eagerly compute sub_relations again

compiler/rustc_trait_selection/src/solve/delegate.rs

@@ -126,13 +126,12 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
             }
             ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, .. })
             | ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
-                if self.shallow_resolve(a).is_ty_var() && self.shallow_resolve(b).is_ty_var() {
-                    // FIXME: We also need to register a subtype relation between these vars
-                    // when those are added, and if they aren't in the same sub root then
-                    // we should mark this goal as `has_changed`.
-                    Some(Certainty::AMBIGUOUS)
-                } else {
-                    None
+                match (self.shallow_resolve(a).kind(), self.shallow_resolve(b).kind()) {
+                    (&ty::Infer(ty::TyVar(a_vid)), &ty::Infer(ty::TyVar(b_vid))) => {
+                        self.sub_ty_vids_raw(a_vid, b_vid);
+                        Some(Certainty::AMBIGUOUS)
+                    }
+                    _ => None,
                 }
             }
             ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, _)) => {