1 //! Support code for rustdoc and external tools . You really don't
2 //! want to be using this unless you need to.
6 use std::collections::hash_map::Entry;
7 use std::collections::VecDeque;
9 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
10 use crate::infer::InferCtxt;
11 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
13 use crate::ty::fold::TypeFolder;
14 use crate::ty::{Region, RegionVid};
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<'a, 'tcx: 'a> {
51 tcx: TyCtxt<'a, 'tcx, 'tcx>,
54 impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
55 pub fn new(tcx: TyCtxt<'a, 'tcx, '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 for<'i> Fn(&InferCtxt<'_, 'tcx, 'i>, AutoTraitInfo<'i>) -> A,
83 ) -> AutoTraitResult<A> {
86 let trait_ref = ty::TraitRef {
88 substs: tcx.mk_substs_trait(ty, &[]),
91 let trait_pred = ty::Binder::bind(trait_ref);
93 let bail_out = tcx.infer_ctxt().enter(|infcx| {
94 let mut selcx = SelectionContext::with_negative(&infcx, true);
95 let result = selcx.select(&Obligation::new(
96 ObligationCause::dummy(),
98 trait_pred.to_poly_trait_predicate(),
102 Ok(Some(Vtable::VtableImpl(_))) => {
104 "find_auto_trait_generics({:?}): \
105 manual impl found, bailing out",
114 // If an explicit impl exists, it always takes priority over an auto impl
116 return AutoTraitResult::ExplicitImpl;
119 return tcx.infer_ctxt().enter(|mut infcx| {
120 let mut fresh_preds = FxHashSet::default();
122 // Due to the way projections are handled by SelectionContext, we need to run
123 // evaluate_predicates twice: once on the original param env, and once on the result of
124 // the first evaluate_predicates call.
126 // The problem is this: most of rustc, including SelectionContext and traits::project,
127 // are designed to work with a concrete usage of a type (e.g., Vec<u8>
128 // fn<T>() { Vec<T> }. This information will generally never change - given
129 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
130 // If we're unable to prove that 'T' implements a particular trait, we're done -
131 // there's nothing left to do but error out.
133 // However, synthesizing an auto trait impl works differently. Here, we start out with
134 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
135 // with - and progressively discover the conditions we need to fulfill for it to
136 // implement a certain auto trait. This ends up breaking two assumptions made by trait
137 // selection and projection:
139 // * We can always cache the result of a particular trait selection for the lifetime of
141 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
142 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
144 // We fix the first assumption by manually clearing out all of the InferCtxt's caches
145 // in between calls to SelectionContext.select. This allows us to keep all of the
146 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
147 // them between calls.
149 // We fix the second assumption by reprocessing the result of our first call to
150 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
151 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
152 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
153 // SelectionContext to return it back to us.
155 let (new_env, user_env) = match self.evaluate_predicates(
165 None => return AutoTraitResult::NegativeImpl,
168 let (full_env, full_user_env) = self.evaluate_predicates(
176 ).unwrap_or_else(|| {
178 "Failed to fully process: {:?} {:?} {:?}",
179 ty, trait_did, orig_env
184 "find_auto_trait_generics({:?}): fulfilling \
188 infcx.clear_caches();
190 // At this point, we already have all of the bounds we need. FulfillmentContext is used
191 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
192 // an additional sanity check.
193 let mut fulfill = FulfillmentContext::new();
194 fulfill.register_bound(
199 ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID),
201 fulfill.select_all_or_error(&infcx).unwrap_or_else(|e| {
203 "Unable to fulfill trait {:?} for '{:?}': {:?}",
208 let body_id_map: FxHashMap<_, _> = infcx
212 .map(|&(id, _)| (id, vec![]))
215 infcx.process_registered_region_obligations(&body_id_map, None, full_env);
217 let region_data = infcx
218 .borrow_region_constraints()
219 .region_constraint_data()
222 let vid_to_region = self.map_vid_to_region(®ion_data);
224 let info = AutoTraitInfo {
230 return AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info));
235 impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
236 // The core logic responsible for computing the bounds for our synthesized impl.
238 // To calculate the bounds, we call SelectionContext.select in a loop. Like FulfillmentContext,
239 // we recursively select the nested obligations of predicates we encounter. However, whenever we
240 // encounter an UnimplementedError involving a type parameter, we add it to our ParamEnv. Since
241 // our goal is to determine when a particular type implements an auto trait, Unimplemented
242 // errors tell us what conditions need to be met.
244 // This method ends up working somewhat similarly to FulfillmentContext, but with a few key
245 // differences. FulfillmentContext works under the assumption that it's dealing with concrete
246 // user code. According, it considers all possible ways that a Predicate could be met - which
247 // isn't always what we want for a synthesized impl. For example, given the predicate 'T:
248 // Iterator', FulfillmentContext can end up reporting an Unimplemented error for T:
249 // IntoIterator - since there's an implementation of Iteratpr where T: IntoIterator,
250 // FulfillmentContext will drive SelectionContext to consider that impl before giving up. If we
251 // were to rely on FulfillmentContext's decision, we might end up synthesizing an impl like
253 // 'impl<T> Send for Foo<T> where T: IntoIterator'
255 // While it might be technically true that Foo implements Send where T: IntoIterator,
256 // the bound is overly restrictive - it's really only necessary that T: Iterator.
258 // For this reason, evaluate_predicates handles predicates with type variables specially. When
259 // we encounter an Unimplemented error for a bound such as 'T: Iterator', we immediately add it
260 // to our ParamEnv, and add it to our stack for recursive evaluation. When we later select it,
261 // we'll pick up any nested bounds, without ever inferring that 'T: IntoIterator' needs to
264 // One additional consideration is supertrait bounds. Normally, a ParamEnv is only ever
265 // constructed once for a given type. As part of the construction process, the ParamEnv will
266 // have any supertrait bounds normalized - e.g., if we have a type 'struct Foo<T: Copy>', the
267 // ParamEnv will contain 'T: Copy' and 'T: Clone', since 'Copy: Clone'. When we construct our
268 // own ParamEnv, we need to do this ourselves, through traits::elaborate_predicates, or else
269 // SelectionContext will choke on the missing predicates. However, this should never show up in
270 // the final synthesized generics: we don't want our generated docs page to contain something
271 // like 'T: Copy + Clone', as that's redundant. Therefore, we keep track of a separate
272 // 'user_env', which only holds the predicates that will actually be displayed to the user.
273 fn evaluate_predicates<'b, 'gcx, 'c>(
275 infcx: &InferCtxt<'b, 'tcx, 'c>,
278 param_env: ty::ParamEnv<'c>,
279 user_env: ty::ParamEnv<'c>,
280 fresh_preds: &mut FxHashSet<ty::Predicate<'c>>,
281 only_projections: bool,
282 ) -> Option<(ty::ParamEnv<'c>, ty::ParamEnv<'c>)> {
285 let mut select = SelectionContext::with_negative(&infcx, true);
287 let mut already_visited = FxHashSet::default();
288 let mut predicates = VecDeque::new();
289 predicates.push_back(ty::Binder::bind(ty::TraitPredicate {
290 trait_ref: ty::TraitRef {
292 substs: infcx.tcx.mk_substs_trait(ty, &[]),
296 let mut computed_preds: FxHashSet<_> = param_env.caller_bounds.iter().cloned().collect();
297 let mut user_computed_preds: FxHashSet<_> =
298 user_env.caller_bounds.iter().cloned().collect();
300 let mut new_env = param_env;
301 let dummy_cause = ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID);
303 while let Some(pred) = predicates.pop_front() {
304 infcx.clear_caches();
306 if !already_visited.insert(pred) {
310 // Call infcx.resolve_vars_if_possible to see if we can
311 // get rid of any inference variables.
312 let obligation = infcx.resolve_vars_if_possible(
313 &Obligation::new(dummy_cause.clone(), new_env, pred)
315 let result = select.select(&obligation);
318 &Ok(Some(ref vtable)) => {
319 // If we see an explicit negative impl (e.g., 'impl !Send for MyStruct'),
320 // we immediately bail out, since it's impossible for us to continue.
322 Vtable::VtableImpl(VtableImplData { impl_def_id, .. }) => {
323 // Blame tidy for the weird bracket placement
324 if infcx.tcx.impl_polarity(*impl_def_id) == hir::ImplPolarity::Negative
326 debug!("evaluate_nested_obligations: Found explicit negative impl\
327 {:?}, bailing out", impl_def_id);
334 let obligations = vtable.clone().nested_obligations().into_iter();
336 if !self.evaluate_nested_obligations(
339 &mut user_computed_preds,
349 &Err(SelectionError::Unimplemented) => {
350 if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
351 already_visited.remove(&pred);
353 &mut user_computed_preds,
354 ty::Predicate::Trait(pred),
356 predicates.push_back(pred);
359 "evaluate_nested_obligations: Unimplemented found, bailing: \
363 pred.skip_binder().trait_ref.substs
368 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
371 computed_preds.extend(user_computed_preds.iter().cloned());
372 let normalized_preds =
373 elaborate_predicates(tcx, computed_preds.iter().cloned().collect());
374 new_env = ty::ParamEnv::new(
375 tcx.mk_predicates(normalized_preds),
381 let final_user_env = ty::ParamEnv::new(
382 tcx.mk_predicates(user_computed_preds.into_iter()),
387 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
389 ty, trait_did, new_env, final_user_env
392 return Some((new_env, final_user_env));
395 // This method is designed to work around the following issue:
396 // When we compute auto trait bounds, we repeatedly call SelectionContext.select,
397 // progressively building a ParamEnv based on the results we get.
398 // However, our usage of SelectionContext differs from its normal use within the compiler,
399 // in that we capture and re-reprocess predicates from Unimplemented errors.
401 // This can lead to a corner case when dealing with region parameters.
402 // During our selection loop in evaluate_predicates, we might end up with
403 // two trait predicates that differ only in their region parameters:
404 // one containing a HRTB lifetime parameter, and one containing a 'normal'
405 // lifetime parameter. For example:
408 // T as MyTrait<'static>
410 // If we put both of these predicates in our computed ParamEnv, we'll
411 // confuse SelectionContext, since it will (correctly) view both as being applicable.
413 // To solve this, we pick the 'more strict' lifetime bound - i.e., the HRTB
414 // Our end goal is to generate a user-visible description of the conditions
415 // under which a type implements an auto trait. A trait predicate involving
416 // a HRTB means that the type needs to work with any choice of lifetime,
417 // not just one specific lifetime (e.g., 'static).
418 fn add_user_pred<'c>(
420 user_computed_preds: &mut FxHashSet<ty::Predicate<'c>>,
421 new_pred: ty::Predicate<'c>,
423 let mut should_add_new = true;
424 user_computed_preds.retain(|&old_pred| {
425 match (&new_pred, old_pred) {
426 (&ty::Predicate::Trait(new_trait), ty::Predicate::Trait(old_trait)) => {
427 if new_trait.def_id() == old_trait.def_id() {
428 let new_substs = new_trait.skip_binder().trait_ref.substs;
429 let old_substs = old_trait.skip_binder().trait_ref.substs;
431 if !new_substs.types().eq(old_substs.types()) {
432 // We can't compare lifetimes if the types are different,
433 // so skip checking old_pred
437 for (new_region, old_region) in
438 new_substs.regions().zip(old_substs.regions())
440 match (new_region, old_region) {
441 // If both predicates have an 'ReLateBound' (a HRTB) in the
442 // same spot, we do nothing
444 ty::RegionKind::ReLateBound(_, _),
445 ty::RegionKind::ReLateBound(_, _),
448 (ty::RegionKind::ReLateBound(_, _), _) |
449 (_, ty::RegionKind::ReVar(_)) => {
450 // One of these is true:
451 // The new predicate has a HRTB in a spot where the old
452 // predicate does not (if they both had a HRTB, the previous
453 // match arm would have executed). A HRBT is a 'stricter'
454 // bound than anything else, so we want to keep the newer
455 // predicate (with the HRBT) in place of the old predicate.
459 // The old predicate has a region variable where the new
460 // predicate has some other kind of region. An region
461 // variable isn't something we can actually display to a user,
462 // so we choose ther new predicate (which doesn't have a region
465 // In both cases, we want to remove the old predicate,
466 // from user_computed_preds, and replace it with the new
467 // one. Having both the old and the new
468 // predicate in a ParamEnv would confuse SelectionContext
470 // We're currently in the predicate passed to 'retain',
471 // so we return 'false' to remove the old predicate from
472 // user_computed_preds
475 (_, ty::RegionKind::ReLateBound(_, _)) |
476 (ty::RegionKind::ReVar(_), _) => {
477 // This is the opposite situation as the previous arm.
478 // One of these is true:
480 // The old predicate has a HRTB lifetime in a place where the
481 // new predicate does not.
485 // The new predicate has a region variable where the old
486 // predicate has some other type of region.
488 // We want to leave the old
489 // predicate in user_computed_preds, and skip adding
490 // new_pred to user_computed_params.
491 should_add_new = false
504 user_computed_preds.insert(new_pred);
508 // This is very similar to handle_lifetimes. However, instead of matching ty::Region's
509 // to each other, we match ty::RegionVid's to ty::Region's
510 fn map_vid_to_region<'cx>(
512 regions: &RegionConstraintData<'cx>,
513 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
514 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
515 let mut finished_map = FxHashMap::default();
517 for constraint in regions.constraints.keys() {
519 &Constraint::VarSubVar(r1, r2) => {
521 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
522 deps1.larger.insert(RegionTarget::RegionVid(r2));
525 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
526 deps2.smaller.insert(RegionTarget::RegionVid(r1));
528 &Constraint::RegSubVar(region, vid) => {
530 let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
531 deps1.larger.insert(RegionTarget::RegionVid(vid));
534 let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
535 deps2.smaller.insert(RegionTarget::Region(region));
537 &Constraint::VarSubReg(vid, region) => {
538 finished_map.insert(vid, region);
540 &Constraint::RegSubReg(r1, r2) => {
542 let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
543 deps1.larger.insert(RegionTarget::Region(r2));
546 let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
547 deps2.smaller.insert(RegionTarget::Region(r1));
552 while !vid_map.is_empty() {
553 let target = vid_map.keys().next().expect("Keys somehow empty").clone();
554 let deps = vid_map.remove(&target).expect("Entry somehow missing");
556 for smaller in deps.smaller.iter() {
557 for larger in deps.larger.iter() {
558 match (smaller, larger) {
559 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
560 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
561 let smaller_deps = v.into_mut();
562 smaller_deps.larger.insert(*larger);
563 smaller_deps.larger.remove(&target);
566 if let Entry::Occupied(v) = vid_map.entry(*larger) {
567 let larger_deps = v.into_mut();
568 larger_deps.smaller.insert(*smaller);
569 larger_deps.smaller.remove(&target);
572 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
573 finished_map.insert(v1, r1);
575 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
576 // Do nothing - we don't care about regions that are smaller than vids
578 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
579 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
580 let smaller_deps = v.into_mut();
581 smaller_deps.larger.insert(*larger);
582 smaller_deps.larger.remove(&target);
585 if let Entry::Occupied(v) = vid_map.entry(*larger) {
586 let larger_deps = v.into_mut();
587 larger_deps.smaller.insert(*smaller);
588 larger_deps.smaller.remove(&target);
598 fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
599 return self.is_of_param(substs.type_at(0)) &&
600 !substs.types().any(|t| t.has_infer_types());
603 pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
604 return match ty.sty {
605 ty::Param(_) => true,
606 ty::Projection(p) => self.is_of_param(p.self_ty()),
611 fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
612 match p.ty().skip_binder().sty {
613 ty::Projection(proj) if proj == p.skip_binder().projection_ty => {
620 fn evaluate_nested_obligations<
625 T: Iterator<Item = Obligation<'cx, ty::Predicate<'cx>>>,
630 computed_preds: &'b mut FxHashSet<ty::Predicate<'cx>>,
631 fresh_preds: &'b mut FxHashSet<ty::Predicate<'cx>>,
632 predicates: &'b mut VecDeque<ty::PolyTraitPredicate<'cx>>,
633 select: &mut SelectionContext<'c, 'd, 'cx>,
634 only_projections: bool,
636 let dummy_cause = ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID);
638 for (obligation, mut predicate) in nested
639 .map(|o| (o.clone(), o.predicate))
642 fresh_preds.insert(self.clean_pred(select.infcx(), predicate));
644 // Resolve any inference variables that we can, to help selection succeed
645 predicate = select.infcx().resolve_vars_if_possible(&predicate);
647 // We only add a predicate as a user-displayable bound if
648 // it involves a generic parameter, and doesn't contain
649 // any inference variables.
651 // Displaying a bound involving a concrete type (instead of a generic
652 // parameter) would be pointless, since it's always true
654 // Displaying an inference variable is impossible, since they're
655 // an internal compiler detail without a defined visual representation
657 // We check this by calling is_of_param on the relevant types
658 // from the various possible predicates
660 &ty::Predicate::Trait(p) => {
661 if self.is_param_no_infer(p.skip_binder().trait_ref.substs)
665 self.add_user_pred(computed_preds, predicate);
667 predicates.push_back(p);
669 &ty::Predicate::Projection(p) => {
670 debug!("evaluate_nested_obligations: examining projection predicate {:?}",
673 // As described above, we only want to display
674 // bounds which include a generic parameter but don't include
675 // an inference variable.
676 // Additionally, we check if we've seen this predicate before,
677 // to avoid rendering duplicate bounds to the user.
678 if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
679 && !p.ty().skip_binder().has_infer_types()
681 debug!("evaluate_nested_obligations: adding projection predicate\
682 to computed_preds: {:?}", predicate);
684 // Under unusual circumstances, we can end up with a self-refeential
685 // projection predicate. For example:
686 // <T as MyType>::Value == <T as MyType>::Value
687 // Not only is displaying this to the user pointless,
688 // having it in the ParamEnv will cause an issue if we try to call
689 // poly_project_and_unify_type on the predicate, since this kind of
690 // predicate will normally never end up in a ParamEnv.
692 // For these reasons, we ignore these weird predicates,
693 // ensuring that we're able to properly synthesize an auto trait impl
694 if self.is_self_referential_projection(p) {
695 debug!("evaluate_nested_obligations: encountered a projection
696 predicate equating a type with itself! Skipping");
699 self.add_user_pred(computed_preds, predicate);
703 // There are three possible cases when we project a predicate:
705 // 1. We encounter an error. This means that it's impossible for
706 // our current type to implement the auto trait - there's bound
707 // that we could add to our ParamEnv that would 'fix' this kind
708 // of error, as it's not caused by an unimplemented type.
710 // 2. We succesfully project the predicate (Ok(Some(_))), generating
711 // some subobligations. We then process these subobligations
712 // like any other generated sub-obligations.
714 // 3. We receieve an 'ambiguous' result (Ok(None))
715 // If we were actually trying to compile a crate,
716 // we would need to re-process this obligation later.
717 // However, all we care about is finding out what bounds
718 // are needed for our type to implement a particular auto trait.
719 // We've already added this obligation to our computed ParamEnv
720 // above (if it was necessary). Therefore, we don't need
721 // to do any further processing of the obligation.
723 // Note that we *must* try to project *all* projection predicates
724 // we encounter, even ones without inference variable.
725 // This ensures that we detect any projection errors,
726 // which indicate that our type can *never* implement the given
727 // auto trait. In that case, we will generate an explicit negative
728 // impl (e.g. 'impl !Send for MyType'). However, we don't
729 // try to process any of the generated subobligations -
730 // they contain no new information, since we already know
731 // that our type implements the projected-through trait,
732 // and can lead to weird region issues.
734 // Normally, we'll generate a negative impl as a result of encountering
735 // a type with an explicit negative impl of an auto trait
736 // (for example, raw pointers have !Send and !Sync impls)
737 // However, through some **interesting** manipulations of the type
738 // system, it's actually possible to write a type that never
739 // implements an auto trait due to a projection error, not a normal
740 // negative impl error. To properly handle this case, we need
741 // to ensure that we catch any potential projection errors,
742 // and turn them into an explicit negative impl for our type.
743 debug!("Projecting and unifying projection predicate {:?}",
746 match poly_project_and_unify_type(select, &obligation.with(p)) {
749 "evaluate_nested_obligations: Unable to unify predicate \
750 '{:?}' '{:?}', bailing out",
756 // We only care about sub-obligations
757 // when we started out trying to unify
758 // some inference variables. See the comment above
759 // for more infomration
760 if p.ty().skip_binder().has_infer_types() {
761 if !self.evaluate_nested_obligations(
763 v.clone().iter().cloned(),
775 // It's ok not to make progress when hvave no inference variables -
776 // in that case, we were only performing unifcation to check if an
777 // error occured (which would indicate that it's impossible for our
778 // type to implement the auto trait).
779 // However, we should always make progress (either by generating
780 // subobligations or getting an error) when we started off with
781 // inference variables
782 if p.ty().skip_binder().has_infer_types() {
783 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
788 &ty::Predicate::RegionOutlives(ref binder) => {
791 .region_outlives_predicate(&dummy_cause, binder)
797 &ty::Predicate::TypeOutlives(ref binder) => {
799 binder.no_bound_vars(),
800 binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
802 (None, Some(t_a)) => {
803 select.infcx().register_region_obligation_with_cause(
805 select.infcx().tcx.lifetimes.re_static,
809 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
810 select.infcx().register_region_obligation_with_cause(
819 _ => panic!("Unexpected predicate {:?} {:?}", ty, predicate),
825 pub fn clean_pred<'c, 'd, 'cx>(
827 infcx: &InferCtxt<'c, 'd, 'cx>,
828 p: ty::Predicate<'cx>,
829 ) -> ty::Predicate<'cx> {
834 // Replaces all ReVars in a type with ty::Region's, using the provided map
835 pub struct RegionReplacer<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
836 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
837 tcx: TyCtxt<'a, 'gcx, 'tcx>,
840 impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionReplacer<'a, 'gcx, 'tcx> {
841 fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> {
845 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
847 &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
849 }).unwrap_or_else(|| r.super_fold_with(self))