error[E0499]: cannot borrow `i` as mutable more than once at a time
  --> $DIR/E0499.rs:4:17
   |
LL |     let mut x = &mut i;
   |                 ------ first mutable borrow occurs here
LL |     let mut a = &mut i;
   |                 ^^^^^^ second mutable borrow occurs here
LL |     a.use_mut();
LL |     x.use_mut();
   |     - first borrow later used here

error: aborting due to 1 previous error

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