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
   |
help: borrow this binding in the pattern to avoid moving the value
   |
LL |     let ref a @ ref b = U;
   |         +++

error: aborting due to 1 previous error