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 crate::traits::project::ProjectAndUnifyResult;
9 use rustc_middle::ty::fold::TypeFolder;
10 use rustc_middle::ty::{Region, RegionVid, Term};
12 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
14 use std::collections::hash_map::Entry;
15 use std::collections::VecDeque;
18 // FIXME(twk): this is obviously not nice to duplicate like that
19 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
20 pub enum RegionTarget<'tcx> {
25 #[derive(Default, Debug, Clone)]
26 pub struct RegionDeps<'tcx> {
27 larger: FxHashSet<RegionTarget<'tcx>>,
28 smaller: FxHashSet<RegionTarget<'tcx>>,
31 pub enum AutoTraitResult<A> {
38 impl<A> AutoTraitResult<A> {
39 fn is_auto(&self) -> bool {
40 matches!(self, AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl)
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 mut auto_trait_callback: impl FnMut(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::dummy(trait_ref);
90 let bail_out = tcx.infer_ctxt().enter(|infcx| {
91 let mut selcx = SelectionContext::new(&infcx);
92 let result = selcx.select(&Obligation::new(
93 ObligationCause::dummy(),
95 trait_pred.to_poly_trait_predicate(),
99 Ok(Some(ImplSource::UserDefined(_))) => {
101 "find_auto_trait_generics({:?}): \
102 manual impl found, bailing out",
110 let result = selcx.select(&Obligation::new(
111 ObligationCause::dummy(),
113 trait_pred.to_poly_trait_predicate_negative_polarity(),
117 Ok(Some(ImplSource::UserDefined(_))) => {
119 "find_auto_trait_generics({:?}): \
120 manual impl found, bailing out",
129 // If an explicit impl exists, it always takes priority over an auto impl
131 return AutoTraitResult::ExplicitImpl;
134 tcx.infer_ctxt().enter(|infcx| {
135 let mut fresh_preds = FxHashSet::default();
137 // Due to the way projections are handled by SelectionContext, we need to run
138 // evaluate_predicates twice: once on the original param env, and once on the result of
139 // the first evaluate_predicates call.
141 // The problem is this: most of rustc, including SelectionContext and traits::project,
142 // are designed to work with a concrete usage of a type (e.g., Vec<u8>
143 // fn<T>() { Vec<T> }. This information will generally never change - given
144 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
145 // If we're unable to prove that 'T' implements a particular trait, we're done -
146 // there's nothing left to do but error out.
148 // However, synthesizing an auto trait impl works differently. Here, we start out with
149 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
150 // with - and progressively discover the conditions we need to fulfill for it to
151 // implement a certain auto trait. This ends up breaking two assumptions made by trait
152 // selection and projection:
154 // * We can always cache the result of a particular trait selection for the lifetime of
156 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
157 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
159 // We fix the first assumption by manually clearing out all of the InferCtxt's caches
160 // in between calls to SelectionContext.select. This allows us to keep all of the
161 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
162 // them between calls.
164 // We fix the second assumption by reprocessing the result of our first call to
165 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
166 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
167 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
168 // SelectionContext to return it back to us.
170 let Some((new_env, user_env)) = self.evaluate_predicates(
179 return AutoTraitResult::NegativeImpl;
182 let (full_env, full_user_env) = self
183 .evaluate_predicates(
193 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
197 "find_auto_trait_generics({:?}): fulfilling \
201 infcx.clear_caches();
203 // At this point, we already have all of the bounds we need. FulfillmentContext is used
204 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
205 // an additional sanity check.
206 let mut fulfill = FulfillmentContext::new();
207 fulfill.register_bound(&infcx, full_env, ty, trait_did, ObligationCause::dummy());
208 let errors = fulfill.select_all_or_error(&infcx);
210 if !errors.is_empty() {
211 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
214 let body_id_map: FxHashMap<_, _> = infcx
217 .region_obligations()
219 .map(|&(id, _)| (id, vec![]))
222 infcx.process_registered_region_obligations(&body_id_map, None, full_env);
224 let region_data = infcx
227 .unwrap_region_constraints()
228 .region_constraint_data()
231 let vid_to_region = self.map_vid_to_region(®ion_data);
233 let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
235 AutoTraitResult::PositiveImpl(auto_trait_callback(info))
240 impl<'tcx> AutoTraitFinder<'tcx> {
241 /// The core logic responsible for computing the bounds for our synthesized impl.
243 /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
244 /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
245 /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
246 /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
247 /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
249 /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
250 /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
251 /// user code. According, it considers all possible ways that a `Predicate` could be met, which
252 /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
253 /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
254 /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
255 /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
256 /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
259 /// impl<T> Send for Foo<T> where T: IntoIterator
261 /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
262 /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
264 /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
265 /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
266 /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
267 /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
270 /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
271 /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
272 /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
273 /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
274 /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
275 /// else `SelectionContext` will choke on the missing predicates. However, this should never
276 /// show up in the final synthesized generics: we don't want our generated docs page to contain
277 /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
278 /// separate `user_env`, which only holds the predicates that will actually be displayed to the
280 fn evaluate_predicates(
282 infcx: &InferCtxt<'_, 'tcx>,
285 param_env: ty::ParamEnv<'tcx>,
286 user_env: ty::ParamEnv<'tcx>,
287 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
288 only_projections: bool,
289 ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
292 // Don't try to process any nested obligations involving predicates
293 // that are already in the `ParamEnv` (modulo regions): we already
294 // know that they must hold.
295 for predicate in param_env.caller_bounds() {
296 fresh_preds.insert(self.clean_pred(infcx, predicate));
299 let mut select = SelectionContext::new(&infcx);
301 let mut already_visited = FxHashSet::default();
302 let mut predicates = VecDeque::new();
303 predicates.push_back(ty::Binder::dummy(ty::TraitPredicate {
304 trait_ref: ty::TraitRef {
306 substs: infcx.tcx.mk_substs_trait(ty, &[]),
308 constness: ty::BoundConstness::NotConst,
309 // Auto traits are positive
310 polarity: ty::ImplPolarity::Positive,
313 let computed_preds = param_env.caller_bounds().iter();
314 let mut user_computed_preds: FxHashSet<_> = user_env.caller_bounds().iter().collect();
316 let mut new_env = param_env;
317 let dummy_cause = ObligationCause::dummy();
319 while let Some(pred) = predicates.pop_front() {
320 infcx.clear_caches();
322 if !already_visited.insert(pred) {
326 // Call `infcx.resolve_vars_if_possible` to see if we can
327 // get rid of any inference variables.
329 infcx.resolve_vars_if_possible(Obligation::new(dummy_cause.clone(), new_env, pred));
330 let result = select.select(&obligation);
333 Ok(Some(ref impl_source)) => {
334 // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
335 // we immediately bail out, since it's impossible for us to continue.
337 if let ImplSource::UserDefined(ImplSourceUserDefinedData {
341 // Blame 'tidy' for the weird bracket placement.
342 if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
344 "evaluate_nested_obligations: found explicit negative impl\
352 let obligations = impl_source.clone().nested_obligations().into_iter();
354 if !self.evaluate_nested_obligations(
357 &mut user_computed_preds,
367 Err(SelectionError::Unimplemented) => {
368 if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
369 already_visited.remove(&pred);
370 self.add_user_pred(&mut user_computed_preds, pred.to_predicate(self.tcx));
371 predicates.push_back(pred);
374 "evaluate_nested_obligations: `Unimplemented` found, bailing: \
378 pred.skip_binder().trait_ref.substs
383 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
386 let normalized_preds = elaborate_predicates(
388 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
390 .map(|o| o.predicate);
391 new_env = ty::ParamEnv::new(
392 tcx.mk_predicates(normalized_preds),
394 param_env.constness(),
398 let final_user_env = ty::ParamEnv::new(
399 tcx.mk_predicates(user_computed_preds.into_iter()),
401 user_env.constness(),
404 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
406 ty, trait_did, new_env, final_user_env
409 Some((new_env, final_user_env))
412 /// This method is designed to work around the following issue:
413 /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
414 /// progressively building a `ParamEnv` based on the results we get.
415 /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
416 /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
418 /// This can lead to a corner case when dealing with region parameters.
419 /// During our selection loop in `evaluate_predicates`, we might end up with
420 /// two trait predicates that differ only in their region parameters:
421 /// one containing a HRTB lifetime parameter, and one containing a 'normal'
422 /// lifetime parameter. For example:
425 /// T as MyTrait<'static>
427 /// If we put both of these predicates in our computed `ParamEnv`, we'll
428 /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
430 /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
431 /// Our end goal is to generate a user-visible description of the conditions
432 /// under which a type implements an auto trait. A trait predicate involving
433 /// a HRTB means that the type needs to work with any choice of lifetime,
434 /// not just one specific lifetime (e.g., `'static`).
437 user_computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
438 new_pred: ty::Predicate<'tcx>,
440 let mut should_add_new = true;
441 user_computed_preds.retain(|&old_pred| {
442 if let (ty::PredicateKind::Trait(new_trait), ty::PredicateKind::Trait(old_trait)) =
443 (new_pred.kind().skip_binder(), old_pred.kind().skip_binder())
445 if new_trait.def_id() == old_trait.def_id() {
446 let new_substs = new_trait.trait_ref.substs;
447 let old_substs = old_trait.trait_ref.substs;
449 if !new_substs.types().eq(old_substs.types()) {
450 // We can't compare lifetimes if the types are different,
451 // so skip checking `old_pred`.
455 for (new_region, old_region) in
456 iter::zip(new_substs.regions(), old_substs.regions())
458 match (*new_region, *old_region) {
459 // If both predicates have an `ReLateBound` (a HRTB) in the
460 // same spot, we do nothing.
461 (ty::ReLateBound(_, _), ty::ReLateBound(_, _)) => {}
463 (ty::ReLateBound(_, _), _) | (_, ty::ReVar(_)) => {
464 // One of these is true:
465 // The new predicate has a HRTB in a spot where the old
466 // predicate does not (if they both had a HRTB, the previous
467 // match arm would have executed). A HRBT is a 'stricter'
468 // bound than anything else, so we want to keep the newer
469 // predicate (with the HRBT) in place of the old predicate.
473 // The old predicate has a region variable where the new
474 // predicate has some other kind of region. An region
475 // variable isn't something we can actually display to a user,
476 // so we choose their new predicate (which doesn't have a region
479 // In both cases, we want to remove the old predicate,
480 // from `user_computed_preds`, and replace it with the new
481 // one. Having both the old and the new
482 // predicate in a `ParamEnv` would confuse `SelectionContext`.
484 // We're currently in the predicate passed to 'retain',
485 // so we return `false` to remove the old predicate from
486 // `user_computed_preds`.
489 (_, ty::ReLateBound(_, _)) | (ty::ReVar(_), _) => {
490 // This is the opposite situation as the previous arm.
491 // One of these is true:
493 // The old predicate has a HRTB lifetime in a place where the
494 // new predicate does not.
498 // The new predicate has a region variable where the old
499 // predicate has some other type of region.
501 // We want to leave the old
502 // predicate in `user_computed_preds`, and skip adding
503 // new_pred to `user_computed_params`.
504 should_add_new = false
515 user_computed_preds.insert(new_pred);
519 /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
520 /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
521 fn map_vid_to_region<'cx>(
523 regions: &RegionConstraintData<'cx>,
524 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
525 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
526 let mut finished_map = FxHashMap::default();
528 for constraint in regions.constraints.keys() {
530 &Constraint::VarSubVar(r1, r2) => {
532 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
533 deps1.larger.insert(RegionTarget::RegionVid(r2));
536 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
537 deps2.smaller.insert(RegionTarget::RegionVid(r1));
539 &Constraint::RegSubVar(region, vid) => {
541 let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
542 deps1.larger.insert(RegionTarget::RegionVid(vid));
545 let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
546 deps2.smaller.insert(RegionTarget::Region(region));
548 &Constraint::VarSubReg(vid, region) => {
549 finished_map.insert(vid, region);
551 &Constraint::RegSubReg(r1, r2) => {
553 let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
554 deps1.larger.insert(RegionTarget::Region(r2));
557 let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
558 deps2.smaller.insert(RegionTarget::Region(r1));
563 while !vid_map.is_empty() {
564 let target = *vid_map.keys().next().expect("Keys somehow empty");
565 let deps = vid_map.remove(&target).expect("Entry somehow missing");
567 for smaller in deps.smaller.iter() {
568 for larger in deps.larger.iter() {
569 match (smaller, larger) {
570 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
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);
583 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
584 finished_map.insert(v1, r1);
586 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
587 // Do nothing; we don't care about regions that are smaller than vids.
589 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
590 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
591 let smaller_deps = v.into_mut();
592 smaller_deps.larger.insert(*larger);
593 smaller_deps.larger.remove(&target);
596 if let Entry::Occupied(v) = vid_map.entry(*larger) {
597 let larger_deps = v.into_mut();
598 larger_deps.smaller.insert(*smaller);
599 larger_deps.smaller.remove(&target);
609 fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
610 self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
613 pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
615 ty::Param(_) => true,
616 ty::Projection(p) => self.is_of_param(p.self_ty()),
621 fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
622 if let Term::Ty(ty) = p.term().skip_binder() {
623 matches!(ty.kind(), ty::Projection(proj) if proj == &p.skip_binder().projection_ty)
629 fn evaluate_nested_obligations(
632 nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
633 computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
634 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
635 predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
636 select: &mut SelectionContext<'_, 'tcx>,
637 only_projections: bool,
639 let dummy_cause = ObligationCause::dummy();
641 for obligation in nested {
643 fresh_preds.insert(self.clean_pred(select.infcx(), obligation.predicate));
645 // Resolve any inference variables that we can, to help selection succeed
646 let predicate = select.infcx().resolve_vars_if_possible(obligation.predicate);
648 // We only add a predicate as a user-displayable bound if
649 // it involves a generic parameter, and doesn't contain
650 // any inference variables.
652 // Displaying a bound involving a concrete type (instead of a generic
653 // parameter) would be pointless, since it's always true
655 // Displaying an inference variable is impossible, since they're
656 // an internal compiler detail without a defined visual representation
658 // We check this by calling is_of_param on the relevant types
659 // from the various possible predicates
661 let bound_predicate = predicate.kind();
662 match bound_predicate.skip_binder() {
663 ty::PredicateKind::Trait(p) => {
664 // Add this to `predicates` so that we end up calling `select`
665 // with it. If this predicate ends up being unimplemented,
666 // then `evaluate_predicates` will handle adding it the `ParamEnv`
668 predicates.push_back(bound_predicate.rebind(p));
670 ty::PredicateKind::Projection(p) => {
671 let p = bound_predicate.rebind(p);
673 "evaluate_nested_obligations: examining projection predicate {:?}",
677 // As described above, we only want to display
678 // bounds which include a generic parameter but don't include
679 // an inference variable.
680 // Additionally, we check if we've seen this predicate before,
681 // to avoid rendering duplicate bounds to the user.
682 if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
683 && !p.term().skip_binder().has_infer_types()
687 "evaluate_nested_obligations: adding projection predicate \
688 to computed_preds: {:?}",
692 // Under unusual circumstances, we can end up with a self-referential
693 // projection predicate. For example:
694 // <T as MyType>::Value == <T as MyType>::Value
695 // Not only is displaying this to the user pointless,
696 // having it in the ParamEnv will cause an issue if we try to call
697 // poly_project_and_unify_type on the predicate, since this kind of
698 // predicate will normally never end up in a ParamEnv.
700 // For these reasons, we ignore these weird predicates,
701 // ensuring that we're able to properly synthesize an auto trait impl
702 if self.is_self_referential_projection(p) {
704 "evaluate_nested_obligations: encountered a projection
705 predicate equating a type with itself! Skipping"
708 self.add_user_pred(computed_preds, predicate);
712 // There are three possible cases when we project a predicate:
714 // 1. We encounter an error. This means that it's impossible for
715 // our current type to implement the auto trait - there's bound
716 // that we could add to our ParamEnv that would 'fix' this kind
717 // of error, as it's not caused by an unimplemented type.
719 // 2. We successfully project the predicate (Ok(Some(_))), generating
720 // some subobligations. We then process these subobligations
721 // like any other generated sub-obligations.
723 // 3. We receive an 'ambiguous' result (Ok(None))
724 // If we were actually trying to compile a crate,
725 // we would need to re-process this obligation later.
726 // However, all we care about is finding out what bounds
727 // are needed for our type to implement a particular auto trait.
728 // We've already added this obligation to our computed ParamEnv
729 // above (if it was necessary). Therefore, we don't need
730 // to do any further processing of the obligation.
732 // Note that we *must* try to project *all* projection predicates
733 // we encounter, even ones without inference variable.
734 // This ensures that we detect any projection errors,
735 // which indicate that our type can *never* implement the given
736 // auto trait. In that case, we will generate an explicit negative
737 // impl (e.g. 'impl !Send for MyType'). However, we don't
738 // try to process any of the generated subobligations -
739 // they contain no new information, since we already know
740 // that our type implements the projected-through trait,
741 // and can lead to weird region issues.
743 // Normally, we'll generate a negative impl as a result of encountering
744 // a type with an explicit negative impl of an auto trait
745 // (for example, raw pointers have !Send and !Sync impls)
746 // However, through some **interesting** manipulations of the type
747 // system, it's actually possible to write a type that never
748 // implements an auto trait due to a projection error, not a normal
749 // negative impl error. To properly handle this case, we need
750 // to ensure that we catch any potential projection errors,
751 // and turn them into an explicit negative impl for our type.
752 debug!("Projecting and unifying projection predicate {:?}", predicate);
754 match project::poly_project_and_unify_type(select, &obligation.with(p)) {
755 ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
757 "evaluate_nested_obligations: Unable to unify predicate \
758 '{:?}' '{:?}', bailing out",
763 ProjectAndUnifyResult::Recursive => {
764 debug!("evaluate_nested_obligations: recursive projection predicate");
767 ProjectAndUnifyResult::Holds(v) => {
768 // We only care about sub-obligations
769 // when we started out trying to unify
770 // some inference variables. See the comment above
771 // for more information
772 if p.term().skip_binder().has_infer_types() {
773 if !self.evaluate_nested_obligations(
786 ProjectAndUnifyResult::FailedNormalization => {
787 // It's ok not to make progress when have no inference variables -
788 // in that case, we were only performing unification to check if an
789 // error occurred (which would indicate that it's impossible for our
790 // type to implement the auto trait).
791 // However, we should always make progress (either by generating
792 // subobligations or getting an error) when we started off with
793 // inference variables
794 if p.term().skip_binder().has_infer_types() {
795 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
800 ty::PredicateKind::RegionOutlives(binder) => {
801 let binder = bound_predicate.rebind(binder);
802 if select.infcx().region_outlives_predicate(&dummy_cause, binder).is_err() {
806 ty::PredicateKind::TypeOutlives(binder) => {
807 let binder = bound_predicate.rebind(binder);
809 binder.no_bound_vars(),
810 binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
812 (None, Some(t_a)) => {
813 select.infcx().register_region_obligation_with_cause(
815 select.infcx().tcx.lifetimes.re_static,
819 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
820 select.infcx().register_region_obligation_with_cause(
829 ty::PredicateKind::ConstEquate(c1, c2) => {
830 let evaluate = |c: ty::Const<'tcx>| {
831 if let ty::ConstKind::Unevaluated(unevaluated) = c.val() {
832 match select.infcx().const_eval_resolve(
833 obligation.param_env,
835 Some(obligation.cause.span),
837 Ok(val) => Ok(ty::Const::from_value(select.tcx(), val, c.ty())),
838 Err(err) => Err(err),
845 match (evaluate(c1), evaluate(c2)) {
846 (Ok(c1), Ok(c2)) => {
849 .at(&obligation.cause, obligation.param_env)
853 Err(_) => return false,
859 // There's not really much we can do with these predicates -
860 // we start out with a `ParamEnv` with no inference variables,
861 // and these don't correspond to adding any new bounds to
863 ty::PredicateKind::WellFormed(..)
864 | ty::PredicateKind::ObjectSafe(..)
865 | ty::PredicateKind::ClosureKind(..)
866 | ty::PredicateKind::Subtype(..)
867 | ty::PredicateKind::ConstEvaluatable(..)
868 | ty::PredicateKind::Coerce(..)
869 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
877 infcx: &InferCtxt<'_, 'tcx>,
878 p: ty::Predicate<'tcx>,
879 ) -> ty::Predicate<'tcx> {
884 // Replaces all ReVars in a type with ty::Region's, using the provided map
885 pub struct RegionReplacer<'a, 'tcx> {
886 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
890 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
891 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
895 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
897 ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
900 .unwrap_or_else(|| r.super_fold_with(self))