error[E0005]: refutable pattern in function argument
  --> $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`
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 previous error

For more information about this error, try `rustc --explain E0005`.