Auto merge of #64564 - jonas-schievink:cowardly-default, r=nikomatsakis

Deny specializing items not in the parent impl

Part of https://github.com/rust-lang/rust/issues/29661 (https://github.com/rust-lang/rfcs/pull/2532). At least sort of?

This was discussed in https://github.com/rust-lang/rust/pull/61812#discussion_r300504114 and is needed for that PR to make progress (fixing an unsoundness).

One annoyance with doing this is that it sometimes requires users to copy-paste a provided trait method into an impl just to mark it `default` (ie. there is no syntax to forward this impl method to the provided trait method).

cc @Centril and @arielb1

src/librustc/ty/query/config.rs

@@ -73,6 +73,17 @@ impl<'tcx, M: QueryAccessors<'tcx, Key = DefId>> QueryDescription<'tcx> for M {
             format!("processing {:?} with query `{}`", def_id, name).into()
         }
     }
+
+    default fn cache_on_disk(_: TyCtxt<'tcx>, _: Self::Key, _: Option<&Self::Value>) -> bool {
+        false
+    }
+
+    default fn try_load_from_disk(
+        _: TyCtxt<'tcx>,
+        _: SerializedDepNodeIndex,
+    ) -> Option<Self::Value> {
+        bug!("QueryDescription::load_from_disk() called for an unsupported query.")
+    }
 }
 
 impl<'tcx> QueryDescription<'tcx> for queries::analysis<'tcx> {