rustc_metadata: don't encode an empty slice for inferred_outlives.

src/librustc_metadata/decoder.rs

@@ -671,7 +671,9 @@ impl<'a, 'tcx> CrateMetadata {
         item_id: DefIndex,
         tcx: TyCtxt<'tcx>,
     ) -> &'tcx [(ty::Predicate<'tcx>, Span)] {
-        self.root.per_def.inferred_outlives.get(self, item_id).unwrap().decode((self, tcx))
+        self.root.per_def.inferred_outlives.get(self, item_id).map(|predicates| {
+            predicates.decode((self, tcx))
+        }).unwrap_or_default()
     }
 
     crate fn get_super_predicates(

src/librustc_metadata/encoder.rs

@@ -842,8 +842,10 @@ impl EncodeContext<'tcx> {
 
     fn encode_inferred_outlives(&mut self, def_id: DefId) {
         debug!("EncodeContext::encode_inferred_outlives({:?})", def_id);
-        record!(self.per_def.inferred_outlives[def_id] <-
-            self.tcx.inferred_outlives_of(def_id));
+        let inferred_outlives = self.tcx.inferred_outlives_of(def_id);
+        if !inferred_outlives.is_empty() {
+            record!(self.per_def.inferred_outlives[def_id] <- inferred_outlives);
+        }
     }
 
     fn encode_super_predicates(&mut self, def_id: DefId) {

src/librustc_metadata/schema.rs

@@ -249,6 +249,7 @@ crate struct LazyPerDefTables<'tcx> {
     // doesn't handle shorthands in its own (de)serialization impls,
     // as it's an `enum` for which we want to derive (de)serialization,
     // so the `ty::codec` APIs handle the whole `&'tcx [...]` at once.
+    // Also, as an optimization, a missing entry indicates an empty `&[]`.
     pub inferred_outlives: Lazy!(PerDefTable<Lazy!(&'tcx [(ty::Predicate<'tcx>, Span)])>),
     pub super_predicates: Lazy!(PerDefTable<Lazy!(ty::GenericPredicates<'tcx>)>),