error[E0005]: refutable pattern in function argument: `_` not covered --> $DIR/refutable-pattern-in-fn-arg.rs:2:14 | LL | let f = |3: isize| println!("hello"); | ^ pattern `_` not covered | = note: the matched value is of type `isize` error: aborting due to previous error For more information about this error, try `rustc --explain E0005`.