Mark an unreachable match arm as such

Synchronize which elements have code paths and which are unreachable betwen def collector and build reduced graph

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -1474,7 +1474,9 @@ fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
                 return;
             }
 
-            AssocItemKind::DelegationMac(..) => bug!(),
+            AssocItemKind::DelegationMac(..) => {
+                span_bug!(item.span, "delegation mac should already have been removed")
+            }
         };
         let vis = self.resolve_visibility(&item.vis);
         let feed = self.r.feed(item.id);

compiler/rustc_resolve/src/def_collector.rs

@@ -358,9 +358,12 @@ fn visit_assoc_item(&mut self, i: &'a AssocItem, ctxt: visit::AssocCtxt) {
                 },
             ),
             AssocItemKind::Type(box TyAlias { ident, .. }) => (*ident, DefKind::AssocTy),
-            AssocItemKind::MacCall(..) | AssocItemKind::DelegationMac(..) => {
+            AssocItemKind::MacCall(..) => {
                 return self.visit_macro_invoc(i.id);
             }
+            AssocItemKind::DelegationMac(..) => {
+                span_bug!(i.span, "degation mac invoc should have already been handled")
+            }
         };
 
         let def = self.create_def(i.id, Some(ident.name), def_kind, i.span);