Prevent leaking Feeds into query results

compiler/rustc_middle/src/ty/context.rs

@@ -520,6 +520,10 @@ pub struct TyCtxtFeed<'tcx, KEY: Copy> {
     key: KEY,
 }
 
+/// Never return a `Feed` from a query. Only queries that create a `DefId` are
+/// allowed to feed queries for that `DefId`.
+impl<KEY: Copy, CTX> !HashStable<CTX> for TyCtxtFeed<'_, KEY> {}
+
 /// The same as `TyCtxtFeed`, but does not contain a `TyCtxt`.
 /// Use this to pass around when you have a `TyCtxt` elsewhere.
 /// Just an optimization to save space and not store hundreds of
@@ -531,6 +535,10 @@ pub struct Feed<'tcx, KEY: Copy> {
     key: KEY,
 }
 
+/// Never return a `Feed` from a query. Only queries that create a `DefId` are
+/// allowed to feed queries for that `DefId`.
+impl<KEY: Copy, CTX> !HashStable<CTX> for Feed<'_, KEY> {}
+
 impl<T: fmt::Debug + Copy> fmt::Debug for Feed<'_, T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.key.fmt(f)