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

error: aborting due to previous error

error: Could not compile `clippy_tests`.

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