error[E0502]: cannot borrow `x` as mutable because it is also borrowed as immutable
 --> test.rs:4:9
  |
3 | let y = &x;
  |         -- immutable borrow occurs here
4 | let z = &mut x;
  |         ^^^^^^ mutable borrow occurs here
...
7 | dbg!(y);
  |      - immutable borrow later used here