Add test for wild fields

tests/compile-fail/unneeded_field_pattern.rs

@@ -0,0 +1,26 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#![deny(unneeded_field_pattern)]
+#[allow(dead_code, unused)]
+
+struct Foo {
+    a: i32,
+    b: i32,
+    c: i32,
+}
+
+fn main() {
+    let f = Foo { a: 0, b: 0, c: 0 };
+
+    match f {
+        Foo { a: _, b: 0, .. } => {} //~ERROR You matched a field with a wildcard pattern
+                                     //~^ HELP Try with `Foo { b: 0, .. }`
+        Foo { a: _, b: _, c: _ } => {} //~ERROR All the struct fields are matched to a
+                                       //~^ HELP Try with `Foo { .. }`
+    }
+    match f {
+        Foo { b: 0, .. } => {} // should be OK
+        Foo { .. } => {} // and the Force might be with this one
+    }
+}