error[E0499]: cannot borrow `slice` as mutable more than once at a time
 --> $DIR/mut2.rs:5:18
  |
4 |     let first = slice.first_mut();
  |                 ----- first mutable borrow occurs here
5 |     let second = slice.first_mut();
  |                  ^^^^^ second mutable borrow occurs here
6 |     assert_eq!(first as *mut _, second as *mut _);
  |                ----- first borrow later used here