error: type alias is never used: `Unused`
  --> $DIR/impl-trait.rs:12:1
   |
LL | type Unused = ();
   | ^^^^^^^^^^^^^^^^^
   |
note: the lint level is defined here
  --> $DIR/impl-trait.rs:1:9
   |
LL | #![deny(dead_code)]
   |         ^^^^^^^^^

error: aborting due to previous error