Use unused_generic_params from crate metadata

compiler/rustc_monomorphize/src/polymorphize.rs

@@ -36,6 +36,8 @@ fn unused_generic_params<'tcx>(
     tcx: TyCtxt<'tcx>,
     instance: ty::InstanceDef<'tcx>,
 ) -> UnusedGenericParams {
+    assert!(instance.def_id().is_local());
+
     if !tcx.sess.opts.unstable_opts.polymorphize {
         // If polymorphization disabled, then all parameters are used.
         return UnusedGenericParams::new_all_used();
@@ -100,13 +102,6 @@ fn should_polymorphize<'tcx>(
         return false;
     }
 
-    // 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 false;
-    }
-
     // Foreign items have no bodies to analyze.
     if tcx.is_foreign_item(def_id) {
         return false;