aborting due to previous error