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::{ImplPolarity, Region, RegionVid};
15 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
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: FxIndexSet<RegionTarget<'tcx>>,
31 smaller: FxIndexSet<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
70 /// type). The analysis attempts to account for custom impls as well as other complex cases.
71 /// This 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 = tcx.mk_trait_ref(trait_did, [ty]);
91 let infcx = tcx.infer_ctxt().build();
92 let mut selcx = SelectionContext::new(&infcx);
93 for polarity in [true, false] {
94 let result = selcx.select(&Obligation::new(
96 ObligationCause::dummy(),
98 ty::Binder::dummy(ty::TraitPredicate {
100 constness: ty::BoundConstness::NotConst,
101 polarity: if polarity {
102 ImplPolarity::Positive
104 ImplPolarity::Negative
108 if let Ok(Some(ImplSource::UserDefined(_))) = result {
110 "find_auto_trait_generics({:?}): \
111 manual impl found, bailing out",
114 // If an explicit impl exists, it always takes priority over an auto impl
115 return AutoTraitResult::ExplicitImpl;
119 let infcx = tcx.infer_ctxt().build();
120 let mut fresh_preds = FxHashSet::default();
122 // Due to the way projections are handled by SelectionContext, we need to run
123 // evaluate_predicates twice: once on the original param env, and once on the result of
124 // the first evaluate_predicates call.
126 // The problem is this: most of rustc, including SelectionContext and traits::project,
127 // are designed to work with a concrete usage of a type (e.g., Vec<u8>
128 // fn<T>() { Vec<T> }. This information will generally never change - given
129 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
130 // If we're unable to prove that 'T' implements a particular trait, we're done -
131 // there's nothing left to do but error out.
133 // However, synthesizing an auto trait impl works differently. Here, we start out with
134 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
135 // with - and progressively discover the conditions we need to fulfill for it to
136 // implement a certain auto trait. This ends up breaking two assumptions made by trait
137 // selection and projection:
139 // * We can always cache the result of a particular trait selection for the lifetime of
141 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
142 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
144 // We fix the first assumption by manually clearing out all of the InferCtxt's caches
145 // in between calls to SelectionContext.select. This allows us to keep all of the
146 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
147 // them between calls.
149 // We fix the second assumption by reprocessing the result of our first call to
150 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
151 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
152 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
153 // SelectionContext to return it back to us.
155 let Some((new_env, user_env)) = self.evaluate_predicates(
164 return AutoTraitResult::NegativeImpl;
167 let (full_env, full_user_env) = self
168 .evaluate_predicates(&infcx, trait_did, ty, new_env, user_env, &mut fresh_preds, true)
170 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
174 "find_auto_trait_generics({:?}): fulfilling \
178 infcx.clear_caches();
180 // At this point, we already have all of the bounds we need. FulfillmentContext is used
181 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
182 // an additional sanity check.
184 super::fully_solve_bound(&infcx, ObligationCause::dummy(), full_env, ty, trait_did);
185 if !errors.is_empty() {
186 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
189 infcx.process_registered_region_obligations(&Default::default(), full_env);
192 infcx.inner.borrow_mut().unwrap_region_constraints().region_constraint_data().clone();
194 let vid_to_region = self.map_vid_to_region(®ion_data);
196 let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
198 AutoTraitResult::PositiveImpl(auto_trait_callback(info))
202 impl<'tcx> AutoTraitFinder<'tcx> {
203 /// The core logic responsible for computing the bounds for our synthesized impl.
205 /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
206 /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
207 /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
208 /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
209 /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
211 /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
212 /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
213 /// user code. According, it considers all possible ways that a `Predicate` could be met, which
214 /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
215 /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
216 /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
217 /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
218 /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
220 /// ```ignore (illustrative)
221 /// impl<T> Send for Foo<T> where T: IntoIterator
223 /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
224 /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
226 /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
227 /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
228 /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
229 /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
232 /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
233 /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
234 /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
235 /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
236 /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
237 /// else `SelectionContext` will choke on the missing predicates. However, this should never
238 /// show up in the final synthesized generics: we don't want our generated docs page to contain
239 /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
240 /// separate `user_env`, which only holds the predicates that will actually be displayed to the
242 fn evaluate_predicates(
244 infcx: &InferCtxt<'tcx>,
247 param_env: ty::ParamEnv<'tcx>,
248 user_env: ty::ParamEnv<'tcx>,
249 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
250 only_projections: bool,
251 ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
254 // Don't try to process any nested obligations involving predicates
255 // that are already in the `ParamEnv` (modulo regions): we already
256 // know that they must hold.
257 for predicate in param_env.caller_bounds() {
258 fresh_preds.insert(self.clean_pred(infcx, predicate));
261 let mut select = SelectionContext::new(&infcx);
263 let mut already_visited = FxHashSet::default();
264 let mut predicates = VecDeque::new();
265 predicates.push_back(ty::Binder::dummy(ty::TraitPredicate {
266 trait_ref: infcx.tcx.mk_trait_ref(trait_did, [ty]),
268 constness: ty::BoundConstness::NotConst,
269 // Auto traits are positive
270 polarity: ty::ImplPolarity::Positive,
273 let computed_preds = param_env.caller_bounds().iter();
274 let mut user_computed_preds: FxIndexSet<_> = user_env.caller_bounds().iter().collect();
276 let mut new_env = param_env;
277 let dummy_cause = ObligationCause::dummy();
279 while let Some(pred) = predicates.pop_front() {
280 infcx.clear_caches();
282 if !already_visited.insert(pred) {
286 // Call `infcx.resolve_vars_if_possible` to see if we can
287 // get rid of any inference variables.
288 let obligation = infcx.resolve_vars_if_possible(Obligation::new(
294 let result = select.select(&obligation);
297 Ok(Some(ref impl_source)) => {
298 // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
299 // we immediately bail out, since it's impossible for us to continue.
301 if let ImplSource::UserDefined(ImplSourceUserDefinedData {
305 // Blame 'tidy' for the weird bracket placement.
306 if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
308 "evaluate_nested_obligations: found explicit negative impl\
316 let obligations = impl_source.borrow_nested_obligations().iter().cloned();
318 if !self.evaluate_nested_obligations(
321 &mut user_computed_preds,
331 Err(SelectionError::Unimplemented) => {
332 if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
333 already_visited.remove(&pred);
334 self.add_user_pred(&mut user_computed_preds, pred.to_predicate(self.tcx));
335 predicates.push_back(pred);
338 "evaluate_nested_obligations: `Unimplemented` found, bailing: \
342 pred.skip_binder().trait_ref.substs
347 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
350 let normalized_preds = elaborate_predicates(
352 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
354 .map(|o| o.predicate);
355 new_env = ty::ParamEnv::new(
356 tcx.mk_predicates(normalized_preds),
358 param_env.constness(),
362 let final_user_env = ty::ParamEnv::new(
363 tcx.mk_predicates(user_computed_preds.into_iter()),
365 user_env.constness(),
368 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
370 ty, trait_did, new_env, final_user_env
373 Some((new_env, final_user_env))
376 /// This method is designed to work around the following issue:
377 /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
378 /// progressively building a `ParamEnv` based on the results we get.
379 /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
380 /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
382 /// This can lead to a corner case when dealing with region parameters.
383 /// During our selection loop in `evaluate_predicates`, we might end up with
384 /// two trait predicates that differ only in their region parameters:
385 /// one containing a HRTB lifetime parameter, and one containing a 'normal'
386 /// lifetime parameter. For example:
387 /// ```ignore (illustrative)
389 /// T as MyTrait<'static>
391 /// If we put both of these predicates in our computed `ParamEnv`, we'll
392 /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
394 /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
395 /// Our end goal is to generate a user-visible description of the conditions
396 /// under which a type implements an auto trait. A trait predicate involving
397 /// a HRTB means that the type needs to work with any choice of lifetime,
398 /// not just one specific lifetime (e.g., `'static`).
401 user_computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>,
402 new_pred: ty::Predicate<'tcx>,
404 let mut should_add_new = true;
405 user_computed_preds.retain(|&old_pred| {
407 ty::PredicateKind::Clause(ty::Clause::Trait(new_trait)),
408 ty::PredicateKind::Clause(ty::Clause::Trait(old_trait)),
409 ) = (new_pred.kind().skip_binder(), old_pred.kind().skip_binder())
411 if new_trait.def_id() == old_trait.def_id() {
412 let new_substs = new_trait.trait_ref.substs;
413 let old_substs = old_trait.trait_ref.substs;
415 if !new_substs.types().eq(old_substs.types()) {
416 // We can't compare lifetimes if the types are different,
417 // so skip checking `old_pred`.
421 for (new_region, old_region) in
422 iter::zip(new_substs.regions(), old_substs.regions())
424 match (*new_region, *old_region) {
425 // If both predicates have an `ReLateBound` (a HRTB) in the
426 // same spot, we do nothing.
427 (ty::ReLateBound(_, _), ty::ReLateBound(_, _)) => {}
429 (ty::ReLateBound(_, _), _) | (_, ty::ReVar(_)) => {
430 // One of these is true:
431 // The new predicate has a HRTB in a spot where the old
432 // predicate does not (if they both had a HRTB, the previous
433 // match arm would have executed). A HRBT is a 'stricter'
434 // bound than anything else, so we want to keep the newer
435 // predicate (with the HRBT) in place of the old predicate.
439 // The old predicate has a region variable where the new
440 // predicate has some other kind of region. An region
441 // variable isn't something we can actually display to a user,
442 // so we choose their new predicate (which doesn't have a region
445 // In both cases, we want to remove the old predicate,
446 // from `user_computed_preds`, and replace it with the new
447 // one. Having both the old and the new
448 // predicate in a `ParamEnv` would confuse `SelectionContext`.
450 // We're currently in the predicate passed to 'retain',
451 // so we return `false` to remove the old predicate from
452 // `user_computed_preds`.
455 (_, ty::ReLateBound(_, _)) | (ty::ReVar(_), _) => {
456 // This is the opposite situation as the previous arm.
457 // One of these is true:
459 // The old predicate has a HRTB lifetime in a place where the
460 // new predicate does not.
464 // The new predicate has a region variable where the old
465 // predicate has some other type of region.
467 // We want to leave the old
468 // predicate in `user_computed_preds`, and skip adding
469 // new_pred to `user_computed_params`.
470 should_add_new = false
481 user_computed_preds.insert(new_pred);
485 /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
486 /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
487 fn map_vid_to_region<'cx>(
489 regions: &RegionConstraintData<'cx>,
490 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
491 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
492 let mut finished_map = FxHashMap::default();
494 for constraint in regions.constraints.keys() {
496 &Constraint::VarSubVar(r1, r2) => {
498 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
499 deps1.larger.insert(RegionTarget::RegionVid(r2));
502 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
503 deps2.smaller.insert(RegionTarget::RegionVid(r1));
505 &Constraint::RegSubVar(region, vid) => {
507 let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
508 deps1.larger.insert(RegionTarget::RegionVid(vid));
511 let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
512 deps2.smaller.insert(RegionTarget::Region(region));
514 &Constraint::VarSubReg(vid, region) => {
515 finished_map.insert(vid, region);
517 &Constraint::RegSubReg(r1, r2) => {
519 let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
520 deps1.larger.insert(RegionTarget::Region(r2));
523 let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
524 deps2.smaller.insert(RegionTarget::Region(r1));
529 while !vid_map.is_empty() {
530 let target = *vid_map.keys().next().expect("Keys somehow empty");
531 let deps = vid_map.remove(&target).expect("Entry somehow missing");
533 for smaller in deps.smaller.iter() {
534 for larger in deps.larger.iter() {
535 match (smaller, larger) {
536 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
537 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
538 let smaller_deps = v.into_mut();
539 smaller_deps.larger.insert(*larger);
540 smaller_deps.larger.remove(&target);
543 if let Entry::Occupied(v) = vid_map.entry(*larger) {
544 let larger_deps = v.into_mut();
545 larger_deps.smaller.insert(*smaller);
546 larger_deps.smaller.remove(&target);
549 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
550 finished_map.insert(v1, r1);
552 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
553 // Do nothing; we don't care about regions that are smaller than vids.
555 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
556 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
557 let smaller_deps = v.into_mut();
558 smaller_deps.larger.insert(*larger);
559 smaller_deps.larger.remove(&target);
562 if let Entry::Occupied(v) = vid_map.entry(*larger) {
563 let larger_deps = v.into_mut();
564 larger_deps.smaller.insert(*smaller);
565 larger_deps.smaller.remove(&target);
575 fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
576 self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
579 pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
581 ty::Param(_) => true,
582 ty::Projection(p) => self.is_of_param(p.self_ty()),
587 fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
588 if let Some(ty) = p.term().skip_binder().ty() {
589 matches!(ty.kind(), ty::Projection(proj) if proj == &p.skip_binder().projection_ty)
595 fn evaluate_nested_obligations(
598 nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
599 computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>,
600 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
601 predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
602 selcx: &mut SelectionContext<'_, 'tcx>,
603 only_projections: bool,
605 let dummy_cause = ObligationCause::dummy();
607 for obligation in nested {
609 fresh_preds.insert(self.clean_pred(selcx.infcx, obligation.predicate));
611 // Resolve any inference variables that we can, to help selection succeed
612 let predicate = selcx.infcx.resolve_vars_if_possible(obligation.predicate);
614 // We only add a predicate as a user-displayable bound if
615 // it involves a generic parameter, and doesn't contain
616 // any inference variables.
618 // Displaying a bound involving a concrete type (instead of a generic
619 // parameter) would be pointless, since it's always true
621 // Displaying an inference variable is impossible, since they're
622 // an internal compiler detail without a defined visual representation
624 // We check this by calling is_of_param on the relevant types
625 // from the various possible predicates
627 let bound_predicate = predicate.kind();
628 match bound_predicate.skip_binder() {
629 ty::PredicateKind::Clause(ty::Clause::Trait(p)) => {
630 // Add this to `predicates` so that we end up calling `select`
631 // with it. If this predicate ends up being unimplemented,
632 // then `evaluate_predicates` will handle adding it the `ParamEnv`
634 predicates.push_back(bound_predicate.rebind(p));
636 ty::PredicateKind::Clause(ty::Clause::Projection(p)) => {
637 let p = bound_predicate.rebind(p);
639 "evaluate_nested_obligations: examining projection predicate {:?}",
643 // As described above, we only want to display
644 // bounds which include a generic parameter but don't include
645 // an inference variable.
646 // Additionally, we check if we've seen this predicate before,
647 // to avoid rendering duplicate bounds to the user.
648 if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
649 && !p.term().skip_binder().has_infer_types()
653 "evaluate_nested_obligations: adding projection predicate \
654 to computed_preds: {:?}",
658 // Under unusual circumstances, we can end up with a self-referential
659 // projection predicate. For example:
660 // <T as MyType>::Value == <T as MyType>::Value
661 // Not only is displaying this to the user pointless,
662 // having it in the ParamEnv will cause an issue if we try to call
663 // poly_project_and_unify_type on the predicate, since this kind of
664 // predicate will normally never end up in a ParamEnv.
666 // For these reasons, we ignore these weird predicates,
667 // ensuring that we're able to properly synthesize an auto trait impl
668 if self.is_self_referential_projection(p) {
670 "evaluate_nested_obligations: encountered a projection
671 predicate equating a type with itself! Skipping"
674 self.add_user_pred(computed_preds, predicate);
678 // There are three possible cases when we project a predicate:
680 // 1. We encounter an error. This means that it's impossible for
681 // our current type to implement the auto trait - there's bound
682 // that we could add to our ParamEnv that would 'fix' this kind
683 // of error, as it's not caused by an unimplemented type.
685 // 2. We successfully project the predicate (Ok(Some(_))), generating
686 // some subobligations. We then process these subobligations
687 // like any other generated sub-obligations.
689 // 3. We receive an 'ambiguous' result (Ok(None))
690 // If we were actually trying to compile a crate,
691 // we would need to re-process this obligation later.
692 // However, all we care about is finding out what bounds
693 // are needed for our type to implement a particular auto trait.
694 // We've already added this obligation to our computed ParamEnv
695 // above (if it was necessary). Therefore, we don't need
696 // to do any further processing of the obligation.
698 // Note that we *must* try to project *all* projection predicates
699 // we encounter, even ones without inference variable.
700 // This ensures that we detect any projection errors,
701 // which indicate that our type can *never* implement the given
702 // auto trait. In that case, we will generate an explicit negative
703 // impl (e.g. 'impl !Send for MyType'). However, we don't
704 // try to process any of the generated subobligations -
705 // they contain no new information, since we already know
706 // that our type implements the projected-through trait,
707 // and can lead to weird region issues.
709 // Normally, we'll generate a negative impl as a result of encountering
710 // a type with an explicit negative impl of an auto trait
711 // (for example, raw pointers have !Send and !Sync impls)
712 // However, through some **interesting** manipulations of the type
713 // system, it's actually possible to write a type that never
714 // implements an auto trait due to a projection error, not a normal
715 // negative impl error. To properly handle this case, we need
716 // to ensure that we catch any potential projection errors,
717 // and turn them into an explicit negative impl for our type.
718 debug!("Projecting and unifying projection predicate {:?}", predicate);
720 match project::poly_project_and_unify_type(selcx, &obligation.with(self.tcx, p))
722 ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
724 "evaluate_nested_obligations: Unable to unify predicate \
725 '{:?}' '{:?}', bailing out",
730 ProjectAndUnifyResult::Recursive => {
731 debug!("evaluate_nested_obligations: recursive projection predicate");
734 ProjectAndUnifyResult::Holds(v) => {
735 // We only care about sub-obligations
736 // when we started out trying to unify
737 // some inference variables. See the comment above
738 // for more information
739 if p.term().skip_binder().has_infer_types() {
740 if !self.evaluate_nested_obligations(
753 ProjectAndUnifyResult::FailedNormalization => {
754 // It's ok not to make progress when have no inference variables -
755 // in that case, we were only performing unification to check if an
756 // error occurred (which would indicate that it's impossible for our
757 // type to implement the auto trait).
758 // However, we should always make progress (either by generating
759 // subobligations or getting an error) when we started off with
760 // inference variables
761 if p.term().skip_binder().has_infer_types() {
762 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
767 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(binder)) => {
768 let binder = bound_predicate.rebind(binder);
769 selcx.infcx.region_outlives_predicate(&dummy_cause, binder)
771 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(binder)) => {
772 let binder = bound_predicate.rebind(binder);
774 binder.no_bound_vars(),
775 binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
777 (None, Some(t_a)) => {
778 selcx.infcx.register_region_obligation_with_cause(
780 selcx.infcx.tcx.lifetimes.re_static,
784 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
785 selcx.infcx.register_region_obligation_with_cause(
794 ty::PredicateKind::ConstEquate(c1, c2) => {
795 let evaluate = |c: ty::Const<'tcx>| {
796 if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
797 match selcx.infcx.const_eval_resolve(
798 obligation.param_env,
800 Some(obligation.cause.span),
802 Ok(Some(valtree)) => Ok(selcx.tcx().mk_const(valtree, c.ty())),
805 let def_id = unevaluated.def.did;
807 tcx.sess.emit_err(UnableToConstructConstantValue {
808 span: tcx.def_span(def_id),
809 unevaluated: unevaluated,
811 Err(ErrorHandled::Reported(reported))
813 Err(err) => Err(err),
820 match (evaluate(c1), evaluate(c2)) {
821 (Ok(c1), Ok(c2)) => {
822 match selcx.infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
825 Err(_) => return false,
831 // There's not really much we can do with these predicates -
832 // we start out with a `ParamEnv` with no inference variables,
833 // and these don't correspond to adding any new bounds to
835 ty::PredicateKind::WellFormed(..)
836 | ty::PredicateKind::ObjectSafe(..)
837 | ty::PredicateKind::ClosureKind(..)
838 | ty::PredicateKind::Subtype(..)
839 | ty::PredicateKind::ConstEvaluatable(..)
840 | ty::PredicateKind::Coerce(..)
841 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
842 ty::PredicateKind::Ambiguous => return false,
850 infcx: &InferCtxt<'tcx>,
851 p: ty::Predicate<'tcx>,
852 ) -> ty::Predicate<'tcx> {
857 /// Replaces all ReVars in a type with ty::Region's, using the provided map
858 pub struct RegionReplacer<'a, 'tcx> {
859 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
863 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
864 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
868 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
870 ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
873 .unwrap_or_else(|| r.super_fold_with(self))