error[E0499]: cannot borrow `x` as mutable more than once at a time
 --> test.rs:4:9
  |
3 | let y = &mut x;
  |         ------ first mutable borrow occurs here
4 | let z = &mut x;
  |         ^^^^^^ second mutable borrow occurs here
...
7 | dbg!(y);
  |      - first borrow later used here