1 //! Support code for rustdoc and external tools.
2 //! You really don't want to be using this unless you need to.
6 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
7 use crate::infer::InferCtxt;
8 use rustc_middle::ty::fold::TypeFolder;
9 use rustc_middle::ty::{Region, RegionVid};
11 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
13 use std::collections::hash_map::Entry;
14 use std::collections::VecDeque;
16 // FIXME(twk): this is obviously not nice to duplicate like that
17 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
18 pub enum RegionTarget<'tcx> {
23 #[derive(Default, Debug, Clone)]
24 pub struct RegionDeps<'tcx> {
25 larger: FxHashSet<RegionTarget<'tcx>>,
26 smaller: FxHashSet<RegionTarget<'tcx>>,
29 pub enum AutoTraitResult<A> {
35 impl<A> AutoTraitResult<A> {
36 fn is_auto(&self) -> bool {
38 AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl => true,
44 pub struct AutoTraitInfo<'cx> {
45 pub full_user_env: ty::ParamEnv<'cx>,
46 pub region_data: RegionConstraintData<'cx>,
47 pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>,
50 pub struct AutoTraitFinder<'tcx> {
54 impl<'tcx> AutoTraitFinder<'tcx> {
55 pub fn new(tcx: TyCtxt<'tcx>) -> Self {
56 AutoTraitFinder { tcx }
59 /// Makes a best effort to determine whether and under which conditions an auto trait is
60 /// implemented for a type. For example, if you have
63 /// struct Foo<T> { data: Box<T> }
66 /// then this might return that Foo<T>: Send if T: Send (encoded in the AutoTraitResult type).
67 /// The analysis attempts to account for custom impls as well as other complex cases. This
68 /// result is intended for use by rustdoc and other such consumers.
70 /// (Note that due to the coinductive nature of Send, the full and correct result is actually
71 /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
72 /// types are all Send. So, in our example, we might have that Foo<T>: Send if Box<T>: Send.
73 /// But this is often not the best way to present to the user.)
75 /// Warning: The API should be considered highly unstable, and it may be refactored or removed
77 pub fn find_auto_trait_generics<A>(
80 orig_env: ty::ParamEnv<'tcx>,
82 auto_trait_callback: impl Fn(&InferCtxt<'_, 'tcx>, AutoTraitInfo<'tcx>) -> A,
83 ) -> AutoTraitResult<A> {
86 let trait_ref = ty::TraitRef { def_id: trait_did, substs: tcx.mk_substs_trait(ty, &[]) };
88 let trait_pred = ty::Binder::bind(trait_ref);
90 let bail_out = tcx.infer_ctxt().enter(|infcx| {
91 let mut selcx = SelectionContext::with_negative(&infcx, true);
92 let result = selcx.select(&Obligation::new(
93 ObligationCause::dummy(),
95 trait_pred.to_poly_trait_predicate(),
99 Ok(Some(ImplSource::ImplSourceUserDefined(_))) => {
101 "find_auto_trait_generics({:?}): \
102 manual impl found, bailing out",
111 // If an explicit impl exists, it always takes priority over an auto impl
113 return AutoTraitResult::ExplicitImpl;
116 tcx.infer_ctxt().enter(|infcx| {
117 let mut fresh_preds = FxHashSet::default();
119 // Due to the way projections are handled by SelectionContext, we need to run
120 // evaluate_predicates twice: once on the original param env, and once on the result of
121 // the first evaluate_predicates call.
123 // The problem is this: most of rustc, including SelectionContext and traits::project,
124 // are designed to work with a concrete usage of a type (e.g., Vec<u8>
125 // fn<T>() { Vec<T> }. This information will generally never change - given
126 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
127 // If we're unable to prove that 'T' implements a particular trait, we're done -
128 // there's nothing left to do but error out.
130 // However, synthesizing an auto trait impl works differently. Here, we start out with
131 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
132 // with - and progressively discover the conditions we need to fulfill for it to
133 // implement a certain auto trait. This ends up breaking two assumptions made by trait
134 // selection and projection:
136 // * We can always cache the result of a particular trait selection for the lifetime of
138 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
139 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
141 // We fix the first assumption by manually clearing out all of the InferCtxt's caches
142 // in between calls to SelectionContext.select. This allows us to keep all of the
143 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
144 // them between calls.
146 // We fix the second assumption by reprocessing the result of our first call to
147 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
148 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
149 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
150 // SelectionContext to return it back to us.
152 let (new_env, user_env) = match self.evaluate_predicates(
162 None => return AutoTraitResult::NegativeImpl,
165 let (full_env, full_user_env) = self
166 .evaluate_predicates(
176 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
180 "find_auto_trait_generics({:?}): fulfilling \
184 infcx.clear_caches();
186 // At this point, we already have all of the bounds we need. FulfillmentContext is used
187 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
188 // an additional sanity check.
189 let mut fulfill = FulfillmentContext::new();
190 fulfill.register_bound(&infcx, full_env, ty, trait_did, ObligationCause::dummy());
191 fulfill.select_all_or_error(&infcx).unwrap_or_else(|e| {
192 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, e)
195 let body_id_map: FxHashMap<_, _> = infcx
198 .region_obligations()
200 .map(|&(id, _)| (id, vec![]))
203 infcx.process_registered_region_obligations(&body_id_map, None, full_env);
205 let region_data = infcx
208 .unwrap_region_constraints()
209 .region_constraint_data()
212 let vid_to_region = self.map_vid_to_region(®ion_data);
214 let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
216 AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info))
221 impl AutoTraitFinder<'tcx> {
222 /// The core logic responsible for computing the bounds for our synthesized impl.
224 /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
225 /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
226 /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
227 /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
228 /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
230 /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
231 /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
232 /// user code. According, it considers all possible ways that a `Predicate` could be met, which
233 /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
234 /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
235 /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
236 /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
237 /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
240 /// impl<T> Send for Foo<T> where T: IntoIterator
242 /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
243 /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
245 /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
246 /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
247 /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
248 /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
251 /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
252 /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
253 /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
254 /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
255 /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
256 /// else `SelectionContext` will choke on the missing predicates. However, this should never
257 /// show up in the final synthesized generics: we don't want our generated docs page to contain
258 /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
259 /// separate `user_env`, which only holds the predicates that will actually be displayed to the
261 fn evaluate_predicates(
263 infcx: &InferCtxt<'_, 'tcx>,
266 param_env: ty::ParamEnv<'tcx>,
267 user_env: ty::ParamEnv<'tcx>,
268 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
269 only_projections: bool,
270 ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
273 let mut select = SelectionContext::with_negative(&infcx, true);
275 let mut already_visited = FxHashSet::default();
276 let mut predicates = VecDeque::new();
277 predicates.push_back(ty::Binder::bind(ty::TraitPredicate {
278 trait_ref: ty::TraitRef {
280 substs: infcx.tcx.mk_substs_trait(ty, &[]),
284 let computed_preds = param_env.caller_bounds().iter();
285 let mut user_computed_preds: FxHashSet<_> = user_env.caller_bounds().iter().collect();
287 let mut new_env = param_env;
288 let dummy_cause = ObligationCause::dummy();
290 while let Some(pred) = predicates.pop_front() {
291 infcx.clear_caches();
293 if !already_visited.insert(pred) {
297 // Call `infcx.resolve_vars_if_possible` to see if we can
298 // get rid of any inference variables.
299 let obligation = infcx.resolve_vars_if_possible(&Obligation::new(
304 let result = select.select(&obligation);
307 &Ok(Some(ref impl_source)) => {
308 // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
309 // we immediately bail out, since it's impossible for us to continue.
311 if let ImplSource::ImplSourceUserDefined(ImplSourceUserDefinedData {
316 // Blame 'tidy' for the weird bracket placement.
317 if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
319 "evaluate_nested_obligations: found explicit negative impl\
327 let obligations = impl_source.clone().nested_obligations().into_iter();
329 if !self.evaluate_nested_obligations(
332 &mut user_computed_preds,
342 &Err(SelectionError::Unimplemented) => {
343 if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
344 already_visited.remove(&pred);
346 &mut user_computed_preds,
347 pred.without_const().to_predicate(self.tcx),
349 predicates.push_back(pred);
352 "evaluate_nested_obligations: `Unimplemented` found, bailing: \
356 pred.skip_binder().trait_ref.substs
361 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
364 let normalized_preds = elaborate_predicates(
366 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
368 .map(|o| o.predicate);
370 ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal(), None);
373 let final_user_env = ty::ParamEnv::new(
374 tcx.mk_predicates(user_computed_preds.into_iter()),
379 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
381 ty, trait_did, new_env, final_user_env
384 Some((new_env, final_user_env))
387 /// This method is designed to work around the following issue:
388 /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
389 /// progressively building a `ParamEnv` based on the results we get.
390 /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
391 /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
393 /// This can lead to a corner case when dealing with region parameters.
394 /// During our selection loop in `evaluate_predicates`, we might end up with
395 /// two trait predicates that differ only in their region parameters:
396 /// one containing a HRTB lifetime parameter, and one containing a 'normal'
397 /// lifetime parameter. For example:
400 /// T as MyTrait<'static>
402 /// If we put both of these predicates in our computed `ParamEnv`, we'll
403 /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
405 /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
406 /// Our end goal is to generate a user-visible description of the conditions
407 /// under which a type implements an auto trait. A trait predicate involving
408 /// a HRTB means that the type needs to work with any choice of lifetime,
409 /// not just one specific lifetime (e.g., `'static`).
412 user_computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
413 new_pred: ty::Predicate<'tcx>,
415 let mut should_add_new = true;
416 user_computed_preds.retain(|&old_pred| {
418 ty::PredicateKind::Trait(new_trait, _),
419 ty::PredicateKind::Trait(old_trait, _),
421 new_pred.ignore_qualifiers(self.tcx).skip_binder().kind(),
422 old_pred.ignore_qualifiers(self.tcx).skip_binder().kind(),
424 if new_trait.def_id() == old_trait.def_id() {
425 let new_substs = new_trait.trait_ref.substs;
426 let old_substs = old_trait.trait_ref.substs;
428 if !new_substs.types().eq(old_substs.types()) {
429 // We can't compare lifetimes if the types are different,
430 // so skip checking `old_pred`.
434 for (new_region, old_region) in new_substs.regions().zip(old_substs.regions()) {
435 match (new_region, old_region) {
436 // If both predicates have an `ReLateBound` (a HRTB) in the
437 // same spot, we do nothing.
439 ty::RegionKind::ReLateBound(_, _),
440 ty::RegionKind::ReLateBound(_, _),
443 (ty::RegionKind::ReLateBound(_, _), _)
444 | (_, ty::RegionKind::ReVar(_)) => {
445 // One of these is true:
446 // The new predicate has a HRTB in a spot where the old
447 // predicate does not (if they both had a HRTB, the previous
448 // match arm would have executed). A HRBT is a 'stricter'
449 // bound than anything else, so we want to keep the newer
450 // predicate (with the HRBT) in place of the old predicate.
454 // The old predicate has a region variable where the new
455 // predicate has some other kind of region. An region
456 // variable isn't something we can actually display to a user,
457 // so we choose their new predicate (which doesn't have a region
460 // In both cases, we want to remove the old predicate,
461 // from `user_computed_preds`, and replace it with the new
462 // one. Having both the old and the new
463 // predicate in a `ParamEnv` would confuse `SelectionContext`.
465 // We're currently in the predicate passed to 'retain',
466 // so we return `false` to remove the old predicate from
467 // `user_computed_preds`.
470 (_, ty::RegionKind::ReLateBound(_, _))
471 | (ty::RegionKind::ReVar(_), _) => {
472 // This is the opposite situation as the previous arm.
473 // One of these is true:
475 // The old predicate has a HRTB lifetime in a place where the
476 // new predicate does not.
480 // The new predicate has a region variable where the old
481 // predicate has some other type of region.
483 // We want to leave the old
484 // predicate in `user_computed_preds`, and skip adding
485 // new_pred to `user_computed_params`.
486 should_add_new = false
497 user_computed_preds.insert(new_pred);
501 /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
502 /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
503 fn map_vid_to_region<'cx>(
505 regions: &RegionConstraintData<'cx>,
506 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
507 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
508 let mut finished_map = FxHashMap::default();
510 for constraint in regions.constraints.keys() {
512 &Constraint::VarSubVar(r1, r2) => {
514 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
515 deps1.larger.insert(RegionTarget::RegionVid(r2));
518 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
519 deps2.smaller.insert(RegionTarget::RegionVid(r1));
521 &Constraint::RegSubVar(region, vid) => {
523 let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
524 deps1.larger.insert(RegionTarget::RegionVid(vid));
527 let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
528 deps2.smaller.insert(RegionTarget::Region(region));
530 &Constraint::VarSubReg(vid, region) => {
531 finished_map.insert(vid, region);
533 &Constraint::RegSubReg(r1, r2) => {
535 let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
536 deps1.larger.insert(RegionTarget::Region(r2));
539 let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
540 deps2.smaller.insert(RegionTarget::Region(r1));
545 while !vid_map.is_empty() {
546 let target = *vid_map.keys().next().expect("Keys somehow empty");
547 let deps = vid_map.remove(&target).expect("Entry somehow missing");
549 for smaller in deps.smaller.iter() {
550 for larger in deps.larger.iter() {
551 match (smaller, larger) {
552 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
553 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
554 let smaller_deps = v.into_mut();
555 smaller_deps.larger.insert(*larger);
556 smaller_deps.larger.remove(&target);
559 if let Entry::Occupied(v) = vid_map.entry(*larger) {
560 let larger_deps = v.into_mut();
561 larger_deps.smaller.insert(*smaller);
562 larger_deps.smaller.remove(&target);
565 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
566 finished_map.insert(v1, r1);
568 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
569 // Do nothing; we don't care about regions that are smaller than vids.
571 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
572 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
573 let smaller_deps = v.into_mut();
574 smaller_deps.larger.insert(*larger);
575 smaller_deps.larger.remove(&target);
578 if let Entry::Occupied(v) = vid_map.entry(*larger) {
579 let larger_deps = v.into_mut();
580 larger_deps.smaller.insert(*smaller);
581 larger_deps.smaller.remove(&target);
591 fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
592 self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
595 pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
597 ty::Param(_) => true,
598 ty::Projection(p) => self.is_of_param(p.self_ty()),
603 fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
604 match p.ty().skip_binder().kind {
605 ty::Projection(proj) if proj == p.skip_binder().projection_ty => true,
610 fn evaluate_nested_obligations(
613 nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
614 computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
615 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
616 predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
617 select: &mut SelectionContext<'_, 'tcx>,
618 only_projections: bool,
620 let dummy_cause = ObligationCause::dummy();
622 for obligation in nested {
624 fresh_preds.insert(self.clean_pred(select.infcx(), obligation.predicate));
626 // Resolve any inference variables that we can, to help selection succeed
627 let predicate = select.infcx().resolve_vars_if_possible(&obligation.predicate);
629 // We only add a predicate as a user-displayable bound if
630 // it involves a generic parameter, and doesn't contain
631 // any inference variables.
633 // Displaying a bound involving a concrete type (instead of a generic
634 // parameter) would be pointless, since it's always true
636 // Displaying an inference variable is impossible, since they're
637 // an internal compiler detail without a defined visual representation
639 // We check this by calling is_of_param on the relevant types
640 // from the various possible predicates
643 match predicate.ignore_qualifiers(self.tcx).skip_binder().kind() {
644 &ty::PredicateKind::Trait(p, _) => {
645 if self.is_param_no_infer(p.trait_ref.substs)
649 self.add_user_pred(computed_preds, predicate);
651 predicates.push_back(ty::Binder::bind(p));
653 &ty::PredicateKind::Projection(p) => {
654 let p = ty::Binder::bind(p);
656 "evaluate_nested_obligations: examining projection predicate {:?}",
660 // As described above, we only want to display
661 // bounds which include a generic parameter but don't include
662 // an inference variable.
663 // Additionally, we check if we've seen this predicate before,
664 // to avoid rendering duplicate bounds to the user.
665 if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
666 && !p.ty().skip_binder().has_infer_types()
670 "evaluate_nested_obligations: adding projection predicate\
671 to computed_preds: {:?}",
675 // Under unusual circumstances, we can end up with a self-refeential
676 // projection predicate. For example:
677 // <T as MyType>::Value == <T as MyType>::Value
678 // Not only is displaying this to the user pointless,
679 // having it in the ParamEnv will cause an issue if we try to call
680 // poly_project_and_unify_type on the predicate, since this kind of
681 // predicate will normally never end up in a ParamEnv.
683 // For these reasons, we ignore these weird predicates,
684 // ensuring that we're able to properly synthesize an auto trait impl
685 if self.is_self_referential_projection(p) {
687 "evaluate_nested_obligations: encountered a projection
688 predicate equating a type with itself! Skipping"
691 self.add_user_pred(computed_preds, predicate);
695 // There are three possible cases when we project a predicate:
697 // 1. We encounter an error. This means that it's impossible for
698 // our current type to implement the auto trait - there's bound
699 // that we could add to our ParamEnv that would 'fix' this kind
700 // of error, as it's not caused by an unimplemented type.
702 // 2. We successfully project the predicate (Ok(Some(_))), generating
703 // some subobligations. We then process these subobligations
704 // like any other generated sub-obligations.
706 // 3. We receive an 'ambiguous' result (Ok(None))
707 // If we were actually trying to compile a crate,
708 // we would need to re-process this obligation later.
709 // However, all we care about is finding out what bounds
710 // are needed for our type to implement a particular auto trait.
711 // We've already added this obligation to our computed ParamEnv
712 // above (if it was necessary). Therefore, we don't need
713 // to do any further processing of the obligation.
715 // Note that we *must* try to project *all* projection predicates
716 // we encounter, even ones without inference variable.
717 // This ensures that we detect any projection errors,
718 // which indicate that our type can *never* implement the given
719 // auto trait. In that case, we will generate an explicit negative
720 // impl (e.g. 'impl !Send for MyType'). However, we don't
721 // try to process any of the generated subobligations -
722 // they contain no new information, since we already know
723 // that our type implements the projected-through trait,
724 // and can lead to weird region issues.
726 // Normally, we'll generate a negative impl as a result of encountering
727 // a type with an explicit negative impl of an auto trait
728 // (for example, raw pointers have !Send and !Sync impls)
729 // However, through some **interesting** manipulations of the type
730 // system, it's actually possible to write a type that never
731 // implements an auto trait due to a projection error, not a normal
732 // negative impl error. To properly handle this case, we need
733 // to ensure that we catch any potential projection errors,
734 // and turn them into an explicit negative impl for our type.
735 debug!("Projecting and unifying projection predicate {:?}", predicate);
737 match poly_project_and_unify_type(select, &obligation.with(p)) {
740 "evaluate_nested_obligations: Unable to unify predicate \
741 '{:?}' '{:?}', bailing out",
747 // We only care about sub-obligations
748 // when we started out trying to unify
749 // some inference variables. See the comment above
750 // for more infomration
751 if p.ty().skip_binder().has_infer_types() {
752 if !self.evaluate_nested_obligations(
766 // It's ok not to make progress when have no inference variables -
767 // in that case, we were only performing unifcation to check if an
768 // error occurred (which would indicate that it's impossible for our
769 // type to implement the auto trait).
770 // However, we should always make progress (either by generating
771 // subobligations or getting an error) when we started off with
772 // inference variables
773 if p.ty().skip_binder().has_infer_types() {
774 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
779 &ty::PredicateKind::RegionOutlives(binder) => {
780 let binder = ty::Binder::bind(binder);
781 if select.infcx().region_outlives_predicate(&dummy_cause, binder).is_err() {
785 &ty::PredicateKind::TypeOutlives(binder) => {
786 let binder = ty::Binder::bind(binder);
788 binder.no_bound_vars(),
789 binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
791 (None, Some(t_a)) => {
792 select.infcx().register_region_obligation_with_cause(
794 select.infcx().tcx.lifetimes.re_static,
798 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
799 select.infcx().register_region_obligation_with_cause(
808 _ => panic!("Unexpected predicate {:?} {:?}", ty, predicate),
816 infcx: &InferCtxt<'_, 'tcx>,
817 p: ty::Predicate<'tcx>,
818 ) -> ty::Predicate<'tcx> {
823 // Replaces all ReVars in a type with ty::Region's, using the provided map
824 pub struct RegionReplacer<'a, 'tcx> {
825 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
829 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
830 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
834 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
836 &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
839 .unwrap_or_else(|| r.super_fold_with(self))