error[E0502]: cannot borrow `s` as mutable because it is also borrowed as immutable
 --> test.rs:5:5
  |
3 |     let c = s.as_str();
  |             - immutable borrow occurs here
4 |     
5 |     s.push_str(" World!");
  |     ^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
6 | 
7 |     dbg!(c);
  |          - immutable borrow later used here