$ cargo run
   Compiling collections v0.1.0 (file:///projects/collections)
error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
 --> src/main.rs:6:5
  |
4 |     let first = &v[0];
  |                  - immutable borrow occurs here
5 |
6 |     v.push(6);
  |     ^^^^^^^^^ mutable borrow occurs here
7 |
8 |     println!("Pierwszy element to: {first}");
  |                                     ----- immutable borrow later used here

For more information about this error, try `rustc --explain E0502`.
error: could not compile `collections` due to previous error
