1 error[E0412]: cannot find type `Oper` in this scope
2 --> $DIR/issue-54966.rs:3:27
4 LL | fn generate_duration() -> Oper<impl FnMut()> {}
5 | ^^^^ not found in this scope
7 error: aborting due to previous error
9 For more information about this error, try `rustc --explain E0412`.