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 middle::const_val::ConstVal;
32 use mir::interpret::{GlobalId};
33 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
34 use syntax::symbol::Symbol;
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 trans 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::*;
142 *self = Single(candidate);
146 // Duplicates can happen inside ParamEnv. In the case, we
147 // perform a lazy deduplication.
148 if current == &candidate {
151 // Prefer where-clauses. As in select, if there are multiple
152 // candidates, we prefer where-clause candidates over impls. This
153 // may seem a bit surprising, since impls are the source of
154 // "truth" in some sense, but in fact some of the impls that SEEM
155 // applicable are not, because of nested obligations. Where
156 // clauses are the safer choice. See the comment on
157 // `select::SelectionCandidate` and #21974 for more details.
158 match (current, candidate) {
159 (ParamEnv(..), ParamEnv(..)) => { *self = Ambiguous; }
160 (ParamEnv(..), _) => {}
161 (_, ParamEnv(..)) => { unreachable!(); }
162 (_, _) => { *self = Ambiguous; }
166 Ambiguous | Error(..) => {
173 /// Evaluates constraints of the form:
175 /// for<...> <T as Trait>::U == V
177 /// If successful, this may result in additional obligations. Also returns
178 /// the projection cache key used to track these additional obligations.
179 pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
180 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
181 obligation: &PolyProjectionObligation<'tcx>)
182 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
183 MismatchedProjectionTypes<'tcx>>
185 debug!("poly_project_and_unify_type(obligation={:?})",
188 let infcx = selcx.infcx();
189 infcx.commit_if_ok(|snapshot| {
190 let (skol_predicate, skol_map) =
191 infcx.skolemize_late_bound_regions(&obligation.predicate, snapshot);
193 let skol_obligation = obligation.with(skol_predicate);
194 let r = match project_and_unify_type(selcx, &skol_obligation) {
196 let span = obligation.cause.span;
197 match infcx.leak_check(false, span, &skol_map, snapshot) {
198 Ok(()) => Ok(infcx.plug_leaks(skol_map, snapshot, result)),
199 Err(e) => Err(MismatchedProjectionTypes { err: e }),
211 /// Evaluates constraints of the form:
213 /// <T as Trait>::U == V
215 /// If successful, this may result in additional obligations.
216 fn project_and_unify_type<'cx, 'gcx, 'tcx>(
217 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
218 obligation: &ProjectionObligation<'tcx>)
219 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
220 MismatchedProjectionTypes<'tcx>>
222 debug!("project_and_unify_type(obligation={:?})",
225 let Normalized { value: normalized_ty, mut obligations } =
226 match opt_normalize_projection_type(selcx,
227 obligation.param_env,
228 obligation.predicate.projection_ty,
229 obligation.cause.clone(),
230 obligation.recursion_depth) {
232 None => return Ok(None),
235 debug!("project_and_unify_type: normalized_ty={:?} obligations={:?}",
239 let infcx = selcx.infcx();
240 match infcx.at(&obligation.cause, obligation.param_env)
241 .eq(normalized_ty, obligation.predicate.ty) {
242 Ok(InferOk { obligations: inferred_obligations, value: () }) => {
243 obligations.extend(inferred_obligations);
244 Ok(Some(obligations))
246 Err(err) => Err(MismatchedProjectionTypes { err: err }),
250 /// Normalizes any associated type projections in `value`, replacing
251 /// them with a fully resolved type where possible. The return value
252 /// combines the normalized result and any additional obligations that
253 /// were incurred as result.
254 pub fn normalize<'a, 'b, 'gcx, 'tcx, T>(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
255 param_env: ty::ParamEnv<'tcx>,
256 cause: ObligationCause<'tcx>,
258 -> Normalized<'tcx, T>
259 where T : TypeFoldable<'tcx>
261 normalize_with_depth(selcx, param_env, cause, 0, value)
264 /// As `normalize`, but with a custom depth.
265 pub fn normalize_with_depth<'a, 'b, 'gcx, 'tcx, T>(
266 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
267 param_env: ty::ParamEnv<'tcx>,
268 cause: ObligationCause<'tcx>,
271 -> Normalized<'tcx, T>
273 where T : TypeFoldable<'tcx>
275 debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
276 let mut normalizer = AssociatedTypeNormalizer::new(selcx, param_env, cause, depth);
277 let result = normalizer.fold(value);
278 debug!("normalize_with_depth: depth={} result={:?} with {} obligations",
279 depth, result, normalizer.obligations.len());
280 debug!("normalize_with_depth: depth={} obligations={:?}",
281 depth, normalizer.obligations);
284 obligations: normalizer.obligations,
288 struct AssociatedTypeNormalizer<'a, 'b: 'a, 'gcx: 'b+'tcx, 'tcx: 'b> {
289 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
290 param_env: ty::ParamEnv<'tcx>,
291 cause: ObligationCause<'tcx>,
292 obligations: Vec<PredicateObligation<'tcx>>,
296 impl<'a, 'b, 'gcx, 'tcx> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
297 fn new(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
298 param_env: ty::ParamEnv<'tcx>,
299 cause: ObligationCause<'tcx>,
301 -> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx>
303 AssociatedTypeNormalizer {
312 fn fold<T:TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
313 let value = self.selcx.infcx().resolve_type_vars_if_possible(value);
315 if !value.has_projections() {
318 value.fold_with(self)
323 impl<'a, 'b, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
324 fn tcx<'c>(&'c self) -> TyCtxt<'c, 'gcx, 'tcx> {
328 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
329 // We don't want to normalize associated types that occur inside of region
330 // binders, because they may contain bound regions, and we can't cope with that.
334 // for<'a> fn(<T as Foo<&'a>>::A)
336 // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
337 // normalize it when we instantiate those bound regions (which
338 // should occur eventually).
340 let ty = ty.super_fold_with(self);
342 ty::TyAnon(def_id, substs) if !substs.has_escaping_regions() => { // (*)
343 // Only normalize `impl Trait` after type-checking, usually in trans.
344 match self.param_env.reveal {
345 Reveal::UserFacing => ty,
348 let recursion_limit = *self.tcx().sess.recursion_limit.get();
349 if self.depth >= recursion_limit {
350 let obligation = Obligation::with_depth(
356 self.selcx.infcx().report_overflow_error(&obligation, true);
359 let generic_ty = self.tcx().type_of(def_id);
360 let concrete_ty = generic_ty.subst(self.tcx(), substs);
362 let folded_ty = self.fold_ty(concrete_ty);
369 ty::TyProjection(ref data) if !data.has_escaping_regions() => { // (*)
371 // (*) This is kind of hacky -- we need to be able to
372 // handle normalization within binders because
373 // otherwise we wind up a need to normalize when doing
374 // trait matching (since you can have a trait
375 // obligation like `for<'a> T::B : Fn(&'a int)`), but
376 // we can't normalize with bound regions in scope. So
377 // far now we just ignore binders but only normalize
378 // if all bound regions are gone (and then we still
379 // have to renormalize whenever we instantiate a
380 // binder). It would be better to normalize in a
381 // binding-aware fashion.
383 let Normalized { value: normalized_ty, obligations } =
384 normalize_projection_type(self.selcx,
389 debug!("AssociatedTypeNormalizer: depth={} normalized {:?} to {:?} \
390 with {} add'l obligations",
391 self.depth, ty, normalized_ty, obligations.len());
392 self.obligations.extend(obligations);
402 fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
403 if let ConstVal::Unevaluated(def_id, substs) = constant.val {
404 let tcx = self.selcx.tcx().global_tcx();
405 if let Some(param_env) = self.tcx().lift_to_global(&self.param_env) {
406 if substs.needs_infer() {
407 let identity_substs = Substs::identity_for_item(tcx, def_id);
408 let instance = ty::Instance::resolve(tcx, param_env, def_id, identity_substs);
409 if let Some(instance) = instance {
414 match tcx.const_eval(param_env.and(cid)) {
416 let evaluated = evaluated.subst(self.tcx(), substs);
417 return self.fold_const(evaluated);
423 if let Some(substs) = self.tcx().lift_to_global(&substs) {
424 let instance = ty::Instance::resolve(tcx, param_env, def_id, substs);
425 if let Some(instance) = instance {
430 match tcx.const_eval(param_env.and(cid)) {
431 Ok(evaluated) => return self.fold_const(evaluated),
444 pub struct Normalized<'tcx,T> {
446 pub obligations: Vec<PredicateObligation<'tcx>>,
449 pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
451 impl<'tcx,T> Normalized<'tcx,T> {
452 pub fn with<U>(self, value: U) -> Normalized<'tcx,U> {
453 Normalized { value: value, obligations: self.obligations }
457 /// The guts of `normalize`: normalize a specific projection like `<T
458 /// as Trait>::Item`. The result is always a type (and possibly
459 /// additional obligations). If ambiguity arises, which implies that
460 /// there are unresolved type variables in the projection, we will
461 /// substitute a fresh type variable `$X` and generate a new
462 /// obligation `<T as Trait>::Item == $X` for later.
463 pub fn normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
464 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
465 param_env: ty::ParamEnv<'tcx>,
466 projection_ty: ty::ProjectionTy<'tcx>,
467 cause: ObligationCause<'tcx>,
469 -> NormalizedTy<'tcx>
471 opt_normalize_projection_type(selcx, param_env, projection_ty.clone(), cause.clone(), depth)
472 .unwrap_or_else(move || {
473 // if we bottom out in ambiguity, create a type variable
474 // and a deferred predicate to resolve this when more type
475 // information is available.
477 let tcx = selcx.infcx().tcx;
478 let def_id = projection_ty.item_def_id;
479 let ty_var = selcx.infcx().next_ty_var(
480 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
481 let projection = ty::Binder::dummy(ty::ProjectionPredicate {
485 let obligation = Obligation::with_depth(
486 cause, depth + 1, param_env, projection.to_predicate());
489 obligations: vec![obligation]
494 /// The guts of `normalize`: normalize a specific projection like `<T
495 /// as Trait>::Item`. The result is always a type (and possibly
496 /// additional obligations). Returns `None` in the case of ambiguity,
497 /// which indicates that there are unbound type variables.
498 fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
499 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
500 param_env: ty::ParamEnv<'tcx>,
501 projection_ty: ty::ProjectionTy<'tcx>,
502 cause: ObligationCause<'tcx>,
504 -> Option<NormalizedTy<'tcx>>
506 let infcx = selcx.infcx();
508 let projection_ty = infcx.resolve_type_vars_if_possible(&projection_ty);
509 let cache_key = ProjectionCacheKey { ty: projection_ty };
511 debug!("opt_normalize_projection_type(\
512 projection_ty={:?}, \
517 // FIXME(#20304) For now, I am caching here, which is good, but it
518 // means we don't capture the type variables that are created in
519 // the case of ambiguity. Which means we may create a large stream
520 // of such variables. OTOH, if we move the caching up a level, we
521 // would not benefit from caching when proving `T: Trait<U=Foo>`
522 // bounds. It might be the case that we want two distinct caches,
523 // or else another kind of cache entry.
525 let cache_result = infcx.projection_cache.borrow_mut().try_start(cache_key);
528 Err(ProjectionCacheEntry::Ambiguous) => {
529 // If we found ambiguity the last time, that generally
530 // means we will continue to do so until some type in the
531 // key changes (and we know it hasn't, because we just
532 // fully resolved it). One exception though is closure
533 // types, which can transition from having a fixed kind to
534 // no kind with no visible change in the key.
536 // FIXME(#32286) refactor this so that closure type
538 debug!("opt_normalize_projection_type: \
539 found cache entry: ambiguous");
540 if !projection_ty.has_closure_types() {
544 Err(ProjectionCacheEntry::InProgress) => {
545 // If while normalized A::B, we are asked to normalize
546 // A::B, just return A::B itself. This is a conservative
547 // answer, in the sense that A::B *is* clearly equivalent
548 // to A::B, though there may be a better value we can
551 // Under lazy normalization, this can arise when
552 // bootstrapping. That is, imagine an environment with a
553 // where-clause like `A::B == u32`. Now, if we are asked
554 // to normalize `A::B`, we will want to check the
555 // where-clauses in scope. So we will try to unify `A::B`
556 // with `A::B`, which can trigger a recursive
557 // normalization. In that case, I think we will want this code:
560 // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
561 // projection_ty.substs;
562 // return Some(NormalizedTy { value: v, obligations: vec![] });
565 debug!("opt_normalize_projection_type: \
566 found cache entry: in-progress");
568 // But for now, let's classify this as an overflow:
569 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
570 let obligation = Obligation::with_depth(cause.clone(),
574 selcx.infcx().report_overflow_error(&obligation, false);
576 Err(ProjectionCacheEntry::NormalizedTy(mut ty)) => {
577 // If we find the value in the cache, then return it along
578 // with the obligations that went along with it. Note
579 // that, when using a fulfillment context, these
580 // obligations could in principle be ignored: they have
581 // already been registered when the cache entry was
582 // created (and hence the new ones will quickly be
583 // discarded as duplicated). But when doing trait
584 // evaluation this is not the case, and dropping the trait
585 // evaluations can causes ICEs (e.g. #43132).
586 debug!("opt_normalize_projection_type: \
587 found normalized ty `{:?}`",
590 // Once we have inferred everything we need to know, we
591 // can ignore the `obligations` from that point on.
592 if !infcx.any_unresolved_type_vars(&ty.value) {
593 infcx.projection_cache.borrow_mut().complete(cache_key);
594 ty.obligations = vec![];
597 push_paranoid_cache_value_obligation(infcx,
606 Err(ProjectionCacheEntry::Error) => {
607 debug!("opt_normalize_projection_type: \
609 return Some(normalize_to_error(selcx, param_env, projection_ty, cause, depth));
613 let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
614 match project_type(selcx, &obligation) {
615 Ok(ProjectedTy::Progress(Progress { ty: projected_ty, mut obligations })) => {
616 // if projection succeeded, then what we get out of this
617 // is also non-normalized (consider: it was derived from
618 // an impl, where-clause etc) and hence we must
621 debug!("opt_normalize_projection_type: \
629 let result = if projected_ty.has_projections() {
630 let mut normalizer = AssociatedTypeNormalizer::new(selcx,
634 let normalized_ty = normalizer.fold(&projected_ty);
636 debug!("opt_normalize_projection_type: \
637 normalized_ty={:?} depth={}",
641 obligations.extend(normalizer.obligations);
643 value: normalized_ty,
653 let cache_value = prune_cache_value_obligations(infcx, &result);
654 infcx.projection_cache.borrow_mut().insert_ty(cache_key, cache_value);
658 Ok(ProjectedTy::NoProgress(projected_ty)) => {
659 debug!("opt_normalize_projection_type: \
660 projected_ty={:?} no progress",
662 let result = Normalized {
666 infcx.projection_cache.borrow_mut().insert_ty(cache_key, result.clone());
669 Err(ProjectionTyError::TooManyCandidates) => {
670 debug!("opt_normalize_projection_type: \
671 too many candidates");
672 infcx.projection_cache.borrow_mut()
673 .ambiguous(cache_key);
676 Err(ProjectionTyError::TraitSelectionError(_)) => {
677 debug!("opt_normalize_projection_type: ERROR");
678 // if we got an error processing the `T as Trait` part,
679 // just return `ty::err` but add the obligation `T :
680 // Trait`, which when processed will cause the error to be
683 infcx.projection_cache.borrow_mut()
685 Some(normalize_to_error(selcx, param_env, projection_ty, cause, depth))
690 /// If there are unresolved type variables, then we need to include
691 /// any subobligations that bind them, at least until those type
692 /// variables are fully resolved.
693 fn prune_cache_value_obligations<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
694 result: &NormalizedTy<'tcx>)
695 -> NormalizedTy<'tcx> {
696 if !infcx.any_unresolved_type_vars(&result.value) {
697 return NormalizedTy { value: result.value, obligations: vec![] };
700 let mut obligations: Vec<_> =
703 .filter(|obligation| match obligation.predicate {
704 // We found a `T: Foo<X = U>` predicate, let's check
705 // if `U` references any unresolved type
706 // variables. In principle, we only care if this
707 // projection can help resolve any of the type
708 // variables found in `result.value` -- but we just
709 // check for any type variables here, for fear of
710 // indirect obligations (e.g., we project to `?0`,
711 // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
713 ty::Predicate::Projection(ref data) =>
714 infcx.any_unresolved_type_vars(&data.ty()),
716 // We are only interested in `T: Foo<X = U>` predicates, whre
717 // `U` references one of `unresolved_type_vars`. =)
723 obligations.shrink_to_fit();
725 NormalizedTy { value: result.value, obligations }
728 /// Whenever we give back a cache result for a projection like `<T as
729 /// Trait>::Item ==> X`, we *always* include the obligation to prove
730 /// that `T: Trait` (we may also include some other obligations). This
731 /// may or may not be necessary -- in principle, all the obligations
732 /// that must be proven to show that `T: Trait` were also returned
733 /// when the cache was first populated. But there are some vague concerns,
734 /// and so we take the precatuionary measure of including `T: Trait` in
737 /// Concern #1. The current setup is fragile. Perhaps someone could
738 /// have failed to prove the concerns from when the cache was
739 /// populated, but also not have used a snapshot, in which case the
740 /// cache could remain populated even though `T: Trait` has not been
741 /// shown. In this case, the "other code" is at fault -- when you
742 /// project something, you are supposed to either have a snapshot or
743 /// else prove all the resulting obligations -- but it's still easy to
746 /// Concern #2. Even within the snapshot, if those original
747 /// obligations are not yet proven, then we are able to do projections
748 /// that may yet turn out to be wrong. This *may* lead to some sort
749 /// of trouble, though we don't have a concrete example of how that
750 /// can occur yet. But it seems risky at best.
751 fn push_paranoid_cache_value_obligation<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
752 param_env: ty::ParamEnv<'tcx>,
753 projection_ty: ty::ProjectionTy<'tcx>,
754 cause: ObligationCause<'tcx>,
756 result: &mut NormalizedTy<'tcx>)
758 let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
759 let trait_obligation = Obligation { cause,
760 recursion_depth: depth,
762 predicate: trait_ref.to_predicate() };
763 result.obligations.push(trait_obligation);
766 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
767 /// hold. In various error cases, we cannot generate a valid
768 /// normalized projection. Therefore, we create an inference variable
769 /// return an associated obligation that, when fulfilled, will lead to
772 /// Note that we used to return `TyError` here, but that was quite
773 /// dubious -- the premise was that an error would *eventually* be
774 /// reported, when the obligation was processed. But in general once
775 /// you see a `TyError` you are supposed to be able to assume that an
776 /// error *has been* reported, so that you can take whatever heuristic
777 /// paths you want to take. To make things worse, it was possible for
778 /// cycles to arise, where you basically had a setup like `<MyType<$0>
779 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
780 /// Trait>::Foo> to `[type error]` would lead to an obligation of
781 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
782 /// an error for this obligation, but we legitimately should not,
783 /// because it contains `[type error]`. Yuck! (See issue #29857 for
784 /// one case where this arose.)
785 fn normalize_to_error<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
786 param_env: ty::ParamEnv<'tcx>,
787 projection_ty: ty::ProjectionTy<'tcx>,
788 cause: ObligationCause<'tcx>,
790 -> NormalizedTy<'tcx>
792 let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
793 let trait_obligation = Obligation { cause,
794 recursion_depth: depth,
796 predicate: trait_ref.to_predicate() };
797 let tcx = selcx.infcx().tcx;
798 let def_id = projection_ty.item_def_id;
799 let new_value = selcx.infcx().next_ty_var(
800 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
803 obligations: vec![trait_obligation]
807 enum ProjectedTy<'tcx> {
808 Progress(Progress<'tcx>),
809 NoProgress(Ty<'tcx>),
812 struct Progress<'tcx> {
814 obligations: Vec<PredicateObligation<'tcx>>,
817 impl<'tcx> Progress<'tcx> {
818 fn error<'a,'gcx>(tcx: TyCtxt<'a,'gcx,'tcx>) -> Self {
825 fn with_addl_obligations(mut self,
826 mut obligations: Vec<PredicateObligation<'tcx>>)
828 debug!("with_addl_obligations: self.obligations.len={} obligations.len={}",
829 self.obligations.len(), obligations.len());
831 debug!("with_addl_obligations: self.obligations={:?} obligations={:?}",
832 self.obligations, obligations);
834 self.obligations.append(&mut obligations);
839 /// Compute the result of a projection type (if we can).
842 /// - `obligation` must be fully normalized
843 fn project_type<'cx, 'gcx, 'tcx>(
844 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
845 obligation: &ProjectionTyObligation<'tcx>)
846 -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>>
848 debug!("project(obligation={:?})",
851 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
852 if obligation.recursion_depth >= recursion_limit {
853 debug!("project: overflow!");
854 selcx.infcx().report_overflow_error(&obligation, true);
857 let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
859 debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
861 if obligation_trait_ref.references_error() {
862 return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
865 let mut candidates = ProjectionTyCandidateSet::None;
867 // Make sure that the following procedures are kept in order. ParamEnv
868 // needs to be first because it has highest priority, and Select checks
869 // the return value of push_candidate which assumes it's ran at last.
870 assemble_candidates_from_param_env(selcx,
872 &obligation_trait_ref,
875 assemble_candidates_from_trait_def(selcx,
877 &obligation_trait_ref,
880 assemble_candidates_from_impls(selcx,
882 &obligation_trait_ref,
886 ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
887 confirm_candidate(selcx,
889 &obligation_trait_ref,
891 ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
892 selcx.tcx().mk_projection(
893 obligation.predicate.item_def_id,
894 obligation.predicate.substs))),
895 // Error occurred while trying to processing impls.
896 ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
897 // Inherent ambiguity that prevents us from even enumerating the
899 ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
904 /// The first thing we have to do is scan through the parameter
905 /// environment to see whether there are any projection predicates
906 /// there that can answer this question.
907 fn assemble_candidates_from_param_env<'cx, 'gcx, 'tcx>(
908 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
909 obligation: &ProjectionTyObligation<'tcx>,
910 obligation_trait_ref: &ty::TraitRef<'tcx>,
911 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
913 debug!("assemble_candidates_from_param_env(..)");
914 assemble_candidates_from_predicates(selcx,
916 obligation_trait_ref,
918 ProjectionTyCandidate::ParamEnv,
919 obligation.param_env.caller_bounds.iter().cloned());
922 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
923 /// that the definition of `Foo` has some clues:
927 /// type FooT : Bar<BarT=i32>
931 /// Here, for example, we could conclude that the result is `i32`.
932 fn assemble_candidates_from_trait_def<'cx, 'gcx, 'tcx>(
933 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
934 obligation: &ProjectionTyObligation<'tcx>,
935 obligation_trait_ref: &ty::TraitRef<'tcx>,
936 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
938 debug!("assemble_candidates_from_trait_def(..)");
940 let tcx = selcx.tcx();
941 // Check whether the self-type is itself a projection.
942 let (def_id, substs) = match obligation_trait_ref.self_ty().sty {
943 ty::TyProjection(ref data) => {
944 (data.trait_ref(tcx).def_id, data.substs)
946 ty::TyAnon(def_id, substs) => (def_id, substs),
947 ty::TyInfer(ty::TyVar(_)) => {
948 // If the self-type is an inference variable, then it MAY wind up
949 // being a projected type, so induce an ambiguity.
950 candidate_set.mark_ambiguous();
956 // If so, extract what we know from the trait and try to come up with a good answer.
957 let trait_predicates = tcx.predicates_of(def_id);
958 let bounds = trait_predicates.instantiate(tcx, substs);
959 let bounds = elaborate_predicates(tcx, bounds.predicates);
960 assemble_candidates_from_predicates(selcx,
962 obligation_trait_ref,
964 ProjectionTyCandidate::TraitDef,
968 fn assemble_candidates_from_predicates<'cx, 'gcx, 'tcx, I>(
969 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
970 obligation: &ProjectionTyObligation<'tcx>,
971 obligation_trait_ref: &ty::TraitRef<'tcx>,
972 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
973 ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
975 where I: IntoIterator<Item=ty::Predicate<'tcx>>
977 debug!("assemble_candidates_from_predicates(obligation={:?})",
979 let infcx = selcx.infcx();
980 for predicate in env_predicates {
981 debug!("assemble_candidates_from_predicates: predicate={:?}",
984 ty::Predicate::Projection(data) => {
985 let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
987 let is_match = same_def_id && infcx.probe(|_| {
988 let data_poly_trait_ref =
989 data.to_poly_trait_ref(infcx.tcx);
990 let obligation_poly_trait_ref =
991 obligation_trait_ref.to_poly_trait_ref();
992 infcx.at(&obligation.cause, obligation.param_env)
993 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
994 .map(|InferOk { obligations: _, value: () }| {
995 // FIXME(#32730) -- do we need to take obligations
996 // into account in any way? At the moment, no.
1001 debug!("assemble_candidates_from_predicates: candidate={:?} \
1002 is_match={} same_def_id={}",
1003 data, is_match, same_def_id);
1006 candidate_set.push_candidate(ctor(data));
1014 fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
1015 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1016 obligation: &ProjectionTyObligation<'tcx>,
1017 obligation_trait_ref: &ty::TraitRef<'tcx>,
1018 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
1020 // If we are resolving `<T as TraitRef<...>>::Item == Type`,
1021 // start out by selecting the predicate `T as TraitRef<...>`:
1022 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1023 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1024 let _ = selcx.infcx().commit_if_ok(|_| {
1025 let vtable = match selcx.select(&trait_obligation) {
1026 Ok(Some(vtable)) => vtable,
1028 candidate_set.mark_ambiguous();
1032 debug!("assemble_candidates_from_impls: selection error {:?}",
1034 candidate_set.mark_error(e);
1039 let eligible = match &vtable {
1040 super::VtableClosure(_) |
1041 super::VtableGenerator(_) |
1042 super::VtableFnPointer(_) |
1043 super::VtableObject(_) => {
1044 debug!("assemble_candidates_from_impls: vtable={:?}",
1048 super::VtableImpl(impl_data) => {
1049 // We have to be careful when projecting out of an
1050 // impl because of specialization. If we are not in
1051 // trans (i.e., projection mode is not "any"), and the
1052 // impl's type is declared as default, then we disable
1053 // projection (even if the trait ref is fully
1054 // monomorphic). In the case where trait ref is not
1055 // fully monomorphic (i.e., includes type parameters),
1056 // this is because those type parameters may
1057 // ultimately be bound to types from other crates that
1058 // may have specialized impls we can't see. In the
1059 // case where the trait ref IS fully monomorphic, this
1060 // is a policy decision that we made in the RFC in
1061 // order to preserve flexibility for the crate that
1062 // defined the specializable impl to specialize later
1063 // for existing types.
1065 // In either case, we handle this by not adding a
1066 // candidate for an impl if it contains a `default`
1068 let node_item = assoc_ty_def(selcx,
1069 impl_data.impl_def_id,
1070 obligation.predicate.item_def_id);
1072 let is_default = if node_item.node.is_from_trait() {
1073 // If true, the impl inherited a `type Foo = Bar`
1074 // given in the trait, which is implicitly default.
1075 // Otherwise, the impl did not specify `type` and
1076 // neither did the trait:
1079 // trait Foo { type T; }
1080 // impl Foo for Bar { }
1083 // This is an error, but it will be
1084 // reported in `check_impl_items_against_trait`.
1085 // We accept it here but will flag it as
1086 // an error when we confirm the candidate
1087 // (which will ultimately lead to `normalize_to_error`
1089 node_item.item.defaultness.has_value()
1091 node_item.item.defaultness.is_default() ||
1092 selcx.tcx().impl_is_default(node_item.node.def_id())
1095 // Only reveal a specializable default if we're past type-checking
1096 // and the obligations is monomorphic, otherwise passes such as
1097 // transmute checking and polymorphic MIR optimizations could
1098 // get a result which isn't correct for all monomorphizations.
1101 } else if obligation.param_env.reveal == Reveal::All {
1102 assert!(!poly_trait_ref.needs_infer());
1103 if !poly_trait_ref.needs_subst() {
1112 super::VtableParam(..) => {
1113 // This case tell us nothing about the value of an
1114 // associated type. Consider:
1117 // trait SomeTrait { type Foo; }
1118 // fn foo<T:SomeTrait>(...) { }
1121 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1122 // : SomeTrait` binding does not help us decide what the
1123 // type `Foo` is (at least, not more specifically than
1124 // what we already knew).
1126 // But wait, you say! What about an example like this:
1129 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1132 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1133 // resolve `T::Foo`? And of course it does, but in fact
1134 // that single predicate is desugared into two predicates
1135 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1136 // projection. And the projection where clause is handled
1137 // in `assemble_candidates_from_param_env`.
1140 super::VtableAutoImpl(..) |
1141 super::VtableBuiltin(..) => {
1142 // These traits have no associated types.
1144 obligation.cause.span,
1145 "Cannot project an associated type from `{:?}`",
1151 if candidate_set.push_candidate(ProjectionTyCandidate::Select(vtable)) {
1162 fn confirm_candidate<'cx, 'gcx, 'tcx>(
1163 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1164 obligation: &ProjectionTyObligation<'tcx>,
1165 obligation_trait_ref: &ty::TraitRef<'tcx>,
1166 candidate: ProjectionTyCandidate<'tcx>)
1169 debug!("confirm_candidate(candidate={:?}, obligation={:?})",
1174 ProjectionTyCandidate::ParamEnv(poly_projection) |
1175 ProjectionTyCandidate::TraitDef(poly_projection) => {
1176 confirm_param_env_candidate(selcx, obligation, poly_projection)
1179 ProjectionTyCandidate::Select(vtable) => {
1180 confirm_select_candidate(selcx, obligation, obligation_trait_ref, vtable)
1185 fn confirm_select_candidate<'cx, 'gcx, 'tcx>(
1186 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1187 obligation: &ProjectionTyObligation<'tcx>,
1188 obligation_trait_ref: &ty::TraitRef<'tcx>,
1189 vtable: Selection<'tcx>)
1193 super::VtableImpl(data) =>
1194 confirm_impl_candidate(selcx, obligation, data),
1195 super::VtableGenerator(data) =>
1196 confirm_generator_candidate(selcx, obligation, data),
1197 super::VtableClosure(data) =>
1198 confirm_closure_candidate(selcx, obligation, data),
1199 super::VtableFnPointer(data) =>
1200 confirm_fn_pointer_candidate(selcx, obligation, data),
1201 super::VtableObject(_) =>
1202 confirm_object_candidate(selcx, obligation, obligation_trait_ref),
1203 super::VtableAutoImpl(..) |
1204 super::VtableParam(..) |
1205 super::VtableBuiltin(..) =>
1206 // we don't create Select candidates with this kind of resolution
1208 obligation.cause.span,
1209 "Cannot project an associated type from `{:?}`",
1214 fn confirm_object_candidate<'cx, 'gcx, 'tcx>(
1215 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1216 obligation: &ProjectionTyObligation<'tcx>,
1217 obligation_trait_ref: &ty::TraitRef<'tcx>)
1220 let self_ty = obligation_trait_ref.self_ty();
1221 let object_ty = selcx.infcx().shallow_resolve(self_ty);
1222 debug!("confirm_object_candidate(object_ty={:?})",
1224 let data = match object_ty.sty {
1225 ty::TyDynamic(ref data, ..) => data,
1228 obligation.cause.span,
1229 "confirm_object_candidate called with non-object: {:?}",
1233 let env_predicates = data.projection_bounds().map(|p| {
1234 p.with_self_ty(selcx.tcx(), object_ty).to_predicate()
1236 let env_predicate = {
1237 let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1239 // select only those projections that are actually projecting an
1240 // item with the correct name
1241 let env_predicates = env_predicates.filter_map(|p| match p {
1242 ty::Predicate::Projection(data) =>
1243 if data.projection_def_id() == obligation.predicate.item_def_id {
1251 // select those with a relevant trait-ref
1252 let mut env_predicates = env_predicates.filter(|data| {
1253 let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1254 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1255 selcx.infcx().probe(|_| {
1256 selcx.infcx().at(&obligation.cause, obligation.param_env)
1257 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1262 // select the first matching one; there really ought to be one or
1263 // else the object type is not WF, since an object type should
1264 // include all of its projections explicitly
1265 match env_predicates.next() {
1266 Some(env_predicate) => env_predicate,
1268 debug!("confirm_object_candidate: no env-predicate \
1269 found in object type `{:?}`; ill-formed",
1271 return Progress::error(selcx.tcx());
1276 confirm_param_env_candidate(selcx, obligation, env_predicate)
1279 fn confirm_generator_candidate<'cx, 'gcx, 'tcx>(
1280 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1281 obligation: &ProjectionTyObligation<'tcx>,
1282 vtable: VtableGeneratorData<'tcx, PredicateObligation<'tcx>>)
1285 let gen_sig = vtable.substs.generator_poly_sig(vtable.closure_def_id, selcx.tcx());
1289 } = normalize_with_depth(selcx,
1290 obligation.param_env,
1291 obligation.cause.clone(),
1292 obligation.recursion_depth+1,
1295 debug!("confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1300 let tcx = selcx.tcx();
1302 let gen_def_id = tcx.lang_items().gen_trait().unwrap();
1305 tcx.generator_trait_ref_and_outputs(gen_def_id,
1306 obligation.predicate.self_ty(),
1308 .map_bound(|(trait_ref, yield_ty, return_ty)| {
1309 let name = tcx.associated_item(obligation.predicate.item_def_id).name;
1310 let ty = if name == Symbol::intern("Return") {
1312 } else if name == Symbol::intern("Yield") {
1318 ty::ProjectionPredicate {
1319 projection_ty: ty::ProjectionTy {
1320 substs: trait_ref.substs,
1321 item_def_id: obligation.predicate.item_def_id,
1327 confirm_param_env_candidate(selcx, obligation, predicate)
1328 .with_addl_obligations(vtable.nested)
1329 .with_addl_obligations(obligations)
1332 fn confirm_fn_pointer_candidate<'cx, 'gcx, 'tcx>(
1333 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1334 obligation: &ProjectionTyObligation<'tcx>,
1335 fn_pointer_vtable: VtableFnPointerData<'tcx, PredicateObligation<'tcx>>)
1338 let fn_type = selcx.infcx().shallow_resolve(fn_pointer_vtable.fn_ty);
1339 let sig = fn_type.fn_sig(selcx.tcx());
1343 } = normalize_with_depth(selcx,
1344 obligation.param_env,
1345 obligation.cause.clone(),
1346 obligation.recursion_depth+1,
1349 confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1350 .with_addl_obligations(fn_pointer_vtable.nested)
1351 .with_addl_obligations(obligations)
1354 fn confirm_closure_candidate<'cx, 'gcx, 'tcx>(
1355 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1356 obligation: &ProjectionTyObligation<'tcx>,
1357 vtable: VtableClosureData<'tcx, PredicateObligation<'tcx>>)
1360 let tcx = selcx.tcx();
1361 let infcx = selcx.infcx();
1362 let closure_sig_ty = vtable.substs.closure_sig_ty(vtable.closure_def_id, tcx);
1363 let closure_sig = infcx.shallow_resolve(&closure_sig_ty).fn_sig(tcx);
1367 } = normalize_with_depth(selcx,
1368 obligation.param_env,
1369 obligation.cause.clone(),
1370 obligation.recursion_depth+1,
1373 debug!("confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1378 confirm_callable_candidate(selcx,
1381 util::TupleArgumentsFlag::No)
1382 .with_addl_obligations(vtable.nested)
1383 .with_addl_obligations(obligations)
1386 fn confirm_callable_candidate<'cx, 'gcx, 'tcx>(
1387 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1388 obligation: &ProjectionTyObligation<'tcx>,
1389 fn_sig: ty::PolyFnSig<'tcx>,
1390 flag: util::TupleArgumentsFlag)
1393 let tcx = selcx.tcx();
1395 debug!("confirm_callable_candidate({:?},{:?})",
1399 // the `Output` associated type is declared on `FnOnce`
1400 let fn_once_def_id = tcx.lang_items().fn_once_trait().unwrap();
1403 tcx.closure_trait_ref_and_return_type(fn_once_def_id,
1404 obligation.predicate.self_ty(),
1407 .map_bound(|(trait_ref, ret_type)| {
1408 ty::ProjectionPredicate {
1409 projection_ty: ty::ProjectionTy::from_ref_and_name(
1412 Symbol::intern(FN_OUTPUT_NAME),
1418 confirm_param_env_candidate(selcx, obligation, predicate)
1421 fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
1422 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1423 obligation: &ProjectionTyObligation<'tcx>,
1424 poly_projection: ty::PolyProjectionPredicate<'tcx>)
1427 let infcx = selcx.infcx();
1428 let cause = obligation.cause.clone();
1429 let param_env = obligation.param_env;
1430 let trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1431 match infcx.match_poly_projection_predicate(cause, param_env, poly_projection, trait_ref) {
1432 Ok(InferOk { value: ty_match, obligations }) => {
1440 obligation.cause.span,
1441 "Failed to unify obligation `{:?}` \
1442 with poly_projection `{:?}`: {:?}",
1450 fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
1451 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1452 obligation: &ProjectionTyObligation<'tcx>,
1453 impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>)
1456 let VtableImplData { substs, nested, impl_def_id } = impl_vtable;
1458 let tcx = selcx.tcx();
1459 let param_env = obligation.param_env;
1460 let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_def_id);
1462 let ty = if !assoc_ty.item.defaultness.has_value() {
1463 // This means that the impl is missing a definition for the
1464 // associated type. This error will be reported by the type
1465 // checker method `check_impl_items_against_trait`, so here we
1466 // just return TyError.
1467 debug!("confirm_impl_candidate: no associated type {:?} for {:?}",
1469 obligation.predicate);
1472 tcx.type_of(assoc_ty.item.def_id)
1474 let substs = translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.node);
1476 ty: ty.subst(tcx, substs),
1477 obligations: nested,
1481 /// Locate the definition of an associated type in the specialization hierarchy,
1482 /// starting from the given impl.
1484 /// Based on the "projection mode", this lookup may in fact only examine the
1485 /// topmost impl. See the comments for `Reveal` for more details.
1486 fn assoc_ty_def<'cx, 'gcx, 'tcx>(
1487 selcx: &SelectionContext<'cx, 'gcx, 'tcx>,
1489 assoc_ty_def_id: DefId)
1490 -> specialization_graph::NodeItem<ty::AssociatedItem>
1492 let tcx = selcx.tcx();
1493 let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).name;
1494 let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1495 let trait_def = tcx.trait_def(trait_def_id);
1497 // This function may be called while we are still building the
1498 // specialization graph that is queried below (via TraidDef::ancestors()),
1499 // so, in order to avoid unnecessary infinite recursion, we manually look
1500 // for the associated item at the given impl.
1501 // If there is no such item in that impl, this function will fail with a
1502 // cycle error if the specialization graph is currently being built.
1503 let impl_node = specialization_graph::Node::Impl(impl_def_id);
1504 for item in impl_node.items(tcx) {
1505 if item.kind == ty::AssociatedKind::Type &&
1506 tcx.hygienic_eq(item.name, assoc_ty_name, trait_def_id) {
1507 return specialization_graph::NodeItem {
1508 node: specialization_graph::Node::Impl(impl_def_id),
1514 if let Some(assoc_item) = trait_def
1515 .ancestors(tcx, impl_def_id)
1516 .defs(tcx, assoc_ty_name, ty::AssociatedKind::Type, trait_def_id)
1520 // This is saying that neither the trait nor
1521 // the impl contain a definition for this
1522 // associated type. Normally this situation
1523 // could only arise through a compiler bug --
1524 // if the user wrote a bad item name, it
1525 // should have failed in astconv.
1526 bug!("No associated type `{}` for {}",
1528 tcx.item_path_str(impl_def_id))
1534 /// The projection cache. Unlike the standard caches, this can
1535 /// include infcx-dependent type variables - therefore, we have to roll
1536 /// the cache back each time we roll a snapshot back, to avoid assumptions
1537 /// on yet-unresolved inference variables. Types with skolemized regions
1538 /// also have to be removed when the respective snapshot ends.
1540 /// Because of that, projection cache entries can be "stranded" and left
1541 /// inaccessible when type variables inside the key are resolved. We make no
1542 /// attempt to recover or remove "stranded" entries, but rather let them be
1543 /// (for the lifetime of the infcx).
1545 /// Entries in the projection cache might contain inference variables
1546 /// that will be resolved by obligations on the projection cache entry - e.g.
1547 /// when a type parameter in the associated type is constrained through
1548 /// an "RFC 447" projection on the impl.
1550 /// When working with a fulfillment context, the derived obligations of each
1551 /// projection cache entry will be registered on the fulfillcx, so any users
1552 /// that can wait for a fulfillcx fixed point need not care about this. However,
1553 /// users that don't wait for a fixed point (e.g. trait evaluation) have to
1554 /// resolve the obligations themselves to make sure the projected result is
1555 /// ok and avoid issues like #43132.
1557 /// If that is done, after evaluation the obligations, it is a good idea to
1558 /// call `ProjectionCache::complete` to make sure the obligations won't be
1559 /// re-evaluated and avoid an exponential worst-case.
1561 /// FIXME: we probably also want some sort of cross-infcx cache here to
1562 /// reduce the amount of duplication. Let's see what we get with the Chalk
1564 pub struct ProjectionCache<'tcx> {
1565 map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
1568 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
1569 pub struct ProjectionCacheKey<'tcx> {
1570 ty: ty::ProjectionTy<'tcx>
1573 impl<'cx, 'gcx, 'tcx> ProjectionCacheKey<'tcx> {
1574 pub fn from_poly_projection_predicate(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1575 predicate: &ty::PolyProjectionPredicate<'tcx>)
1578 let infcx = selcx.infcx();
1579 // We don't do cross-snapshot caching of obligations with escaping regions,
1580 // so there's no cache key to use
1581 predicate.no_late_bound_regions()
1582 .map(|predicate| ProjectionCacheKey {
1583 // We don't attempt to match up with a specific type-variable state
1584 // from a specific call to `opt_normalize_projection_type` - if
1585 // there's no precise match, the original cache entry is "stranded"
1587 ty: infcx.resolve_type_vars_if_possible(&predicate.projection_ty)
1592 #[derive(Clone, Debug)]
1593 enum ProjectionCacheEntry<'tcx> {
1597 NormalizedTy(NormalizedTy<'tcx>),
1600 // NB: intentionally not Clone
1601 pub struct ProjectionCacheSnapshot {
1605 impl<'tcx> ProjectionCache<'tcx> {
1606 pub fn new() -> Self {
1608 map: SnapshotMap::new()
1612 pub fn clear(&mut self) {
1616 pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
1617 ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
1620 pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
1621 self.map.rollback_to(snapshot.snapshot);
1624 pub fn rollback_skolemized(&mut self, snapshot: &ProjectionCacheSnapshot) {
1625 self.map.partial_rollback(&snapshot.snapshot, &|k| k.ty.has_re_skol());
1628 pub fn commit(&mut self, snapshot: ProjectionCacheSnapshot) {
1629 self.map.commit(snapshot.snapshot);
1632 /// Try to start normalize `key`; returns an error if
1633 /// normalization already occurred (this error corresponds to a
1634 /// cache hit, so it's actually a good thing).
1635 fn try_start(&mut self, key: ProjectionCacheKey<'tcx>)
1636 -> Result<(), ProjectionCacheEntry<'tcx>> {
1637 if let Some(entry) = self.map.get(&key) {
1638 return Err(entry.clone());
1641 self.map.insert(key, ProjectionCacheEntry::InProgress);
1645 /// Indicates that `key` was normalized to `value`.
1646 fn insert_ty(&mut self, key: ProjectionCacheKey<'tcx>, value: NormalizedTy<'tcx>) {
1647 debug!("ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
1649 let fresh_key = self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
1650 assert!(!fresh_key, "never started projecting `{:?}`", key);
1653 /// Mark the relevant projection cache key as having its derived obligations
1654 /// complete, so they won't have to be re-computed (this is OK to do in a
1655 /// snapshot - if the snapshot is rolled back, the obligations will be
1656 /// marked as incomplete again).
1657 pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
1658 let ty = match self.map.get(&key) {
1659 Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
1660 debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}",
1665 // Type inference could "strand behind" old cache entries. Leave
1666 // them alone for now.
1667 debug!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}",
1673 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1679 /// Indicates that trying to normalize `key` resulted in
1680 /// ambiguity. No point in trying it again then until we gain more
1681 /// type information (in which case, the "fully resolved" key will
1683 fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
1684 let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
1685 assert!(!fresh, "never started projecting `{:?}`", key);
1688 /// Indicates that trying to normalize `key` resulted in
1690 fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
1691 let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
1692 assert!(!fresh, "never started projecting `{:?}`", key);