1 use crate::infer::{InferCtxt, TyOrConstInferVar};
2 use rustc_data_structures::fx::FxHashMap;
3 use rustc_data_structures::obligation_forest::ProcessResult;
4 use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
5 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
6 use rustc_infer::traits::ProjectionCacheKey;
7 use rustc_infer::traits::{SelectionError, TraitEngine, TraitEngineExt as _, TraitObligation};
8 use rustc_middle::mir::interpret::ErrorHandled;
9 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
10 use rustc_middle::ty::error::{ExpectedFound, TypeError};
11 use rustc_middle::ty::subst::SubstsRef;
12 use rustc_middle::ty::ToPredicate;
13 use rustc_middle::ty::{self, Binder, Const, Ty, TypeVisitable};
14 use std::marker::PhantomData;
16 use super::const_evaluatable;
17 use super::project::{self, ProjectAndUnifyResult};
18 use super::select::SelectionContext;
20 use super::CodeAmbiguity;
21 use super::CodeProjectionError;
22 use super::CodeSelectionError;
23 use super::EvaluationResult;
24 use super::Unimplemented;
25 use super::{FulfillmentError, FulfillmentErrorCode};
26 use super::{ObligationCause, PredicateObligation};
28 use crate::traits::error_reporting::InferCtxtExt as _;
29 use crate::traits::project::PolyProjectionObligation;
30 use crate::traits::project::ProjectionCacheKeyExt as _;
31 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
33 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
34 /// Note that we include both the `ParamEnv` and the `Predicate`,
35 /// as the `ParamEnv` can influence whether fulfillment succeeds
37 type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
39 fn as_cache_key(&self) -> Self::CacheKey {
40 self.obligation.param_env.and(self.obligation.predicate)
44 /// The fulfillment context is used to drive trait resolution. It
45 /// consists of a list of obligations that must be (eventually)
46 /// satisfied. The job is to track which are satisfied, which yielded
47 /// errors, and which are still pending. At any point, users can call
48 /// `select_where_possible`, and the fulfillment context will try to do
49 /// selection, retaining only those obligations that remain
50 /// ambiguous. This may be helpful in pushing type inference
51 /// along. Once all type inference constraints have been generated, the
52 /// method `select_all_or_error` can be used to report any remaining
53 /// ambiguous cases as errors.
54 pub struct FulfillmentContext<'tcx> {
55 // A list of all obligations that have been registered with this
56 // fulfillment context.
57 predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
59 relationships: FxHashMap<ty::TyVid, ty::FoundRelationships>,
61 // Is it OK to register obligations into this infcx inside
64 // The "primary fulfillment" in many cases in typeck lives
65 // outside of any snapshot, so any use of it inside a snapshot
66 // will lead to trouble and therefore is checked against, but
67 // other fulfillment contexts sometimes do live inside of
68 // a snapshot (they don't *straddle* a snapshot, so there
69 // is no trouble there).
70 usable_in_snapshot: bool,
73 #[derive(Clone, Debug)]
74 pub struct PendingPredicateObligation<'tcx> {
75 pub obligation: PredicateObligation<'tcx>,
76 // This is far more often read than modified, meaning that we
77 // should mostly optimize for reading speed, while modifying is not as relevant.
79 // For whatever reason using a boxed slice is slower than using a `Vec` here.
80 pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
83 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
84 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
85 static_assert_size!(PendingPredicateObligation<'_>, 72);
87 impl<'a, 'tcx> FulfillmentContext<'tcx> {
88 /// Creates a new fulfillment context.
89 pub fn new() -> FulfillmentContext<'tcx> {
91 predicates: ObligationForest::new(),
92 relationships: FxHashMap::default(),
93 usable_in_snapshot: false,
97 pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
99 predicates: ObligationForest::new(),
100 relationships: FxHashMap::default(),
101 usable_in_snapshot: true,
105 /// Attempts to select obligations using `selcx`.
106 fn select(&mut self, selcx: &mut SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
107 let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
108 let _enter = span.enter();
110 // Process pending obligations.
111 let outcome: Outcome<_, _> =
112 self.predicates.process_obligations(&mut FulfillProcessor { selcx });
114 // FIXME: if we kept the original cache key, we could mark projection
115 // obligations as complete for the projection cache here.
117 let errors: Vec<FulfillmentError<'tcx>> =
118 outcome.errors.into_iter().map(to_fulfillment_error).collect();
121 "select({} predicates remaining, {} errors) done",
122 self.predicates.len(),
130 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
131 /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
132 /// creating a fresh type variable `$0` as well as a projection
133 /// predicate `<SomeType as SomeTrait>::X == $0`. When the
134 /// inference engine runs, it will attempt to find an impl of
135 /// `SomeTrait` or a where-clause that lets us unify `$0` with
136 /// something concrete. If this fails, we'll unify `$0` with
137 /// `projection_ty` again.
138 #[instrument(level = "debug", skip(self, infcx, param_env, cause))]
139 fn normalize_projection_type(
141 infcx: &InferCtxt<'_, 'tcx>,
142 param_env: ty::ParamEnv<'tcx>,
143 projection_ty: ty::ProjectionTy<'tcx>,
144 cause: ObligationCause<'tcx>,
146 debug_assert!(!projection_ty.has_escaping_bound_vars());
148 // FIXME(#20304) -- cache
150 let mut selcx = SelectionContext::new(infcx);
151 let mut obligations = vec![];
152 let normalized_ty = project::normalize_projection_type(
160 self.register_predicate_obligations(infcx, obligations);
162 debug!(?normalized_ty);
164 normalized_ty.ty().unwrap()
167 fn register_predicate_obligation(
169 infcx: &InferCtxt<'_, 'tcx>,
170 obligation: PredicateObligation<'tcx>,
172 // this helps to reduce duplicate errors, as well as making
173 // debug output much nicer to read and so on.
174 let obligation = infcx.resolve_vars_if_possible(obligation);
176 debug!(?obligation, "register_predicate_obligation");
178 assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
180 super::relationships::update(self, infcx, &obligation);
183 .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
186 fn select_all_or_error(&mut self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
188 let errors = self.select_where_possible(infcx);
189 if !errors.is_empty() {
194 self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
197 fn select_where_possible(
199 infcx: &InferCtxt<'_, 'tcx>,
200 ) -> Vec<FulfillmentError<'tcx>> {
201 let mut selcx = SelectionContext::new(infcx);
202 self.select(&mut selcx)
205 fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
206 self.predicates.map_pending_obligations(|o| o.obligation.clone())
209 fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
210 &mut self.relationships
214 struct FulfillProcessor<'a, 'b, 'tcx> {
215 selcx: &'a mut SelectionContext<'b, 'tcx>,
218 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
220 .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
224 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
225 type Obligation = PendingPredicateObligation<'tcx>;
226 type Error = FulfillmentErrorCode<'tcx>;
228 /// Identifies whether a predicate obligation needs processing.
230 /// This is always inlined, despite its size, because it has a single
231 /// callsite and it is called *very* frequently.
233 fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
234 // If we were stalled on some unresolved variables, first check whether
235 // any of them have been resolved; if not, don't bother doing more work
237 match pending_obligation.stalled_on.len() {
238 // Match arms are in order of frequency, which matters because this
239 // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
241 let infer_var = pending_obligation.stalled_on[0];
242 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
245 // In this case we haven't changed, but wish to make a change.
249 // This `for` loop was once a call to `all()`, but this lower-level
250 // form was a perf win. See #64545 for details.
252 for &infer_var in &pending_obligation.stalled_on {
253 if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
263 /// Processes a predicate obligation and returns either:
264 /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
265 /// - `Unchanged` if we don't have enough info to be sure
266 /// - `Error(e)` if the predicate does not hold
268 /// This is called much less often than `needs_process_obligation`, so we
271 #[instrument(level = "debug", skip(self, pending_obligation))]
272 fn process_obligation(
274 pending_obligation: &mut PendingPredicateObligation<'tcx>,
275 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
276 pending_obligation.stalled_on.truncate(0);
278 let obligation = &mut pending_obligation.obligation;
280 debug!(?obligation, "pre-resolve");
282 if obligation.predicate.has_infer_types_or_consts() {
283 obligation.predicate =
284 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
287 let obligation = &pending_obligation.obligation;
289 let infcx = self.selcx.infcx();
291 if obligation.predicate.has_projections() {
292 let mut obligations = Vec::new();
293 let predicate = crate::traits::project::try_normalize_with_depth_to(
295 obligation.param_env,
296 obligation.cause.clone(),
297 obligation.recursion_depth + 1,
298 obligation.predicate,
301 if predicate != obligation.predicate {
302 obligations.push(obligation.with(predicate));
303 return ProcessResult::Changed(mk_pending(obligations));
306 let binder = obligation.predicate.kind();
307 match binder.no_bound_vars() {
308 None => match binder.skip_binder() {
309 // Evaluation will discard candidates using the leak check.
310 // This means we need to pass it the bound version of our
312 ty::PredicateKind::Trait(trait_ref) => {
313 let trait_obligation = obligation.with(binder.rebind(trait_ref));
315 self.process_trait_obligation(
318 &mut pending_obligation.stalled_on,
321 ty::PredicateKind::Projection(data) => {
322 let project_obligation = obligation.with(binder.rebind(data));
324 self.process_projection_obligation(
327 &mut pending_obligation.stalled_on,
330 ty::PredicateKind::RegionOutlives(_)
331 | ty::PredicateKind::TypeOutlives(_)
332 | ty::PredicateKind::WellFormed(_)
333 | ty::PredicateKind::ObjectSafe(_)
334 | ty::PredicateKind::ClosureKind(..)
335 | ty::PredicateKind::Subtype(_)
336 | ty::PredicateKind::Coerce(_)
337 | ty::PredicateKind::ConstEvaluatable(..)
338 | ty::PredicateKind::ConstEquate(..) => {
340 ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
341 ProcessResult::Changed(mk_pending(vec![
342 obligation.with(pred.to_predicate(self.selcx.tcx())),
345 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
346 bug!("TypeWellFormedFromEnv is only used for Chalk")
349 Some(pred) => match pred {
350 ty::PredicateKind::Trait(data) => {
351 let trait_obligation = obligation.with(Binder::dummy(data));
353 self.process_trait_obligation(
356 &mut pending_obligation.stalled_on,
360 ty::PredicateKind::RegionOutlives(data) => {
361 if infcx.considering_regions || data.has_placeholders() {
362 infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
365 ProcessResult::Changed(vec![])
368 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
369 if infcx.considering_regions {
370 infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
372 ProcessResult::Changed(vec![])
375 ty::PredicateKind::Projection(ref data) => {
376 let project_obligation = obligation.with(Binder::dummy(*data));
378 self.process_projection_obligation(
381 &mut pending_obligation.stalled_on,
385 ty::PredicateKind::ObjectSafe(trait_def_id) => {
386 if !self.selcx.tcx().is_object_safe(trait_def_id) {
387 ProcessResult::Error(CodeSelectionError(Unimplemented))
389 ProcessResult::Changed(vec![])
393 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
394 match self.selcx.infcx().closure_kind(closure_substs) {
395 Some(closure_kind) => {
396 if closure_kind.extends(kind) {
397 ProcessResult::Changed(vec![])
399 ProcessResult::Error(CodeSelectionError(Unimplemented))
402 None => ProcessResult::Unchanged,
406 ty::PredicateKind::WellFormed(arg) => {
407 match wf::obligations(
409 obligation.param_env,
410 obligation.cause.body_id,
411 obligation.recursion_depth + 1,
413 obligation.cause.span,
416 pending_obligation.stalled_on =
417 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
418 ProcessResult::Unchanged
420 Some(os) => ProcessResult::Changed(mk_pending(os)),
424 ty::PredicateKind::Subtype(subtype) => {
425 match self.selcx.infcx().subtype_predicate(
427 obligation.param_env,
428 Binder::dummy(subtype),
431 // None means that both are unresolved.
432 pending_obligation.stalled_on =
433 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
434 ProcessResult::Unchanged
436 Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
439 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
440 ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
448 ty::PredicateKind::Coerce(coerce) => {
449 match self.selcx.infcx().coerce_predicate(
451 obligation.param_env,
452 Binder::dummy(coerce),
455 // None means that both are unresolved.
456 pending_obligation.stalled_on =
457 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
458 ProcessResult::Unchanged
460 Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
462 let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
463 ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
471 ty::PredicateKind::ConstEvaluatable(uv) => {
472 match const_evaluatable::is_const_evaluatable(
475 obligation.param_env,
476 obligation.cause.span,
478 Ok(()) => ProcessResult::Changed(vec![]),
479 Err(NotConstEvaluatable::MentionsInfer) => {
480 pending_obligation.stalled_on.clear();
481 pending_obligation.stalled_on.extend(
484 .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
486 ProcessResult::Unchanged
489 e @ NotConstEvaluatable::MentionsParam
490 | e @ NotConstEvaluatable::Error(_),
491 ) => ProcessResult::Error(CodeSelectionError(
492 SelectionError::NotConstEvaluatable(e),
497 ty::PredicateKind::ConstEquate(c1, c2) => {
498 debug!(?c1, ?c2, "equating consts");
499 let tcx = self.selcx.tcx();
500 if tcx.features().generic_const_exprs {
501 // FIXME: we probably should only try to unify abstract constants
502 // if the constants depend on generic parameters.
504 // Let's just see where this breaks :shrug:
505 if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
506 (c1.kind(), c2.kind())
508 if infcx.try_unify_abstract_consts(a, b, obligation.param_env) {
509 return ProcessResult::Changed(vec![]);
514 let stalled_on = &mut pending_obligation.stalled_on;
516 let mut evaluate = |c: Const<'tcx>| {
517 if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
518 match self.selcx.infcx().try_const_eval_resolve(
519 obligation.param_env,
522 Some(obligation.cause.span),
526 ErrorHandled::TooGeneric => {
528 unevaluated.substs.iter().filter_map(
529 TyOrConstInferVar::maybe_from_generic_arg,
532 Err(ErrorHandled::TooGeneric)
542 match (evaluate(c1), evaluate(c2)) {
543 (Ok(c1), Ok(c2)) => {
547 .at(&obligation.cause, obligation.param_env)
550 Ok(_) => ProcessResult::Changed(vec![]),
551 Err(err) => ProcessResult::Error(
552 FulfillmentErrorCode::CodeConstEquateError(
553 ExpectedFound::new(true, c1, c2),
559 (Err(ErrorHandled::Reported(reported)), _)
560 | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
561 CodeSelectionError(SelectionError::NotConstEvaluatable(
562 NotConstEvaluatable::Error(reported),
565 (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
567 obligation.cause.span(),
568 "ConstEquate: const_eval_resolve returned an unexpected error"
571 (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
572 if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
573 ProcessResult::Unchanged
575 // Two different constants using generic parameters ~> error.
576 let expected_found = ExpectedFound::new(true, c1, c2);
577 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
579 TypeError::ConstMismatch(expected_found),
585 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
586 bug!("TypeWellFormedFromEnv is only used for Chalk")
593 fn process_backedge<'c, I>(
596 _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
598 I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
600 if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
601 debug!("process_child_obligations: coinductive match");
603 let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
604 self.selcx.infcx().report_overflow_error_cycle(&cycle);
609 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
610 #[instrument(level = "debug", skip(self, obligation, stalled_on))]
611 fn process_trait_obligation(
613 obligation: &PredicateObligation<'tcx>,
614 trait_obligation: TraitObligation<'tcx>,
615 stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
616 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
617 let infcx = self.selcx.infcx();
618 if obligation.predicate.is_global() {
619 // no type variables present, can use evaluation for better caching.
620 // FIXME: consider caching errors too.
621 if infcx.predicate_must_hold_considering_regions(obligation) {
623 "selecting trait at depth {} evaluated to holds",
624 obligation.recursion_depth
626 return ProcessResult::Changed(vec![]);
630 match self.selcx.select(&trait_obligation) {
631 Ok(Some(impl_source)) => {
632 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
633 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
636 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
638 // This is a bit subtle: for the most part, the
639 // only reason we can fail to make progress on
640 // trait selection is because we don't have enough
641 // information about the types in the trait.
643 stalled_on.extend(substs_infer_vars(
645 trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
649 "process_predicate: pending obligation {:?} now stalled on {:?}",
650 infcx.resolve_vars_if_possible(obligation.clone()),
654 ProcessResult::Unchanged
656 Err(selection_err) => {
657 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
659 ProcessResult::Error(CodeSelectionError(selection_err))
664 fn process_projection_obligation(
666 obligation: &PredicateObligation<'tcx>,
667 project_obligation: PolyProjectionObligation<'tcx>,
668 stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
669 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
670 let tcx = self.selcx.tcx();
672 if obligation.predicate.is_global() {
673 // no type variables present, can use evaluation for better caching.
674 // FIXME: consider caching errors too.
675 if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
676 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
678 project_obligation.predicate,
680 // If `predicate_must_hold_considering_regions` succeeds, then we've
681 // evaluated all sub-obligations. We can therefore mark the 'root'
682 // obligation as complete, and skip evaluating sub-obligations.
688 .complete(key, EvaluationResult::EvaluatedToOk);
690 return ProcessResult::Changed(vec![]);
692 debug!("Does NOT hold: {:?}", obligation);
696 match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
697 ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
698 ProjectAndUnifyResult::FailedNormalization => {
700 stalled_on.extend(substs_infer_vars(
702 project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
704 ProcessResult::Unchanged
706 // Let the caller handle the recursion
707 ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
708 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
710 ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
711 ProcessResult::Error(CodeProjectionError(e))
717 /// Returns the set of inference variables contained in `substs`.
718 fn substs_infer_vars<'a, 'tcx>(
719 selcx: &mut SelectionContext<'a, 'tcx>,
720 substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
721 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
724 .resolve_vars_if_possible(substs)
725 .skip_binder() // ok because this check doesn't care about regions
727 .filter(|arg| arg.has_infer_types_or_consts())
729 let mut walker = arg.walk();
730 while let Some(c) = walker.next() {
731 if !c.has_infer_types_or_consts() {
732 walker.visited.remove(&c);
733 walker.skip_current_subtree();
736 walker.visited.into_iter()
738 .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
741 fn to_fulfillment_error<'tcx>(
742 error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
743 ) -> FulfillmentError<'tcx> {
744 let mut iter = error.backtrace.into_iter();
745 let obligation = iter.next().unwrap().obligation;
746 // The root obligation is the last item in the backtrace - if there's only
747 // one item, then it's the same as the main obligation
748 let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
749 FulfillmentError::new(obligation, error.error, root_obligation)