error[E0596]: cannot borrow data in a dereference of `UniquePtr<ffi::Opaque>` as mutable
  --> $DIR/unique_ptr_as_mut.rs:22:31
   |
22 |     let _: &mut ffi::Opaque = &mut opaque;
   |                               ^^^^^^^^^^^ cannot borrow as mutable
   |
   = help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `UniquePtr<ffi::Opaque>`