make pointer_structural_match warn-by-default

Author: RalfJung

core/src/marker.rs

@@ -247,6 +247,7 @@ marker_impls! {
 ///
 /// const CFN: Wrap<fn(&())> = Wrap(higher_order);
 ///
+/// #[allow(pointer_structural_match)]
 /// fn main() {
 ///     match CFN {
 ///         CFN => {}