error: borrow of moved value
  --> $DIR/borrowck-pat-by-move-and-ref-inverse-promotion.rs:6:9
   |
LL |     let a @ ref b = U;
   |         -^^^-----
   |         |   |
   |         |   value borrowed here after move
   |         value moved into `a` here
   |         move occurs because `a` has type `U` which does not implement the `Copy` trait

error: aborting due to previous error