Provide a witness pattern for non-exhaustive patterns

Fixed #4321

src/test/compile-fail/refutable-pattern-in-fn-arg.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 fn main() {
-    let f = |3: int| println!("hello");  //~ ERROR refutable pattern
+    let f = |3: int| println!("hello");
+    //~^ ERROR refutable pattern in function argument: _ not covered
     f(4);
 }