error[E0507]: cannot move out of a shared reference
  --> $DIR/move-ref-patterns-default-binding-modes.rs:8:22
   |
LL |     let (a, mut b) = &p;
   |             -----    ^^
   |             |
   |             data moved here
   |             move occurs because `b` has type `U`, which does not implement the `Copy` trait
   |
help: consider borrowing the pattern binding
   |
LL |     let (a, ref mut b) = &p;
   |             +++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0507`.