1 use crate::infer::{InferCtxt, TyOrConstInferVar};
2 use rustc_data_structures::obligation_forest::ProcessResult;
3 use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
4 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
5 use rustc_infer::traits::ProjectionCacheKey;
6 use rustc_infer::traits::{SelectionError, TraitEngine, TraitObligation};
7 use rustc_middle::mir::interpret::ErrorHandled;
8 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
9 use rustc_middle::ty::error::{ExpectedFound, TypeError};
10 use rustc_middle::ty::subst::SubstsRef;
11 use rustc_middle::ty::{self, Binder, Const, TypeVisitable};
12 use std::marker::PhantomData;
14 use super::const_evaluatable;
15 use super::project::{self, ProjectAndUnifyResult};
16 use super::select::SelectionContext;
18 use super::CodeAmbiguity;
19 use super::CodeProjectionError;
20 use super::CodeSelectionError;
21 use super::EvaluationResult;
22 use super::PredicateObligation;
23 use super::Unimplemented;
24 use super::{FulfillmentError, FulfillmentErrorCode};
26 use crate::traits::project::PolyProjectionObligation;
27 use crate::traits::project::ProjectionCacheKeyExt as _;
28 use crate::traits::query::evaluate_obligation::InferCtxtExt;
30 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
31 /// Note that we include both the `ParamEnv` and the `Predicate`,
32 /// as the `ParamEnv` can influence whether fulfillment succeeds
34 type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
36 fn as_cache_key(&self) -> Self::CacheKey {
37 self.obligation.param_env.and(self.obligation.predicate)
41 /// The fulfillment context is used to drive trait resolution. It
42 /// consists of a list of obligations that must be (eventually)
43 /// satisfied. The job is to track which are satisfied, which yielded
44 /// errors, and which are still pending. At any point, users can call
45 /// `select_where_possible`, and the fulfillment context will try to do
46 /// selection, retaining only those obligations that remain
47 /// ambiguous. This may be helpful in pushing type inference
48 /// along. Once all type inference constraints have been generated, the
49 /// method `select_all_or_error` can be used to report any remaining
50 /// ambiguous cases as errors.
51 pub struct FulfillmentContext<'tcx> {
52 // A list of all obligations that have been registered with this
53 // fulfillment context.
54 predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
56 // Is it OK to register obligations into this infcx inside
59 // The "primary fulfillment" in many cases in typeck lives
60 // outside of any snapshot, so any use of it inside a snapshot
61 // will lead to trouble and therefore is checked against, but
62 // other fulfillment contexts sometimes do live inside of
63 // a snapshot (they don't *straddle* a snapshot, so there
64 // is no trouble there).
65 usable_in_snapshot: bool,
68 #[derive(Clone, Debug)]
69 pub struct PendingPredicateObligation<'tcx> {
70 pub obligation: PredicateObligation<'tcx>,
71 // This is far more often read than modified, meaning that we
72 // should mostly optimize for reading speed, while modifying is not as relevant.
74 // For whatever reason using a boxed slice is slower than using a `Vec` here.
75 pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
78 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
79 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
80 static_assert_size!(PendingPredicateObligation<'_>, 72);
82 impl<'a, 'tcx> FulfillmentContext<'tcx> {
83 /// Creates a new fulfillment context.
84 pub(super) fn new() -> FulfillmentContext<'tcx> {
85 FulfillmentContext { predicates: ObligationForest::new(), usable_in_snapshot: false }
88 pub(super) fn new_in_snapshot() -> FulfillmentContext<'tcx> {
89 FulfillmentContext { predicates: ObligationForest::new(), usable_in_snapshot: true }
92 /// Attempts to select obligations using `selcx`.
93 fn select(&mut self, selcx: SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
94 let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
95 let _enter = span.enter();
97 // Process pending obligations.
98 let outcome: Outcome<_, _> =
99 self.predicates.process_obligations(&mut FulfillProcessor { selcx });
101 // FIXME: if we kept the original cache key, we could mark projection
102 // obligations as complete for the projection cache here.
104 let errors: Vec<FulfillmentError<'tcx>> =
105 outcome.errors.into_iter().map(to_fulfillment_error).collect();
108 "select({} predicates remaining, {} errors) done",
109 self.predicates.len(),
117 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
118 fn register_predicate_obligation(
120 infcx: &InferCtxt<'tcx>,
121 obligation: PredicateObligation<'tcx>,
123 // this helps to reduce duplicate errors, as well as making
124 // debug output much nicer to read and so on.
125 let obligation = infcx.resolve_vars_if_possible(obligation);
127 debug!(?obligation, "register_predicate_obligation");
129 assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
132 .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
135 fn collect_remaining_errors(&mut self) -> Vec<FulfillmentError<'tcx>> {
136 self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
139 fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
140 let selcx = SelectionContext::new(infcx);
144 fn drain_unstalled_obligations(
146 infcx: &InferCtxt<'tcx>,
147 ) -> Vec<PredicateObligation<'tcx>> {
148 let mut processor = DrainProcessor { removed_predicates: Vec::new(), infcx };
149 let outcome: Outcome<_, _> = self.predicates.process_obligations(&mut processor);
150 assert!(outcome.errors.is_empty());
151 return processor.removed_predicates;
153 struct DrainProcessor<'a, 'tcx> {
154 infcx: &'a InferCtxt<'tcx>,
155 removed_predicates: Vec<PredicateObligation<'tcx>>,
158 impl<'tcx> ObligationProcessor for DrainProcessor<'_, 'tcx> {
159 type Obligation = PendingPredicateObligation<'tcx>;
161 type OUT = Outcome<Self::Obligation, Self::Error>;
163 fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
167 .any(|&var| self.infcx.ty_or_const_infer_var_changed(var))
170 fn process_obligation(
172 pending_obligation: &mut PendingPredicateObligation<'tcx>,
173 ) -> ProcessResult<PendingPredicateObligation<'tcx>, !> {
174 assert!(self.needs_process_obligation(pending_obligation));
175 self.removed_predicates.push(pending_obligation.obligation.clone());
176 ProcessResult::Changed(vec![])
179 fn process_backedge<'c, I>(
182 _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
185 I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
187 self.removed_predicates.extend(cycle.map(|c| c.obligation.clone()));
193 fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
194 self.predicates.map_pending_obligations(|o| o.obligation.clone())
198 struct FulfillProcessor<'a, 'tcx> {
199 selcx: SelectionContext<'a, 'tcx>,
202 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
204 .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
208 impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
209 type Obligation = PendingPredicateObligation<'tcx>;
210 type Error = FulfillmentErrorCode<'tcx>;
211 type OUT = Outcome<Self::Obligation, Self::Error>;
213 /// Identifies whether a predicate obligation needs processing.
215 /// This is always inlined, despite its size, because it has a single
216 /// callsite and it is called *very* frequently.
218 fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
219 // If we were stalled on some unresolved variables, first check whether
220 // any of them have been resolved; if not, don't bother doing more work
222 match pending_obligation.stalled_on.len() {
223 // Match arms are in order of frequency, which matters because this
224 // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
226 let infer_var = pending_obligation.stalled_on[0];
227 self.selcx.infcx.ty_or_const_infer_var_changed(infer_var)
230 // In this case we haven't changed, but wish to make a change.
234 // This `for` loop was once a call to `all()`, but this lower-level
235 // form was a perf win. See #64545 for details.
237 for &infer_var in &pending_obligation.stalled_on {
238 if self.selcx.infcx.ty_or_const_infer_var_changed(infer_var) {
248 /// Processes a predicate obligation and returns either:
249 /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
250 /// - `Unchanged` if we don't have enough info to be sure
251 /// - `Error(e)` if the predicate does not hold
253 /// This is called much less often than `needs_process_obligation`, so we
256 #[instrument(level = "debug", skip(self, pending_obligation))]
257 fn process_obligation(
259 pending_obligation: &mut PendingPredicateObligation<'tcx>,
260 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
261 pending_obligation.stalled_on.truncate(0);
263 let obligation = &mut pending_obligation.obligation;
265 debug!(?obligation, "pre-resolve");
267 if obligation.predicate.has_non_region_infer() {
268 obligation.predicate = self.selcx.infcx.resolve_vars_if_possible(obligation.predicate);
271 let obligation = &pending_obligation.obligation;
273 let infcx = self.selcx.infcx;
275 if obligation.predicate.has_projections() {
276 let mut obligations = Vec::new();
277 let predicate = crate::traits::project::try_normalize_with_depth_to(
279 obligation.param_env,
280 obligation.cause.clone(),
281 obligation.recursion_depth + 1,
282 obligation.predicate,
285 if predicate != obligation.predicate {
286 obligations.push(obligation.with(infcx.tcx, predicate));
287 return ProcessResult::Changed(mk_pending(obligations));
290 let binder = obligation.predicate.kind();
291 match binder.no_bound_vars() {
292 None => match binder.skip_binder() {
293 // Evaluation will discard candidates using the leak check.
294 // This means we need to pass it the bound version of our
296 ty::PredicateKind::Clause(ty::Clause::Trait(trait_ref)) => {
297 let trait_obligation = obligation.with(infcx.tcx, binder.rebind(trait_ref));
299 self.process_trait_obligation(
302 &mut pending_obligation.stalled_on,
305 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
306 let project_obligation = obligation.with(infcx.tcx, binder.rebind(data));
308 self.process_projection_obligation(
311 &mut pending_obligation.stalled_on,
314 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(_))
315 | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(_))
316 | ty::PredicateKind::WellFormed(_)
317 | ty::PredicateKind::ObjectSafe(_)
318 | ty::PredicateKind::ClosureKind(..)
319 | ty::PredicateKind::Subtype(_)
320 | ty::PredicateKind::Coerce(_)
321 | ty::PredicateKind::ConstEvaluatable(..)
322 | ty::PredicateKind::ConstEquate(..) => {
324 ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
325 ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)]))
327 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
328 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
329 bug!("TypeWellFormedFromEnv is only used for Chalk")
332 Some(pred) => match pred {
333 ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
334 let trait_obligation = obligation.with(infcx.tcx, Binder::dummy(data));
336 self.process_trait_obligation(
339 &mut pending_obligation.stalled_on,
343 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(data)) => {
344 if infcx.considering_regions {
345 infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
348 ProcessResult::Changed(vec![])
351 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
355 if infcx.considering_regions {
356 infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
358 ProcessResult::Changed(vec![])
361 ty::PredicateKind::Clause(ty::Clause::Projection(ref data)) => {
362 let project_obligation = obligation.with(infcx.tcx, Binder::dummy(*data));
364 self.process_projection_obligation(
367 &mut pending_obligation.stalled_on,
371 ty::PredicateKind::ObjectSafe(trait_def_id) => {
372 if !self.selcx.tcx().is_object_safe(trait_def_id) {
373 ProcessResult::Error(CodeSelectionError(Unimplemented))
375 ProcessResult::Changed(vec![])
379 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
380 match self.selcx.infcx.closure_kind(closure_substs) {
381 Some(closure_kind) => {
382 if closure_kind.extends(kind) {
383 ProcessResult::Changed(vec![])
385 ProcessResult::Error(CodeSelectionError(Unimplemented))
388 None => ProcessResult::Unchanged,
392 ty::PredicateKind::WellFormed(arg) => {
393 match wf::obligations(
395 obligation.param_env,
396 obligation.cause.body_id,
397 obligation.recursion_depth + 1,
399 obligation.cause.span,
402 pending_obligation.stalled_on =
403 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
404 ProcessResult::Unchanged
406 Some(os) => ProcessResult::Changed(mk_pending(os)),
410 ty::PredicateKind::Subtype(subtype) => {
411 match self.selcx.infcx.subtype_predicate(
413 obligation.param_env,
414 Binder::dummy(subtype),
417 // None means that both are unresolved.
418 pending_obligation.stalled_on =
419 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
420 ProcessResult::Unchanged
422 Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
425 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
426 ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
434 ty::PredicateKind::Coerce(coerce) => {
435 match self.selcx.infcx.coerce_predicate(
437 obligation.param_env,
438 Binder::dummy(coerce),
441 // None means that both are unresolved.
442 pending_obligation.stalled_on =
443 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
444 ProcessResult::Unchanged
446 Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
448 let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
449 ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
457 ty::PredicateKind::ConstEvaluatable(uv) => {
458 match const_evaluatable::is_const_evaluatable(
461 obligation.param_env,
462 obligation.cause.span,
464 Ok(()) => ProcessResult::Changed(vec![]),
465 Err(NotConstEvaluatable::MentionsInfer) => {
466 pending_obligation.stalled_on.clear();
467 pending_obligation.stalled_on.extend(
468 uv.walk().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
470 ProcessResult::Unchanged
473 e @ NotConstEvaluatable::MentionsParam
474 | e @ NotConstEvaluatable::Error(_),
475 ) => ProcessResult::Error(CodeSelectionError(
476 SelectionError::NotConstEvaluatable(e),
481 ty::PredicateKind::ConstEquate(c1, c2) => {
482 let tcx = self.selcx.tcx();
484 tcx.features().generic_const_exprs,
485 "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
487 // FIXME: we probably should only try to unify abstract constants
488 // if the constants depend on generic parameters.
490 // Let's just see where this breaks :shrug:
492 let c1 = tcx.expand_abstract_consts(c1);
493 let c2 = tcx.expand_abstract_consts(c2);
494 debug!("equating consts:\nc1= {:?}\nc2= {:?}", c1, c2);
496 use rustc_hir::def::DefKind;
497 use ty::ConstKind::Unevaluated;
498 match (c1.kind(), c2.kind()) {
499 (Unevaluated(a), Unevaluated(b))
500 if a.def.did == b.def.did
501 && tcx.def_kind(a.def.did) == DefKind::AssocConst =>
503 if let Ok(new_obligations) = infcx
504 .at(&obligation.cause, obligation.param_env)
506 .eq(a.substs, b.substs)
508 return ProcessResult::Changed(mk_pending(
509 new_obligations.into_obligations(),
513 (_, Unevaluated(_)) | (Unevaluated(_), _) => (),
515 if let Ok(new_obligations) =
516 infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
518 return ProcessResult::Changed(mk_pending(
519 new_obligations.into_obligations(),
526 let stalled_on = &mut pending_obligation.stalled_on;
528 let mut evaluate = |c: Const<'tcx>| {
529 if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
530 match self.selcx.infcx.try_const_eval_resolve(
531 obligation.param_env,
534 Some(obligation.cause.span),
538 ErrorHandled::TooGeneric => {
540 unevaluated.substs.iter().filter_map(
541 TyOrConstInferVar::maybe_from_generic_arg,
544 Err(ErrorHandled::TooGeneric)
554 match (evaluate(c1), evaluate(c2)) {
555 (Ok(c1), Ok(c2)) => {
559 .at(&obligation.cause, obligation.param_env)
563 ProcessResult::Changed(mk_pending(inf_ok.into_obligations()))
565 Err(err) => ProcessResult::Error(
566 FulfillmentErrorCode::CodeConstEquateError(
567 ExpectedFound::new(true, c1, c2),
573 (Err(ErrorHandled::Reported(reported)), _)
574 | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
575 CodeSelectionError(SelectionError::NotConstEvaluatable(
576 NotConstEvaluatable::Error(reported),
579 (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
580 if c1.has_non_region_infer() || c2.has_non_region_infer() {
581 ProcessResult::Unchanged
583 // Two different constants using generic parameters ~> error.
584 let expected_found = ExpectedFound::new(true, c1, c2);
585 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
587 TypeError::ConstMismatch(expected_found),
593 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
594 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
595 bug!("TypeWellFormedFromEnv is only used for Chalk")
602 fn process_backedge<'c, I>(
605 _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
606 ) -> Result<(), FulfillmentErrorCode<'tcx>>
608 I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
610 if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
611 debug!("process_child_obligations: coinductive match");
614 let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
615 Err(FulfillmentErrorCode::CodeCycle(cycle))
620 impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> {
621 #[instrument(level = "debug", skip(self, obligation, stalled_on))]
622 fn process_trait_obligation(
624 obligation: &PredicateObligation<'tcx>,
625 trait_obligation: TraitObligation<'tcx>,
626 stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
627 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
628 let infcx = self.selcx.infcx;
629 if obligation.predicate.is_global() {
630 // no type variables present, can use evaluation for better caching.
631 // FIXME: consider caching errors too.
632 if infcx.predicate_must_hold_considering_regions(obligation) {
634 "selecting trait at depth {} evaluated to holds",
635 obligation.recursion_depth
637 return ProcessResult::Changed(vec![]);
641 match self.selcx.select(&trait_obligation) {
642 Ok(Some(impl_source)) => {
643 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
644 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
647 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
649 // This is a bit subtle: for the most part, the
650 // only reason we can fail to make progress on
651 // trait selection is because we don't have enough
652 // information about the types in the trait.
654 stalled_on.extend(substs_infer_vars(
656 trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
660 "process_predicate: pending obligation {:?} now stalled on {:?}",
661 infcx.resolve_vars_if_possible(obligation.clone()),
665 ProcessResult::Unchanged
667 Err(selection_err) => {
668 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
670 ProcessResult::Error(CodeSelectionError(selection_err))
675 fn process_projection_obligation(
677 obligation: &PredicateObligation<'tcx>,
678 project_obligation: PolyProjectionObligation<'tcx>,
679 stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
680 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
681 let tcx = self.selcx.tcx();
683 if obligation.predicate.is_global() {
684 // no type variables present, can use evaluation for better caching.
685 // FIXME: consider caching errors too.
686 if self.selcx.infcx.predicate_must_hold_considering_regions(obligation) {
687 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
689 project_obligation.predicate,
691 // If `predicate_must_hold_considering_regions` succeeds, then we've
692 // evaluated all sub-obligations. We can therefore mark the 'root'
693 // obligation as complete, and skip evaluating sub-obligations.
699 .complete(key, EvaluationResult::EvaluatedToOk);
701 return ProcessResult::Changed(vec![]);
703 debug!("Does NOT hold: {:?}", obligation);
707 match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) {
708 ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
709 ProjectAndUnifyResult::FailedNormalization => {
711 stalled_on.extend(substs_infer_vars(
713 project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
715 ProcessResult::Unchanged
717 // Let the caller handle the recursion
718 ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
719 project_obligation.with(tcx, project_obligation.predicate),
721 ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
722 ProcessResult::Error(CodeProjectionError(e))
728 /// Returns the set of inference variables contained in `substs`.
729 fn substs_infer_vars<'a, 'tcx>(
730 selcx: &SelectionContext<'a, 'tcx>,
731 substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
732 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
735 .resolve_vars_if_possible(substs)
736 .skip_binder() // ok because this check doesn't care about regions
738 .filter(|arg| arg.has_non_region_infer())
740 let mut walker = arg.walk();
741 while let Some(c) = walker.next() {
742 if !c.has_non_region_infer() {
743 walker.visited.remove(&c);
744 walker.skip_current_subtree();
747 walker.visited.into_iter()
749 .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
752 fn to_fulfillment_error<'tcx>(
753 error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
754 ) -> FulfillmentError<'tcx> {
755 let mut iter = error.backtrace.into_iter();
756 let obligation = iter.next().unwrap().obligation;
757 // The root obligation is the last item in the backtrace - if there's only
758 // one item, then it's the same as the main obligation
759 let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
760 FulfillmentError::new(obligation, error.error, root_obligation)