Split relate_substs into two functions

One for the case with variances, and one without.
All callers use an explicit Option for the variable anyway.

compiler/rustc_infer/src/infer/combine.rs

@@ -567,11 +567,17 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
             // Avoid fetching the variance if we are in an invariant
             // context; no need, and it can induce dependency cycles
             // (e.g., #41849).
-            relate::relate_substs(self, None, a_subst, b_subst)
+            relate::relate_substs(self, a_subst, b_subst)
         } else {
             let tcx = self.tcx();
             let opt_variances = tcx.variances_of(item_def_id);
-            relate::relate_substs(self, Some((item_def_id, &opt_variances)), a_subst, b_subst)
+            relate::relate_substs_with_variances(
+                self,
+                item_def_id,
+                &opt_variances,
+                a_subst,
+                b_subst,
+            )
         }
     }
 

compiler/rustc_infer/src/infer/equate.rs

@@ -53,7 +53,7 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
         // performing trait matching (which then performs equality
         // unification).
 
-        relate::relate_substs(self, None, a_subst, b_subst)
+        relate::relate_substs(self, a_subst, b_subst)
     }
 
     fn relate_with_variance<T: Relate<'tcx>>(