Sync from rust 730d5d4

Author: bjorn3

example/mini_core_hello_world.rs

@@ -585,6 +585,7 @@ pub enum E2<X> {
 V4,
 }
 
+#[allow(unreachable_patterns)]
 fncheck_niche_behavior(){
 ifletE1::V2{ .. } = (E1::V1{f:true}){
  intrinsics::abort();