error: needlessly taken reference of both operands
  --> $DIR/op_ref.rs:13:15
   |
13 |     let foo = &5 - &6;
   |               ^^^^^^^
   |
   = note: `-D op-ref` implied by `-D warnings`
help: use the values directly
   |
13 |     let foo = 5 - 6;
   |               ^