Don't throw away information just to recompute it again

compiler/rustc_infer/src/infer/mod.rs

@@ -1022,7 +1022,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         cause: &ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         predicate: ty::PolyCoercePredicate<'tcx>,
-    ) -> Option<InferResult<'tcx, ()>> {
+    ) -> Result<InferResult<'tcx, ()>, (TyVid, TyVid)> {
         let subtype_predicate = predicate.map_bound(|p| ty::SubtypePredicate {
             a_is_expected: false, // when coercing from `a` to `b`, `b` is expected
             a: p.a,
@@ -1036,7 +1036,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         cause: &ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         predicate: ty::PolySubtypePredicate<'tcx>,
-    ) -> Option<InferResult<'tcx, ()>> {
+    ) -> Result<InferResult<'tcx, ()>, (TyVid, TyVid)> {
         // Check for two unresolved inference variables, in which case we can
         // make no progress. This is partly a micro-optimization, but it's
         // also an opportunity to "sub-unify" the variables. This isn't
@@ -1055,12 +1055,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         match (r_a.kind(), r_b.kind()) {
             (&ty::Infer(ty::TyVar(a_vid)), &ty::Infer(ty::TyVar(b_vid))) => {
                 self.inner.borrow_mut().type_variables().sub(a_vid, b_vid);
-                return None;
+                return Err((a_vid, b_vid));
             }
             _ => {}
         }
 
-        Some(self.commit_if_ok(|_snapshot| {
+        Ok(self.commit_if_ok(|_snapshot| {
             let ty::SubtypePredicate { a_is_expected, a, b } =
                 self.replace_bound_vars_with_placeholders(predicate);
 
@@ -1848,7 +1848,7 @@ impl<'tcx> TyOrConstInferVar<'tcx> {
 
     /// Tries to extract an inference variable from a type, returns `None`
     /// for types other than `ty::Infer(_)` (or `InferTy::Fresh*`).
-    pub fn maybe_from_ty(ty: Ty<'tcx>) -> Option<Self> {
+    fn maybe_from_ty(ty: Ty<'tcx>) -> Option<Self> {
         match *ty.kind() {
             ty::Infer(ty::TyVar(v)) => Some(TyOrConstInferVar::Ty(v)),
             ty::Infer(ty::IntVar(v)) => Some(TyOrConstInferVar::TyInt(v)),
@@ -1859,7 +1859,7 @@ impl<'tcx> TyOrConstInferVar<'tcx> {
 
     /// Tries to extract an inference variable from a constant, returns `None`
     /// for constants other than `ty::ConstKind::Infer(_)` (or `InferConst::Fresh`).
-    pub fn maybe_from_const(ct: ty::Const<'tcx>) -> Option<Self> {
+    fn maybe_from_const(ct: ty::Const<'tcx>) -> Option<Self> {
         match ct.kind() {
             ty::ConstKind::Infer(InferConst::Var(v)) => Some(TyOrConstInferVar::Const(v)),
             _ => None,

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -427,16 +427,14 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                         obligation.param_env,
                         Binder::dummy(subtype),
                     ) {
-                        None => {
+                        Err((a, b)) => {
                             // None means that both are unresolved.
-                            pending_obligation.stalled_on = vec![
-                                TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
-                                TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
-                            ];
+                            pending_obligation.stalled_on =
+                                vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
                             ProcessResult::Unchanged
                         }
-                        Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
-                        Some(Err(err)) => {
+                        Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
+                        Ok(Err(err)) => {
                             let expected_found =
                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
@@ -453,16 +451,14 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                         obligation.param_env,
                         Binder::dummy(coerce),
                     ) {
-                        None => {
+                        Err((a, b)) => {
                             // None means that both are unresolved.
-                            pending_obligation.stalled_on = vec![
-                                TyOrConstInferVar::maybe_from_ty(coerce.a).unwrap(),
-                                TyOrConstInferVar::maybe_from_ty(coerce.b).unwrap(),
-                            ];
+                            pending_obligation.stalled_on =
+                                vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
                             ProcessResult::Unchanged
                         }
-                        Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
-                        Some(Err(err)) => {
+                        Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
+                        Ok(Err(err)) => {
                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
                                 expected_found,

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -462,31 +462,31 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     let p = bound_predicate.rebind(p);
                     // Does this code ever run?
                     match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
-                        Some(Ok(InferOk { mut obligations, .. })) => {
+                        Ok(Ok(InferOk { mut obligations, .. })) => {
                             self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
                             self.evaluate_predicates_recursively(
                                 previous_stack,
                                 obligations.into_iter(),
                             )
                         }
-                        Some(Err(_)) => Ok(EvaluatedToErr),
-                        None => Ok(EvaluatedToAmbig),
+                        Ok(Err(_)) => Ok(EvaluatedToErr),
+                        Err(..) => Ok(EvaluatedToAmbig),
                     }
                 }
 
                 ty::PredicateKind::Coerce(p) => {
                     let p = bound_predicate.rebind(p);
                     // Does this code ever run?
                     match self.infcx.coerce_predicate(&obligation.cause, obligation.param_env, p) {
-                        Some(Ok(InferOk { mut obligations, .. })) => {
+                        Ok(Ok(InferOk { mut obligations, .. })) => {
                             self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
                             self.evaluate_predicates_recursively(
                                 previous_stack,
                                 obligations.into_iter(),
                             )
                         }
-                        Some(Err(_)) => Ok(EvaluatedToErr),
-                        None => Ok(EvaluatedToAmbig),
+                        Ok(Err(_)) => Ok(EvaluatedToErr),
+                        Err(..) => Ok(EvaluatedToAmbig),
                     }
                 }