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, Term};
11 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
13 use std::collections::hash_map::Entry;
14 use std::collections::VecDeque;
17 // FIXME(twk): this is obviously not nice to duplicate like that
18 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
19 pub enum RegionTarget<'tcx> {
24 #[derive(Default, Debug, Clone)]
25 pub struct RegionDeps<'tcx> {
26 larger: FxHashSet<RegionTarget<'tcx>>,
27 smaller: FxHashSet<RegionTarget<'tcx>>,
30 pub enum AutoTraitResult<A> {
37 impl<A> AutoTraitResult<A> {
38 fn is_auto(&self) -> bool {
39 matches!(self, AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl)
43 pub struct AutoTraitInfo<'cx> {
44 pub full_user_env: ty::ParamEnv<'cx>,
45 pub region_data: RegionConstraintData<'cx>,
46 pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>,
49 pub struct AutoTraitFinder<'tcx> {
53 impl<'tcx> AutoTraitFinder<'tcx> {
54 pub fn new(tcx: TyCtxt<'tcx>) -> Self {
55 AutoTraitFinder { tcx }
58 /// Makes a best effort to determine whether and under which conditions an auto trait is
59 /// implemented for a type. For example, if you have
62 /// struct Foo<T> { data: Box<T> }
65 /// then this might return that Foo<T>: Send if T: Send (encoded in the AutoTraitResult type).
66 /// The analysis attempts to account for custom impls as well as other complex cases. This
67 /// result is intended for use by rustdoc and other such consumers.
69 /// (Note that due to the coinductive nature of Send, the full and correct result is actually
70 /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
71 /// types are all Send. So, in our example, we might have that Foo<T>: Send if Box<T>: Send.
72 /// But this is often not the best way to present to the user.)
74 /// Warning: The API should be considered highly unstable, and it may be refactored or removed
76 pub fn find_auto_trait_generics<A>(
79 orig_env: ty::ParamEnv<'tcx>,
81 mut auto_trait_callback: impl FnMut(AutoTraitInfo<'tcx>) -> A,
82 ) -> AutoTraitResult<A> {
85 let trait_ref = ty::TraitRef { def_id: trait_did, substs: tcx.mk_substs_trait(ty, &[]) };
87 let trait_pred = ty::Binder::dummy(trait_ref);
89 let bail_out = tcx.infer_ctxt().enter(|infcx| {
90 let mut selcx = SelectionContext::with_negative(&infcx, true);
91 let result = selcx.select(&Obligation::new(
92 ObligationCause::dummy(),
94 trait_pred.to_poly_trait_predicate(),
98 Ok(Some(ImplSource::UserDefined(_))) => {
100 "find_auto_trait_generics({:?}): \
101 manual impl found, bailing out",
110 // If an explicit impl exists, it always takes priority over an auto impl
112 return AutoTraitResult::ExplicitImpl;
115 tcx.infer_ctxt().enter(|infcx| {
116 let mut fresh_preds = FxHashSet::default();
118 // Due to the way projections are handled by SelectionContext, we need to run
119 // evaluate_predicates twice: once on the original param env, and once on the result of
120 // the first evaluate_predicates call.
122 // The problem is this: most of rustc, including SelectionContext and traits::project,
123 // are designed to work with a concrete usage of a type (e.g., Vec<u8>
124 // fn<T>() { Vec<T> }. This information will generally never change - given
125 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
126 // If we're unable to prove that 'T' implements a particular trait, we're done -
127 // there's nothing left to do but error out.
129 // However, synthesizing an auto trait impl works differently. Here, we start out with
130 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
131 // with - and progressively discover the conditions we need to fulfill for it to
132 // implement a certain auto trait. This ends up breaking two assumptions made by trait
133 // selection and projection:
135 // * We can always cache the result of a particular trait selection for the lifetime of
137 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
138 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
140 // We fix the first assumption by manually clearing out all of the InferCtxt's caches
141 // in between calls to SelectionContext.select. This allows us to keep all of the
142 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
143 // them between calls.
145 // We fix the second assumption by reprocessing the result of our first call to
146 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
147 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
148 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
149 // SelectionContext to return it back to us.
151 let (new_env, user_env) = match self.evaluate_predicates(
161 None => return AutoTraitResult::NegativeImpl,
164 let (full_env, full_user_env) = self
165 .evaluate_predicates(
175 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
179 "find_auto_trait_generics({:?}): fulfilling \
183 infcx.clear_caches();
185 // At this point, we already have all of the bounds we need. FulfillmentContext is used
186 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
187 // an additional sanity check.
188 let mut fulfill = FulfillmentContext::new();
189 fulfill.register_bound(&infcx, full_env, ty, trait_did, ObligationCause::dummy());
190 let errors = fulfill.select_all_or_error(&infcx);
192 if !errors.is_empty() {
193 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
196 let body_id_map: FxHashMap<_, _> = infcx
199 .region_obligations()
201 .map(|&(id, _)| (id, vec![]))
204 infcx.process_registered_region_obligations(&body_id_map, None, full_env);
206 let region_data = infcx
209 .unwrap_region_constraints()
210 .region_constraint_data()
213 let vid_to_region = self.map_vid_to_region(®ion_data);
215 let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
217 AutoTraitResult::PositiveImpl(auto_trait_callback(info))
222 impl<'tcx> AutoTraitFinder<'tcx> {
223 /// The core logic responsible for computing the bounds for our synthesized impl.
225 /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
226 /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
227 /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
228 /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
229 /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
231 /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
232 /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
233 /// user code. According, it considers all possible ways that a `Predicate` could be met, which
234 /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
235 /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
236 /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
237 /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
238 /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
241 /// impl<T> Send for Foo<T> where T: IntoIterator
243 /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
244 /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
246 /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
247 /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
248 /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
249 /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
252 /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
253 /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
254 /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
255 /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
256 /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
257 /// else `SelectionContext` will choke on the missing predicates. However, this should never
258 /// show up in the final synthesized generics: we don't want our generated docs page to contain
259 /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
260 /// separate `user_env`, which only holds the predicates that will actually be displayed to the
262 fn evaluate_predicates(
264 infcx: &InferCtxt<'_, 'tcx>,
267 param_env: ty::ParamEnv<'tcx>,
268 user_env: ty::ParamEnv<'tcx>,
269 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
270 only_projections: bool,
271 ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
274 // Don't try to proess any nested obligations involving predicates
275 // that are already in the `ParamEnv` (modulo regions): we already
276 // know that they must hold.
277 for predicate in param_env.caller_bounds() {
278 fresh_preds.insert(self.clean_pred(infcx, predicate));
281 let mut select = SelectionContext::with_negative(&infcx, true);
283 let mut already_visited = FxHashSet::default();
284 let mut predicates = VecDeque::new();
285 predicates.push_back(ty::Binder::dummy(ty::TraitPredicate {
286 trait_ref: ty::TraitRef {
288 substs: infcx.tcx.mk_substs_trait(ty, &[]),
290 constness: ty::BoundConstness::NotConst,
291 // Auto traits are positive
292 polarity: ty::ImplPolarity::Positive,
295 let computed_preds = param_env.caller_bounds().iter();
296 let mut user_computed_preds: FxHashSet<_> = user_env.caller_bounds().iter().collect();
298 let mut new_env = param_env;
299 let dummy_cause = ObligationCause::dummy();
301 while let Some(pred) = predicates.pop_front() {
302 infcx.clear_caches();
304 if !already_visited.insert(pred) {
308 // Call `infcx.resolve_vars_if_possible` to see if we can
309 // get rid of any inference variables.
311 infcx.resolve_vars_if_possible(Obligation::new(dummy_cause.clone(), new_env, pred));
312 let result = select.select(&obligation);
315 Ok(Some(ref impl_source)) => {
316 // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
317 // we immediately bail out, since it's impossible for us to continue.
319 if let ImplSource::UserDefined(ImplSourceUserDefinedData {
323 // Blame 'tidy' for the weird bracket placement.
324 if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
326 "evaluate_nested_obligations: found explicit negative impl\
334 let obligations = impl_source.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);
352 self.add_user_pred(&mut user_computed_preds, pred.to_predicate(self.tcx));
353 predicates.push_back(pred);
356 "evaluate_nested_obligations: `Unimplemented` found, bailing: \
360 pred.skip_binder().trait_ref.substs
365 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
368 let normalized_preds = elaborate_predicates(
370 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
372 .map(|o| o.predicate);
373 new_env = ty::ParamEnv::new(
374 tcx.mk_predicates(normalized_preds),
376 param_env.constness(),
380 let final_user_env = ty::ParamEnv::new(
381 tcx.mk_predicates(user_computed_preds.into_iter()),
383 user_env.constness(),
386 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
388 ty, trait_did, new_env, final_user_env
391 Some((new_env, final_user_env))
394 /// This method is designed to work around the following issue:
395 /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
396 /// progressively building a `ParamEnv` based on the results we get.
397 /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
398 /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
400 /// This can lead to a corner case when dealing with region parameters.
401 /// During our selection loop in `evaluate_predicates`, we might end up with
402 /// two trait predicates that differ only in their region parameters:
403 /// one containing a HRTB lifetime parameter, and one containing a 'normal'
404 /// lifetime parameter. For example:
407 /// T as MyTrait<'static>
409 /// If we put both of these predicates in our computed `ParamEnv`, we'll
410 /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
412 /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
413 /// Our end goal is to generate a user-visible description of the conditions
414 /// under which a type implements an auto trait. A trait predicate involving
415 /// a HRTB means that the type needs to work with any choice of lifetime,
416 /// not just one specific lifetime (e.g., `'static`).
419 user_computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
420 new_pred: ty::Predicate<'tcx>,
422 let mut should_add_new = true;
423 user_computed_preds.retain(|&old_pred| {
424 if let (ty::PredicateKind::Trait(new_trait), ty::PredicateKind::Trait(old_trait)) =
425 (new_pred.kind().skip_binder(), old_pred.kind().skip_binder())
427 if new_trait.def_id() == old_trait.def_id() {
428 let new_substs = new_trait.trait_ref.substs;
429 let old_substs = old_trait.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 iter::zip(new_substs.regions(), 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 their 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
502 user_computed_preds.insert(new_pred);
506 /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
507 /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
508 fn map_vid_to_region<'cx>(
510 regions: &RegionConstraintData<'cx>,
511 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
512 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
513 let mut finished_map = FxHashMap::default();
515 for constraint in regions.constraints.keys() {
517 &Constraint::VarSubVar(r1, r2) => {
519 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
520 deps1.larger.insert(RegionTarget::RegionVid(r2));
523 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
524 deps2.smaller.insert(RegionTarget::RegionVid(r1));
526 &Constraint::RegSubVar(region, vid) => {
528 let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
529 deps1.larger.insert(RegionTarget::RegionVid(vid));
532 let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
533 deps2.smaller.insert(RegionTarget::Region(region));
535 &Constraint::VarSubReg(vid, region) => {
536 finished_map.insert(vid, region);
538 &Constraint::RegSubReg(r1, r2) => {
540 let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
541 deps1.larger.insert(RegionTarget::Region(r2));
544 let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
545 deps2.smaller.insert(RegionTarget::Region(r1));
550 while !vid_map.is_empty() {
551 let target = *vid_map.keys().next().expect("Keys somehow empty");
552 let deps = vid_map.remove(&target).expect("Entry somehow missing");
554 for smaller in deps.smaller.iter() {
555 for larger in deps.larger.iter() {
556 match (smaller, larger) {
557 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
558 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
559 let smaller_deps = v.into_mut();
560 smaller_deps.larger.insert(*larger);
561 smaller_deps.larger.remove(&target);
564 if let Entry::Occupied(v) = vid_map.entry(*larger) {
565 let larger_deps = v.into_mut();
566 larger_deps.smaller.insert(*smaller);
567 larger_deps.smaller.remove(&target);
570 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
571 finished_map.insert(v1, r1);
573 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
574 // Do nothing; we don't care about regions that are smaller than vids.
576 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
577 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
578 let smaller_deps = v.into_mut();
579 smaller_deps.larger.insert(*larger);
580 smaller_deps.larger.remove(&target);
583 if let Entry::Occupied(v) = vid_map.entry(*larger) {
584 let larger_deps = v.into_mut();
585 larger_deps.smaller.insert(*smaller);
586 larger_deps.smaller.remove(&target);
596 fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
597 self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
600 pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
602 ty::Param(_) => true,
603 ty::Projection(p) => self.is_of_param(p.self_ty()),
608 fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
609 if let Term::Ty(ty) = p.term().skip_binder() {
610 matches!(ty.kind(), ty::Projection(proj) if proj == &p.skip_binder().projection_ty)
616 fn evaluate_nested_obligations(
619 nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
620 computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
621 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
622 predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
623 select: &mut SelectionContext<'_, 'tcx>,
624 only_projections: bool,
626 let dummy_cause = ObligationCause::dummy();
628 for obligation in nested {
630 fresh_preds.insert(self.clean_pred(select.infcx(), obligation.predicate));
632 // Resolve any inference variables that we can, to help selection succeed
633 let predicate = select.infcx().resolve_vars_if_possible(obligation.predicate);
635 // We only add a predicate as a user-displayable bound if
636 // it involves a generic parameter, and doesn't contain
637 // any inference variables.
639 // Displaying a bound involving a concrete type (instead of a generic
640 // parameter) would be pointless, since it's always true
642 // Displaying an inference variable is impossible, since they're
643 // an internal compiler detail without a defined visual representation
645 // We check this by calling is_of_param on the relevant types
646 // from the various possible predicates
648 let bound_predicate = predicate.kind();
649 match bound_predicate.skip_binder() {
650 ty::PredicateKind::Trait(p) => {
651 // Add this to `predicates` so that we end up calling `select`
652 // with it. If this predicate ends up being unimplemented,
653 // then `evaluate_predicates` will handle adding it the `ParamEnv`
655 predicates.push_back(bound_predicate.rebind(p));
657 ty::PredicateKind::Projection(p) => {
658 let p = bound_predicate.rebind(p);
660 "evaluate_nested_obligations: examining projection predicate {:?}",
664 // As described above, we only want to display
665 // bounds which include a generic parameter but don't include
666 // an inference variable.
667 // Additionally, we check if we've seen this predicate before,
668 // to avoid rendering duplicate bounds to the user.
669 if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
670 && !p.term().skip_binder().has_infer_types()
674 "evaluate_nested_obligations: adding projection predicate\
675 to computed_preds: {:?}",
679 // Under unusual circumstances, we can end up with a self-refeential
680 // projection predicate. For example:
681 // <T as MyType>::Value == <T as MyType>::Value
682 // Not only is displaying this to the user pointless,
683 // having it in the ParamEnv will cause an issue if we try to call
684 // poly_project_and_unify_type on the predicate, since this kind of
685 // predicate will normally never end up in a ParamEnv.
687 // For these reasons, we ignore these weird predicates,
688 // ensuring that we're able to properly synthesize an auto trait impl
689 if self.is_self_referential_projection(p) {
691 "evaluate_nested_obligations: encountered a projection
692 predicate equating a type with itself! Skipping"
695 self.add_user_pred(computed_preds, predicate);
699 // There are three possible cases when we project a predicate:
701 // 1. We encounter an error. This means that it's impossible for
702 // our current type to implement the auto trait - there's bound
703 // that we could add to our ParamEnv that would 'fix' this kind
704 // of error, as it's not caused by an unimplemented type.
706 // 2. We successfully project the predicate (Ok(Some(_))), generating
707 // some subobligations. We then process these subobligations
708 // like any other generated sub-obligations.
710 // 3. We receive an 'ambiguous' result (Ok(None))
711 // If we were actually trying to compile a crate,
712 // we would need to re-process this obligation later.
713 // However, all we care about is finding out what bounds
714 // are needed for our type to implement a particular auto trait.
715 // We've already added this obligation to our computed ParamEnv
716 // above (if it was necessary). Therefore, we don't need
717 // to do any further processing of the obligation.
719 // Note that we *must* try to project *all* projection predicates
720 // we encounter, even ones without inference variable.
721 // This ensures that we detect any projection errors,
722 // which indicate that our type can *never* implement the given
723 // auto trait. In that case, we will generate an explicit negative
724 // impl (e.g. 'impl !Send for MyType'). However, we don't
725 // try to process any of the generated subobligations -
726 // they contain no new information, since we already know
727 // that our type implements the projected-through trait,
728 // and can lead to weird region issues.
730 // Normally, we'll generate a negative impl as a result of encountering
731 // a type with an explicit negative impl of an auto trait
732 // (for example, raw pointers have !Send and !Sync impls)
733 // However, through some **interesting** manipulations of the type
734 // system, it's actually possible to write a type that never
735 // implements an auto trait due to a projection error, not a normal
736 // negative impl error. To properly handle this case, we need
737 // to ensure that we catch any potential projection errors,
738 // and turn them into an explicit negative impl for our type.
739 debug!("Projecting and unifying projection predicate {:?}", predicate);
741 match project::poly_project_and_unify_type(select, &obligation.with(p)) {
744 "evaluate_nested_obligations: Unable to unify predicate \
745 '{:?}' '{:?}', bailing out",
750 Ok(Err(project::InProgress)) => {
751 debug!("evaluate_nested_obligations: recursive projection predicate");
755 // We only care about sub-obligations
756 // when we started out trying to unify
757 // some inference variables. See the comment above
758 // for more infomration
759 if p.term().skip_binder().has_infer_types() {
760 if !self.evaluate_nested_obligations(
774 // It's ok not to make progress when have no inference variables -
775 // in that case, we were only performing unifcation to check if an
776 // error occurred (which would indicate that it's impossible for our
777 // type to implement the auto trait).
778 // However, we should always make progress (either by generating
779 // subobligations or getting an error) when we started off with
780 // inference variables
781 if p.term().skip_binder().has_infer_types() {
782 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
787 ty::PredicateKind::RegionOutlives(binder) => {
788 let binder = bound_predicate.rebind(binder);
789 if select.infcx().region_outlives_predicate(&dummy_cause, binder).is_err() {
793 ty::PredicateKind::TypeOutlives(binder) => {
794 let binder = bound_predicate.rebind(binder);
796 binder.no_bound_vars(),
797 binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
799 (None, Some(t_a)) => {
800 select.infcx().register_region_obligation_with_cause(
802 select.infcx().tcx.lifetimes.re_static,
806 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
807 select.infcx().register_region_obligation_with_cause(
816 ty::PredicateKind::ConstEquate(c1, c2) => {
817 let evaluate = |c: &'tcx ty::Const<'tcx>| {
818 if let ty::ConstKind::Unevaluated(unevaluated) = c.val {
819 match select.infcx().const_eval_resolve(
820 obligation.param_env,
822 Some(obligation.cause.span),
824 Ok(val) => Ok(ty::Const::from_value(select.tcx(), val, c.ty)),
825 Err(err) => Err(err),
832 match (evaluate(c1), evaluate(c2)) {
833 (Ok(c1), Ok(c2)) => {
836 .at(&obligation.cause, obligation.param_env)
840 Err(_) => return false,
846 // There's not really much we can do with these predicates -
847 // we start out with a `ParamEnv` with no inference variables,
848 // and these don't correspond to adding any new bounds to
850 ty::PredicateKind::WellFormed(..)
851 | ty::PredicateKind::ObjectSafe(..)
852 | ty::PredicateKind::ClosureKind(..)
853 | ty::PredicateKind::Subtype(..)
854 | ty::PredicateKind::ConstEvaluatable(..)
855 | ty::PredicateKind::Coerce(..)
856 | ty::PredicateKind::OpaqueType(..)
857 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
865 infcx: &InferCtxt<'_, 'tcx>,
866 p: ty::Predicate<'tcx>,
867 ) -> ty::Predicate<'tcx> {
872 // Replaces all ReVars in a type with ty::Region's, using the provided map
873 pub struct RegionReplacer<'a, 'tcx> {
874 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
878 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
879 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
883 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
885 ty::ReVar(vid) => self.vid_to_region.get(vid).cloned(),
888 .unwrap_or_else(|| r.super_fold_with(self))