error[E0005]: refutable pattern in function argument --> $DIR/refutable-pattern-in-fn-arg.rs:2:14 | LL | let f = |3: isize| println!("hello"); | ^ patterns `..=2_isize` and `4_isize..` not covered | = note: the matched value is of type `isize` help: alternatively, you could prepend the pattern with an underscore to define a new named variable; identifiers cannot begin with digits | LL | let f = |_3: isize| println!("hello"); | + error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0005`.