error[E0502]: cannot borrow `vec` as mutable because it is also borrowed as immutable
 --> src/main.rs:4:9
  |
3 |     for value in &vec {
  |                  ----
  |                  |
  |                  immutable borrow occurs here
  |                  immutable borrow later used here
4 |         vec.push(*value);
  |         ^^^^^^^^^^^^^^^^ mutable borrow occurs here