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::{PolyTraitRef, 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 = ty::TraitRef { def_id: trait_did, substs: tcx.mk_substs_trait(ty, &[]) };
91 let trait_pred = ty::Binder::dummy(trait_ref);
93 let infcx = tcx.infer_ctxt().build();
94 let mut selcx = SelectionContext::new(&infcx);
96 PolyTraitRef::to_poly_trait_predicate,
97 PolyTraitRef::to_poly_trait_predicate_negative_polarity,
100 selcx.select(&Obligation::new(ObligationCause::dummy(), orig_env, f(&trait_pred)));
101 if let Ok(Some(ImplSource::UserDefined(_))) = result {
103 "find_auto_trait_generics({:?}): \
104 manual impl found, bailing out",
107 // If an explicit impl exists, it always takes priority over an auto impl
108 return AutoTraitResult::ExplicitImpl;
112 let infcx = tcx.infer_ctxt().build();
113 let mut fresh_preds = FxHashSet::default();
115 // Due to the way projections are handled by SelectionContext, we need to run
116 // evaluate_predicates twice: once on the original param env, and once on the result of
117 // the first evaluate_predicates call.
119 // The problem is this: most of rustc, including SelectionContext and traits::project,
120 // are designed to work with a concrete usage of a type (e.g., Vec<u8>
121 // fn<T>() { Vec<T> }. This information will generally never change - given
122 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
123 // If we're unable to prove that 'T' implements a particular trait, we're done -
124 // there's nothing left to do but error out.
126 // However, synthesizing an auto trait impl works differently. Here, we start out with
127 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
128 // with - and progressively discover the conditions we need to fulfill for it to
129 // implement a certain auto trait. This ends up breaking two assumptions made by trait
130 // selection and projection:
132 // * We can always cache the result of a particular trait selection for the lifetime of
134 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
135 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
137 // We fix the first assumption by manually clearing out all of the InferCtxt's caches
138 // in between calls to SelectionContext.select. This allows us to keep all of the
139 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
140 // them between calls.
142 // We fix the second assumption by reprocessing the result of our first call to
143 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
144 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
145 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
146 // SelectionContext to return it back to us.
148 let Some((new_env, user_env)) = self.evaluate_predicates(
157 return AutoTraitResult::NegativeImpl;
160 let (full_env, full_user_env) = self
161 .evaluate_predicates(&infcx, trait_did, ty, new_env, user_env, &mut fresh_preds, true)
163 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
167 "find_auto_trait_generics({:?}): fulfilling \
171 infcx.clear_caches();
173 // At this point, we already have all of the bounds we need. FulfillmentContext is used
174 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
175 // an additional sanity check.
177 super::fully_solve_bound(&infcx, ObligationCause::dummy(), full_env, ty, trait_did);
178 if !errors.is_empty() {
179 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
182 infcx.process_registered_region_obligations(&Default::default(), full_env);
185 infcx.inner.borrow_mut().unwrap_region_constraints().region_constraint_data().clone();
187 let vid_to_region = self.map_vid_to_region(®ion_data);
189 let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
191 AutoTraitResult::PositiveImpl(auto_trait_callback(info))
195 impl<'tcx> AutoTraitFinder<'tcx> {
196 /// The core logic responsible for computing the bounds for our synthesized impl.
198 /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
199 /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
200 /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
201 /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
202 /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
204 /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
205 /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
206 /// user code. According, it considers all possible ways that a `Predicate` could be met, which
207 /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
208 /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
209 /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
210 /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
211 /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
213 /// ```ignore (illustrative)
214 /// impl<T> Send for Foo<T> where T: IntoIterator
216 /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
217 /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
219 /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
220 /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
221 /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
222 /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
225 /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
226 /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
227 /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
228 /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
229 /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
230 /// else `SelectionContext` will choke on the missing predicates. However, this should never
231 /// show up in the final synthesized generics: we don't want our generated docs page to contain
232 /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
233 /// separate `user_env`, which only holds the predicates that will actually be displayed to the
235 fn evaluate_predicates(
237 infcx: &InferCtxt<'tcx>,
240 param_env: ty::ParamEnv<'tcx>,
241 user_env: ty::ParamEnv<'tcx>,
242 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
243 only_projections: bool,
244 ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
247 // Don't try to process any nested obligations involving predicates
248 // that are already in the `ParamEnv` (modulo regions): we already
249 // know that they must hold.
250 for predicate in param_env.caller_bounds() {
251 fresh_preds.insert(self.clean_pred(infcx, predicate));
254 let mut select = SelectionContext::new(&infcx);
256 let mut already_visited = FxHashSet::default();
257 let mut predicates = VecDeque::new();
258 predicates.push_back(ty::Binder::dummy(ty::TraitPredicate {
259 trait_ref: ty::TraitRef {
261 substs: infcx.tcx.mk_substs_trait(ty, &[]),
263 constness: ty::BoundConstness::NotConst,
264 // Auto traits are positive
265 polarity: ty::ImplPolarity::Positive,
268 let computed_preds = param_env.caller_bounds().iter();
269 let mut user_computed_preds: FxIndexSet<_> = user_env.caller_bounds().iter().collect();
271 let mut new_env = param_env;
272 let dummy_cause = ObligationCause::dummy();
274 while let Some(pred) = predicates.pop_front() {
275 infcx.clear_caches();
277 if !already_visited.insert(pred) {
281 // Call `infcx.resolve_vars_if_possible` to see if we can
282 // get rid of any inference variables.
284 infcx.resolve_vars_if_possible(Obligation::new(dummy_cause.clone(), new_env, pred));
285 let result = select.select(&obligation);
288 Ok(Some(ref impl_source)) => {
289 // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
290 // we immediately bail out, since it's impossible for us to continue.
292 if let ImplSource::UserDefined(ImplSourceUserDefinedData {
296 // Blame 'tidy' for the weird bracket placement.
297 if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
299 "evaluate_nested_obligations: found explicit negative impl\
307 let obligations = impl_source.borrow_nested_obligations().iter().cloned();
309 if !self.evaluate_nested_obligations(
312 &mut user_computed_preds,
322 Err(SelectionError::Unimplemented) => {
323 if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
324 already_visited.remove(&pred);
325 self.add_user_pred(&mut user_computed_preds, pred.to_predicate(self.tcx));
326 predicates.push_back(pred);
329 "evaluate_nested_obligations: `Unimplemented` found, bailing: \
333 pred.skip_binder().trait_ref.substs
338 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
341 let normalized_preds = elaborate_predicates(
343 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
345 .map(|o| o.predicate);
346 new_env = ty::ParamEnv::new(
347 tcx.mk_predicates(normalized_preds),
349 param_env.constness(),
353 let final_user_env = ty::ParamEnv::new(
354 tcx.mk_predicates(user_computed_preds.into_iter()),
356 user_env.constness(),
359 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
361 ty, trait_did, new_env, final_user_env
364 Some((new_env, final_user_env))
367 /// This method is designed to work around the following issue:
368 /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
369 /// progressively building a `ParamEnv` based on the results we get.
370 /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
371 /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
373 /// This can lead to a corner case when dealing with region parameters.
374 /// During our selection loop in `evaluate_predicates`, we might end up with
375 /// two trait predicates that differ only in their region parameters:
376 /// one containing a HRTB lifetime parameter, and one containing a 'normal'
377 /// lifetime parameter. For example:
378 /// ```ignore (illustrative)
380 /// T as MyTrait<'static>
382 /// If we put both of these predicates in our computed `ParamEnv`, we'll
383 /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
385 /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
386 /// Our end goal is to generate a user-visible description of the conditions
387 /// under which a type implements an auto trait. A trait predicate involving
388 /// a HRTB means that the type needs to work with any choice of lifetime,
389 /// not just one specific lifetime (e.g., `'static`).
392 user_computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>,
393 new_pred: ty::Predicate<'tcx>,
395 let mut should_add_new = true;
396 user_computed_preds.retain(|&old_pred| {
397 if let (ty::PredicateKind::Trait(new_trait), ty::PredicateKind::Trait(old_trait)) =
398 (new_pred.kind().skip_binder(), old_pred.kind().skip_binder())
400 if new_trait.def_id() == old_trait.def_id() {
401 let new_substs = new_trait.trait_ref.substs;
402 let old_substs = old_trait.trait_ref.substs;
404 if !new_substs.types().eq(old_substs.types()) {
405 // We can't compare lifetimes if the types are different,
406 // so skip checking `old_pred`.
410 for (new_region, old_region) in
411 iter::zip(new_substs.regions(), old_substs.regions())
413 match (*new_region, *old_region) {
414 // If both predicates have an `ReLateBound` (a HRTB) in the
415 // same spot, we do nothing.
416 (ty::ReLateBound(_, _), ty::ReLateBound(_, _)) => {}
418 (ty::ReLateBound(_, _), _) | (_, ty::ReVar(_)) => {
419 // One of these is true:
420 // The new predicate has a HRTB in a spot where the old
421 // predicate does not (if they both had a HRTB, the previous
422 // match arm would have executed). A HRBT is a 'stricter'
423 // bound than anything else, so we want to keep the newer
424 // predicate (with the HRBT) in place of the old predicate.
428 // The old predicate has a region variable where the new
429 // predicate has some other kind of region. An region
430 // variable isn't something we can actually display to a user,
431 // so we choose their new predicate (which doesn't have a region
434 // In both cases, we want to remove the old predicate,
435 // from `user_computed_preds`, and replace it with the new
436 // one. Having both the old and the new
437 // predicate in a `ParamEnv` would confuse `SelectionContext`.
439 // We're currently in the predicate passed to 'retain',
440 // so we return `false` to remove the old predicate from
441 // `user_computed_preds`.
444 (_, ty::ReLateBound(_, _)) | (ty::ReVar(_), _) => {
445 // This is the opposite situation as the previous arm.
446 // One of these is true:
448 // The old predicate has a HRTB lifetime in a place where the
449 // new predicate does not.
453 // The new predicate has a region variable where the old
454 // predicate has some other type of region.
456 // We want to leave the old
457 // predicate in `user_computed_preds`, and skip adding
458 // new_pred to `user_computed_params`.
459 should_add_new = false
470 user_computed_preds.insert(new_pred);
474 /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
475 /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
476 fn map_vid_to_region<'cx>(
478 regions: &RegionConstraintData<'cx>,
479 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
480 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
481 let mut finished_map = FxHashMap::default();
483 for constraint in regions.constraints.keys() {
485 &Constraint::VarSubVar(r1, r2) => {
487 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
488 deps1.larger.insert(RegionTarget::RegionVid(r2));
491 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
492 deps2.smaller.insert(RegionTarget::RegionVid(r1));
494 &Constraint::RegSubVar(region, vid) => {
496 let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
497 deps1.larger.insert(RegionTarget::RegionVid(vid));
500 let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
501 deps2.smaller.insert(RegionTarget::Region(region));
503 &Constraint::VarSubReg(vid, region) => {
504 finished_map.insert(vid, region);
506 &Constraint::RegSubReg(r1, r2) => {
508 let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
509 deps1.larger.insert(RegionTarget::Region(r2));
512 let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
513 deps2.smaller.insert(RegionTarget::Region(r1));
518 while !vid_map.is_empty() {
519 let target = *vid_map.keys().next().expect("Keys somehow empty");
520 let deps = vid_map.remove(&target).expect("Entry somehow missing");
522 for smaller in deps.smaller.iter() {
523 for larger in deps.larger.iter() {
524 match (smaller, larger) {
525 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
526 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
527 let smaller_deps = v.into_mut();
528 smaller_deps.larger.insert(*larger);
529 smaller_deps.larger.remove(&target);
532 if let Entry::Occupied(v) = vid_map.entry(*larger) {
533 let larger_deps = v.into_mut();
534 larger_deps.smaller.insert(*smaller);
535 larger_deps.smaller.remove(&target);
538 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
539 finished_map.insert(v1, r1);
541 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
542 // Do nothing; we don't care about regions that are smaller than vids.
544 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
545 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
546 let smaller_deps = v.into_mut();
547 smaller_deps.larger.insert(*larger);
548 smaller_deps.larger.remove(&target);
551 if let Entry::Occupied(v) = vid_map.entry(*larger) {
552 let larger_deps = v.into_mut();
553 larger_deps.smaller.insert(*smaller);
554 larger_deps.smaller.remove(&target);
564 fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
565 self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
568 pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
570 ty::Param(_) => true,
571 ty::Projection(p) => self.is_of_param(p.self_ty()),
576 fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
577 if let Some(ty) = p.term().skip_binder().ty() {
578 matches!(ty.kind(), ty::Projection(proj) if proj == &p.skip_binder().projection_ty)
584 fn evaluate_nested_obligations(
587 nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
588 computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>,
589 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
590 predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
591 select: &mut SelectionContext<'_, 'tcx>,
592 only_projections: bool,
594 let dummy_cause = ObligationCause::dummy();
596 for obligation in nested {
598 fresh_preds.insert(self.clean_pred(select.infcx(), obligation.predicate));
600 // Resolve any inference variables that we can, to help selection succeed
601 let predicate = select.infcx().resolve_vars_if_possible(obligation.predicate);
603 // We only add a predicate as a user-displayable bound if
604 // it involves a generic parameter, and doesn't contain
605 // any inference variables.
607 // Displaying a bound involving a concrete type (instead of a generic
608 // parameter) would be pointless, since it's always true
610 // Displaying an inference variable is impossible, since they're
611 // an internal compiler detail without a defined visual representation
613 // We check this by calling is_of_param on the relevant types
614 // from the various possible predicates
616 let bound_predicate = predicate.kind();
617 match bound_predicate.skip_binder() {
618 ty::PredicateKind::Trait(p) => {
619 // Add this to `predicates` so that we end up calling `select`
620 // with it. If this predicate ends up being unimplemented,
621 // then `evaluate_predicates` will handle adding it the `ParamEnv`
623 predicates.push_back(bound_predicate.rebind(p));
625 ty::PredicateKind::Projection(p) => {
626 let p = bound_predicate.rebind(p);
628 "evaluate_nested_obligations: examining projection predicate {:?}",
632 // As described above, we only want to display
633 // bounds which include a generic parameter but don't include
634 // an inference variable.
635 // Additionally, we check if we've seen this predicate before,
636 // to avoid rendering duplicate bounds to the user.
637 if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
638 && !p.term().skip_binder().has_infer_types()
642 "evaluate_nested_obligations: adding projection predicate \
643 to computed_preds: {:?}",
647 // Under unusual circumstances, we can end up with a self-referential
648 // projection predicate. For example:
649 // <T as MyType>::Value == <T as MyType>::Value
650 // Not only is displaying this to the user pointless,
651 // having it in the ParamEnv will cause an issue if we try to call
652 // poly_project_and_unify_type on the predicate, since this kind of
653 // predicate will normally never end up in a ParamEnv.
655 // For these reasons, we ignore these weird predicates,
656 // ensuring that we're able to properly synthesize an auto trait impl
657 if self.is_self_referential_projection(p) {
659 "evaluate_nested_obligations: encountered a projection
660 predicate equating a type with itself! Skipping"
663 self.add_user_pred(computed_preds, predicate);
667 // There are three possible cases when we project a predicate:
669 // 1. We encounter an error. This means that it's impossible for
670 // our current type to implement the auto trait - there's bound
671 // that we could add to our ParamEnv that would 'fix' this kind
672 // of error, as it's not caused by an unimplemented type.
674 // 2. We successfully project the predicate (Ok(Some(_))), generating
675 // some subobligations. We then process these subobligations
676 // like any other generated sub-obligations.
678 // 3. We receive an 'ambiguous' result (Ok(None))
679 // If we were actually trying to compile a crate,
680 // we would need to re-process this obligation later.
681 // However, all we care about is finding out what bounds
682 // are needed for our type to implement a particular auto trait.
683 // We've already added this obligation to our computed ParamEnv
684 // above (if it was necessary). Therefore, we don't need
685 // to do any further processing of the obligation.
687 // Note that we *must* try to project *all* projection predicates
688 // we encounter, even ones without inference variable.
689 // This ensures that we detect any projection errors,
690 // which indicate that our type can *never* implement the given
691 // auto trait. In that case, we will generate an explicit negative
692 // impl (e.g. 'impl !Send for MyType'). However, we don't
693 // try to process any of the generated subobligations -
694 // they contain no new information, since we already know
695 // that our type implements the projected-through trait,
696 // and can lead to weird region issues.
698 // Normally, we'll generate a negative impl as a result of encountering
699 // a type with an explicit negative impl of an auto trait
700 // (for example, raw pointers have !Send and !Sync impls)
701 // However, through some **interesting** manipulations of the type
702 // system, it's actually possible to write a type that never
703 // implements an auto trait due to a projection error, not a normal
704 // negative impl error. To properly handle this case, we need
705 // to ensure that we catch any potential projection errors,
706 // and turn them into an explicit negative impl for our type.
707 debug!("Projecting and unifying projection predicate {:?}", predicate);
709 match project::poly_project_and_unify_type(select, &obligation.with(p)) {
710 ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
712 "evaluate_nested_obligations: Unable to unify predicate \
713 '{:?}' '{:?}', bailing out",
718 ProjectAndUnifyResult::Recursive => {
719 debug!("evaluate_nested_obligations: recursive projection predicate");
722 ProjectAndUnifyResult::Holds(v) => {
723 // We only care about sub-obligations
724 // when we started out trying to unify
725 // some inference variables. See the comment above
726 // for more information
727 if p.term().skip_binder().has_infer_types() {
728 if !self.evaluate_nested_obligations(
741 ProjectAndUnifyResult::FailedNormalization => {
742 // It's ok not to make progress when have no inference variables -
743 // in that case, we were only performing unification to check if an
744 // error occurred (which would indicate that it's impossible for our
745 // type to implement the auto trait).
746 // However, we should always make progress (either by generating
747 // subobligations or getting an error) when we started off with
748 // inference variables
749 if p.term().skip_binder().has_infer_types() {
750 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
755 ty::PredicateKind::RegionOutlives(binder) => {
756 let binder = bound_predicate.rebind(binder);
757 select.infcx().region_outlives_predicate(&dummy_cause, binder)
759 ty::PredicateKind::TypeOutlives(binder) => {
760 let binder = bound_predicate.rebind(binder);
762 binder.no_bound_vars(),
763 binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
765 (None, Some(t_a)) => {
766 select.infcx().register_region_obligation_with_cause(
768 select.infcx().tcx.lifetimes.re_static,
772 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
773 select.infcx().register_region_obligation_with_cause(
782 ty::PredicateKind::ConstEquate(c1, c2) => {
783 let evaluate = |c: ty::Const<'tcx>| {
784 if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
785 match select.infcx().const_eval_resolve(
786 obligation.param_env,
788 Some(obligation.cause.span),
790 Ok(Some(valtree)) => {
791 Ok(ty::Const::from_value(select.tcx(), valtree, c.ty()))
795 let def_id = unevaluated.def.did;
797 tcx.sess.emit_err(UnableToConstructConstantValue {
798 span: tcx.def_span(def_id),
799 unevaluated: unevaluated,
801 Err(ErrorHandled::Reported(reported))
803 Err(err) => Err(err),
810 match (evaluate(c1), evaluate(c2)) {
811 (Ok(c1), Ok(c2)) => {
814 .at(&obligation.cause, obligation.param_env)
818 Err(_) => return false,
824 // There's not really much we can do with these predicates -
825 // we start out with a `ParamEnv` with no inference variables,
826 // and these don't correspond to adding any new bounds to
828 ty::PredicateKind::WellFormed(..)
829 | ty::PredicateKind::ObjectSafe(..)
830 | ty::PredicateKind::ClosureKind(..)
831 | ty::PredicateKind::Subtype(..)
832 | ty::PredicateKind::ConstEvaluatable(..)
833 | ty::PredicateKind::Coerce(..)
834 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
842 infcx: &InferCtxt<'tcx>,
843 p: ty::Predicate<'tcx>,
844 ) -> ty::Predicate<'tcx> {
849 // Replaces all ReVars in a type with ty::Region's, using the provided map
850 pub struct RegionReplacer<'a, 'tcx> {
851 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
855 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
856 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
860 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
862 ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
865 .unwrap_or_else(|| r.super_fold_with(self))