error[E0502]: cannot borrow `val` as mutable because it is also borrowed as immutable
  --> $DIR/invalid_borrow.rs:8:13
   |
7  |     let borrow = val.borrow::<i32>();
   |                  ------------------- immutable borrow occurs here
8  |     let _ = val.borrow_mut::<i32>();
   |             ^^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
9  |
10 |     println!("Borrowed over mutable: {}", borrow);
   |                                           ------ immutable borrow later used here