1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Code for projecting associated types out of trait references.
13 use super::elaborate_predicates;
14 use super::specialization_graph;
15 use super::translate_substs;
16 use super::Obligation;
17 use super::ObligationCause;
18 use super::PredicateObligation;
20 use super::SelectionContext;
21 use super::SelectionError;
22 use super::VtableClosureData;
23 use super::VtableGeneratorData;
24 use super::VtableFnPointerData;
25 use super::VtableImplData;
28 use hir::def_id::DefId;
29 use infer::{InferCtxt, InferOk};
30 use infer::type_variable::TypeVariableOrigin;
31 use mir::interpret::ConstValue;
32 use mir::interpret::{GlobalId};
33 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
34 use syntax::ast::Ident;
35 use ty::subst::{Subst, Substs};
36 use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt};
37 use ty::fold::{TypeFoldable, TypeFolder};
38 use util::common::FN_OUTPUT_NAME;
40 /// Depending on the stage of compilation, we want projection to be
41 /// more or less conservative.
42 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
44 /// At type-checking time, we refuse to project any associated
45 /// type that is marked `default`. Non-`default` ("final") types
46 /// are always projected. This is necessary in general for
47 /// soundness of specialization. However, we *could* allow
48 /// projections in fully-monomorphic cases. We choose not to,
49 /// because we prefer for `default type` to force the type
50 /// definition to be treated abstractly by any consumers of the
51 /// impl. Concretely, that means that the following example will
59 /// impl<T> Assoc for T {
60 /// default type Output = bool;
64 /// let <() as Assoc>::Output = true;
68 /// At codegen time, all monomorphic projections will succeed.
69 /// Also, `impl Trait` is normalized to the concrete type,
70 /// which has to be already collected by type-checking.
72 /// NOTE: As `impl Trait`'s concrete type should *never*
73 /// be observable directly by the user, `Reveal::All`
74 /// should not be used by checks which may expose
75 /// type equality or type contents to the user.
76 /// There are some exceptions, e.g. around OIBITS and
77 /// transmute-checking, which expose some details, but
78 /// not the whole concrete type of the `impl Trait`.
82 pub type PolyProjectionObligation<'tcx> =
83 Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
85 pub type ProjectionObligation<'tcx> =
86 Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
88 pub type ProjectionTyObligation<'tcx> =
89 Obligation<'tcx, ty::ProjectionTy<'tcx>>;
91 /// When attempting to resolve `<T as TraitRef>::Name` ...
93 pub enum ProjectionTyError<'tcx> {
94 /// ...we found multiple sources of information and couldn't resolve the ambiguity.
97 /// ...an error occurred matching `T : TraitRef`
98 TraitSelectionError(SelectionError<'tcx>),
102 pub struct MismatchedProjectionTypes<'tcx> {
103 pub err: ty::error::TypeError<'tcx>
106 #[derive(PartialEq, Eq, Debug)]
107 enum ProjectionTyCandidate<'tcx> {
108 // from a where-clause in the env or object type
109 ParamEnv(ty::PolyProjectionPredicate<'tcx>),
111 // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
112 TraitDef(ty::PolyProjectionPredicate<'tcx>),
114 // from a "impl" (or a "pseudo-impl" returned by select)
115 Select(Selection<'tcx>),
118 enum ProjectionTyCandidateSet<'tcx> {
120 Single(ProjectionTyCandidate<'tcx>),
122 Error(SelectionError<'tcx>),
125 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
126 fn mark_ambiguous(&mut self) {
127 *self = ProjectionTyCandidateSet::Ambiguous;
130 fn mark_error(&mut self, err: SelectionError<'tcx>) {
131 *self = ProjectionTyCandidateSet::Error(err);
134 // Returns true if the push was successful, or false if the candidate
135 // was discarded -- this could be because of ambiguity, or because
136 // a higher-priority candidate is already there.
137 fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
138 use self::ProjectionTyCandidateSet::*;
139 use self::ProjectionTyCandidate::*;
141 // This wacky variable is just used to try and
142 // make code readable and avoid confusing paths.
143 // It is assigned a "value" of `()` only on those
144 // paths in which we wish to convert `*self` to
145 // ambiguous (and return false, because the candidate
146 // was not used). On other paths, it is not assigned,
147 // and hence if those paths *could* reach the code that
148 // comes after the match, this fn would not compile.
149 let convert_to_ambiguous;
153 *self = Single(candidate);
158 // Duplicates can happen inside ParamEnv. In the case, we
159 // perform a lazy deduplication.
160 if current == &candidate {
164 // Prefer where-clauses. As in select, if there are multiple
165 // candidates, we prefer where-clause candidates over impls. This
166 // may seem a bit surprising, since impls are the source of
167 // "truth" in some sense, but in fact some of the impls that SEEM
168 // applicable are not, because of nested obligations. Where
169 // clauses are the safer choice. See the comment on
170 // `select::SelectionCandidate` and #21974 for more details.
171 match (current, candidate) {
172 (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
173 (ParamEnv(..), _) => return false,
174 (_, ParamEnv(..)) => { unreachable!(); }
175 (_, _) => convert_to_ambiguous = (),
179 Ambiguous | Error(..) => {
184 // We only ever get here when we moved from a single candidate
186 let () = convert_to_ambiguous;
192 /// Evaluates constraints of the form:
194 /// for<...> <T as Trait>::U == V
196 /// If successful, this may result in additional obligations. Also returns
197 /// the projection cache key used to track these additional obligations.
198 pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
199 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
200 obligation: &PolyProjectionObligation<'tcx>)
201 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
202 MismatchedProjectionTypes<'tcx>>
204 debug!("poly_project_and_unify_type(obligation={:?})",
207 let infcx = selcx.infcx();
208 infcx.commit_if_ok(|snapshot| {
209 let (skol_predicate, skol_map) =
210 infcx.skolemize_late_bound_regions(&obligation.predicate);
212 let skol_obligation = obligation.with(skol_predicate);
213 let r = match project_and_unify_type(selcx, &skol_obligation) {
215 let span = obligation.cause.span;
216 match infcx.leak_check(false, span, &skol_map, snapshot) {
217 Ok(()) => Ok(infcx.plug_leaks(skol_map, snapshot, result)),
219 debug!("poly_project_and_unify_type: leak check encountered error {:?}", e);
220 Err(MismatchedProjectionTypes { err: e })
233 /// Evaluates constraints of the form:
235 /// <T as Trait>::U == V
237 /// If successful, this may result in additional obligations.
238 fn project_and_unify_type<'cx, 'gcx, 'tcx>(
239 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
240 obligation: &ProjectionObligation<'tcx>)
241 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
242 MismatchedProjectionTypes<'tcx>>
244 debug!("project_and_unify_type(obligation={:?})",
247 let mut obligations = vec![];
249 match opt_normalize_projection_type(selcx,
250 obligation.param_env,
251 obligation.predicate.projection_ty,
252 obligation.cause.clone(),
253 obligation.recursion_depth,
256 None => return Ok(None),
259 debug!("project_and_unify_type: normalized_ty={:?} obligations={:?}",
263 let infcx = selcx.infcx();
264 match infcx.at(&obligation.cause, obligation.param_env)
265 .eq(normalized_ty, obligation.predicate.ty) {
266 Ok(InferOk { obligations: inferred_obligations, value: () }) => {
267 obligations.extend(inferred_obligations);
268 Ok(Some(obligations))
271 debug!("project_and_unify_type: equating types encountered error {:?}", err);
272 Err(MismatchedProjectionTypes { err: err })
277 /// Normalizes any associated type projections in `value`, replacing
278 /// them with a fully resolved type where possible. The return value
279 /// combines the normalized result and any additional obligations that
280 /// were incurred as result.
281 pub fn normalize<'a, 'b, 'gcx, 'tcx, T>(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
282 param_env: ty::ParamEnv<'tcx>,
283 cause: ObligationCause<'tcx>,
285 -> Normalized<'tcx, T>
286 where T : TypeFoldable<'tcx>
288 normalize_with_depth(selcx, param_env, cause, 0, value)
291 /// As `normalize`, but with a custom depth.
292 pub fn normalize_with_depth<'a, 'b, 'gcx, 'tcx, T>(
293 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
294 param_env: ty::ParamEnv<'tcx>,
295 cause: ObligationCause<'tcx>,
298 -> Normalized<'tcx, T>
300 where T : TypeFoldable<'tcx>
302 debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
303 let mut normalizer = AssociatedTypeNormalizer::new(selcx, param_env, cause, depth);
304 let result = normalizer.fold(value);
305 debug!("normalize_with_depth: depth={} result={:?} with {} obligations",
306 depth, result, normalizer.obligations.len());
307 debug!("normalize_with_depth: depth={} obligations={:?}",
308 depth, normalizer.obligations);
311 obligations: normalizer.obligations,
315 struct AssociatedTypeNormalizer<'a, 'b: 'a, 'gcx: 'b+'tcx, 'tcx: 'b> {
316 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
317 param_env: ty::ParamEnv<'tcx>,
318 cause: ObligationCause<'tcx>,
319 obligations: Vec<PredicateObligation<'tcx>>,
323 impl<'a, 'b, 'gcx, 'tcx> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
324 fn new(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
325 param_env: ty::ParamEnv<'tcx>,
326 cause: ObligationCause<'tcx>,
328 -> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx>
330 AssociatedTypeNormalizer {
339 fn fold<T:TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
340 let value = self.selcx.infcx().resolve_type_vars_if_possible(value);
342 if !value.has_projections() {
345 value.fold_with(self)
350 impl<'a, 'b, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
351 fn tcx<'c>(&'c self) -> TyCtxt<'c, 'gcx, 'tcx> {
355 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
356 // We don't want to normalize associated types that occur inside of region
357 // binders, because they may contain bound regions, and we can't cope with that.
361 // for<'a> fn(<T as Foo<&'a>>::A)
363 // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
364 // normalize it when we instantiate those bound regions (which
365 // should occur eventually).
367 let ty = ty.super_fold_with(self);
369 ty::TyAnon(def_id, substs) if !substs.has_escaping_regions() => { // (*)
370 // Only normalize `impl Trait` after type-checking, usually in codegen.
371 match self.param_env.reveal {
372 Reveal::UserFacing => ty,
375 let recursion_limit = *self.tcx().sess.recursion_limit.get();
376 if self.depth >= recursion_limit {
377 let obligation = Obligation::with_depth(
383 self.selcx.infcx().report_overflow_error(&obligation, true);
386 let generic_ty = self.tcx().type_of(def_id);
387 let concrete_ty = generic_ty.subst(self.tcx(), substs);
389 let folded_ty = self.fold_ty(concrete_ty);
396 ty::TyProjection(ref data) if !data.has_escaping_regions() => { // (*)
398 // (*) This is kind of hacky -- we need to be able to
399 // handle normalization within binders because
400 // otherwise we wind up a need to normalize when doing
401 // trait matching (since you can have a trait
402 // obligation like `for<'a> T::B : Fn(&'a int)`), but
403 // we can't normalize with bound regions in scope. So
404 // far now we just ignore binders but only normalize
405 // if all bound regions are gone (and then we still
406 // have to renormalize whenever we instantiate a
407 // binder). It would be better to normalize in a
408 // binding-aware fashion.
410 let normalized_ty = normalize_projection_type(self.selcx,
415 &mut self.obligations);
416 debug!("AssociatedTypeNormalizer: depth={} normalized {:?} to {:?}, \
417 now with {} obligations",
418 self.depth, ty, normalized_ty, self.obligations.len());
428 fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
429 if let ConstValue::Unevaluated(def_id, substs) = constant.val {
430 let tcx = self.selcx.tcx().global_tcx();
431 if let Some(param_env) = self.tcx().lift_to_global(&self.param_env) {
432 if substs.needs_infer() || substs.has_skol() {
433 let identity_substs = Substs::identity_for_item(tcx, def_id);
434 let instance = ty::Instance::resolve(tcx, param_env, def_id, identity_substs);
435 if let Some(instance) = instance {
440 match tcx.const_eval(param_env.and(cid)) {
442 let evaluated = evaluated.subst(self.tcx(), substs);
443 return self.fold_const(evaluated);
449 if let Some(substs) = self.tcx().lift_to_global(&substs) {
450 let instance = ty::Instance::resolve(tcx, param_env, def_id, substs);
451 if let Some(instance) = instance {
456 match tcx.const_eval(param_env.and(cid)) {
457 Ok(evaluated) => return self.fold_const(evaluated),
470 pub struct Normalized<'tcx,T> {
472 pub obligations: Vec<PredicateObligation<'tcx>>,
475 pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
477 impl<'tcx,T> Normalized<'tcx,T> {
478 pub fn with<U>(self, value: U) -> Normalized<'tcx,U> {
479 Normalized { value: value, obligations: self.obligations }
483 /// The guts of `normalize`: normalize a specific projection like `<T
484 /// as Trait>::Item`. The result is always a type (and possibly
485 /// additional obligations). If ambiguity arises, which implies that
486 /// there are unresolved type variables in the projection, we will
487 /// substitute a fresh type variable `$X` and generate a new
488 /// obligation `<T as Trait>::Item == $X` for later.
489 pub fn normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
490 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
491 param_env: ty::ParamEnv<'tcx>,
492 projection_ty: ty::ProjectionTy<'tcx>,
493 cause: ObligationCause<'tcx>,
495 obligations: &mut Vec<PredicateObligation<'tcx>>)
498 opt_normalize_projection_type(selcx, param_env, projection_ty.clone(), cause.clone(), depth,
500 .unwrap_or_else(move || {
501 // if we bottom out in ambiguity, create a type variable
502 // and a deferred predicate to resolve this when more type
503 // information is available.
505 let tcx = selcx.infcx().tcx;
506 let def_id = projection_ty.item_def_id;
507 let ty_var = selcx.infcx().next_ty_var(
508 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
509 let projection = ty::Binder::dummy(ty::ProjectionPredicate {
513 let obligation = Obligation::with_depth(
514 cause, depth + 1, param_env, projection.to_predicate());
515 obligations.push(obligation);
520 /// The guts of `normalize`: normalize a specific projection like `<T
521 /// as Trait>::Item`. The result is always a type (and possibly
522 /// additional obligations). Returns `None` in the case of ambiguity,
523 /// which indicates that there are unbound type variables.
525 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
526 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
527 /// often immediately appended to another obligations vector. So now this
528 /// function takes an obligations vector and appends to it directly, which is
529 /// slightly uglier but avoids the need for an extra short-lived allocation.
530 fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
531 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
532 param_env: ty::ParamEnv<'tcx>,
533 projection_ty: ty::ProjectionTy<'tcx>,
534 cause: ObligationCause<'tcx>,
536 obligations: &mut Vec<PredicateObligation<'tcx>>)
539 let infcx = selcx.infcx();
541 let projection_ty = infcx.resolve_type_vars_if_possible(&projection_ty);
542 let cache_key = ProjectionCacheKey { ty: projection_ty };
544 debug!("opt_normalize_projection_type(\
545 projection_ty={:?}, \
550 // FIXME(#20304) For now, I am caching here, which is good, but it
551 // means we don't capture the type variables that are created in
552 // the case of ambiguity. Which means we may create a large stream
553 // of such variables. OTOH, if we move the caching up a level, we
554 // would not benefit from caching when proving `T: Trait<U=Foo>`
555 // bounds. It might be the case that we want two distinct caches,
556 // or else another kind of cache entry.
558 let cache_result = infcx.projection_cache.borrow_mut().try_start(cache_key);
561 Err(ProjectionCacheEntry::Ambiguous) => {
562 // If we found ambiguity the last time, that generally
563 // means we will continue to do so until some type in the
564 // key changes (and we know it hasn't, because we just
565 // fully resolved it). One exception though is closure
566 // types, which can transition from having a fixed kind to
567 // no kind with no visible change in the key.
569 // FIXME(#32286) refactor this so that closure type
571 debug!("opt_normalize_projection_type: \
572 found cache entry: ambiguous");
573 if !projection_ty.has_closure_types() {
577 Err(ProjectionCacheEntry::InProgress) => {
578 // If while normalized A::B, we are asked to normalize
579 // A::B, just return A::B itself. This is a conservative
580 // answer, in the sense that A::B *is* clearly equivalent
581 // to A::B, though there may be a better value we can
584 // Under lazy normalization, this can arise when
585 // bootstrapping. That is, imagine an environment with a
586 // where-clause like `A::B == u32`. Now, if we are asked
587 // to normalize `A::B`, we will want to check the
588 // where-clauses in scope. So we will try to unify `A::B`
589 // with `A::B`, which can trigger a recursive
590 // normalization. In that case, I think we will want this code:
593 // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
594 // projection_ty.substs;
595 // return Some(NormalizedTy { value: v, obligations: vec![] });
598 debug!("opt_normalize_projection_type: \
599 found cache entry: in-progress");
601 // But for now, let's classify this as an overflow:
602 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
603 let obligation = Obligation::with_depth(cause.clone(),
607 selcx.infcx().report_overflow_error(&obligation, false);
609 Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
610 // This is the hottest path in this function.
612 // If we find the value in the cache, then return it along
613 // with the obligations that went along with it. Note
614 // that, when using a fulfillment context, these
615 // obligations could in principle be ignored: they have
616 // already been registered when the cache entry was
617 // created (and hence the new ones will quickly be
618 // discarded as duplicated). But when doing trait
619 // evaluation this is not the case, and dropping the trait
620 // evaluations can causes ICEs (e.g. #43132).
621 debug!("opt_normalize_projection_type: \
622 found normalized ty `{:?}`",
625 // Once we have inferred everything we need to know, we
626 // can ignore the `obligations` from that point on.
627 if !infcx.any_unresolved_type_vars(&ty.value) {
628 infcx.projection_cache.borrow_mut().complete_normalized(cache_key, &ty);
629 // No need to extend `obligations`.
631 obligations.extend(ty.obligations);
634 obligations.push(get_paranoid_cache_value_obligation(infcx,
639 return Some(ty.value);
641 Err(ProjectionCacheEntry::Error) => {
642 debug!("opt_normalize_projection_type: \
644 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
645 obligations.extend(result.obligations);
646 return Some(result.value)
650 let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
651 match project_type(selcx, &obligation) {
652 Ok(ProjectedTy::Progress(Progress { ty: projected_ty,
653 obligations: mut projected_obligations })) => {
654 // if projection succeeded, then what we get out of this
655 // is also non-normalized (consider: it was derived from
656 // an impl, where-clause etc) and hence we must
659 debug!("opt_normalize_projection_type: \
662 projected_obligations={:?}",
665 projected_obligations);
667 let result = if projected_ty.has_projections() {
668 let mut normalizer = AssociatedTypeNormalizer::new(selcx,
672 let normalized_ty = normalizer.fold(&projected_ty);
674 debug!("opt_normalize_projection_type: \
675 normalized_ty={:?} depth={}",
679 projected_obligations.extend(normalizer.obligations);
681 value: normalized_ty,
682 obligations: projected_obligations,
687 obligations: projected_obligations,
691 let cache_value = prune_cache_value_obligations(infcx, &result);
692 infcx.projection_cache.borrow_mut().insert_ty(cache_key, cache_value);
693 obligations.extend(result.obligations);
696 Ok(ProjectedTy::NoProgress(projected_ty)) => {
697 debug!("opt_normalize_projection_type: \
698 projected_ty={:?} no progress",
700 let result = Normalized {
704 infcx.projection_cache.borrow_mut().insert_ty(cache_key, result.clone());
705 // No need to extend `obligations`.
708 Err(ProjectionTyError::TooManyCandidates) => {
709 debug!("opt_normalize_projection_type: \
710 too many candidates");
711 infcx.projection_cache.borrow_mut()
712 .ambiguous(cache_key);
715 Err(ProjectionTyError::TraitSelectionError(_)) => {
716 debug!("opt_normalize_projection_type: ERROR");
717 // if we got an error processing the `T as Trait` part,
718 // just return `ty::err` but add the obligation `T :
719 // Trait`, which when processed will cause the error to be
722 infcx.projection_cache.borrow_mut()
724 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
725 obligations.extend(result.obligations);
731 /// If there are unresolved type variables, then we need to include
732 /// any subobligations that bind them, at least until those type
733 /// variables are fully resolved.
734 fn prune_cache_value_obligations<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
735 result: &NormalizedTy<'tcx>)
736 -> NormalizedTy<'tcx> {
737 if !infcx.any_unresolved_type_vars(&result.value) {
738 return NormalizedTy { value: result.value, obligations: vec![] };
741 let mut obligations: Vec<_> =
744 .filter(|obligation| match obligation.predicate {
745 // We found a `T: Foo<X = U>` predicate, let's check
746 // if `U` references any unresolved type
747 // variables. In principle, we only care if this
748 // projection can help resolve any of the type
749 // variables found in `result.value` -- but we just
750 // check for any type variables here, for fear of
751 // indirect obligations (e.g., we project to `?0`,
752 // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
754 ty::Predicate::Projection(ref data) =>
755 infcx.any_unresolved_type_vars(&data.ty()),
757 // We are only interested in `T: Foo<X = U>` predicates, whre
758 // `U` references one of `unresolved_type_vars`. =)
764 obligations.shrink_to_fit();
766 NormalizedTy { value: result.value, obligations }
769 /// Whenever we give back a cache result for a projection like `<T as
770 /// Trait>::Item ==> X`, we *always* include the obligation to prove
771 /// that `T: Trait` (we may also include some other obligations). This
772 /// may or may not be necessary -- in principle, all the obligations
773 /// that must be proven to show that `T: Trait` were also returned
774 /// when the cache was first populated. But there are some vague concerns,
775 /// and so we take the precautionary measure of including `T: Trait` in
778 /// Concern #1. The current setup is fragile. Perhaps someone could
779 /// have failed to prove the concerns from when the cache was
780 /// populated, but also not have used a snapshot, in which case the
781 /// cache could remain populated even though `T: Trait` has not been
782 /// shown. In this case, the "other code" is at fault -- when you
783 /// project something, you are supposed to either have a snapshot or
784 /// else prove all the resulting obligations -- but it's still easy to
787 /// Concern #2. Even within the snapshot, if those original
788 /// obligations are not yet proven, then we are able to do projections
789 /// that may yet turn out to be wrong. This *may* lead to some sort
790 /// of trouble, though we don't have a concrete example of how that
791 /// can occur yet. But it seems risky at best.
792 fn get_paranoid_cache_value_obligation<'a, 'gcx, 'tcx>(
793 infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
794 param_env: ty::ParamEnv<'tcx>,
795 projection_ty: ty::ProjectionTy<'tcx>,
796 cause: ObligationCause<'tcx>,
798 -> PredicateObligation<'tcx>
800 let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
803 recursion_depth: depth,
805 predicate: trait_ref.to_predicate(),
809 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
810 /// hold. In various error cases, we cannot generate a valid
811 /// normalized projection. Therefore, we create an inference variable
812 /// return an associated obligation that, when fulfilled, will lead to
815 /// Note that we used to return `TyError` here, but that was quite
816 /// dubious -- the premise was that an error would *eventually* be
817 /// reported, when the obligation was processed. But in general once
818 /// you see a `TyError` you are supposed to be able to assume that an
819 /// error *has been* reported, so that you can take whatever heuristic
820 /// paths you want to take. To make things worse, it was possible for
821 /// cycles to arise, where you basically had a setup like `<MyType<$0>
822 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
823 /// Trait>::Foo> to `[type error]` would lead to an obligation of
824 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
825 /// an error for this obligation, but we legitimately should not,
826 /// because it contains `[type error]`. Yuck! (See issue #29857 for
827 /// one case where this arose.)
828 fn normalize_to_error<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
829 param_env: ty::ParamEnv<'tcx>,
830 projection_ty: ty::ProjectionTy<'tcx>,
831 cause: ObligationCause<'tcx>,
833 -> NormalizedTy<'tcx>
835 let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
836 let trait_obligation = Obligation { cause,
837 recursion_depth: depth,
839 predicate: trait_ref.to_predicate() };
840 let tcx = selcx.infcx().tcx;
841 let def_id = projection_ty.item_def_id;
842 let new_value = selcx.infcx().next_ty_var(
843 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
846 obligations: vec![trait_obligation]
850 enum ProjectedTy<'tcx> {
851 Progress(Progress<'tcx>),
852 NoProgress(Ty<'tcx>),
855 struct Progress<'tcx> {
857 obligations: Vec<PredicateObligation<'tcx>>,
860 impl<'tcx> Progress<'tcx> {
861 fn error<'a,'gcx>(tcx: TyCtxt<'a,'gcx,'tcx>) -> Self {
868 fn with_addl_obligations(mut self,
869 mut obligations: Vec<PredicateObligation<'tcx>>)
871 debug!("with_addl_obligations: self.obligations.len={} obligations.len={}",
872 self.obligations.len(), obligations.len());
874 debug!("with_addl_obligations: self.obligations={:?} obligations={:?}",
875 self.obligations, obligations);
877 self.obligations.append(&mut obligations);
882 /// Compute the result of a projection type (if we can).
885 /// - `obligation` must be fully normalized
886 fn project_type<'cx, 'gcx, 'tcx>(
887 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
888 obligation: &ProjectionTyObligation<'tcx>)
889 -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>>
891 debug!("project(obligation={:?})",
894 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
895 if obligation.recursion_depth >= recursion_limit {
896 debug!("project: overflow!");
897 selcx.infcx().report_overflow_error(&obligation, true);
900 let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
902 debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
904 if obligation_trait_ref.references_error() {
905 return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
908 let mut candidates = ProjectionTyCandidateSet::None;
910 // Make sure that the following procedures are kept in order. ParamEnv
911 // needs to be first because it has highest priority, and Select checks
912 // the return value of push_candidate which assumes it's ran at last.
913 assemble_candidates_from_param_env(selcx,
915 &obligation_trait_ref,
918 assemble_candidates_from_trait_def(selcx,
920 &obligation_trait_ref,
923 assemble_candidates_from_impls(selcx,
925 &obligation_trait_ref,
929 ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
930 confirm_candidate(selcx,
932 &obligation_trait_ref,
934 ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
935 selcx.tcx().mk_projection(
936 obligation.predicate.item_def_id,
937 obligation.predicate.substs))),
938 // Error occurred while trying to processing impls.
939 ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
940 // Inherent ambiguity that prevents us from even enumerating the
942 ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
947 /// The first thing we have to do is scan through the parameter
948 /// environment to see whether there are any projection predicates
949 /// there that can answer this question.
950 fn assemble_candidates_from_param_env<'cx, 'gcx, 'tcx>(
951 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
952 obligation: &ProjectionTyObligation<'tcx>,
953 obligation_trait_ref: &ty::TraitRef<'tcx>,
954 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
956 debug!("assemble_candidates_from_param_env(..)");
957 assemble_candidates_from_predicates(selcx,
959 obligation_trait_ref,
961 ProjectionTyCandidate::ParamEnv,
962 obligation.param_env.caller_bounds.iter().cloned());
965 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
966 /// that the definition of `Foo` has some clues:
970 /// type FooT : Bar<BarT=i32>
974 /// Here, for example, we could conclude that the result is `i32`.
975 fn assemble_candidates_from_trait_def<'cx, 'gcx, 'tcx>(
976 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
977 obligation: &ProjectionTyObligation<'tcx>,
978 obligation_trait_ref: &ty::TraitRef<'tcx>,
979 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
981 debug!("assemble_candidates_from_trait_def(..)");
983 let tcx = selcx.tcx();
984 // Check whether the self-type is itself a projection.
985 let (def_id, substs) = match obligation_trait_ref.self_ty().sty {
986 ty::TyProjection(ref data) => {
987 (data.trait_ref(tcx).def_id, data.substs)
989 ty::TyAnon(def_id, substs) => (def_id, substs),
990 ty::TyInfer(ty::TyVar(_)) => {
991 // If the self-type is an inference variable, then it MAY wind up
992 // being a projected type, so induce an ambiguity.
993 candidate_set.mark_ambiguous();
999 // If so, extract what we know from the trait and try to come up with a good answer.
1000 let trait_predicates = tcx.predicates_of(def_id);
1001 let bounds = trait_predicates.instantiate(tcx, substs);
1002 let bounds = elaborate_predicates(tcx, bounds.predicates);
1003 assemble_candidates_from_predicates(selcx,
1005 obligation_trait_ref,
1007 ProjectionTyCandidate::TraitDef,
1011 fn assemble_candidates_from_predicates<'cx, 'gcx, 'tcx, I>(
1012 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1013 obligation: &ProjectionTyObligation<'tcx>,
1014 obligation_trait_ref: &ty::TraitRef<'tcx>,
1015 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
1016 ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
1018 where I: IntoIterator<Item=ty::Predicate<'tcx>>
1020 debug!("assemble_candidates_from_predicates(obligation={:?})",
1022 let infcx = selcx.infcx();
1023 for predicate in env_predicates {
1024 debug!("assemble_candidates_from_predicates: predicate={:?}",
1027 ty::Predicate::Projection(data) => {
1028 let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
1030 let is_match = same_def_id && infcx.probe(|_| {
1031 let data_poly_trait_ref =
1032 data.to_poly_trait_ref(infcx.tcx);
1033 let obligation_poly_trait_ref =
1034 obligation_trait_ref.to_poly_trait_ref();
1035 infcx.at(&obligation.cause, obligation.param_env)
1036 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1037 .map(|InferOk { obligations: _, value: () }| {
1038 // FIXME(#32730) -- do we need to take obligations
1039 // into account in any way? At the moment, no.
1044 debug!("assemble_candidates_from_predicates: candidate={:?} \
1045 is_match={} same_def_id={}",
1046 data, is_match, same_def_id);
1049 candidate_set.push_candidate(ctor(data));
1057 fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
1058 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1059 obligation: &ProjectionTyObligation<'tcx>,
1060 obligation_trait_ref: &ty::TraitRef<'tcx>,
1061 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
1063 // If we are resolving `<T as TraitRef<...>>::Item == Type`,
1064 // start out by selecting the predicate `T as TraitRef<...>`:
1065 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1066 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1067 let _ = selcx.infcx().commit_if_ok(|_| {
1068 let vtable = match selcx.select(&trait_obligation) {
1069 Ok(Some(vtable)) => vtable,
1071 candidate_set.mark_ambiguous();
1075 debug!("assemble_candidates_from_impls: selection error {:?}",
1077 candidate_set.mark_error(e);
1082 let eligible = match &vtable {
1083 super::VtableClosure(_) |
1084 super::VtableGenerator(_) |
1085 super::VtableFnPointer(_) |
1086 super::VtableObject(_) => {
1087 debug!("assemble_candidates_from_impls: vtable={:?}",
1091 super::VtableImpl(impl_data) => {
1092 // We have to be careful when projecting out of an
1093 // impl because of specialization. If we are not in
1094 // codegen (i.e., projection mode is not "any"), and the
1095 // impl's type is declared as default, then we disable
1096 // projection (even if the trait ref is fully
1097 // monomorphic). In the case where trait ref is not
1098 // fully monomorphic (i.e., includes type parameters),
1099 // this is because those type parameters may
1100 // ultimately be bound to types from other crates that
1101 // may have specialized impls we can't see. In the
1102 // case where the trait ref IS fully monomorphic, this
1103 // is a policy decision that we made in the RFC in
1104 // order to preserve flexibility for the crate that
1105 // defined the specializable impl to specialize later
1106 // for existing types.
1108 // In either case, we handle this by not adding a
1109 // candidate for an impl if it contains a `default`
1111 let node_item = assoc_ty_def(selcx,
1112 impl_data.impl_def_id,
1113 obligation.predicate.item_def_id);
1115 let is_default = if node_item.node.is_from_trait() {
1116 // If true, the impl inherited a `type Foo = Bar`
1117 // given in the trait, which is implicitly default.
1118 // Otherwise, the impl did not specify `type` and
1119 // neither did the trait:
1122 // trait Foo { type T; }
1123 // impl Foo for Bar { }
1126 // This is an error, but it will be
1127 // reported in `check_impl_items_against_trait`.
1128 // We accept it here but will flag it as
1129 // an error when we confirm the candidate
1130 // (which will ultimately lead to `normalize_to_error`
1132 node_item.item.defaultness.has_value()
1134 node_item.item.defaultness.is_default() ||
1135 selcx.tcx().impl_is_default(node_item.node.def_id())
1138 // Only reveal a specializable default if we're past type-checking
1139 // and the obligations is monomorphic, otherwise passes such as
1140 // transmute checking and polymorphic MIR optimizations could
1141 // get a result which isn't correct for all monomorphizations.
1144 } else if obligation.param_env.reveal == Reveal::All {
1145 assert!(!poly_trait_ref.needs_infer());
1146 if !poly_trait_ref.needs_subst() {
1155 super::VtableParam(..) => {
1156 // This case tell us nothing about the value of an
1157 // associated type. Consider:
1160 // trait SomeTrait { type Foo; }
1161 // fn foo<T:SomeTrait>(...) { }
1164 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1165 // : SomeTrait` binding does not help us decide what the
1166 // type `Foo` is (at least, not more specifically than
1167 // what we already knew).
1169 // But wait, you say! What about an example like this:
1172 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1175 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1176 // resolve `T::Foo`? And of course it does, but in fact
1177 // that single predicate is desugared into two predicates
1178 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1179 // projection. And the projection where clause is handled
1180 // in `assemble_candidates_from_param_env`.
1183 super::VtableAutoImpl(..) |
1184 super::VtableBuiltin(..) => {
1185 // These traits have no associated types.
1187 obligation.cause.span,
1188 "Cannot project an associated type from `{:?}`",
1194 if candidate_set.push_candidate(ProjectionTyCandidate::Select(vtable)) {
1205 fn confirm_candidate<'cx, 'gcx, 'tcx>(
1206 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1207 obligation: &ProjectionTyObligation<'tcx>,
1208 obligation_trait_ref: &ty::TraitRef<'tcx>,
1209 candidate: ProjectionTyCandidate<'tcx>)
1212 debug!("confirm_candidate(candidate={:?}, obligation={:?})",
1217 ProjectionTyCandidate::ParamEnv(poly_projection) |
1218 ProjectionTyCandidate::TraitDef(poly_projection) => {
1219 confirm_param_env_candidate(selcx, obligation, poly_projection)
1222 ProjectionTyCandidate::Select(vtable) => {
1223 confirm_select_candidate(selcx, obligation, obligation_trait_ref, vtable)
1228 fn confirm_select_candidate<'cx, 'gcx, 'tcx>(
1229 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1230 obligation: &ProjectionTyObligation<'tcx>,
1231 obligation_trait_ref: &ty::TraitRef<'tcx>,
1232 vtable: Selection<'tcx>)
1236 super::VtableImpl(data) =>
1237 confirm_impl_candidate(selcx, obligation, data),
1238 super::VtableGenerator(data) =>
1239 confirm_generator_candidate(selcx, obligation, data),
1240 super::VtableClosure(data) =>
1241 confirm_closure_candidate(selcx, obligation, data),
1242 super::VtableFnPointer(data) =>
1243 confirm_fn_pointer_candidate(selcx, obligation, data),
1244 super::VtableObject(_) =>
1245 confirm_object_candidate(selcx, obligation, obligation_trait_ref),
1246 super::VtableAutoImpl(..) |
1247 super::VtableParam(..) |
1248 super::VtableBuiltin(..) =>
1249 // we don't create Select candidates with this kind of resolution
1251 obligation.cause.span,
1252 "Cannot project an associated type from `{:?}`",
1257 fn confirm_object_candidate<'cx, 'gcx, 'tcx>(
1258 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1259 obligation: &ProjectionTyObligation<'tcx>,
1260 obligation_trait_ref: &ty::TraitRef<'tcx>)
1263 let self_ty = obligation_trait_ref.self_ty();
1264 let object_ty = selcx.infcx().shallow_resolve(self_ty);
1265 debug!("confirm_object_candidate(object_ty={:?})",
1267 let data = match object_ty.sty {
1268 ty::TyDynamic(ref data, ..) => data,
1271 obligation.cause.span,
1272 "confirm_object_candidate called with non-object: {:?}",
1276 let env_predicates = data.projection_bounds().map(|p| {
1277 p.with_self_ty(selcx.tcx(), object_ty).to_predicate()
1279 let env_predicate = {
1280 let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1282 // select only those projections that are actually projecting an
1283 // item with the correct name
1284 let env_predicates = env_predicates.filter_map(|p| match p {
1285 ty::Predicate::Projection(data) =>
1286 if data.projection_def_id() == obligation.predicate.item_def_id {
1294 // select those with a relevant trait-ref
1295 let mut env_predicates = env_predicates.filter(|data| {
1296 let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1297 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1298 selcx.infcx().probe(|_| {
1299 selcx.infcx().at(&obligation.cause, obligation.param_env)
1300 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1305 // select the first matching one; there really ought to be one or
1306 // else the object type is not WF, since an object type should
1307 // include all of its projections explicitly
1308 match env_predicates.next() {
1309 Some(env_predicate) => env_predicate,
1311 debug!("confirm_object_candidate: no env-predicate \
1312 found in object type `{:?}`; ill-formed",
1314 return Progress::error(selcx.tcx());
1319 confirm_param_env_candidate(selcx, obligation, env_predicate)
1322 fn confirm_generator_candidate<'cx, 'gcx, 'tcx>(
1323 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1324 obligation: &ProjectionTyObligation<'tcx>,
1325 vtable: VtableGeneratorData<'tcx, PredicateObligation<'tcx>>)
1328 let gen_sig = vtable.substs.poly_sig(vtable.generator_def_id, selcx.tcx());
1332 } = normalize_with_depth(selcx,
1333 obligation.param_env,
1334 obligation.cause.clone(),
1335 obligation.recursion_depth+1,
1338 debug!("confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1343 let tcx = selcx.tcx();
1345 let gen_def_id = tcx.lang_items().gen_trait().unwrap();
1348 tcx.generator_trait_ref_and_outputs(gen_def_id,
1349 obligation.predicate.self_ty(),
1351 .map_bound(|(trait_ref, yield_ty, return_ty)| {
1352 let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1353 let ty = if name == "Return" {
1355 } else if name == "Yield" {
1361 ty::ProjectionPredicate {
1362 projection_ty: ty::ProjectionTy {
1363 substs: trait_ref.substs,
1364 item_def_id: obligation.predicate.item_def_id,
1370 confirm_param_env_candidate(selcx, obligation, predicate)
1371 .with_addl_obligations(vtable.nested)
1372 .with_addl_obligations(obligations)
1375 fn confirm_fn_pointer_candidate<'cx, 'gcx, 'tcx>(
1376 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1377 obligation: &ProjectionTyObligation<'tcx>,
1378 fn_pointer_vtable: VtableFnPointerData<'tcx, PredicateObligation<'tcx>>)
1381 let fn_type = selcx.infcx().shallow_resolve(fn_pointer_vtable.fn_ty);
1382 let sig = fn_type.fn_sig(selcx.tcx());
1386 } = normalize_with_depth(selcx,
1387 obligation.param_env,
1388 obligation.cause.clone(),
1389 obligation.recursion_depth+1,
1392 confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1393 .with_addl_obligations(fn_pointer_vtable.nested)
1394 .with_addl_obligations(obligations)
1397 fn confirm_closure_candidate<'cx, 'gcx, 'tcx>(
1398 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1399 obligation: &ProjectionTyObligation<'tcx>,
1400 vtable: VtableClosureData<'tcx, PredicateObligation<'tcx>>)
1403 let tcx = selcx.tcx();
1404 let infcx = selcx.infcx();
1405 let closure_sig_ty = vtable.substs.closure_sig_ty(vtable.closure_def_id, tcx);
1406 let closure_sig = infcx.shallow_resolve(&closure_sig_ty).fn_sig(tcx);
1410 } = normalize_with_depth(selcx,
1411 obligation.param_env,
1412 obligation.cause.clone(),
1413 obligation.recursion_depth+1,
1416 debug!("confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1421 confirm_callable_candidate(selcx,
1424 util::TupleArgumentsFlag::No)
1425 .with_addl_obligations(vtable.nested)
1426 .with_addl_obligations(obligations)
1429 fn confirm_callable_candidate<'cx, 'gcx, 'tcx>(
1430 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1431 obligation: &ProjectionTyObligation<'tcx>,
1432 fn_sig: ty::PolyFnSig<'tcx>,
1433 flag: util::TupleArgumentsFlag)
1436 let tcx = selcx.tcx();
1438 debug!("confirm_callable_candidate({:?},{:?})",
1442 // the `Output` associated type is declared on `FnOnce`
1443 let fn_once_def_id = tcx.lang_items().fn_once_trait().unwrap();
1446 tcx.closure_trait_ref_and_return_type(fn_once_def_id,
1447 obligation.predicate.self_ty(),
1450 .map_bound(|(trait_ref, ret_type)| {
1451 ty::ProjectionPredicate {
1452 projection_ty: ty::ProjectionTy::from_ref_and_name(
1455 Ident::from_str(FN_OUTPUT_NAME),
1461 confirm_param_env_candidate(selcx, obligation, predicate)
1464 fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
1465 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1466 obligation: &ProjectionTyObligation<'tcx>,
1467 poly_projection: ty::PolyProjectionPredicate<'tcx>)
1470 let infcx = selcx.infcx();
1471 let cause = obligation.cause.clone();
1472 let param_env = obligation.param_env;
1473 let trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1474 match infcx.match_poly_projection_predicate(cause, param_env, poly_projection, trait_ref) {
1475 Ok(InferOk { value: ty_match, obligations }) => {
1483 obligation.cause.span,
1484 "Failed to unify obligation `{:?}` \
1485 with poly_projection `{:?}`: {:?}",
1493 fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
1494 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1495 obligation: &ProjectionTyObligation<'tcx>,
1496 impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>)
1499 let VtableImplData { substs, nested, impl_def_id } = impl_vtable;
1501 let tcx = selcx.tcx();
1502 let param_env = obligation.param_env;
1503 let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_def_id);
1505 if !assoc_ty.item.defaultness.has_value() {
1506 // This means that the impl is missing a definition for the
1507 // associated type. This error will be reported by the type
1508 // checker method `check_impl_items_against_trait`, so here we
1509 // just return TyError.
1510 debug!("confirm_impl_candidate: no associated type {:?} for {:?}",
1511 assoc_ty.item.ident,
1512 obligation.predicate);
1515 obligations: nested,
1518 let substs = translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.node);
1519 let ty = if let ty::AssociatedKind::Existential = assoc_ty.item.kind {
1520 let item_substs = Substs::identity_for_item(tcx, assoc_ty.item.def_id);
1521 tcx.mk_anon(assoc_ty.item.def_id, item_substs)
1523 tcx.type_of(assoc_ty.item.def_id)
1526 ty: ty.subst(tcx, substs),
1527 obligations: nested,
1531 /// Locate the definition of an associated type in the specialization hierarchy,
1532 /// starting from the given impl.
1534 /// Based on the "projection mode", this lookup may in fact only examine the
1535 /// topmost impl. See the comments for `Reveal` for more details.
1536 fn assoc_ty_def<'cx, 'gcx, 'tcx>(
1537 selcx: &SelectionContext<'cx, 'gcx, 'tcx>,
1539 assoc_ty_def_id: DefId)
1540 -> specialization_graph::NodeItem<ty::AssociatedItem>
1542 let tcx = selcx.tcx();
1543 let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1544 let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1545 let trait_def = tcx.trait_def(trait_def_id);
1547 // This function may be called while we are still building the
1548 // specialization graph that is queried below (via TraidDef::ancestors()),
1549 // so, in order to avoid unnecessary infinite recursion, we manually look
1550 // for the associated item at the given impl.
1551 // If there is no such item in that impl, this function will fail with a
1552 // cycle error if the specialization graph is currently being built.
1553 let impl_node = specialization_graph::Node::Impl(impl_def_id);
1554 for item in impl_node.items(tcx) {
1555 if item.kind == ty::AssociatedKind::Type &&
1556 tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id) {
1557 return specialization_graph::NodeItem {
1558 node: specialization_graph::Node::Impl(impl_def_id),
1564 if let Some(assoc_item) = trait_def
1565 .ancestors(tcx, impl_def_id)
1566 .defs(tcx, assoc_ty_name, ty::AssociatedKind::Type, trait_def_id)
1570 // This is saying that neither the trait nor
1571 // the impl contain a definition for this
1572 // associated type. Normally this situation
1573 // could only arise through a compiler bug --
1574 // if the user wrote a bad item name, it
1575 // should have failed in astconv.
1576 bug!("No associated type `{}` for {}",
1578 tcx.item_path_str(impl_def_id))
1584 /// The projection cache. Unlike the standard caches, this can
1585 /// include infcx-dependent type variables - therefore, we have to roll
1586 /// the cache back each time we roll a snapshot back, to avoid assumptions
1587 /// on yet-unresolved inference variables. Types with skolemized regions
1588 /// also have to be removed when the respective snapshot ends.
1590 /// Because of that, projection cache entries can be "stranded" and left
1591 /// inaccessible when type variables inside the key are resolved. We make no
1592 /// attempt to recover or remove "stranded" entries, but rather let them be
1593 /// (for the lifetime of the infcx).
1595 /// Entries in the projection cache might contain inference variables
1596 /// that will be resolved by obligations on the projection cache entry - e.g.
1597 /// when a type parameter in the associated type is constrained through
1598 /// an "RFC 447" projection on the impl.
1600 /// When working with a fulfillment context, the derived obligations of each
1601 /// projection cache entry will be registered on the fulfillcx, so any users
1602 /// that can wait for a fulfillcx fixed point need not care about this. However,
1603 /// users that don't wait for a fixed point (e.g. trait evaluation) have to
1604 /// resolve the obligations themselves to make sure the projected result is
1605 /// ok and avoid issues like #43132.
1607 /// If that is done, after evaluation the obligations, it is a good idea to
1608 /// call `ProjectionCache::complete` to make sure the obligations won't be
1609 /// re-evaluated and avoid an exponential worst-case.
1611 /// FIXME: we probably also want some sort of cross-infcx cache here to
1612 /// reduce the amount of duplication. Let's see what we get with the Chalk
1614 pub struct ProjectionCache<'tcx> {
1615 map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
1618 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
1619 pub struct ProjectionCacheKey<'tcx> {
1620 ty: ty::ProjectionTy<'tcx>
1623 impl<'cx, 'gcx, 'tcx> ProjectionCacheKey<'tcx> {
1624 pub fn from_poly_projection_predicate(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1625 predicate: &ty::PolyProjectionPredicate<'tcx>)
1628 let infcx = selcx.infcx();
1629 // We don't do cross-snapshot caching of obligations with escaping regions,
1630 // so there's no cache key to use
1631 predicate.no_late_bound_regions()
1632 .map(|predicate| ProjectionCacheKey {
1633 // We don't attempt to match up with a specific type-variable state
1634 // from a specific call to `opt_normalize_projection_type` - if
1635 // there's no precise match, the original cache entry is "stranded"
1637 ty: infcx.resolve_type_vars_if_possible(&predicate.projection_ty)
1642 #[derive(Clone, Debug)]
1643 enum ProjectionCacheEntry<'tcx> {
1647 NormalizedTy(NormalizedTy<'tcx>),
1650 // NB: intentionally not Clone
1651 pub struct ProjectionCacheSnapshot {
1655 impl<'tcx> ProjectionCache<'tcx> {
1656 pub fn new() -> Self {
1658 map: SnapshotMap::new()
1662 pub fn clear(&mut self) {
1666 pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
1667 ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
1670 pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
1671 self.map.rollback_to(snapshot.snapshot);
1674 pub fn rollback_skolemized(&mut self, snapshot: &ProjectionCacheSnapshot) {
1675 self.map.partial_rollback(&snapshot.snapshot, &|k| k.ty.has_re_skol());
1678 pub fn commit(&mut self, snapshot: ProjectionCacheSnapshot) {
1679 self.map.commit(snapshot.snapshot);
1682 /// Try to start normalize `key`; returns an error if
1683 /// normalization already occurred (this error corresponds to a
1684 /// cache hit, so it's actually a good thing).
1685 fn try_start(&mut self, key: ProjectionCacheKey<'tcx>)
1686 -> Result<(), ProjectionCacheEntry<'tcx>> {
1687 if let Some(entry) = self.map.get(&key) {
1688 return Err(entry.clone());
1691 self.map.insert(key, ProjectionCacheEntry::InProgress);
1695 /// Indicates that `key` was normalized to `value`.
1696 fn insert_ty(&mut self, key: ProjectionCacheKey<'tcx>, value: NormalizedTy<'tcx>) {
1697 debug!("ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
1699 let fresh_key = self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
1700 assert!(!fresh_key, "never started projecting `{:?}`", key);
1703 /// Mark the relevant projection cache key as having its derived obligations
1704 /// complete, so they won't have to be re-computed (this is OK to do in a
1705 /// snapshot - if the snapshot is rolled back, the obligations will be
1706 /// marked as incomplete again).
1707 pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
1708 let ty = match self.map.get(&key) {
1709 Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
1710 debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}",
1715 // Type inference could "strand behind" old cache entries. Leave
1716 // them alone for now.
1717 debug!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}",
1723 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1729 /// A specialized version of `complete` for when the key's value is known
1730 /// to be a NormalizedTy.
1731 pub fn complete_normalized(&mut self, key: ProjectionCacheKey<'tcx>, ty: &NormalizedTy<'tcx>) {
1732 // We want to insert `ty` with no obligations. If the existing value
1733 // already has no obligations (as is common) we can use `insert_noop`
1734 // to do a minimal amount of work -- the HashMap insertion is skipped,
1735 // and minimal changes are made to the undo log.
1736 if ty.obligations.is_empty() {
1737 self.map.insert_noop();
1739 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1746 /// Indicates that trying to normalize `key` resulted in
1747 /// ambiguity. No point in trying it again then until we gain more
1748 /// type information (in which case, the "fully resolved" key will
1750 fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
1751 let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
1752 assert!(!fresh, "never started projecting `{:?}`", key);
1755 /// Indicates that trying to normalize `key` resulted in
1757 fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
1758 let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
1759 assert!(!fresh, "never started projecting `{:?}`", key);