Add and use generics.is_empty() and generics.is_own_empty, rather than using generics' attributes

compiler/rustc_hir_analysis/src/variance/constraints.rs

@@ -99,7 +99,7 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
         debug!("build_constraints_for_item({})", tcx.def_path_str(def_id));
 
         // Skip items with no generics - there's nothing to infer in them.
-        if tcx.generics_of(def_id).count() == 0 {
+        if tcx.generics_of(def_id).is_empty() {
             return;
         }