metadata: skip empty polymorphization bitset

This commit skips encoding empty polymorphization results - while
polymorphization is disabled, this should be every polymorphization
result; but when polymorphization is re-enabled, this would help with
non-generic functions and those which do use all their parameters (most
functions).

Signed-off-by: David Wood <david@davidtw.co>

src/librustc_metadata/rmeta/encoder.rs

@@ -1134,8 +1134,11 @@ impl EncodeContext<'a, 'tcx> {
         debug!("EntryBuilder::encode_mir({:?})", def_id);
         if self.tcx.mir_keys(LOCAL_CRATE).contains(&def_id) {
             record!(self.tables.mir[def_id.to_def_id()] <- self.tcx.optimized_mir(def_id));
-            record!(self.tables.unused_generic_params[def_id.to_def_id()] <-
-                    self.tcx.unused_generic_params(def_id));
+
+            let unused = self.tcx.unused_generic_params(def_id);
+            if !unused.is_empty() {
+                record!(self.tables.unused_generic_params[def_id.to_def_id()] <- unused);
+            }
         }
     }
 

src/librustc_mir/monomorphize/polymorphize.rs

@@ -36,6 +36,13 @@ fn unused_generic_params(tcx: TyCtxt<'_>, def_id: DefId) -> FiniteBitSet<u32> {
         return FiniteBitSet::new_empty();
     }
 
+    // Polymorphization results are stored in cross-crate metadata only when there are unused
+    // parameters, so assume that non-local items must have only used parameters (else this query
+    // would not be invoked, and the cross-crate metadata used instead).
+    if !def_id.is_local() {
+        return FiniteBitSet::new_empty();
+    }
+
     let generics = tcx.generics_of(def_id);
     debug!("unused_generic_params: generics={:?}", generics);