1 //! Code for projecting associated types out of trait references.
3 use super::elaborate_predicates;
4 use super::specialization_graph;
5 use super::translate_substs;
7 use super::ObligationCause;
8 use super::PredicateObligation;
10 use super::SelectionContext;
11 use super::SelectionError;
12 use super::{VtableImplData, VtableClosureData, VtableGeneratorData, VtableFnPointerData};
15 use crate::hir::def_id::DefId;
16 use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
17 use crate::infer::type_variable::TypeVariableOrigin;
18 use crate::mir::interpret::{GlobalId, ConstValue};
19 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
20 use rustc_macros::HashStable;
21 use syntax::ast::Ident;
22 use syntax::symbol::sym;
23 use crate::ty::subst::{Subst, InternalSubsts};
24 use crate::ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt};
25 use crate::ty::fold::{TypeFoldable, TypeFolder};
26 use crate::util::common::FN_OUTPUT_NAME;
28 /// Depending on the stage of compilation, we want projection to be
29 /// more or less conservative.
30 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, HashStable)]
32 /// At type-checking time, we refuse to project any associated
33 /// type that is marked `default`. Non-`default` ("final") types
34 /// are always projected. This is necessary in general for
35 /// soundness of specialization. However, we *could* allow
36 /// projections in fully-monomorphic cases. We choose not to,
37 /// because we prefer for `default type` to force the type
38 /// definition to be treated abstractly by any consumers of the
39 /// impl. Concretely, that means that the following example will
47 /// impl<T> Assoc for T {
48 /// default type Output = bool;
52 /// let <() as Assoc>::Output = true;
56 /// At codegen time, all monomorphic projections will succeed.
57 /// Also, `impl Trait` is normalized to the concrete type,
58 /// which has to be already collected by type-checking.
60 /// NOTE: as `impl Trait`'s concrete type should *never*
61 /// be observable directly by the user, `Reveal::All`
62 /// should not be used by checks which may expose
63 /// type equality or type contents to the user.
64 /// There are some exceptions, e.g., around OIBITS and
65 /// transmute-checking, which expose some details, but
66 /// not the whole concrete type of the `impl Trait`.
70 pub type PolyProjectionObligation<'tcx> =
71 Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
73 pub type ProjectionObligation<'tcx> =
74 Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
76 pub type ProjectionTyObligation<'tcx> =
77 Obligation<'tcx, ty::ProjectionTy<'tcx>>;
79 /// When attempting to resolve `<T as TraitRef>::Name` ...
81 pub enum ProjectionTyError<'tcx> {
82 /// ...we found multiple sources of information and couldn't resolve the ambiguity.
85 /// ...an error occurred matching `T : TraitRef`
86 TraitSelectionError(SelectionError<'tcx>),
90 pub struct MismatchedProjectionTypes<'tcx> {
91 pub err: ty::error::TypeError<'tcx>
94 #[derive(PartialEq, Eq, Debug)]
95 enum ProjectionTyCandidate<'tcx> {
96 // from a where-clause in the env or object type
97 ParamEnv(ty::PolyProjectionPredicate<'tcx>),
99 // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
100 TraitDef(ty::PolyProjectionPredicate<'tcx>),
102 // from a "impl" (or a "pseudo-impl" returned by select)
103 Select(Selection<'tcx>),
106 enum ProjectionTyCandidateSet<'tcx> {
108 Single(ProjectionTyCandidate<'tcx>),
110 Error(SelectionError<'tcx>),
113 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
114 fn mark_ambiguous(&mut self) {
115 *self = ProjectionTyCandidateSet::Ambiguous;
118 fn mark_error(&mut self, err: SelectionError<'tcx>) {
119 *self = ProjectionTyCandidateSet::Error(err);
122 // Returns true if the push was successful, or false if the candidate
123 // was discarded -- this could be because of ambiguity, or because
124 // a higher-priority candidate is already there.
125 fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
126 use self::ProjectionTyCandidateSet::*;
127 use self::ProjectionTyCandidate::*;
129 // This wacky variable is just used to try and
130 // make code readable and avoid confusing paths.
131 // It is assigned a "value" of `()` only on those
132 // paths in which we wish to convert `*self` to
133 // ambiguous (and return false, because the candidate
134 // was not used). On other paths, it is not assigned,
135 // and hence if those paths *could* reach the code that
136 // comes after the match, this fn would not compile.
137 let convert_to_ambiguous;
141 *self = Single(candidate);
146 // Duplicates can happen inside ParamEnv. In the case, we
147 // perform a lazy deduplication.
148 if current == &candidate {
152 // Prefer where-clauses. As in select, if there are multiple
153 // candidates, we prefer where-clause candidates over impls. This
154 // may seem a bit surprising, since impls are the source of
155 // "truth" in some sense, but in fact some of the impls that SEEM
156 // applicable are not, because of nested obligations. Where
157 // clauses are the safer choice. See the comment on
158 // `select::SelectionCandidate` and #21974 for more details.
159 match (current, candidate) {
160 (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
161 (ParamEnv(..), _) => return false,
162 (_, ParamEnv(..)) => unreachable!(),
163 (_, _) => convert_to_ambiguous = (),
167 Ambiguous | Error(..) => {
172 // We only ever get here when we moved from a single candidate
174 let () = convert_to_ambiguous;
180 /// Evaluates constraints of the form:
182 /// for<...> <T as Trait>::U == V
184 /// If successful, this may result in additional obligations. Also returns
185 /// the projection cache key used to track these additional obligations.
186 pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
187 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
188 obligation: &PolyProjectionObligation<'tcx>)
189 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
190 MismatchedProjectionTypes<'tcx>>
192 debug!("poly_project_and_unify_type(obligation={:?})",
195 let infcx = selcx.infcx();
196 infcx.commit_if_ok(|snapshot| {
197 let (placeholder_predicate, placeholder_map) =
198 infcx.replace_bound_vars_with_placeholders(&obligation.predicate);
200 let placeholder_obligation = obligation.with(placeholder_predicate);
201 let result = project_and_unify_type(selcx, &placeholder_obligation)?;
202 infcx.leak_check(false, &placeholder_map, snapshot)
203 .map_err(|err| MismatchedProjectionTypes { err })?;
208 /// Evaluates constraints of the form:
210 /// <T as Trait>::U == V
212 /// If successful, this may result in additional obligations.
213 fn project_and_unify_type<'cx, 'gcx, 'tcx>(
214 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
215 obligation: &ProjectionObligation<'tcx>)
216 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
217 MismatchedProjectionTypes<'tcx>>
219 debug!("project_and_unify_type(obligation={:?})",
222 let mut obligations = vec![];
224 match opt_normalize_projection_type(selcx,
225 obligation.param_env,
226 obligation.predicate.projection_ty,
227 obligation.cause.clone(),
228 obligation.recursion_depth,
231 None => return Ok(None),
234 debug!("project_and_unify_type: normalized_ty={:?} obligations={:?}",
238 let infcx = selcx.infcx();
239 match infcx.at(&obligation.cause, obligation.param_env)
240 .eq(normalized_ty, obligation.predicate.ty) {
241 Ok(InferOk { obligations: inferred_obligations, value: () }) => {
242 obligations.extend(inferred_obligations);
243 Ok(Some(obligations))
246 debug!("project_and_unify_type: equating types encountered error {:?}", err);
247 Err(MismatchedProjectionTypes { err })
252 /// Normalizes any associated type projections in `value`, replacing
253 /// them with a fully resolved type where possible. The return value
254 /// combines the normalized result and any additional obligations that
255 /// were incurred as result.
256 pub fn normalize<'a, 'b, 'gcx, 'tcx, T>(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
257 param_env: ty::ParamEnv<'tcx>,
258 cause: ObligationCause<'tcx>,
260 -> Normalized<'tcx, T>
261 where T : TypeFoldable<'tcx>
263 normalize_with_depth(selcx, param_env, cause, 0, value)
266 /// As `normalize`, but with a custom depth.
267 pub fn normalize_with_depth<'a, 'b, 'gcx, 'tcx, T>(
268 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
269 param_env: ty::ParamEnv<'tcx>,
270 cause: ObligationCause<'tcx>,
273 -> Normalized<'tcx, T>
275 where T : TypeFoldable<'tcx>
277 debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
278 let mut normalizer = AssociatedTypeNormalizer::new(selcx, param_env, cause, depth);
279 let result = normalizer.fold(value);
280 debug!("normalize_with_depth: depth={} result={:?} with {} obligations",
281 depth, result, normalizer.obligations.len());
282 debug!("normalize_with_depth: depth={} obligations={:?}",
283 depth, normalizer.obligations);
286 obligations: normalizer.obligations,
290 struct AssociatedTypeNormalizer<'a, 'b: 'a, 'gcx: 'b+'tcx, 'tcx: 'b> {
291 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
292 param_env: ty::ParamEnv<'tcx>,
293 cause: ObligationCause<'tcx>,
294 obligations: Vec<PredicateObligation<'tcx>>,
298 impl<'a, 'b, 'gcx, 'tcx> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
299 fn new(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
300 param_env: ty::ParamEnv<'tcx>,
301 cause: ObligationCause<'tcx>,
303 -> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx>
305 AssociatedTypeNormalizer {
314 fn fold<T:TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
315 let value = self.selcx.infcx().resolve_type_vars_if_possible(value);
317 if !value.has_projections() {
320 value.fold_with(self)
325 impl<'a, 'b, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
326 fn tcx<'c>(&'c self) -> TyCtxt<'c, 'gcx, 'tcx> {
330 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
331 // We don't want to normalize associated types that occur inside of region
332 // binders, because they may contain bound regions, and we can't cope with that.
336 // for<'a> fn(<T as Foo<&'a>>::A)
338 // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
339 // normalize it when we instantiate those bound regions (which
340 // should occur eventually).
342 let ty = ty.super_fold_with(self);
344 ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => { // (*)
345 // Only normalize `impl Trait` after type-checking, usually in codegen.
346 match self.param_env.reveal {
347 Reveal::UserFacing => ty,
350 let recursion_limit = *self.tcx().sess.recursion_limit.get();
351 if self.depth >= recursion_limit {
352 let obligation = Obligation::with_depth(
358 self.selcx.infcx().report_overflow_error(&obligation, true);
361 let generic_ty = self.tcx().type_of(def_id);
362 let concrete_ty = generic_ty.subst(self.tcx(), substs);
364 let folded_ty = self.fold_ty(concrete_ty);
371 ty::Projection(ref data) if !data.has_escaping_bound_vars() => { // (*)
373 // (*) This is kind of hacky -- we need to be able to
374 // handle normalization within binders because
375 // otherwise we wind up a need to normalize when doing
376 // trait matching (since you can have a trait
377 // obligation like `for<'a> T::B : Fn(&'a int)`), but
378 // we can't normalize with bound regions in scope. So
379 // far now we just ignore binders but only normalize
380 // if all bound regions are gone (and then we still
381 // have to renormalize whenever we instantiate a
382 // binder). It would be better to normalize in a
383 // binding-aware fashion.
385 let normalized_ty = normalize_projection_type(self.selcx,
390 &mut self.obligations);
391 debug!("AssociatedTypeNormalizer: depth={} normalized {:?} to {:?}, \
392 now with {} obligations",
393 self.depth, ty, normalized_ty, self.obligations.len());
401 fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
402 if let ConstValue::Unevaluated(def_id, substs) = constant.val {
403 let tcx = self.selcx.tcx().global_tcx();
404 if let Some(param_env) = self.tcx().lift_to_global(&self.param_env) {
405 if substs.needs_infer() || substs.has_placeholders() {
406 let identity_substs = InternalSubsts::identity_for_item(tcx, def_id);
407 let instance = ty::Instance::resolve(tcx, param_env, def_id, identity_substs);
408 if let Some(instance) = instance {
413 if let Ok(evaluated) = tcx.const_eval(param_env.and(cid)) {
414 let substs = tcx.lift_to_global(&substs).unwrap();
415 let evaluated = tcx.mk_const(evaluated);
416 let evaluated = evaluated.subst(tcx, substs);
421 if let Some(substs) = self.tcx().lift_to_global(&substs) {
422 let instance = ty::Instance::resolve(tcx, param_env, def_id, substs);
423 if let Some(instance) = instance {
428 if let Ok(evaluated) = tcx.const_eval(param_env.and(cid)) {
429 return tcx.mk_const(evaluated);
441 pub struct Normalized<'tcx,T> {
443 pub obligations: Vec<PredicateObligation<'tcx>>,
446 pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
448 impl<'tcx,T> Normalized<'tcx,T> {
449 pub fn with<U>(self, value: U) -> Normalized<'tcx,U> {
450 Normalized { value: value, obligations: self.obligations }
454 /// The guts of `normalize`: normalize a specific projection like `<T
455 /// as Trait>::Item`. The result is always a type (and possibly
456 /// additional obligations). If ambiguity arises, which implies that
457 /// there are unresolved type variables in the projection, we will
458 /// substitute a fresh type variable `$X` and generate a new
459 /// obligation `<T as Trait>::Item == $X` for later.
460 pub fn normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
461 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
462 param_env: ty::ParamEnv<'tcx>,
463 projection_ty: ty::ProjectionTy<'tcx>,
464 cause: ObligationCause<'tcx>,
466 obligations: &mut Vec<PredicateObligation<'tcx>>)
469 opt_normalize_projection_type(selcx, param_env, projection_ty.clone(), cause.clone(), depth,
471 .unwrap_or_else(move || {
472 // if we bottom out in ambiguity, create a type variable
473 // and a deferred predicate to resolve this when more type
474 // information is available.
476 let tcx = selcx.infcx().tcx;
477 let def_id = projection_ty.item_def_id;
478 let ty_var = selcx.infcx().next_ty_var(
479 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
480 let projection = ty::Binder::dummy(ty::ProjectionPredicate {
484 let obligation = Obligation::with_depth(
485 cause, depth + 1, param_env, projection.to_predicate());
486 obligations.push(obligation);
491 /// The guts of `normalize`: normalize a specific projection like `<T
492 /// as Trait>::Item`. The result is always a type (and possibly
493 /// additional obligations). Returns `None` in the case of ambiguity,
494 /// which indicates that there are unbound type variables.
496 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
497 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
498 /// often immediately appended to another obligations vector. So now this
499 /// function takes an obligations vector and appends to it directly, which is
500 /// slightly uglier but avoids the need for an extra short-lived allocation.
501 fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
502 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
503 param_env: ty::ParamEnv<'tcx>,
504 projection_ty: ty::ProjectionTy<'tcx>,
505 cause: ObligationCause<'tcx>,
507 obligations: &mut Vec<PredicateObligation<'tcx>>)
510 let infcx = selcx.infcx();
512 let projection_ty = infcx.resolve_type_vars_if_possible(&projection_ty);
513 let cache_key = ProjectionCacheKey { ty: projection_ty };
515 debug!("opt_normalize_projection_type(\
516 projection_ty={:?}, \
521 // FIXME(#20304) For now, I am caching here, which is good, but it
522 // means we don't capture the type variables that are created in
523 // the case of ambiguity. Which means we may create a large stream
524 // of such variables. OTOH, if we move the caching up a level, we
525 // would not benefit from caching when proving `T: Trait<U=Foo>`
526 // bounds. It might be the case that we want two distinct caches,
527 // or else another kind of cache entry.
529 let cache_result = infcx.projection_cache.borrow_mut().try_start(cache_key);
532 Err(ProjectionCacheEntry::Ambiguous) => {
533 // If we found ambiguity the last time, that generally
534 // means we will continue to do so until some type in the
535 // key changes (and we know it hasn't, because we just
536 // fully resolved it). One exception though is closure
537 // types, which can transition from having a fixed kind to
538 // no kind with no visible change in the key.
540 // FIXME(#32286) refactor this so that closure type
542 debug!("opt_normalize_projection_type: \
543 found cache entry: ambiguous");
544 if !projection_ty.has_closure_types() {
548 Err(ProjectionCacheEntry::InProgress) => {
549 // If while normalized A::B, we are asked to normalize
550 // A::B, just return A::B itself. This is a conservative
551 // answer, in the sense that A::B *is* clearly equivalent
552 // to A::B, though there may be a better value we can
555 // Under lazy normalization, this can arise when
556 // bootstrapping. That is, imagine an environment with a
557 // where-clause like `A::B == u32`. Now, if we are asked
558 // to normalize `A::B`, we will want to check the
559 // where-clauses in scope. So we will try to unify `A::B`
560 // with `A::B`, which can trigger a recursive
561 // normalization. In that case, I think we will want this code:
564 // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
565 // projection_ty.substs;
566 // return Some(NormalizedTy { value: v, obligations: vec![] });
569 debug!("opt_normalize_projection_type: \
570 found cache entry: in-progress");
572 // But for now, let's classify this as an overflow:
573 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
574 let obligation = Obligation::with_depth(cause,
578 selcx.infcx().report_overflow_error(&obligation, false);
580 Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
581 // This is the hottest path in this function.
583 // If we find the value in the cache, then return it along
584 // with the obligations that went along with it. Note
585 // that, when using a fulfillment context, these
586 // obligations could in principle be ignored: they have
587 // already been registered when the cache entry was
588 // created (and hence the new ones will quickly be
589 // discarded as duplicated). But when doing trait
590 // evaluation this is not the case, and dropping the trait
591 // evaluations can causes ICEs (e.g., #43132).
592 debug!("opt_normalize_projection_type: \
593 found normalized ty `{:?}`",
596 // Once we have inferred everything we need to know, we
597 // can ignore the `obligations` from that point on.
598 if infcx.unresolved_type_vars(&ty.value).is_none() {
599 infcx.projection_cache.borrow_mut().complete_normalized(cache_key, &ty);
600 // No need to extend `obligations`.
602 obligations.extend(ty.obligations);
605 obligations.push(get_paranoid_cache_value_obligation(infcx,
610 return Some(ty.value);
612 Err(ProjectionCacheEntry::Error) => {
613 debug!("opt_normalize_projection_type: \
615 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
616 obligations.extend(result.obligations);
617 return Some(result.value)
621 let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
622 match project_type(selcx, &obligation) {
623 Ok(ProjectedTy::Progress(Progress { ty: projected_ty,
624 obligations: mut projected_obligations })) => {
625 // if projection succeeded, then what we get out of this
626 // is also non-normalized (consider: it was derived from
627 // an impl, where-clause etc) and hence we must
630 debug!("opt_normalize_projection_type: \
633 projected_obligations={:?}",
636 projected_obligations);
638 let result = if projected_ty.has_projections() {
639 let mut normalizer = AssociatedTypeNormalizer::new(selcx,
643 let normalized_ty = normalizer.fold(&projected_ty);
645 debug!("opt_normalize_projection_type: \
646 normalized_ty={:?} depth={}",
650 projected_obligations.extend(normalizer.obligations);
652 value: normalized_ty,
653 obligations: projected_obligations,
658 obligations: projected_obligations,
662 let cache_value = prune_cache_value_obligations(infcx, &result);
663 infcx.projection_cache.borrow_mut().insert_ty(cache_key, cache_value);
664 obligations.extend(result.obligations);
667 Ok(ProjectedTy::NoProgress(projected_ty)) => {
668 debug!("opt_normalize_projection_type: \
669 projected_ty={:?} no progress",
671 let result = Normalized {
675 infcx.projection_cache.borrow_mut().insert_ty(cache_key, result.clone());
676 // No need to extend `obligations`.
679 Err(ProjectionTyError::TooManyCandidates) => {
680 debug!("opt_normalize_projection_type: \
681 too many candidates");
682 infcx.projection_cache.borrow_mut()
683 .ambiguous(cache_key);
686 Err(ProjectionTyError::TraitSelectionError(_)) => {
687 debug!("opt_normalize_projection_type: ERROR");
688 // if we got an error processing the `T as Trait` part,
689 // just return `ty::err` but add the obligation `T :
690 // Trait`, which when processed will cause the error to be
693 infcx.projection_cache.borrow_mut()
695 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
696 obligations.extend(result.obligations);
702 /// If there are unresolved type variables, then we need to include
703 /// any subobligations that bind them, at least until those type
704 /// variables are fully resolved.
705 fn prune_cache_value_obligations<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
706 result: &NormalizedTy<'tcx>)
707 -> NormalizedTy<'tcx> {
708 if infcx.unresolved_type_vars(&result.value).is_none() {
709 return NormalizedTy { value: result.value, obligations: vec![] };
712 let mut obligations: Vec<_> =
715 .filter(|obligation| match obligation.predicate {
716 // We found a `T: Foo<X = U>` predicate, let's check
717 // if `U` references any unresolved type
718 // variables. In principle, we only care if this
719 // projection can help resolve any of the type
720 // variables found in `result.value` -- but we just
721 // check for any type variables here, for fear of
722 // indirect obligations (e.g., we project to `?0`,
723 // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
725 ty::Predicate::Projection(ref data) =>
726 infcx.unresolved_type_vars(&data.ty()).is_some(),
728 // We are only interested in `T: Foo<X = U>` predicates, whre
729 // `U` references one of `unresolved_type_vars`. =)
735 obligations.shrink_to_fit();
737 NormalizedTy { value: result.value, obligations }
740 /// Whenever we give back a cache result for a projection like `<T as
741 /// Trait>::Item ==> X`, we *always* include the obligation to prove
742 /// that `T: Trait` (we may also include some other obligations). This
743 /// may or may not be necessary -- in principle, all the obligations
744 /// that must be proven to show that `T: Trait` were also returned
745 /// when the cache was first populated. But there are some vague concerns,
746 /// and so we take the precautionary measure of including `T: Trait` in
749 /// Concern #1. The current setup is fragile. Perhaps someone could
750 /// have failed to prove the concerns from when the cache was
751 /// populated, but also not have used a snapshot, in which case the
752 /// cache could remain populated even though `T: Trait` has not been
753 /// shown. In this case, the "other code" is at fault -- when you
754 /// project something, you are supposed to either have a snapshot or
755 /// else prove all the resulting obligations -- but it's still easy to
758 /// Concern #2. Even within the snapshot, if those original
759 /// obligations are not yet proven, then we are able to do projections
760 /// that may yet turn out to be wrong. This *may* lead to some sort
761 /// of trouble, though we don't have a concrete example of how that
762 /// can occur yet. But it seems risky at best.
763 fn get_paranoid_cache_value_obligation<'a, 'gcx, 'tcx>(
764 infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
765 param_env: ty::ParamEnv<'tcx>,
766 projection_ty: ty::ProjectionTy<'tcx>,
767 cause: ObligationCause<'tcx>,
769 -> PredicateObligation<'tcx>
771 let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
774 recursion_depth: depth,
776 predicate: trait_ref.to_predicate(),
780 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
781 /// hold. In various error cases, we cannot generate a valid
782 /// normalized projection. Therefore, we create an inference variable
783 /// return an associated obligation that, when fulfilled, will lead to
786 /// Note that we used to return `Error` here, but that was quite
787 /// dubious -- the premise was that an error would *eventually* be
788 /// reported, when the obligation was processed. But in general once
789 /// you see a `Error` you are supposed to be able to assume that an
790 /// error *has been* reported, so that you can take whatever heuristic
791 /// paths you want to take. To make things worse, it was possible for
792 /// cycles to arise, where you basically had a setup like `<MyType<$0>
793 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
794 /// Trait>::Foo> to `[type error]` would lead to an obligation of
795 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
796 /// an error for this obligation, but we legitimately should not,
797 /// because it contains `[type error]`. Yuck! (See issue #29857 for
798 /// one case where this arose.)
799 fn normalize_to_error<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
800 param_env: ty::ParamEnv<'tcx>,
801 projection_ty: ty::ProjectionTy<'tcx>,
802 cause: ObligationCause<'tcx>,
804 -> NormalizedTy<'tcx>
806 let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
807 let trait_obligation = Obligation { cause,
808 recursion_depth: depth,
810 predicate: trait_ref.to_predicate() };
811 let tcx = selcx.infcx().tcx;
812 let def_id = projection_ty.item_def_id;
813 let new_value = selcx.infcx().next_ty_var(
814 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
817 obligations: vec![trait_obligation]
821 enum ProjectedTy<'tcx> {
822 Progress(Progress<'tcx>),
823 NoProgress(Ty<'tcx>),
826 struct Progress<'tcx> {
828 obligations: Vec<PredicateObligation<'tcx>>,
831 impl<'tcx> Progress<'tcx> {
832 fn error<'a,'gcx>(tcx: TyCtxt<'a,'gcx,'tcx>) -> Self {
839 fn with_addl_obligations(mut self,
840 mut obligations: Vec<PredicateObligation<'tcx>>)
842 debug!("with_addl_obligations: self.obligations.len={} obligations.len={}",
843 self.obligations.len(), obligations.len());
845 debug!("with_addl_obligations: self.obligations={:?} obligations={:?}",
846 self.obligations, obligations);
848 self.obligations.append(&mut obligations);
853 /// Computes the result of a projection type (if we can).
856 /// - `obligation` must be fully normalized
857 fn project_type<'cx, 'gcx, 'tcx>(
858 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
859 obligation: &ProjectionTyObligation<'tcx>)
860 -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>>
862 debug!("project(obligation={:?})",
865 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
866 if obligation.recursion_depth >= recursion_limit {
867 debug!("project: overflow!");
868 return Err(ProjectionTyError::TraitSelectionError(SelectionError::Overflow));
871 let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
873 debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
875 if obligation_trait_ref.references_error() {
876 return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
879 let mut candidates = ProjectionTyCandidateSet::None;
881 // Make sure that the following procedures are kept in order. ParamEnv
882 // needs to be first because it has highest priority, and Select checks
883 // the return value of push_candidate which assumes it's ran at last.
884 assemble_candidates_from_param_env(selcx,
886 &obligation_trait_ref,
889 assemble_candidates_from_trait_def(selcx,
891 &obligation_trait_ref,
894 assemble_candidates_from_impls(selcx,
896 &obligation_trait_ref,
900 ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
901 confirm_candidate(selcx,
903 &obligation_trait_ref,
905 ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
906 selcx.tcx().mk_projection(
907 obligation.predicate.item_def_id,
908 obligation.predicate.substs))),
909 // Error occurred while trying to processing impls.
910 ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
911 // Inherent ambiguity that prevents us from even enumerating the
913 ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
918 /// The first thing we have to do is scan through the parameter
919 /// environment to see whether there are any projection predicates
920 /// there that can answer this question.
921 fn assemble_candidates_from_param_env<'cx, 'gcx, 'tcx>(
922 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
923 obligation: &ProjectionTyObligation<'tcx>,
924 obligation_trait_ref: &ty::TraitRef<'tcx>,
925 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
927 debug!("assemble_candidates_from_param_env(..)");
928 assemble_candidates_from_predicates(selcx,
930 obligation_trait_ref,
932 ProjectionTyCandidate::ParamEnv,
933 obligation.param_env.caller_bounds.iter().cloned());
936 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
937 /// that the definition of `Foo` has some clues:
941 /// type FooT : Bar<BarT=i32>
945 /// Here, for example, we could conclude that the result is `i32`.
946 fn assemble_candidates_from_trait_def<'cx, 'gcx, 'tcx>(
947 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
948 obligation: &ProjectionTyObligation<'tcx>,
949 obligation_trait_ref: &ty::TraitRef<'tcx>,
950 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
952 debug!("assemble_candidates_from_trait_def(..)");
954 let tcx = selcx.tcx();
955 // Check whether the self-type is itself a projection.
956 let (def_id, substs) = match obligation_trait_ref.self_ty().sty {
957 ty::Projection(ref data) => {
958 (data.trait_ref(tcx).def_id, data.substs)
960 ty::Opaque(def_id, substs) => (def_id, substs),
961 ty::Infer(ty::TyVar(_)) => {
962 // If the self-type is an inference variable, then it MAY wind up
963 // being a projected type, so induce an ambiguity.
964 candidate_set.mark_ambiguous();
970 // If so, extract what we know from the trait and try to come up with a good answer.
971 let trait_predicates = tcx.predicates_of(def_id);
972 let bounds = trait_predicates.instantiate(tcx, substs);
973 let bounds = elaborate_predicates(tcx, bounds.predicates);
974 assemble_candidates_from_predicates(selcx,
976 obligation_trait_ref,
978 ProjectionTyCandidate::TraitDef,
982 fn assemble_candidates_from_predicates<'cx, 'gcx, 'tcx, I>(
983 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
984 obligation: &ProjectionTyObligation<'tcx>,
985 obligation_trait_ref: &ty::TraitRef<'tcx>,
986 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
987 ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
989 where I: IntoIterator<Item=ty::Predicate<'tcx>>
991 debug!("assemble_candidates_from_predicates(obligation={:?})",
993 let infcx = selcx.infcx();
994 for predicate in env_predicates {
995 debug!("assemble_candidates_from_predicates: predicate={:?}",
997 if let ty::Predicate::Projection(data) = predicate {
998 let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
1000 let is_match = same_def_id && infcx.probe(|_| {
1001 let data_poly_trait_ref =
1002 data.to_poly_trait_ref(infcx.tcx);
1003 let obligation_poly_trait_ref =
1004 obligation_trait_ref.to_poly_trait_ref();
1005 infcx.at(&obligation.cause, obligation.param_env)
1006 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1007 .map(|InferOk { obligations: _, value: () }| {
1008 // FIXME(#32730) -- do we need to take obligations
1009 // into account in any way? At the moment, no.
1014 debug!("assemble_candidates_from_predicates: candidate={:?} \
1015 is_match={} same_def_id={}",
1016 data, is_match, same_def_id);
1019 candidate_set.push_candidate(ctor(data));
1025 fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
1026 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1027 obligation: &ProjectionTyObligation<'tcx>,
1028 obligation_trait_ref: &ty::TraitRef<'tcx>,
1029 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
1031 // If we are resolving `<T as TraitRef<...>>::Item == Type`,
1032 // start out by selecting the predicate `T as TraitRef<...>`:
1033 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1034 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1035 let _ = selcx.infcx().commit_if_ok(|_| {
1036 let vtable = match selcx.select(&trait_obligation) {
1037 Ok(Some(vtable)) => vtable,
1039 candidate_set.mark_ambiguous();
1043 debug!("assemble_candidates_from_impls: selection error {:?}", e);
1044 candidate_set.mark_error(e);
1049 let eligible = match &vtable {
1050 super::VtableClosure(_) |
1051 super::VtableGenerator(_) |
1052 super::VtableFnPointer(_) |
1053 super::VtableObject(_) |
1054 super::VtableTraitAlias(_) => {
1055 debug!("assemble_candidates_from_impls: vtable={:?}",
1059 super::VtableImpl(impl_data) => {
1060 // We have to be careful when projecting out of an
1061 // impl because of specialization. If we are not in
1062 // codegen (i.e., projection mode is not "any"), and the
1063 // impl's type is declared as default, then we disable
1064 // projection (even if the trait ref is fully
1065 // monomorphic). In the case where trait ref is not
1066 // fully monomorphic (i.e., includes type parameters),
1067 // this is because those type parameters may
1068 // ultimately be bound to types from other crates that
1069 // may have specialized impls we can't see. In the
1070 // case where the trait ref IS fully monomorphic, this
1071 // is a policy decision that we made in the RFC in
1072 // order to preserve flexibility for the crate that
1073 // defined the specializable impl to specialize later
1074 // for existing types.
1076 // In either case, we handle this by not adding a
1077 // candidate for an impl if it contains a `default`
1079 let node_item = assoc_ty_def(selcx,
1080 impl_data.impl_def_id,
1081 obligation.predicate.item_def_id);
1083 let is_default = if node_item.node.is_from_trait() {
1084 // If true, the impl inherited a `type Foo = Bar`
1085 // given in the trait, which is implicitly default.
1086 // Otherwise, the impl did not specify `type` and
1087 // neither did the trait:
1090 // trait Foo { type T; }
1091 // impl Foo for Bar { }
1094 // This is an error, but it will be
1095 // reported in `check_impl_items_against_trait`.
1096 // We accept it here but will flag it as
1097 // an error when we confirm the candidate
1098 // (which will ultimately lead to `normalize_to_error`
1100 node_item.item.defaultness.has_value()
1102 node_item.item.defaultness.is_default() ||
1103 selcx.tcx().impl_is_default(node_item.node.def_id())
1106 // Only reveal a specializable default if we're past type-checking
1107 // and the obligations is monomorphic, otherwise passes such as
1108 // transmute checking and polymorphic MIR optimizations could
1109 // get a result which isn't correct for all monomorphizations.
1112 } else if obligation.param_env.reveal == Reveal::All {
1113 debug_assert!(!poly_trait_ref.needs_infer());
1114 if !poly_trait_ref.needs_subst() {
1123 super::VtableParam(..) => {
1124 // This case tell us nothing about the value of an
1125 // associated type. Consider:
1128 // trait SomeTrait { type Foo; }
1129 // fn foo<T:SomeTrait>(...) { }
1132 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1133 // : SomeTrait` binding does not help us decide what the
1134 // type `Foo` is (at least, not more specifically than
1135 // what we already knew).
1137 // But wait, you say! What about an example like this:
1140 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1143 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1144 // resolve `T::Foo`? And of course it does, but in fact
1145 // that single predicate is desugared into two predicates
1146 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1147 // projection. And the projection where clause is handled
1148 // in `assemble_candidates_from_param_env`.
1151 super::VtableAutoImpl(..) |
1152 super::VtableBuiltin(..) => {
1153 // These traits have no associated types.
1155 obligation.cause.span,
1156 "Cannot project an associated type from `{:?}`",
1162 if candidate_set.push_candidate(ProjectionTyCandidate::Select(vtable)) {
1173 fn confirm_candidate<'cx, 'gcx, 'tcx>(
1174 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1175 obligation: &ProjectionTyObligation<'tcx>,
1176 obligation_trait_ref: &ty::TraitRef<'tcx>,
1177 candidate: ProjectionTyCandidate<'tcx>)
1180 debug!("confirm_candidate(candidate={:?}, obligation={:?})",
1185 ProjectionTyCandidate::ParamEnv(poly_projection) |
1186 ProjectionTyCandidate::TraitDef(poly_projection) => {
1187 confirm_param_env_candidate(selcx, obligation, poly_projection)
1190 ProjectionTyCandidate::Select(vtable) => {
1191 confirm_select_candidate(selcx, obligation, obligation_trait_ref, vtable)
1196 fn confirm_select_candidate<'cx, 'gcx, 'tcx>(
1197 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1198 obligation: &ProjectionTyObligation<'tcx>,
1199 obligation_trait_ref: &ty::TraitRef<'tcx>,
1200 vtable: Selection<'tcx>)
1204 super::VtableImpl(data) =>
1205 confirm_impl_candidate(selcx, obligation, data),
1206 super::VtableGenerator(data) =>
1207 confirm_generator_candidate(selcx, obligation, data),
1208 super::VtableClosure(data) =>
1209 confirm_closure_candidate(selcx, obligation, data),
1210 super::VtableFnPointer(data) =>
1211 confirm_fn_pointer_candidate(selcx, obligation, data),
1212 super::VtableObject(_) =>
1213 confirm_object_candidate(selcx, obligation, obligation_trait_ref),
1214 super::VtableAutoImpl(..) |
1215 super::VtableParam(..) |
1216 super::VtableBuiltin(..) |
1217 super::VtableTraitAlias(..) =>
1218 // we don't create Select candidates with this kind of resolution
1220 obligation.cause.span,
1221 "Cannot project an associated type from `{:?}`",
1226 fn confirm_object_candidate<'cx, 'gcx, 'tcx>(
1227 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1228 obligation: &ProjectionTyObligation<'tcx>,
1229 obligation_trait_ref: &ty::TraitRef<'tcx>)
1232 let self_ty = obligation_trait_ref.self_ty();
1233 let object_ty = selcx.infcx().shallow_resolve(self_ty);
1234 debug!("confirm_object_candidate(object_ty={:?})",
1236 let data = match object_ty.sty {
1237 ty::Dynamic(ref data, ..) => data,
1240 obligation.cause.span,
1241 "confirm_object_candidate called with non-object: {:?}",
1245 let env_predicates = data.projection_bounds().map(|p| {
1246 p.with_self_ty(selcx.tcx(), object_ty).to_predicate()
1248 let env_predicate = {
1249 let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1251 // select only those projections that are actually projecting an
1252 // item with the correct name
1253 let env_predicates = env_predicates.filter_map(|p| match p {
1254 ty::Predicate::Projection(data) =>
1255 if data.projection_def_id() == obligation.predicate.item_def_id {
1263 // select those with a relevant trait-ref
1264 let mut env_predicates = env_predicates.filter(|data| {
1265 let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1266 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1267 selcx.infcx().probe(|_|
1268 selcx.infcx().at(&obligation.cause, obligation.param_env)
1269 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1274 // select the first matching one; there really ought to be one or
1275 // else the object type is not WF, since an object type should
1276 // include all of its projections explicitly
1277 match env_predicates.next() {
1278 Some(env_predicate) => env_predicate,
1280 debug!("confirm_object_candidate: no env-predicate \
1281 found in object type `{:?}`; ill-formed",
1283 return Progress::error(selcx.tcx());
1288 confirm_param_env_candidate(selcx, obligation, env_predicate)
1291 fn confirm_generator_candidate<'cx, 'gcx, 'tcx>(
1292 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1293 obligation: &ProjectionTyObligation<'tcx>,
1294 vtable: VtableGeneratorData<'tcx, PredicateObligation<'tcx>>)
1297 let gen_sig = vtable.substs.poly_sig(vtable.generator_def_id, selcx.tcx());
1301 } = normalize_with_depth(selcx,
1302 obligation.param_env,
1303 obligation.cause.clone(),
1304 obligation.recursion_depth+1,
1307 debug!("confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1312 let tcx = selcx.tcx();
1314 let gen_def_id = tcx.lang_items().gen_trait().unwrap();
1317 tcx.generator_trait_ref_and_outputs(gen_def_id,
1318 obligation.predicate.self_ty(),
1320 .map_bound(|(trait_ref, yield_ty, return_ty)| {
1321 let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1322 let ty = if name == sym::Return {
1324 } else if name == sym::Yield {
1330 ty::ProjectionPredicate {
1331 projection_ty: ty::ProjectionTy {
1332 substs: trait_ref.substs,
1333 item_def_id: obligation.predicate.item_def_id,
1339 confirm_param_env_candidate(selcx, obligation, predicate)
1340 .with_addl_obligations(vtable.nested)
1341 .with_addl_obligations(obligations)
1344 fn confirm_fn_pointer_candidate<'cx, 'gcx, 'tcx>(
1345 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1346 obligation: &ProjectionTyObligation<'tcx>,
1347 fn_pointer_vtable: VtableFnPointerData<'tcx, PredicateObligation<'tcx>>)
1350 let fn_type = selcx.infcx().shallow_resolve(fn_pointer_vtable.fn_ty);
1351 let sig = fn_type.fn_sig(selcx.tcx());
1355 } = normalize_with_depth(selcx,
1356 obligation.param_env,
1357 obligation.cause.clone(),
1358 obligation.recursion_depth+1,
1361 confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1362 .with_addl_obligations(fn_pointer_vtable.nested)
1363 .with_addl_obligations(obligations)
1366 fn confirm_closure_candidate<'cx, 'gcx, 'tcx>(
1367 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1368 obligation: &ProjectionTyObligation<'tcx>,
1369 vtable: VtableClosureData<'tcx, PredicateObligation<'tcx>>)
1372 let tcx = selcx.tcx();
1373 let infcx = selcx.infcx();
1374 let closure_sig_ty = vtable.substs.closure_sig_ty(vtable.closure_def_id, tcx);
1375 let closure_sig = infcx.shallow_resolve(closure_sig_ty).fn_sig(tcx);
1379 } = normalize_with_depth(selcx,
1380 obligation.param_env,
1381 obligation.cause.clone(),
1382 obligation.recursion_depth+1,
1385 debug!("confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1390 confirm_callable_candidate(selcx,
1393 util::TupleArgumentsFlag::No)
1394 .with_addl_obligations(vtable.nested)
1395 .with_addl_obligations(obligations)
1398 fn confirm_callable_candidate<'cx, 'gcx, 'tcx>(
1399 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1400 obligation: &ProjectionTyObligation<'tcx>,
1401 fn_sig: ty::PolyFnSig<'tcx>,
1402 flag: util::TupleArgumentsFlag)
1405 let tcx = selcx.tcx();
1407 debug!("confirm_callable_candidate({:?},{:?})",
1411 // the `Output` associated type is declared on `FnOnce`
1412 let fn_once_def_id = tcx.lang_items().fn_once_trait().unwrap();
1415 tcx.closure_trait_ref_and_return_type(fn_once_def_id,
1416 obligation.predicate.self_ty(),
1419 .map_bound(|(trait_ref, ret_type)|
1420 ty::ProjectionPredicate {
1421 projection_ty: ty::ProjectionTy::from_ref_and_name(
1424 Ident::from_str(FN_OUTPUT_NAME),
1430 confirm_param_env_candidate(selcx, obligation, predicate)
1433 fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
1434 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1435 obligation: &ProjectionTyObligation<'tcx>,
1436 poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
1437 ) -> Progress<'tcx> {
1438 let infcx = selcx.infcx();
1439 let cause = &obligation.cause;
1440 let param_env = obligation.param_env;
1442 let (cache_entry, _) =
1443 infcx.replace_bound_vars_with_fresh_vars(
1445 LateBoundRegionConversionTime::HigherRankedType,
1448 let cache_trait_ref = cache_entry.projection_ty.trait_ref(infcx.tcx);
1449 let obligation_trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1450 match infcx.at(cause, param_env).eq(cache_trait_ref, obligation_trait_ref) {
1451 Ok(InferOk { value: _, obligations }) => {
1459 "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
1464 debug!("confirm_param_env_candidate: {}", msg);
1465 infcx.tcx.sess.delay_span_bug(obligation.cause.span, &msg);
1467 ty: infcx.tcx.types.err,
1468 obligations: vec![],
1474 fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
1475 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1476 obligation: &ProjectionTyObligation<'tcx>,
1477 impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>)
1480 let VtableImplData { impl_def_id, substs, nested } = impl_vtable;
1482 let tcx = selcx.tcx();
1483 let param_env = obligation.param_env;
1484 let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_def_id);
1486 if !assoc_ty.item.defaultness.has_value() {
1487 // This means that the impl is missing a definition for the
1488 // associated type. This error will be reported by the type
1489 // checker method `check_impl_items_against_trait`, so here we
1490 // just return Error.
1491 debug!("confirm_impl_candidate: no associated type {:?} for {:?}",
1492 assoc_ty.item.ident,
1493 obligation.predicate);
1496 obligations: nested,
1499 let substs = translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.node);
1500 let ty = if let ty::AssociatedKind::Existential = assoc_ty.item.kind {
1501 let item_substs = InternalSubsts::identity_for_item(tcx, assoc_ty.item.def_id);
1502 tcx.mk_opaque(assoc_ty.item.def_id, item_substs)
1504 tcx.type_of(assoc_ty.item.def_id)
1507 ty: ty.subst(tcx, substs),
1508 obligations: nested,
1512 /// Locate the definition of an associated type in the specialization hierarchy,
1513 /// starting from the given impl.
1515 /// Based on the "projection mode", this lookup may in fact only examine the
1516 /// topmost impl. See the comments for `Reveal` for more details.
1517 fn assoc_ty_def<'cx, 'gcx, 'tcx>(
1518 selcx: &SelectionContext<'cx, 'gcx, 'tcx>,
1520 assoc_ty_def_id: DefId)
1521 -> specialization_graph::NodeItem<ty::AssociatedItem>
1523 let tcx = selcx.tcx();
1524 let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1525 let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1526 let trait_def = tcx.trait_def(trait_def_id);
1528 // This function may be called while we are still building the
1529 // specialization graph that is queried below (via TraidDef::ancestors()),
1530 // so, in order to avoid unnecessary infinite recursion, we manually look
1531 // for the associated item at the given impl.
1532 // If there is no such item in that impl, this function will fail with a
1533 // cycle error if the specialization graph is currently being built.
1534 let impl_node = specialization_graph::Node::Impl(impl_def_id);
1535 for item in impl_node.items(tcx) {
1536 if item.kind == ty::AssociatedKind::Type &&
1537 tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id) {
1538 return specialization_graph::NodeItem {
1539 node: specialization_graph::Node::Impl(impl_def_id),
1545 if let Some(assoc_item) = trait_def
1546 .ancestors(tcx, impl_def_id)
1547 .defs(tcx, assoc_ty_name, ty::AssociatedKind::Type, trait_def_id)
1551 // This is saying that neither the trait nor
1552 // the impl contain a definition for this
1553 // associated type. Normally this situation
1554 // could only arise through a compiler bug --
1555 // if the user wrote a bad item name, it
1556 // should have failed in astconv.
1557 bug!("No associated type `{}` for {}",
1559 tcx.def_path_str(impl_def_id))
1565 /// The projection cache. Unlike the standard caches, this can include
1566 /// infcx-dependent type variables, therefore we have to roll the
1567 /// cache back each time we roll a snapshot back, to avoid assumptions
1568 /// on yet-unresolved inference variables. Types with placeholder
1569 /// regions also have to be removed when the respective snapshot ends.
1571 /// Because of that, projection cache entries can be "stranded" and left
1572 /// inaccessible when type variables inside the key are resolved. We make no
1573 /// attempt to recover or remove "stranded" entries, but rather let them be
1574 /// (for the lifetime of the infcx).
1576 /// Entries in the projection cache might contain inference variables
1577 /// that will be resolved by obligations on the projection cache entry (e.g.,
1578 /// when a type parameter in the associated type is constrained through
1579 /// an "RFC 447" projection on the impl).
1581 /// When working with a fulfillment context, the derived obligations of each
1582 /// projection cache entry will be registered on the fulfillcx, so any users
1583 /// that can wait for a fulfillcx fixed point need not care about this. However,
1584 /// users that don't wait for a fixed point (e.g., trait evaluation) have to
1585 /// resolve the obligations themselves to make sure the projected result is
1586 /// ok and avoid issues like #43132.
1588 /// If that is done, after evaluation the obligations, it is a good idea to
1589 /// call `ProjectionCache::complete` to make sure the obligations won't be
1590 /// re-evaluated and avoid an exponential worst-case.
1592 // FIXME: we probably also want some sort of cross-infcx cache here to
1593 // reduce the amount of duplication. Let's see what we get with the Chalk reforms.
1595 pub struct ProjectionCache<'tcx> {
1596 map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
1599 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
1600 pub struct ProjectionCacheKey<'tcx> {
1601 ty: ty::ProjectionTy<'tcx>
1604 impl<'cx, 'gcx, 'tcx> ProjectionCacheKey<'tcx> {
1605 pub fn from_poly_projection_predicate(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1606 predicate: &ty::PolyProjectionPredicate<'tcx>)
1609 let infcx = selcx.infcx();
1610 // We don't do cross-snapshot caching of obligations with escaping regions,
1611 // so there's no cache key to use
1612 predicate.no_bound_vars()
1613 .map(|predicate| ProjectionCacheKey {
1614 // We don't attempt to match up with a specific type-variable state
1615 // from a specific call to `opt_normalize_projection_type` - if
1616 // there's no precise match, the original cache entry is "stranded"
1618 ty: infcx.resolve_type_vars_if_possible(&predicate.projection_ty)
1623 #[derive(Clone, Debug)]
1624 enum ProjectionCacheEntry<'tcx> {
1628 NormalizedTy(NormalizedTy<'tcx>),
1631 // N.B., intentionally not Clone
1632 pub struct ProjectionCacheSnapshot {
1636 impl<'tcx> ProjectionCache<'tcx> {
1637 pub fn clear(&mut self) {
1641 pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
1642 ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
1645 pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
1646 self.map.rollback_to(snapshot.snapshot);
1649 pub fn rollback_placeholder(&mut self, snapshot: &ProjectionCacheSnapshot) {
1650 self.map.partial_rollback(&snapshot.snapshot, &|k| k.ty.has_re_placeholders());
1653 pub fn commit(&mut self, snapshot: ProjectionCacheSnapshot) {
1654 self.map.commit(snapshot.snapshot);
1657 /// Try to start normalize `key`; returns an error if
1658 /// normalization already occurred (this error corresponds to a
1659 /// cache hit, so it's actually a good thing).
1660 fn try_start(&mut self, key: ProjectionCacheKey<'tcx>)
1661 -> Result<(), ProjectionCacheEntry<'tcx>> {
1662 if let Some(entry) = self.map.get(&key) {
1663 return Err(entry.clone());
1666 self.map.insert(key, ProjectionCacheEntry::InProgress);
1670 /// Indicates that `key` was normalized to `value`.
1671 fn insert_ty(&mut self, key: ProjectionCacheKey<'tcx>, value: NormalizedTy<'tcx>) {
1672 debug!("ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
1674 let fresh_key = self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
1675 assert!(!fresh_key, "never started projecting `{:?}`", key);
1678 /// Mark the relevant projection cache key as having its derived obligations
1679 /// complete, so they won't have to be re-computed (this is OK to do in a
1680 /// snapshot - if the snapshot is rolled back, the obligations will be
1681 /// marked as incomplete again).
1682 pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
1683 let ty = match self.map.get(&key) {
1684 Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
1685 debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}",
1690 // Type inference could "strand behind" old cache entries. Leave
1691 // them alone for now.
1692 debug!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}",
1698 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1704 /// A specialized version of `complete` for when the key's value is known
1705 /// to be a NormalizedTy.
1706 pub fn complete_normalized(&mut self, key: ProjectionCacheKey<'tcx>, ty: &NormalizedTy<'tcx>) {
1707 // We want to insert `ty` with no obligations. If the existing value
1708 // already has no obligations (as is common) we don't insert anything.
1709 if !ty.obligations.is_empty() {
1710 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1717 /// Indicates that trying to normalize `key` resulted in
1718 /// ambiguity. No point in trying it again then until we gain more
1719 /// type information (in which case, the "fully resolved" key will
1721 fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
1722 let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
1723 assert!(!fresh, "never started projecting `{:?}`", key);
1726 /// Indicates that trying to normalize `key` resulted in
1728 fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
1729 let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
1730 assert!(!fresh, "never started projecting `{:?}`", key);