error[E0499]: cannot borrow `v` as mutable more than once at a time
  --> $DIR/bad_as_mut.rs:9:5
   |
6  |     let v2 = v.as_mut()
   |              ---------- first mutable borrow occurs here
...
9  |     v.borrow_mut::<i32>();
   |     ^^^^^^^^^^^^^^^^^^^^^ second mutable borrow occurs here
10 |
11 |     println!("{:p}", v2);
   |                      -- first borrow later used here