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>

1 files changed, 5 insertions, 2 deletions

diff --git a/src/librustc_metadata/rmeta/encoder.rs b/src/librustc_metadata/rmeta/encoder.rs
index 352b8bff7e2..aec9e8daa0f 100644
--- a/src/librustc_metadata/rmeta/encoder.rs
+++ b/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);
+            }
         }
     }