error[E0502]: cannot borrow `slice_1` as mutable because it is also borrowed as immutable
 --> compile_fail_tests/mut_lock_1.rs
  |
  |     let slice_2 = slice_1.slice(0..2).unwrap();
  |                   ------------------- immutable borrow occurs here
  |
  |     let a = slice_1.get_mut(3).unwrap().borrow_mut();
  |             ^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
...
  |     let _ = (&slice_1, &slice_2);
  |                        -------- immutable borrow later used here