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::errors::UnableToConstructConstantValue;
7 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
8 use crate::infer::InferCtxt;
9 use crate::traits::project::ProjectAndUnifyResult;
10 use rustc_middle::mir::interpret::ErrorHandled;
11 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
12 use rustc_middle::ty::visit::TypeVisitable;
13 use rustc_middle::ty::{Region, RegionVid};
15 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
17 use std::collections::hash_map::Entry;
18 use std::collections::VecDeque;
21 // FIXME(twk): this is obviously not nice to duplicate like that
22 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
23 pub enum RegionTarget<'tcx> {
28 #[derive(Default, Debug, Clone)]
29 pub struct RegionDeps<'tcx> {
30 larger: FxHashSet<RegionTarget<'tcx>>,
31 smaller: FxHashSet<RegionTarget<'tcx>>,
34 pub enum AutoTraitResult<A> {
41 impl<A> AutoTraitResult<A> {
42 fn is_auto(&self) -> bool {
43 matches!(self, AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl)
47 pub struct AutoTraitInfo<'cx> {
48 pub full_user_env: ty::ParamEnv<'cx>,
49 pub region_data: RegionConstraintData<'cx>,
50 pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>,
53 pub struct AutoTraitFinder<'tcx> {
57 impl<'tcx> AutoTraitFinder<'tcx> {
58 pub fn new(tcx: TyCtxt<'tcx>) -> Self {
59 AutoTraitFinder { tcx }
62 /// Makes a best effort to determine whether and under which conditions an auto trait is
63 /// implemented for a type. For example, if you have
66 /// struct Foo<T> { data: Box<T> }
69 /// then this might return that Foo<T>: Send if T: Send (encoded in the AutoTraitResult type).
70 /// The analysis attempts to account for custom impls as well as other complex cases. This
71 /// result is intended for use by rustdoc and other such consumers.
73 /// (Note that due to the coinductive nature of Send, the full and correct result is actually
74 /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
75 /// types are all Send. So, in our example, we might have that Foo<T>: Send if Box<T>: Send.
76 /// But this is often not the best way to present to the user.)
78 /// Warning: The API should be considered highly unstable, and it may be refactored or removed
80 pub fn find_auto_trait_generics<A>(
83 orig_env: ty::ParamEnv<'tcx>,
85 mut auto_trait_callback: impl FnMut(AutoTraitInfo<'tcx>) -> A,
86 ) -> AutoTraitResult<A> {
89 let trait_ref = ty::TraitRef { def_id: trait_did, substs: tcx.mk_substs_trait(ty, &[]) };
91 let trait_pred = ty::Binder::dummy(trait_ref);
93 let bail_out = tcx.infer_ctxt().enter(|infcx| {
94 let mut selcx = SelectionContext::new(&infcx);
95 let result = selcx.select(&Obligation::new(
96 ObligationCause::dummy(),
98 trait_pred.to_poly_trait_predicate(),
102 Ok(Some(ImplSource::UserDefined(_))) => {
104 "find_auto_trait_generics({:?}): \
105 manual impl found, bailing out",
113 let result = selcx.select(&Obligation::new(
114 ObligationCause::dummy(),
116 trait_pred.to_poly_trait_predicate_negative_polarity(),
120 Ok(Some(ImplSource::UserDefined(_))) => {
122 "find_auto_trait_generics({:?}): \
123 manual impl found, bailing out",
132 // If an explicit impl exists, it always takes priority over an auto impl
134 return AutoTraitResult::ExplicitImpl;
137 tcx.infer_ctxt().enter(|infcx| {
138 let mut fresh_preds = FxHashSet::default();
140 // Due to the way projections are handled by SelectionContext, we need to run
141 // evaluate_predicates twice: once on the original param env, and once on the result of
142 // the first evaluate_predicates call.
144 // The problem is this: most of rustc, including SelectionContext and traits::project,
145 // are designed to work with a concrete usage of a type (e.g., Vec<u8>
146 // fn<T>() { Vec<T> }. This information will generally never change - given
147 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
148 // If we're unable to prove that 'T' implements a particular trait, we're done -
149 // there's nothing left to do but error out.
151 // However, synthesizing an auto trait impl works differently. Here, we start out with
152 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
153 // with - and progressively discover the conditions we need to fulfill for it to
154 // implement a certain auto trait. This ends up breaking two assumptions made by trait
155 // selection and projection:
157 // * We can always cache the result of a particular trait selection for the lifetime of
159 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
160 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
162 // We fix the first assumption by manually clearing out all of the InferCtxt's caches
163 // in between calls to SelectionContext.select. This allows us to keep all of the
164 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
165 // them between calls.
167 // We fix the second assumption by reprocessing the result of our first call to
168 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
169 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
170 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
171 // SelectionContext to return it back to us.
173 let Some((new_env, user_env)) = self.evaluate_predicates(
182 return AutoTraitResult::NegativeImpl;
185 let (full_env, full_user_env) = self
186 .evaluate_predicates(
196 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
200 "find_auto_trait_generics({:?}): fulfilling \
204 infcx.clear_caches();
206 // At this point, we already have all of the bounds we need. FulfillmentContext is used
207 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
208 // an additional sanity check.
210 super::fully_solve_bound(&infcx, ObligationCause::dummy(), full_env, ty, trait_did);
211 if !errors.is_empty() {
212 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
215 infcx.process_registered_region_obligations(&Default::default(), full_env);
217 let region_data = infcx
220 .unwrap_region_constraints()
221 .region_constraint_data()
224 let vid_to_region = self.map_vid_to_region(®ion_data);
226 let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
228 AutoTraitResult::PositiveImpl(auto_trait_callback(info))
233 impl<'tcx> AutoTraitFinder<'tcx> {
234 /// The core logic responsible for computing the bounds for our synthesized impl.
236 /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
237 /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
238 /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
239 /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
240 /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
242 /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
243 /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
244 /// user code. According, it considers all possible ways that a `Predicate` could be met, which
245 /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
246 /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
247 /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
248 /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
249 /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
251 /// ```ignore (illustrative)
252 /// impl<T> Send for Foo<T> where T: IntoIterator
254 /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
255 /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
257 /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
258 /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
259 /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
260 /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
263 /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
264 /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
265 /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
266 /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
267 /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
268 /// else `SelectionContext` will choke on the missing predicates. However, this should never
269 /// show up in the final synthesized generics: we don't want our generated docs page to contain
270 /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
271 /// separate `user_env`, which only holds the predicates that will actually be displayed to the
273 fn evaluate_predicates(
275 infcx: &InferCtxt<'_, 'tcx>,
278 param_env: ty::ParamEnv<'tcx>,
279 user_env: ty::ParamEnv<'tcx>,
280 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
281 only_projections: bool,
282 ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
285 // Don't try to process any nested obligations involving predicates
286 // that are already in the `ParamEnv` (modulo regions): we already
287 // know that they must hold.
288 for predicate in param_env.caller_bounds() {
289 fresh_preds.insert(self.clean_pred(infcx, predicate));
292 let mut select = SelectionContext::new(&infcx);
294 let mut already_visited = FxHashSet::default();
295 let mut predicates = VecDeque::new();
296 predicates.push_back(ty::Binder::dummy(ty::TraitPredicate {
297 trait_ref: ty::TraitRef {
299 substs: infcx.tcx.mk_substs_trait(ty, &[]),
301 constness: ty::BoundConstness::NotConst,
302 // Auto traits are positive
303 polarity: ty::ImplPolarity::Positive,
306 let computed_preds = param_env.caller_bounds().iter();
307 let mut user_computed_preds: FxHashSet<_> = user_env.caller_bounds().iter().collect();
309 let mut new_env = param_env;
310 let dummy_cause = ObligationCause::dummy();
312 while let Some(pred) = predicates.pop_front() {
313 infcx.clear_caches();
315 if !already_visited.insert(pred) {
319 // Call `infcx.resolve_vars_if_possible` to see if we can
320 // get rid of any inference variables.
322 infcx.resolve_vars_if_possible(Obligation::new(dummy_cause.clone(), new_env, pred));
323 let result = select.select(&obligation);
326 Ok(Some(ref impl_source)) => {
327 // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
328 // we immediately bail out, since it's impossible for us to continue.
330 if let ImplSource::UserDefined(ImplSourceUserDefinedData {
334 // Blame 'tidy' for the weird bracket placement.
335 if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
337 "evaluate_nested_obligations: found explicit negative impl\
345 let obligations = impl_source.borrow_nested_obligations().iter().cloned();
347 if !self.evaluate_nested_obligations(
350 &mut user_computed_preds,
360 Err(SelectionError::Unimplemented) => {
361 if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
362 already_visited.remove(&pred);
363 self.add_user_pred(&mut user_computed_preds, pred.to_predicate(self.tcx));
364 predicates.push_back(pred);
367 "evaluate_nested_obligations: `Unimplemented` found, bailing: \
371 pred.skip_binder().trait_ref.substs
376 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
379 let normalized_preds = elaborate_predicates(
381 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
383 .map(|o| o.predicate);
384 new_env = ty::ParamEnv::new(
385 tcx.mk_predicates(normalized_preds),
387 param_env.constness(),
391 let final_user_env = ty::ParamEnv::new(
392 tcx.mk_predicates(user_computed_preds.into_iter()),
394 user_env.constness(),
397 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
399 ty, trait_did, new_env, final_user_env
402 Some((new_env, final_user_env))
405 /// This method is designed to work around the following issue:
406 /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
407 /// progressively building a `ParamEnv` based on the results we get.
408 /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
409 /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
411 /// This can lead to a corner case when dealing with region parameters.
412 /// During our selection loop in `evaluate_predicates`, we might end up with
413 /// two trait predicates that differ only in their region parameters:
414 /// one containing a HRTB lifetime parameter, and one containing a 'normal'
415 /// lifetime parameter. For example:
416 /// ```ignore (illustrative)
418 /// T as MyTrait<'static>
420 /// If we put both of these predicates in our computed `ParamEnv`, we'll
421 /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
423 /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
424 /// Our end goal is to generate a user-visible description of the conditions
425 /// under which a type implements an auto trait. A trait predicate involving
426 /// a HRTB means that the type needs to work with any choice of lifetime,
427 /// not just one specific lifetime (e.g., `'static`).
430 user_computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
431 new_pred: ty::Predicate<'tcx>,
433 let mut should_add_new = true;
434 user_computed_preds.retain(|&old_pred| {
435 if let (ty::PredicateKind::Trait(new_trait), ty::PredicateKind::Trait(old_trait)) =
436 (new_pred.kind().skip_binder(), old_pred.kind().skip_binder())
438 if new_trait.def_id() == old_trait.def_id() {
439 let new_substs = new_trait.trait_ref.substs;
440 let old_substs = old_trait.trait_ref.substs;
442 if !new_substs.types().eq(old_substs.types()) {
443 // We can't compare lifetimes if the types are different,
444 // so skip checking `old_pred`.
448 for (new_region, old_region) in
449 iter::zip(new_substs.regions(), old_substs.regions())
451 match (*new_region, *old_region) {
452 // If both predicates have an `ReLateBound` (a HRTB) in the
453 // same spot, we do nothing.
454 (ty::ReLateBound(_, _), ty::ReLateBound(_, _)) => {}
456 (ty::ReLateBound(_, _), _) | (_, ty::ReVar(_)) => {
457 // One of these is true:
458 // The new predicate has a HRTB in a spot where the old
459 // predicate does not (if they both had a HRTB, the previous
460 // match arm would have executed). A HRBT is a 'stricter'
461 // bound than anything else, so we want to keep the newer
462 // predicate (with the HRBT) in place of the old predicate.
466 // The old predicate has a region variable where the new
467 // predicate has some other kind of region. An region
468 // variable isn't something we can actually display to a user,
469 // so we choose their new predicate (which doesn't have a region
472 // In both cases, we want to remove the old predicate,
473 // from `user_computed_preds`, and replace it with the new
474 // one. Having both the old and the new
475 // predicate in a `ParamEnv` would confuse `SelectionContext`.
477 // We're currently in the predicate passed to 'retain',
478 // so we return `false` to remove the old predicate from
479 // `user_computed_preds`.
482 (_, ty::ReLateBound(_, _)) | (ty::ReVar(_), _) => {
483 // This is the opposite situation as the previous arm.
484 // One of these is true:
486 // The old predicate has a HRTB lifetime in a place where the
487 // new predicate does not.
491 // The new predicate has a region variable where the old
492 // predicate has some other type of region.
494 // We want to leave the old
495 // predicate in `user_computed_preds`, and skip adding
496 // new_pred to `user_computed_params`.
497 should_add_new = false
508 user_computed_preds.insert(new_pred);
512 /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
513 /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
514 fn map_vid_to_region<'cx>(
516 regions: &RegionConstraintData<'cx>,
517 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
518 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
519 let mut finished_map = FxHashMap::default();
521 for constraint in regions.constraints.keys() {
523 &Constraint::VarSubVar(r1, r2) => {
525 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
526 deps1.larger.insert(RegionTarget::RegionVid(r2));
529 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
530 deps2.smaller.insert(RegionTarget::RegionVid(r1));
532 &Constraint::RegSubVar(region, vid) => {
534 let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
535 deps1.larger.insert(RegionTarget::RegionVid(vid));
538 let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
539 deps2.smaller.insert(RegionTarget::Region(region));
541 &Constraint::VarSubReg(vid, region) => {
542 finished_map.insert(vid, region);
544 &Constraint::RegSubReg(r1, r2) => {
546 let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
547 deps1.larger.insert(RegionTarget::Region(r2));
550 let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
551 deps2.smaller.insert(RegionTarget::Region(r1));
556 while !vid_map.is_empty() {
557 let target = *vid_map.keys().next().expect("Keys somehow empty");
558 let deps = vid_map.remove(&target).expect("Entry somehow missing");
560 for smaller in deps.smaller.iter() {
561 for larger in deps.larger.iter() {
562 match (smaller, larger) {
563 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
564 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
565 let smaller_deps = v.into_mut();
566 smaller_deps.larger.insert(*larger);
567 smaller_deps.larger.remove(&target);
570 if let Entry::Occupied(v) = vid_map.entry(*larger) {
571 let larger_deps = v.into_mut();
572 larger_deps.smaller.insert(*smaller);
573 larger_deps.smaller.remove(&target);
576 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
577 finished_map.insert(v1, r1);
579 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
580 // Do nothing; we don't care about regions that are smaller than vids.
582 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
583 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
584 let smaller_deps = v.into_mut();
585 smaller_deps.larger.insert(*larger);
586 smaller_deps.larger.remove(&target);
589 if let Entry::Occupied(v) = vid_map.entry(*larger) {
590 let larger_deps = v.into_mut();
591 larger_deps.smaller.insert(*smaller);
592 larger_deps.smaller.remove(&target);
602 fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
603 self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
606 pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
608 ty::Param(_) => true,
609 ty::Projection(p) => self.is_of_param(p.self_ty()),
614 fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
615 if let Some(ty) = p.term().skip_binder().ty() {
616 matches!(ty.kind(), ty::Projection(proj) if proj == &p.skip_binder().projection_ty)
622 fn evaluate_nested_obligations(
625 nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
626 computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
627 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
628 predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
629 select: &mut SelectionContext<'_, 'tcx>,
630 only_projections: bool,
632 let dummy_cause = ObligationCause::dummy();
634 for obligation in nested {
636 fresh_preds.insert(self.clean_pred(select.infcx(), obligation.predicate));
638 // Resolve any inference variables that we can, to help selection succeed
639 let predicate = select.infcx().resolve_vars_if_possible(obligation.predicate);
641 // We only add a predicate as a user-displayable bound if
642 // it involves a generic parameter, and doesn't contain
643 // any inference variables.
645 // Displaying a bound involving a concrete type (instead of a generic
646 // parameter) would be pointless, since it's always true
648 // Displaying an inference variable is impossible, since they're
649 // an internal compiler detail without a defined visual representation
651 // We check this by calling is_of_param on the relevant types
652 // from the various possible predicates
654 let bound_predicate = predicate.kind();
655 match bound_predicate.skip_binder() {
656 ty::PredicateKind::Trait(p) => {
657 // Add this to `predicates` so that we end up calling `select`
658 // with it. If this predicate ends up being unimplemented,
659 // then `evaluate_predicates` will handle adding it the `ParamEnv`
661 predicates.push_back(bound_predicate.rebind(p));
663 ty::PredicateKind::Projection(p) => {
664 let p = bound_predicate.rebind(p);
666 "evaluate_nested_obligations: examining projection predicate {:?}",
670 // As described above, we only want to display
671 // bounds which include a generic parameter but don't include
672 // an inference variable.
673 // Additionally, we check if we've seen this predicate before,
674 // to avoid rendering duplicate bounds to the user.
675 if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
676 && !p.term().skip_binder().has_infer_types()
680 "evaluate_nested_obligations: adding projection predicate \
681 to computed_preds: {:?}",
685 // Under unusual circumstances, we can end up with a self-referential
686 // projection predicate. For example:
687 // <T as MyType>::Value == <T as MyType>::Value
688 // Not only is displaying this to the user pointless,
689 // having it in the ParamEnv will cause an issue if we try to call
690 // poly_project_and_unify_type on the predicate, since this kind of
691 // predicate will normally never end up in a ParamEnv.
693 // For these reasons, we ignore these weird predicates,
694 // ensuring that we're able to properly synthesize an auto trait impl
695 if self.is_self_referential_projection(p) {
697 "evaluate_nested_obligations: encountered a projection
698 predicate equating a type with itself! Skipping"
701 self.add_user_pred(computed_preds, predicate);
705 // There are three possible cases when we project a predicate:
707 // 1. We encounter an error. This means that it's impossible for
708 // our current type to implement the auto trait - there's bound
709 // that we could add to our ParamEnv that would 'fix' this kind
710 // of error, as it's not caused by an unimplemented type.
712 // 2. We successfully project the predicate (Ok(Some(_))), generating
713 // some subobligations. We then process these subobligations
714 // like any other generated sub-obligations.
716 // 3. We receive an 'ambiguous' result (Ok(None))
717 // If we were actually trying to compile a crate,
718 // we would need to re-process this obligation later.
719 // However, all we care about is finding out what bounds
720 // are needed for our type to implement a particular auto trait.
721 // We've already added this obligation to our computed ParamEnv
722 // above (if it was necessary). Therefore, we don't need
723 // to do any further processing of the obligation.
725 // Note that we *must* try to project *all* projection predicates
726 // we encounter, even ones without inference variable.
727 // This ensures that we detect any projection errors,
728 // which indicate that our type can *never* implement the given
729 // auto trait. In that case, we will generate an explicit negative
730 // impl (e.g. 'impl !Send for MyType'). However, we don't
731 // try to process any of the generated subobligations -
732 // they contain no new information, since we already know
733 // that our type implements the projected-through trait,
734 // and can lead to weird region issues.
736 // Normally, we'll generate a negative impl as a result of encountering
737 // a type with an explicit negative impl of an auto trait
738 // (for example, raw pointers have !Send and !Sync impls)
739 // However, through some **interesting** manipulations of the type
740 // system, it's actually possible to write a type that never
741 // implements an auto trait due to a projection error, not a normal
742 // negative impl error. To properly handle this case, we need
743 // to ensure that we catch any potential projection errors,
744 // and turn them into an explicit negative impl for our type.
745 debug!("Projecting and unifying projection predicate {:?}", predicate);
747 match project::poly_project_and_unify_type(select, &obligation.with(p)) {
748 ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
750 "evaluate_nested_obligations: Unable to unify predicate \
751 '{:?}' '{:?}', bailing out",
756 ProjectAndUnifyResult::Recursive => {
757 debug!("evaluate_nested_obligations: recursive projection predicate");
760 ProjectAndUnifyResult::Holds(v) => {
761 // We only care about sub-obligations
762 // when we started out trying to unify
763 // some inference variables. See the comment above
764 // for more information
765 if p.term().skip_binder().has_infer_types() {
766 if !self.evaluate_nested_obligations(
779 ProjectAndUnifyResult::FailedNormalization => {
780 // It's ok not to make progress when have no inference variables -
781 // in that case, we were only performing unification to check if an
782 // error occurred (which would indicate that it's impossible for our
783 // type to implement the auto trait).
784 // However, we should always make progress (either by generating
785 // subobligations or getting an error) when we started off with
786 // inference variables
787 if p.term().skip_binder().has_infer_types() {
788 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
793 ty::PredicateKind::RegionOutlives(binder) => {
794 let binder = bound_predicate.rebind(binder);
795 select.infcx().region_outlives_predicate(&dummy_cause, binder)
797 ty::PredicateKind::TypeOutlives(binder) => {
798 let binder = bound_predicate.rebind(binder);
800 binder.no_bound_vars(),
801 binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
803 (None, Some(t_a)) => {
804 select.infcx().register_region_obligation_with_cause(
806 select.infcx().tcx.lifetimes.re_static,
810 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
811 select.infcx().register_region_obligation_with_cause(
820 ty::PredicateKind::ConstEquate(c1, c2) => {
821 let evaluate = |c: ty::Const<'tcx>| {
822 if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
823 match select.infcx().const_eval_resolve(
824 obligation.param_env,
826 Some(obligation.cause.span),
828 Ok(Some(valtree)) => {
829 Ok(ty::Const::from_value(select.tcx(), valtree, c.ty()))
833 let def_id = unevaluated.def.did;
835 tcx.sess.emit_err(UnableToConstructConstantValue {
836 span: tcx.def_span(def_id),
837 unevaluated: unevaluated,
839 Err(ErrorHandled::Reported(reported))
841 Err(err) => Err(err),
848 match (evaluate(c1), evaluate(c2)) {
849 (Ok(c1), Ok(c2)) => {
852 .at(&obligation.cause, obligation.param_env)
856 Err(_) => return false,
862 // There's not really much we can do with these predicates -
863 // we start out with a `ParamEnv` with no inference variables,
864 // and these don't correspond to adding any new bounds to
866 ty::PredicateKind::WellFormed(..)
867 | ty::PredicateKind::ObjectSafe(..)
868 | ty::PredicateKind::ClosureKind(..)
869 | ty::PredicateKind::Subtype(..)
870 | ty::PredicateKind::ConstEvaluatable(..)
871 | ty::PredicateKind::Coerce(..)
872 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
880 infcx: &InferCtxt<'_, 'tcx>,
881 p: ty::Predicate<'tcx>,
882 ) -> ty::Predicate<'tcx> {
887 // Replaces all ReVars in a type with ty::Region's, using the provided map
888 pub struct RegionReplacer<'a, 'tcx> {
889 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
893 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
894 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
898 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
900 ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
903 .unwrap_or_else(|| r.super_fold_with(self))