warning: method `clone_first` is never used
  --> $DIR/issue-23485.rs:20:8
   |
LL | trait Iterator {
   |       -------- method in this trait
...
LL |     fn clone_first(mut self) -> Option<<Self::Item as Deref>::Target> where
   |        ^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: 1 warning emitted