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 Some((new_env, user_env)) = self.evaluate_predicates(
160 return AutoTraitResult::NegativeImpl;
163 let (full_env, full_user_env) = self
164 .evaluate_predicates(
174 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
178 "find_auto_trait_generics({:?}): fulfilling \
182 infcx.clear_caches();
184 // At this point, we already have all of the bounds we need. FulfillmentContext is used
185 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
186 // an additional sanity check.
187 let mut fulfill = FulfillmentContext::new();
188 fulfill.register_bound(&infcx, full_env, ty, trait_did, ObligationCause::dummy());
189 let errors = fulfill.select_all_or_error(&infcx);
191 if !errors.is_empty() {
192 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
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(info))
221 impl<'tcx> 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 // Don't try to proess any nested obligations involving predicates
274 // that are already in the `ParamEnv` (modulo regions): we already
275 // know that they must hold.
276 for predicate in param_env.caller_bounds() {
277 fresh_preds.insert(self.clean_pred(infcx, predicate));
280 let mut select = SelectionContext::with_negative(&infcx, true);
282 let mut already_visited = FxHashSet::default();
283 let mut predicates = VecDeque::new();
284 predicates.push_back(ty::Binder::dummy(ty::TraitPredicate {
285 trait_ref: ty::TraitRef {
287 substs: infcx.tcx.mk_substs_trait(ty, &[]),
289 constness: ty::BoundConstness::NotConst,
290 // Auto traits are positive
291 polarity: ty::ImplPolarity::Positive,
294 let computed_preds = param_env.caller_bounds().iter();
295 let mut user_computed_preds: FxHashSet<_> = user_env.caller_bounds().iter().collect();
297 let mut new_env = param_env;
298 let dummy_cause = ObligationCause::dummy();
300 while let Some(pred) = predicates.pop_front() {
301 infcx.clear_caches();
303 if !already_visited.insert(pred) {
307 // Call `infcx.resolve_vars_if_possible` to see if we can
308 // get rid of any inference variables.
310 infcx.resolve_vars_if_possible(Obligation::new(dummy_cause.clone(), new_env, pred));
311 let result = select.select(&obligation);
314 Ok(Some(ref impl_source)) => {
315 // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
316 // we immediately bail out, since it's impossible for us to continue.
318 if let ImplSource::UserDefined(ImplSourceUserDefinedData {
322 // Blame 'tidy' for the weird bracket placement.
323 if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
325 "evaluate_nested_obligations: found explicit negative impl\
333 let obligations = impl_source.clone().nested_obligations().into_iter();
335 if !self.evaluate_nested_obligations(
338 &mut user_computed_preds,
348 Err(SelectionError::Unimplemented) => {
349 if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
350 already_visited.remove(&pred);
351 self.add_user_pred(&mut user_computed_preds, pred.to_predicate(self.tcx));
352 predicates.push_back(pred);
355 "evaluate_nested_obligations: `Unimplemented` found, bailing: \
359 pred.skip_binder().trait_ref.substs
364 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
367 let normalized_preds = elaborate_predicates(
369 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
371 .map(|o| o.predicate);
372 new_env = ty::ParamEnv::new(
373 tcx.mk_predicates(normalized_preds),
375 param_env.constness(),
379 let final_user_env = ty::ParamEnv::new(
380 tcx.mk_predicates(user_computed_preds.into_iter()),
382 user_env.constness(),
385 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
387 ty, trait_did, new_env, final_user_env
390 Some((new_env, final_user_env))
393 /// This method is designed to work around the following issue:
394 /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
395 /// progressively building a `ParamEnv` based on the results we get.
396 /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
397 /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
399 /// This can lead to a corner case when dealing with region parameters.
400 /// During our selection loop in `evaluate_predicates`, we might end up with
401 /// two trait predicates that differ only in their region parameters:
402 /// one containing a HRTB lifetime parameter, and one containing a 'normal'
403 /// lifetime parameter. For example:
406 /// T as MyTrait<'static>
408 /// If we put both of these predicates in our computed `ParamEnv`, we'll
409 /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
411 /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
412 /// Our end goal is to generate a user-visible description of the conditions
413 /// under which a type implements an auto trait. A trait predicate involving
414 /// a HRTB means that the type needs to work with any choice of lifetime,
415 /// not just one specific lifetime (e.g., `'static`).
418 user_computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
419 new_pred: ty::Predicate<'tcx>,
421 let mut should_add_new = true;
422 user_computed_preds.retain(|&old_pred| {
423 if let (ty::PredicateKind::Trait(new_trait), ty::PredicateKind::Trait(old_trait)) =
424 (new_pred.kind().skip_binder(), old_pred.kind().skip_binder())
426 if new_trait.def_id() == old_trait.def_id() {
427 let new_substs = new_trait.trait_ref.substs;
428 let old_substs = old_trait.trait_ref.substs;
430 if !new_substs.types().eq(old_substs.types()) {
431 // We can't compare lifetimes if the types are different,
432 // so skip checking `old_pred`.
436 for (new_region, old_region) in
437 iter::zip(new_substs.regions(), old_substs.regions())
439 match (*new_region, *old_region) {
440 // If both predicates have an `ReLateBound` (a HRTB) in the
441 // same spot, we do nothing.
442 (ty::ReLateBound(_, _), ty::ReLateBound(_, _)) => {}
444 (ty::ReLateBound(_, _), _) | (_, ty::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::ReLateBound(_, _)) | (ty::ReVar(_), _) => {
471 // This is the opposite situation as the previous arm.
472 // One of these is true:
474 // The old predicate has a HRTB lifetime in a place where the
475 // new predicate does not.
479 // The new predicate has a region variable where the old
480 // predicate has some other type of region.
482 // We want to leave the old
483 // predicate in `user_computed_preds`, and skip adding
484 // new_pred to `user_computed_params`.
485 should_add_new = false
496 user_computed_preds.insert(new_pred);
500 /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
501 /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
502 fn map_vid_to_region<'cx>(
504 regions: &RegionConstraintData<'cx>,
505 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
506 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
507 let mut finished_map = FxHashMap::default();
509 for constraint in regions.constraints.keys() {
511 &Constraint::VarSubVar(r1, r2) => {
513 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
514 deps1.larger.insert(RegionTarget::RegionVid(r2));
517 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
518 deps2.smaller.insert(RegionTarget::RegionVid(r1));
520 &Constraint::RegSubVar(region, vid) => {
522 let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
523 deps1.larger.insert(RegionTarget::RegionVid(vid));
526 let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
527 deps2.smaller.insert(RegionTarget::Region(region));
529 &Constraint::VarSubReg(vid, region) => {
530 finished_map.insert(vid, region);
532 &Constraint::RegSubReg(r1, r2) => {
534 let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
535 deps1.larger.insert(RegionTarget::Region(r2));
538 let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
539 deps2.smaller.insert(RegionTarget::Region(r1));
544 while !vid_map.is_empty() {
545 let target = *vid_map.keys().next().expect("Keys somehow empty");
546 let deps = vid_map.remove(&target).expect("Entry somehow missing");
548 for smaller in deps.smaller.iter() {
549 for larger in deps.larger.iter() {
550 match (smaller, larger) {
551 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
552 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
553 let smaller_deps = v.into_mut();
554 smaller_deps.larger.insert(*larger);
555 smaller_deps.larger.remove(&target);
558 if let Entry::Occupied(v) = vid_map.entry(*larger) {
559 let larger_deps = v.into_mut();
560 larger_deps.smaller.insert(*smaller);
561 larger_deps.smaller.remove(&target);
564 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
565 finished_map.insert(v1, r1);
567 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
568 // Do nothing; we don't care about regions that are smaller than vids.
570 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
571 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
572 let smaller_deps = v.into_mut();
573 smaller_deps.larger.insert(*larger);
574 smaller_deps.larger.remove(&target);
577 if let Entry::Occupied(v) = vid_map.entry(*larger) {
578 let larger_deps = v.into_mut();
579 larger_deps.smaller.insert(*smaller);
580 larger_deps.smaller.remove(&target);
590 fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
591 self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
594 pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
596 ty::Param(_) => true,
597 ty::Projection(p) => self.is_of_param(p.self_ty()),
602 fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
603 if let Term::Ty(ty) = p.term().skip_binder() {
604 matches!(ty.kind(), ty::Projection(proj) if proj == &p.skip_binder().projection_ty)
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
642 let bound_predicate = predicate.kind();
643 match bound_predicate.skip_binder() {
644 ty::PredicateKind::Trait(p) => {
645 // Add this to `predicates` so that we end up calling `select`
646 // with it. If this predicate ends up being unimplemented,
647 // then `evaluate_predicates` will handle adding it the `ParamEnv`
649 predicates.push_back(bound_predicate.rebind(p));
651 ty::PredicateKind::Projection(p) => {
652 let p = bound_predicate.rebind(p);
654 "evaluate_nested_obligations: examining projection predicate {:?}",
658 // As described above, we only want to display
659 // bounds which include a generic parameter but don't include
660 // an inference variable.
661 // Additionally, we check if we've seen this predicate before,
662 // to avoid rendering duplicate bounds to the user.
663 if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
664 && !p.term().skip_binder().has_infer_types()
668 "evaluate_nested_obligations: adding projection predicate\
669 to computed_preds: {:?}",
673 // Under unusual circumstances, we can end up with a self-refeential
674 // projection predicate. For example:
675 // <T as MyType>::Value == <T as MyType>::Value
676 // Not only is displaying this to the user pointless,
677 // having it in the ParamEnv will cause an issue if we try to call
678 // poly_project_and_unify_type on the predicate, since this kind of
679 // predicate will normally never end up in a ParamEnv.
681 // For these reasons, we ignore these weird predicates,
682 // ensuring that we're able to properly synthesize an auto trait impl
683 if self.is_self_referential_projection(p) {
685 "evaluate_nested_obligations: encountered a projection
686 predicate equating a type with itself! Skipping"
689 self.add_user_pred(computed_preds, predicate);
693 // There are three possible cases when we project a predicate:
695 // 1. We encounter an error. This means that it's impossible for
696 // our current type to implement the auto trait - there's bound
697 // that we could add to our ParamEnv that would 'fix' this kind
698 // of error, as it's not caused by an unimplemented type.
700 // 2. We successfully project the predicate (Ok(Some(_))), generating
701 // some subobligations. We then process these subobligations
702 // like any other generated sub-obligations.
704 // 3. We receive an 'ambiguous' result (Ok(None))
705 // If we were actually trying to compile a crate,
706 // we would need to re-process this obligation later.
707 // However, all we care about is finding out what bounds
708 // are needed for our type to implement a particular auto trait.
709 // We've already added this obligation to our computed ParamEnv
710 // above (if it was necessary). Therefore, we don't need
711 // to do any further processing of the obligation.
713 // Note that we *must* try to project *all* projection predicates
714 // we encounter, even ones without inference variable.
715 // This ensures that we detect any projection errors,
716 // which indicate that our type can *never* implement the given
717 // auto trait. In that case, we will generate an explicit negative
718 // impl (e.g. 'impl !Send for MyType'). However, we don't
719 // try to process any of the generated subobligations -
720 // they contain no new information, since we already know
721 // that our type implements the projected-through trait,
722 // and can lead to weird region issues.
724 // Normally, we'll generate a negative impl as a result of encountering
725 // a type with an explicit negative impl of an auto trait
726 // (for example, raw pointers have !Send and !Sync impls)
727 // However, through some **interesting** manipulations of the type
728 // system, it's actually possible to write a type that never
729 // implements an auto trait due to a projection error, not a normal
730 // negative impl error. To properly handle this case, we need
731 // to ensure that we catch any potential projection errors,
732 // and turn them into an explicit negative impl for our type.
733 debug!("Projecting and unifying projection predicate {:?}", predicate);
735 match project::poly_project_and_unify_type(select, &obligation.with(p)) {
738 "evaluate_nested_obligations: Unable to unify predicate \
739 '{:?}' '{:?}', bailing out",
744 Ok(Err(project::InProgress)) => {
745 debug!("evaluate_nested_obligations: recursive projection predicate");
749 // We only care about sub-obligations
750 // when we started out trying to unify
751 // some inference variables. See the comment above
752 // for more infomration
753 if p.term().skip_binder().has_infer_types() {
754 if !self.evaluate_nested_obligations(
768 // It's ok not to make progress when have no inference variables -
769 // in that case, we were only performing unifcation to check if an
770 // error occurred (which would indicate that it's impossible for our
771 // type to implement the auto trait).
772 // However, we should always make progress (either by generating
773 // subobligations or getting an error) when we started off with
774 // inference variables
775 if p.term().skip_binder().has_infer_types() {
776 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
781 ty::PredicateKind::RegionOutlives(binder) => {
782 let binder = bound_predicate.rebind(binder);
783 if select.infcx().region_outlives_predicate(&dummy_cause, binder).is_err() {
787 ty::PredicateKind::TypeOutlives(binder) => {
788 let binder = bound_predicate.rebind(binder);
790 binder.no_bound_vars(),
791 binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
793 (None, Some(t_a)) => {
794 select.infcx().register_region_obligation_with_cause(
796 select.infcx().tcx.lifetimes.re_static,
800 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
801 select.infcx().register_region_obligation_with_cause(
810 ty::PredicateKind::ConstEquate(c1, c2) => {
811 let evaluate = |c: ty::Const<'tcx>| {
812 if let ty::ConstKind::Unevaluated(unevaluated) = c.val() {
813 match select.infcx().const_eval_resolve(
814 obligation.param_env,
816 Some(obligation.cause.span),
818 Ok(val) => Ok(ty::Const::from_value(select.tcx(), val, c.ty())),
819 Err(err) => Err(err),
826 match (evaluate(c1), evaluate(c2)) {
827 (Ok(c1), Ok(c2)) => {
830 .at(&obligation.cause, obligation.param_env)
834 Err(_) => return false,
840 // There's not really much we can do with these predicates -
841 // we start out with a `ParamEnv` with no inference variables,
842 // and these don't correspond to adding any new bounds to
844 ty::PredicateKind::WellFormed(..)
845 | ty::PredicateKind::ObjectSafe(..)
846 | ty::PredicateKind::ClosureKind(..)
847 | ty::PredicateKind::Subtype(..)
848 | ty::PredicateKind::ConstEvaluatable(..)
849 | ty::PredicateKind::Coerce(..)
850 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
858 infcx: &InferCtxt<'_, 'tcx>,
859 p: ty::Predicate<'tcx>,
860 ) -> ty::Predicate<'tcx> {
865 // Replaces all ReVars in a type with ty::Region's, using the provided map
866 pub struct RegionReplacer<'a, 'tcx> {
867 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
871 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
872 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
876 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
878 ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
881 .unwrap_or_else(|| r.super_fold_with(self))