fn foo(items: &mut Vec<u8>) { items.sort(); } fn bar() -> impl Iterator<Item = i32> { //~^ ERROR expected `foo` to be a fn item that returns `i32`, but it returns `()` [E0271] let mut x: Vec<Vec<u8>> = vec![vec![0, 2, 1], vec![5, 4, 3]]; x.iter_mut().map(foo) } fn main() { bar(); }