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::ty::fold::TypeFolder;
9 use crate::ty::{Region, RegionVid};
11 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
13 use std::collections::hash_map::Entry;
14 use std::collections::VecDeque;
16 // FIXME(twk): this is obviously not nice to duplicate like that
17 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
18 pub enum RegionTarget<'tcx> {
23 #[derive(Default, Debug, Clone)]
24 pub struct RegionDeps<'tcx> {
25 larger: FxHashSet<RegionTarget<'tcx>>,
26 smaller: FxHashSet<RegionTarget<'tcx>>,
29 pub enum AutoTraitResult<A> {
35 impl<A> AutoTraitResult<A> {
36 fn is_auto(&self) -> bool {
38 AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl => true,
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 auto_trait_callback: impl Fn(&InferCtxt<'_, 'tcx>, 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::bind(trait_ref);
90 let bail_out = tcx.infer_ctxt().enter(|infcx| {
91 let mut selcx = SelectionContext::with_negative(&infcx, true);
92 let result = selcx.select(&Obligation::new(
93 ObligationCause::dummy(),
95 trait_pred.to_poly_trait_predicate(),
99 Ok(Some(Vtable::VtableImpl(_))) => {
101 "find_auto_trait_generics({:?}): \
102 manual impl found, bailing out",
111 // If an explicit impl exists, it always takes priority over an auto impl
113 return AutoTraitResult::ExplicitImpl;
116 return tcx.infer_ctxt().enter(|mut infcx| {
117 let mut fresh_preds = FxHashSet::default();
119 // Due to the way projections are handled by SelectionContext, we need to run
120 // evaluate_predicates twice: once on the original param env, and once on the result of
121 // the first evaluate_predicates call.
123 // The problem is this: most of rustc, including SelectionContext and traits::project,
124 // are designed to work with a concrete usage of a type (e.g., Vec<u8>
125 // fn<T>() { Vec<T> }. This information will generally never change - given
126 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
127 // If we're unable to prove that 'T' implements a particular trait, we're done -
128 // there's nothing left to do but error out.
130 // However, synthesizing an auto trait impl works differently. Here, we start out with
131 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
132 // with - and progressively discover the conditions we need to fulfill for it to
133 // implement a certain auto trait. This ends up breaking two assumptions made by trait
134 // selection and projection:
136 // * We can always cache the result of a particular trait selection for the lifetime of
138 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
139 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
141 // We fix the first assumption by manually clearing out all of the InferCtxt's caches
142 // in between calls to SelectionContext.select. This allows us to keep all of the
143 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
144 // them between calls.
146 // We fix the second assumption by reprocessing the result of our first call to
147 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
148 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
149 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
150 // SelectionContext to return it back to us.
152 let (new_env, user_env) = match self.evaluate_predicates(
162 None => return AutoTraitResult::NegativeImpl,
165 let (full_env, full_user_env) = self
166 .evaluate_predicates(
176 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
180 "find_auto_trait_generics({:?}): fulfilling \
184 infcx.clear_caches();
186 // At this point, we already have all of the bounds we need. FulfillmentContext is used
187 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
188 // an additional sanity check.
189 let mut fulfill = FulfillmentContext::new();
190 fulfill.register_bound(
195 ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID),
197 fulfill.select_all_or_error(&infcx).unwrap_or_else(|e| {
198 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, e)
201 let body_id_map: FxHashMap<_, _> =
202 infcx.region_obligations.borrow().iter().map(|&(id, _)| (id, vec![])).collect();
204 infcx.process_registered_region_obligations(&body_id_map, None, full_env);
206 let region_data = infcx.borrow_region_constraints().region_constraint_data().clone();
208 let vid_to_region = self.map_vid_to_region(®ion_data);
210 let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
212 return AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info));
217 impl AutoTraitFinder<'tcx> {
218 /// The core logic responsible for computing the bounds for our synthesized impl.
220 /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
221 /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
222 /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
223 /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
224 /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
226 /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
227 /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
228 /// user code. According, it considers all possible ways that a `Predicate` could be met, which
229 /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
230 /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
231 /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
232 /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
233 /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
236 /// impl<T> Send for Foo<T> where T: IntoIterator
238 /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
239 /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
241 /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
242 /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
243 /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
244 /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
247 /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
248 /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
249 /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
250 /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
251 /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
252 /// else `SelectionContext` will choke on the missing predicates. However, this should never
253 /// show up in the final synthesized generics: we don't want our generated docs page to contain
254 /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
255 /// separate `user_env`, which only holds the predicates that will actually be displayed to the
257 fn evaluate_predicates(
259 infcx: &InferCtxt<'_, 'tcx>,
262 param_env: ty::ParamEnv<'tcx>,
263 user_env: ty::ParamEnv<'tcx>,
264 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
265 only_projections: bool,
266 ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
269 let mut select = SelectionContext::with_negative(&infcx, true);
271 let mut already_visited = FxHashSet::default();
272 let mut predicates = VecDeque::new();
273 predicates.push_back(ty::Binder::bind(ty::TraitPredicate {
274 trait_ref: ty::TraitRef {
276 substs: infcx.tcx.mk_substs_trait(ty, &[]),
280 let mut computed_preds: FxHashSet<_> = param_env.caller_bounds.iter().cloned().collect();
281 let mut user_computed_preds: FxHashSet<_> =
282 user_env.caller_bounds.iter().cloned().collect();
284 let mut new_env = param_env;
285 let dummy_cause = ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID);
287 while let Some(pred) = predicates.pop_front() {
288 infcx.clear_caches();
290 if !already_visited.insert(pred) {
294 // Call `infcx.resolve_vars_if_possible` to see if we can
295 // get rid of any inference variables.
296 let obligation = infcx.resolve_vars_if_possible(&Obligation::new(
301 let result = select.select(&obligation);
304 &Ok(Some(ref vtable)) => {
305 // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
306 // we immediately bail out, since it's impossible for us to continue.
308 Vtable::VtableImpl(VtableImplData { impl_def_id, .. }) => {
309 // Blame 'tidy' for the weird bracket placement.
310 if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
312 "evaluate_nested_obligations: found explicit negative impl\
322 let obligations = vtable.clone().nested_obligations().into_iter();
324 if !self.evaluate_nested_obligations(
327 &mut user_computed_preds,
337 &Err(SelectionError::Unimplemented) => {
338 if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
339 already_visited.remove(&pred);
340 self.add_user_pred(&mut user_computed_preds, ty::Predicate::Trait(pred));
341 predicates.push_back(pred);
344 "evaluate_nested_obligations: `Unimplemented` found, bailing: \
348 pred.skip_binder().trait_ref.substs
353 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
356 computed_preds.extend(user_computed_preds.iter().cloned());
357 let normalized_preds =
358 elaborate_predicates(tcx, computed_preds.iter().cloned().collect());
360 ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal, None);
363 let final_user_env = ty::ParamEnv::new(
364 tcx.mk_predicates(user_computed_preds.into_iter()),
369 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
371 ty, trait_did, new_env, final_user_env
374 return Some((new_env, final_user_env));
377 /// This method is designed to work around the following issue:
378 /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
379 /// progressively building a `ParamEnv` based on the results we get.
380 /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
381 /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
383 /// This can lead to a corner case when dealing with region parameters.
384 /// During our selection loop in `evaluate_predicates`, we might end up with
385 /// two trait predicates that differ only in their region parameters:
386 /// one containing a HRTB lifetime parameter, and one containing a 'normal'
387 /// lifetime parameter. For example:
390 /// T as MyTrait<'static>
392 /// If we put both of these predicates in our computed `ParamEnv`, we'll
393 /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
395 /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
396 /// Our end goal is to generate a user-visible description of the conditions
397 /// under which a type implements an auto trait. A trait predicate involving
398 /// a HRTB means that the type needs to work with any choice of lifetime,
399 /// not just one specific lifetime (e.g., `'static`).
400 fn add_user_pred<'c>(
402 user_computed_preds: &mut FxHashSet<ty::Predicate<'c>>,
403 new_pred: ty::Predicate<'c>,
405 let mut should_add_new = true;
406 user_computed_preds.retain(|&old_pred| {
407 match (&new_pred, old_pred) {
408 (&ty::Predicate::Trait(new_trait), ty::Predicate::Trait(old_trait)) => {
409 if new_trait.def_id() == old_trait.def_id() {
410 let new_substs = new_trait.skip_binder().trait_ref.substs;
411 let old_substs = old_trait.skip_binder().trait_ref.substs;
413 if !new_substs.types().eq(old_substs.types()) {
414 // We can't compare lifetimes if the types are different,
415 // so skip checking `old_pred`.
419 for (new_region, old_region) in
420 new_substs.regions().zip(old_substs.regions())
422 match (new_region, old_region) {
423 // If both predicates have an `ReLateBound` (a HRTB) in the
424 // same spot, we do nothing.
426 ty::RegionKind::ReLateBound(_, _),
427 ty::RegionKind::ReLateBound(_, _),
430 (ty::RegionKind::ReLateBound(_, _), _)
431 | (_, ty::RegionKind::ReVar(_)) => {
432 // One of these is true:
433 // The new predicate has a HRTB in a spot where the old
434 // predicate does not (if they both had a HRTB, the previous
435 // match arm would have executed). A HRBT is a 'stricter'
436 // bound than anything else, so we want to keep the newer
437 // predicate (with the HRBT) in place of the old predicate.
441 // The old predicate has a region variable where the new
442 // predicate has some other kind of region. An region
443 // variable isn't something we can actually display to a user,
444 // so we choose their new predicate (which doesn't have a region
447 // In both cases, we want to remove the old predicate,
448 // from `user_computed_preds`, and replace it with the new
449 // one. Having both the old and the new
450 // predicate in a `ParamEnv` would confuse `SelectionContext`.
452 // We're currently in the predicate passed to 'retain',
453 // so we return `false` to remove the old predicate from
454 // `user_computed_preds`.
457 (_, ty::RegionKind::ReLateBound(_, _))
458 | (ty::RegionKind::ReVar(_), _) => {
459 // This is the opposite situation as the previous arm.
460 // One of these is true:
462 // The old predicate has a HRTB lifetime in a place where the
463 // new predicate does not.
467 // The new predicate has a region variable where the old
468 // predicate has some other type of region.
470 // We want to leave the old
471 // predicate in `user_computed_preds`, and skip adding
472 // new_pred to `user_computed_params`.
473 should_add_new = false
486 user_computed_preds.insert(new_pred);
490 /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
491 /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
492 fn map_vid_to_region<'cx>(
494 regions: &RegionConstraintData<'cx>,
495 ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
496 let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
497 let mut finished_map = FxHashMap::default();
499 for constraint in regions.constraints.keys() {
501 &Constraint::VarSubVar(r1, r2) => {
503 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
504 deps1.larger.insert(RegionTarget::RegionVid(r2));
507 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
508 deps2.smaller.insert(RegionTarget::RegionVid(r1));
510 &Constraint::RegSubVar(region, vid) => {
512 let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
513 deps1.larger.insert(RegionTarget::RegionVid(vid));
516 let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
517 deps2.smaller.insert(RegionTarget::Region(region));
519 &Constraint::VarSubReg(vid, region) => {
520 finished_map.insert(vid, region);
522 &Constraint::RegSubReg(r1, r2) => {
524 let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
525 deps1.larger.insert(RegionTarget::Region(r2));
528 let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
529 deps2.smaller.insert(RegionTarget::Region(r1));
534 while !vid_map.is_empty() {
535 let target = vid_map.keys().next().expect("Keys somehow empty").clone();
536 let deps = vid_map.remove(&target).expect("Entry somehow missing");
538 for smaller in deps.smaller.iter() {
539 for larger in deps.larger.iter() {
540 match (smaller, larger) {
541 (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
542 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
543 let smaller_deps = v.into_mut();
544 smaller_deps.larger.insert(*larger);
545 smaller_deps.larger.remove(&target);
548 if let Entry::Occupied(v) = vid_map.entry(*larger) {
549 let larger_deps = v.into_mut();
550 larger_deps.smaller.insert(*smaller);
551 larger_deps.smaller.remove(&target);
554 (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
555 finished_map.insert(v1, r1);
557 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
558 // Do nothing; we don't care about regions that are smaller than vids.
560 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
561 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
562 let smaller_deps = v.into_mut();
563 smaller_deps.larger.insert(*larger);
564 smaller_deps.larger.remove(&target);
567 if let Entry::Occupied(v) = vid_map.entry(*larger) {
568 let larger_deps = v.into_mut();
569 larger_deps.smaller.insert(*smaller);
570 larger_deps.smaller.remove(&target);
580 fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
581 return self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types());
584 pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
585 return match ty.kind {
586 ty::Param(_) => true,
587 ty::Projection(p) => self.is_of_param(p.self_ty()),
592 fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
593 match p.ty().skip_binder().kind {
594 ty::Projection(proj) if proj == p.skip_binder().projection_ty => true,
599 fn evaluate_nested_obligations(
602 nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
603 computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
604 fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
605 predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
606 select: &mut SelectionContext<'_, 'tcx>,
607 only_projections: bool,
609 let dummy_cause = ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID);
611 for (obligation, mut predicate) in nested.map(|o| (o.clone(), o.predicate)) {
612 let is_new_pred = fresh_preds.insert(self.clean_pred(select.infcx(), predicate));
614 // Resolve any inference variables that we can, to help selection succeed
615 predicate = select.infcx().resolve_vars_if_possible(&predicate);
617 // We only add a predicate as a user-displayable bound if
618 // it involves a generic parameter, and doesn't contain
619 // any inference variables.
621 // Displaying a bound involving a concrete type (instead of a generic
622 // parameter) would be pointless, since it's always true
624 // Displaying an inference variable is impossible, since they're
625 // an internal compiler detail without a defined visual representation
627 // We check this by calling is_of_param on the relevant types
628 // from the various possible predicates
630 &ty::Predicate::Trait(p) => {
631 if self.is_param_no_infer(p.skip_binder().trait_ref.substs)
635 self.add_user_pred(computed_preds, predicate);
637 predicates.push_back(p);
639 &ty::Predicate::Projection(p) => {
641 "evaluate_nested_obligations: examining projection predicate {:?}",
645 // As described above, we only want to display
646 // bounds which include a generic parameter but don't include
647 // an inference variable.
648 // Additionally, we check if we've seen this predicate before,
649 // to avoid rendering duplicate bounds to the user.
650 if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
651 && !p.ty().skip_binder().has_infer_types()
655 "evaluate_nested_obligations: adding projection predicate\
656 to computed_preds: {:?}",
660 // Under unusual circumstances, we can end up with a self-refeential
661 // projection predicate. For example:
662 // <T as MyType>::Value == <T as MyType>::Value
663 // Not only is displaying this to the user pointless,
664 // having it in the ParamEnv will cause an issue if we try to call
665 // poly_project_and_unify_type on the predicate, since this kind of
666 // predicate will normally never end up in a ParamEnv.
668 // For these reasons, we ignore these weird predicates,
669 // ensuring that we're able to properly synthesize an auto trait impl
670 if self.is_self_referential_projection(p) {
672 "evaluate_nested_obligations: encountered a projection
673 predicate equating a type with itself! Skipping"
676 self.add_user_pred(computed_preds, predicate);
680 // There are three possible cases when we project a predicate:
682 // 1. We encounter an error. This means that it's impossible for
683 // our current type to implement the auto trait - there's bound
684 // that we could add to our ParamEnv that would 'fix' this kind
685 // of error, as it's not caused by an unimplemented type.
687 // 2. We successfully project the predicate (Ok(Some(_))), generating
688 // some subobligations. We then process these subobligations
689 // like any other generated sub-obligations.
691 // 3. We receieve an 'ambiguous' result (Ok(None))
692 // If we were actually trying to compile a crate,
693 // we would need to re-process this obligation later.
694 // However, all we care about is finding out what bounds
695 // are needed for our type to implement a particular auto trait.
696 // We've already added this obligation to our computed ParamEnv
697 // above (if it was necessary). Therefore, we don't need
698 // to do any further processing of the obligation.
700 // Note that we *must* try to project *all* projection predicates
701 // we encounter, even ones without inference variable.
702 // This ensures that we detect any projection errors,
703 // which indicate that our type can *never* implement the given
704 // auto trait. In that case, we will generate an explicit negative
705 // impl (e.g. 'impl !Send for MyType'). However, we don't
706 // try to process any of the generated subobligations -
707 // they contain no new information, since we already know
708 // that our type implements the projected-through trait,
709 // and can lead to weird region issues.
711 // Normally, we'll generate a negative impl as a result of encountering
712 // a type with an explicit negative impl of an auto trait
713 // (for example, raw pointers have !Send and !Sync impls)
714 // However, through some **interesting** manipulations of the type
715 // system, it's actually possible to write a type that never
716 // implements an auto trait due to a projection error, not a normal
717 // negative impl error. To properly handle this case, we need
718 // to ensure that we catch any potential projection errors,
719 // and turn them into an explicit negative impl for our type.
720 debug!("Projecting and unifying projection predicate {:?}", predicate);
722 match poly_project_and_unify_type(select, &obligation.with(p)) {
725 "evaluate_nested_obligations: Unable to unify predicate \
726 '{:?}' '{:?}', bailing out",
732 // We only care about sub-obligations
733 // when we started out trying to unify
734 // some inference variables. See the comment above
735 // for more infomration
736 if p.ty().skip_binder().has_infer_types() {
737 if !self.evaluate_nested_obligations(
739 v.clone().iter().cloned(),
751 // It's ok not to make progress when hvave no inference variables -
752 // in that case, we were only performing unifcation to check if an
753 // error occurred (which would indicate that it's impossible for our
754 // type to implement the auto trait).
755 // However, we should always make progress (either by generating
756 // subobligations or getting an error) when we started off with
757 // inference variables
758 if p.ty().skip_binder().has_infer_types() {
759 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
764 &ty::Predicate::RegionOutlives(ref binder) => {
765 if select.infcx().region_outlives_predicate(&dummy_cause, binder).is_err() {
769 &ty::Predicate::TypeOutlives(ref binder) => {
771 binder.no_bound_vars(),
772 binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
774 (None, Some(t_a)) => {
775 select.infcx().register_region_obligation_with_cause(
777 select.infcx().tcx.lifetimes.re_static,
781 (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
782 select.infcx().register_region_obligation_with_cause(
791 _ => panic!("Unexpected predicate {:?} {:?}", ty, predicate),
799 infcx: &InferCtxt<'_, 'tcx>,
800 p: ty::Predicate<'tcx>,
801 ) -> ty::Predicate<'tcx> {
806 // Replaces all ReVars in a type with ty::Region's, using the provided map
807 pub struct RegionReplacer<'a, 'tcx> {
808 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
812 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
813 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
817 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
819 &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
822 .unwrap_or_else(|| r.super_fold_with(self))