Fixes in various places

Author: Nadrieril

example/mini_core_hello_world.rs

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