error[E0412]: cannot find type `Oper` in this scope
  --> $DIR/issue-54966.rs:3:27
   |
LL | fn generate_duration() -> Oper<impl FnMut()> {}
   |                           ^^^^ not found in this scope

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0412`.