error[E0277]: expected a `FnMut<(_, char)>` closure, found `()` --> $DIR/issue-23966.rs:2:32 | LL | "".chars().fold(|_, _| (), ()); | ---- ^^ expected an `FnMut<(_, char)>` closure, found `()` | | | required by a bound introduced by this call | = help: the trait `FnMut<(_, char)>` is not implemented for `()` note: required by a bound in `fold` --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.