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;
19 use super::SelectionContext;
20 use super::SelectionError;
21 use super::VtableClosureData;
22 use super::VtableFnPointerData;
23 use super::VtableImplData;
26 use hir::def_id::DefId;
28 use infer::type_variable::TypeVariableOrigin;
29 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
31 use syntax::symbol::Symbol;
33 use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt};
34 use ty::fold::{TypeFoldable, TypeFolder};
35 use util::common::FN_OUTPUT_NAME;
37 /// Depending on the stage of compilation, we want projection to be
38 /// more or less conservative.
39 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
42 /// At coherence-checking time, we're still constructing the
43 /// specialization graph, and thus we only project
44 /// non-`default` associated types that are defined directly in
45 /// the applicable impl. (This behavior should be improved over
46 /// time, to allow for successful projections modulo cycles
47 /// between different impls).
49 /// Here's an example that will fail due to the restriction:
56 /// impl<T> Assoc for T {
57 /// type Output = bool;
60 /// impl Assoc for u8 {} // <- inherits the non-default type from above
63 /// impl Foo for u32 {}
64 /// impl Foo for <u8 as Assoc>::Output {} // <- this projection will fail
67 /// The projection would succeed if `Output` had been defined
68 /// directly in the impl for `u8`.
71 /// At type-checking time, we refuse to project any associated
72 /// type that is marked `default`. Non-`default` ("final") types
73 /// are always projected. This is necessary in general for
74 /// soundness of specialization. However, we *could* allow
75 /// projections in fully-monomorphic cases. We choose not to,
76 /// because we prefer for `default type` to force the type
77 /// definition to be treated abstractly by any consumers of the
78 /// impl. Concretely, that means that the following example will
86 /// impl<T> Assoc for T {
87 /// default type Output = bool;
91 /// let <() as Assoc>::Output = true;
95 /// At trans time, all monomorphic projections will succeed.
96 /// Also, `impl Trait` is normalized to the concrete type,
97 /// which has to be already collected by type-checking.
99 /// NOTE: As `impl Trait`'s concrete type should *never*
100 /// be observable directly by the user, `Reveal::All`
101 /// should not be used by checks which may expose
102 /// type equality or type contents to the user.
103 /// There are some exceptions, e.g. around OIBITS and
104 /// transmute-checking, which expose some details, but
105 /// not the whole concrete type of the `impl Trait`.
109 pub type PolyProjectionObligation<'tcx> =
110 Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
112 pub type ProjectionObligation<'tcx> =
113 Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
115 pub type ProjectionTyObligation<'tcx> =
116 Obligation<'tcx, ty::ProjectionTy<'tcx>>;
118 /// When attempting to resolve `<T as TraitRef>::Name` ...
120 pub enum ProjectionTyError<'tcx> {
121 /// ...we found multiple sources of information and couldn't resolve the ambiguity.
124 /// ...an error occurred matching `T : TraitRef`
125 TraitSelectionError(SelectionError<'tcx>),
129 pub struct MismatchedProjectionTypes<'tcx> {
130 pub err: ty::error::TypeError<'tcx>
133 #[derive(PartialEq, Eq, Debug)]
134 enum ProjectionTyCandidate<'tcx> {
135 // from a where-clause in the env or object type
136 ParamEnv(ty::PolyProjectionPredicate<'tcx>),
138 // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
139 TraitDef(ty::PolyProjectionPredicate<'tcx>),
141 // from a "impl" (or a "pseudo-impl" returned by select)
145 struct ProjectionTyCandidateSet<'tcx> {
146 vec: Vec<ProjectionTyCandidate<'tcx>>,
150 /// Evaluates constraints of the form:
152 /// for<...> <T as Trait>::U == V
154 /// If successful, this may result in additional obligations.
155 pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
156 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
157 obligation: &PolyProjectionObligation<'tcx>)
158 -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>>
160 debug!("poly_project_and_unify_type(obligation={:?})",
163 let infcx = selcx.infcx();
164 infcx.commit_if_ok(|snapshot| {
165 let (skol_predicate, skol_map) =
166 infcx.skolemize_late_bound_regions(&obligation.predicate, snapshot);
168 let skol_obligation = obligation.with(skol_predicate);
169 let r = match project_and_unify_type(selcx, &skol_obligation) {
171 let span = obligation.cause.span;
172 match infcx.leak_check(false, span, &skol_map, snapshot) {
173 Ok(()) => Ok(infcx.plug_leaks(skol_map, snapshot, result)),
174 Err(e) => Err(MismatchedProjectionTypes { err: e }),
186 /// Evaluates constraints of the form:
188 /// <T as Trait>::U == V
190 /// If successful, this may result in additional obligations.
191 fn project_and_unify_type<'cx, 'gcx, 'tcx>(
192 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
193 obligation: &ProjectionObligation<'tcx>)
194 -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>>
196 debug!("project_and_unify_type(obligation={:?})",
199 let Normalized { value: normalized_ty, mut obligations } =
200 match opt_normalize_projection_type(selcx,
201 obligation.predicate.projection_ty.clone(),
202 obligation.cause.clone(),
203 obligation.recursion_depth) {
205 None => return Ok(None),
208 debug!("project_and_unify_type: normalized_ty={:?} obligations={:?}",
212 let infcx = selcx.infcx();
213 match infcx.eq_types(true, &obligation.cause, normalized_ty, obligation.predicate.ty) {
214 Ok(InferOk { obligations: inferred_obligations, value: () }) => {
215 obligations.extend(inferred_obligations);
216 Ok(Some(obligations))
218 Err(err) => Err(MismatchedProjectionTypes { err: err }),
222 /// Normalizes any associated type projections in `value`, replacing
223 /// them with a fully resolved type where possible. The return value
224 /// combines the normalized result and any additional obligations that
225 /// were incurred as result.
226 pub fn normalize<'a, 'b, 'gcx, 'tcx, T>(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
227 cause: ObligationCause<'tcx>,
229 -> Normalized<'tcx, T>
230 where T : TypeFoldable<'tcx>
232 normalize_with_depth(selcx, cause, 0, value)
235 /// As `normalize`, but with a custom depth.
236 pub fn normalize_with_depth<'a, 'b, 'gcx, 'tcx, T>(
237 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
238 cause: ObligationCause<'tcx>,
241 -> Normalized<'tcx, T>
243 where T : TypeFoldable<'tcx>
245 debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
246 let mut normalizer = AssociatedTypeNormalizer::new(selcx, cause, depth);
247 let result = normalizer.fold(value);
248 debug!("normalize_with_depth: depth={} result={:?} with {} obligations",
249 depth, result, normalizer.obligations.len());
250 debug!("normalize_with_depth: depth={} obligations={:?}",
251 depth, normalizer.obligations);
254 obligations: normalizer.obligations,
258 struct AssociatedTypeNormalizer<'a, 'b: 'a, 'gcx: 'b+'tcx, 'tcx: 'b> {
259 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
260 cause: ObligationCause<'tcx>,
261 obligations: Vec<PredicateObligation<'tcx>>,
265 impl<'a, 'b, 'gcx, 'tcx> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
266 fn new(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
267 cause: ObligationCause<'tcx>,
269 -> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx>
271 AssociatedTypeNormalizer {
279 fn fold<T:TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
280 let value = self.selcx.infcx().resolve_type_vars_if_possible(value);
282 if !value.has_projection_types() {
285 value.fold_with(self)
290 impl<'a, 'b, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
291 fn tcx<'c>(&'c self) -> TyCtxt<'c, 'gcx, 'tcx> {
295 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
296 // We don't want to normalize associated types that occur inside of region
297 // binders, because they may contain bound regions, and we can't cope with that.
301 // for<'a> fn(<T as Foo<&'a>>::A)
303 // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
304 // normalize it when we instantiate those bound regions (which
305 // should occur eventually).
307 let ty = ty.super_fold_with(self);
309 ty::TyAnon(def_id, substs) if !substs.has_escaping_regions() => { // (*)
310 // Only normalize `impl Trait` after type-checking, usually in trans.
311 if self.selcx.projection_mode() == Reveal::All {
312 let generic_ty = self.tcx().item_type(def_id);
313 let concrete_ty = generic_ty.subst(self.tcx(), substs);
314 self.fold_ty(concrete_ty)
320 ty::TyProjection(ref data) if !data.has_escaping_regions() => { // (*)
322 // (*) This is kind of hacky -- we need to be able to
323 // handle normalization within binders because
324 // otherwise we wind up a need to normalize when doing
325 // trait matching (since you can have a trait
326 // obligation like `for<'a> T::B : Fn(&'a int)`), but
327 // we can't normalize with bound regions in scope. So
328 // far now we just ignore binders but only normalize
329 // if all bound regions are gone (and then we still
330 // have to renormalize whenever we instantiate a
331 // binder). It would be better to normalize in a
332 // binding-aware fashion.
334 let Normalized { value: normalized_ty, obligations } =
335 normalize_projection_type(self.selcx,
339 debug!("AssociatedTypeNormalizer: depth={} normalized {:?} to {:?} \
340 with {} add'l obligations",
341 self.depth, ty, normalized_ty, obligations.len());
342 self.obligations.extend(obligations);
354 pub struct Normalized<'tcx,T> {
356 pub obligations: Vec<PredicateObligation<'tcx>>,
359 pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
361 impl<'tcx,T> Normalized<'tcx,T> {
362 pub fn with<U>(self, value: U) -> Normalized<'tcx,U> {
363 Normalized { value: value, obligations: self.obligations }
367 /// The guts of `normalize`: normalize a specific projection like `<T
368 /// as Trait>::Item`. The result is always a type (and possibly
369 /// additional obligations). If ambiguity arises, which implies that
370 /// there are unresolved type variables in the projection, we will
371 /// substitute a fresh type variable `$X` and generate a new
372 /// obligation `<T as Trait>::Item == $X` for later.
373 pub fn normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
374 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
375 projection_ty: ty::ProjectionTy<'tcx>,
376 cause: ObligationCause<'tcx>,
378 -> NormalizedTy<'tcx>
380 opt_normalize_projection_type(selcx, projection_ty.clone(), cause.clone(), depth)
381 .unwrap_or_else(move || {
382 // if we bottom out in ambiguity, create a type variable
383 // and a deferred predicate to resolve this when more type
384 // information is available.
386 let tcx = selcx.infcx().tcx;
387 let def_id = tcx.associated_items(projection_ty.trait_ref.def_id).find(|i|
388 i.name == projection_ty.item_name && i.kind == ty::AssociatedKind::Type
389 ).map(|i| i.def_id).unwrap();
390 let ty_var = selcx.infcx().next_ty_var(
391 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
392 let projection = ty::Binder(ty::ProjectionPredicate {
393 projection_ty: projection_ty,
396 let obligation = Obligation::with_depth(
397 cause, depth + 1, projection.to_predicate());
400 obligations: vec![obligation]
405 /// The guts of `normalize`: normalize a specific projection like `<T
406 /// as Trait>::Item`. The result is always a type (and possibly
407 /// additional obligations). Returns `None` in the case of ambiguity,
408 /// which indicates that there are unbound type variables.
409 fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
410 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
411 projection_ty: ty::ProjectionTy<'tcx>,
412 cause: ObligationCause<'tcx>,
414 -> Option<NormalizedTy<'tcx>>
416 let infcx = selcx.infcx();
418 let projection_ty = infcx.resolve_type_vars_if_possible(&projection_ty);
420 debug!("opt_normalize_projection_type(\
421 projection_ty={:?}, \
426 // FIXME(#20304) For now, I am caching here, which is good, but it
427 // means we don't capture the type variables that are created in
428 // the case of ambiguity. Which means we may create a large stream
429 // of such variables. OTOH, if we move the caching up a level, we
430 // would not benefit from caching when proving `T: Trait<U=Foo>`
431 // bounds. It might be the case that we want two distinct caches,
432 // or else another kind of cache entry.
434 match infcx.projection_cache.borrow_mut().try_start(projection_ty) {
436 Err(ProjectionCacheEntry::Ambiguous) => {
437 // If we found ambiguity the last time, that generally
438 // means we will continue to do so until some type in the
439 // key changes (and we know it hasn't, because we just
440 // fully resolved it). One exception though is closure
441 // types, which can transition from having a fixed kind to
442 // no kind with no visible change in the key.
444 // FIXME(#32286) refactor this so that closure type
446 debug!("opt_normalize_projection_type: \
447 found cache entry: ambiguous");
448 if !projection_ty.has_closure_types() {
452 Err(ProjectionCacheEntry::InProgress) => {
453 // If while normalized A::B, we are asked to normalize
454 // A::B, just return A::B itself. This is a conservative
455 // answer, in the sense that A::B *is* clearly equivalent
456 // to A::B, though there may be a better value we can
459 // Under lazy normalization, this can arise when
460 // bootstrapping. That is, imagine an environment with a
461 // where-clause like `A::B == u32`. Now, if we are asked
462 // to normalize `A::B`, we will want to check the
463 // where-clauses in scope. So we will try to unify `A::B`
464 // with `A::B`, which can trigger a recursive
465 // normalization. In that case, I think we will want this code:
468 // let ty = selcx.tcx().mk_projection(projection_ty.trait_ref,
469 // projection_ty.item_name);
470 // return Some(NormalizedTy { value: v, obligations: vec![] });
473 debug!("opt_normalize_projection_type: \
474 found cache entry: in-progress");
476 // But for now, let's classify this as an overflow:
477 let recursion_limit = selcx.tcx().sess.recursion_limit.get();
478 let obligation = Obligation::with_depth(cause.clone(),
481 selcx.infcx().report_overflow_error(&obligation, false);
483 Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
484 // If we find the value in the cache, then the obligations
485 // have already been returned from the previous entry (and
486 // should therefore have been honored).
487 debug!("opt_normalize_projection_type: \
488 found normalized ty `{:?}`",
490 return Some(NormalizedTy { value: ty, obligations: vec![] });
492 Err(ProjectionCacheEntry::Error) => {
493 debug!("opt_normalize_projection_type: \
495 return Some(normalize_to_error(selcx, projection_ty, cause, depth));
499 let obligation = Obligation::with_depth(cause.clone(), depth, projection_ty.clone());
500 match project_type(selcx, &obligation) {
501 Ok(ProjectedTy::Progress(Progress { ty: projected_ty,
504 // if projection succeeded, then what we get out of this
505 // is also non-normalized (consider: it was derived from
506 // an impl, where-clause etc) and hence we must
509 debug!("opt_normalize_projection_type: \
519 let result = if projected_ty.has_projection_types() {
520 let mut normalizer = AssociatedTypeNormalizer::new(selcx, cause, depth+1);
521 let normalized_ty = normalizer.fold(&projected_ty);
523 debug!("opt_normalize_projection_type: \
524 normalized_ty={:?} depth={}",
528 obligations.extend(normalizer.obligations);
530 value: normalized_ty,
531 obligations: obligations,
536 obligations: obligations,
539 infcx.projection_cache.borrow_mut()
540 .complete(projection_ty, &result, cacheable);
543 Ok(ProjectedTy::NoProgress(projected_ty)) => {
544 debug!("opt_normalize_projection_type: \
545 projected_ty={:?} no progress",
547 let result = Normalized {
551 infcx.projection_cache.borrow_mut()
552 .complete(projection_ty, &result, true);
555 Err(ProjectionTyError::TooManyCandidates) => {
556 debug!("opt_normalize_projection_type: \
557 too many candidates");
558 infcx.projection_cache.borrow_mut()
559 .ambiguous(projection_ty);
562 Err(ProjectionTyError::TraitSelectionError(_)) => {
563 debug!("opt_normalize_projection_type: ERROR");
564 // if we got an error processing the `T as Trait` part,
565 // just return `ty::err` but add the obligation `T :
566 // Trait`, which when processed will cause the error to be
569 infcx.projection_cache.borrow_mut()
570 .error(projection_ty);
571 Some(normalize_to_error(selcx, projection_ty, cause, depth))
576 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
577 /// hold. In various error cases, we cannot generate a valid
578 /// normalized projection. Therefore, we create an inference variable
579 /// return an associated obligation that, when fulfilled, will lead to
582 /// Note that we used to return `TyError` here, but that was quite
583 /// dubious -- the premise was that an error would *eventually* be
584 /// reported, when the obligation was processed. But in general once
585 /// you see a `TyError` you are supposed to be able to assume that an
586 /// error *has been* reported, so that you can take whatever heuristic
587 /// paths you want to take. To make things worse, it was possible for
588 /// cycles to arise, where you basically had a setup like `<MyType<$0>
589 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
590 /// Trait>::Foo> to `[type error]` would lead to an obligation of
591 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
592 /// an error for this obligation, but we legitimately should not,
593 /// because it contains `[type error]`. Yuck! (See issue #29857 for
594 /// one case where this arose.)
595 fn normalize_to_error<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
596 projection_ty: ty::ProjectionTy<'tcx>,
597 cause: ObligationCause<'tcx>,
599 -> NormalizedTy<'tcx>
601 let trait_ref = projection_ty.trait_ref.to_poly_trait_ref();
602 let trait_obligation = Obligation { cause: cause,
603 recursion_depth: depth,
604 predicate: trait_ref.to_predicate() };
605 let tcx = selcx.infcx().tcx;
606 let def_id = tcx.associated_items(projection_ty.trait_ref.def_id).find(|i|
607 i.name == projection_ty.item_name && i.kind == ty::AssociatedKind::Type
608 ).map(|i| i.def_id).unwrap();
609 let new_value = selcx.infcx().next_ty_var(
610 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
613 obligations: vec![trait_obligation]
617 enum ProjectedTy<'tcx> {
618 Progress(Progress<'tcx>),
619 NoProgress(Ty<'tcx>),
622 struct Progress<'tcx> {
624 obligations: Vec<PredicateObligation<'tcx>>,
628 impl<'tcx> Progress<'tcx> {
629 fn error<'a,'gcx>(tcx: TyCtxt<'a,'gcx,'tcx>) -> Self {
637 fn with_addl_obligations(mut self,
638 mut obligations: Vec<PredicateObligation<'tcx>>)
640 debug!("with_addl_obligations: self.obligations.len={} obligations.len={}",
641 self.obligations.len(), obligations.len());
643 debug!("with_addl_obligations: self.obligations={:?} obligations={:?}",
644 self.obligations, obligations);
646 self.obligations.append(&mut obligations);
651 /// Compute the result of a projection type (if we can).
654 /// - `obligation` must be fully normalized
655 fn project_type<'cx, 'gcx, 'tcx>(
656 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
657 obligation: &ProjectionTyObligation<'tcx>)
658 -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>>
660 debug!("project(obligation={:?})",
663 let recursion_limit = selcx.tcx().sess.recursion_limit.get();
664 if obligation.recursion_depth >= recursion_limit {
665 debug!("project: overflow!");
666 selcx.infcx().report_overflow_error(&obligation, true);
669 let obligation_trait_ref = &obligation.predicate.trait_ref;
671 debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
673 if obligation_trait_ref.references_error() {
674 return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
677 let mut candidates = ProjectionTyCandidateSet {
682 assemble_candidates_from_param_env(selcx,
684 &obligation_trait_ref,
687 assemble_candidates_from_trait_def(selcx,
689 &obligation_trait_ref,
692 if let Err(e) = assemble_candidates_from_impls(selcx,
694 &obligation_trait_ref,
696 return Err(ProjectionTyError::TraitSelectionError(e));
699 debug!("{} candidates, ambiguous={}",
700 candidates.vec.len(),
701 candidates.ambiguous);
703 // Inherent ambiguity that prevents us from even enumerating the
705 if candidates.ambiguous {
706 return Err(ProjectionTyError::TooManyCandidates);
711 // Note: `candidates.vec` seems to be on the critical path of the
712 // compiler. Replacing it with an hash set was also tried, which would
713 // render the following dedup unnecessary. It led to cleaner code but
714 // prolonged compiling time of `librustc` from 5m30s to 6m in one test, or
715 // ~9% performance lost.
716 if candidates.vec.len() > 1 {
718 while i < candidates.vec.len() {
719 let has_dup = (0..i).any(|j| candidates.vec[i] == candidates.vec[j]);
721 candidates.vec.swap_remove(i);
728 // Prefer where-clauses. As in select, if there are multiple
729 // candidates, we prefer where-clause candidates over impls. This
730 // may seem a bit surprising, since impls are the source of
731 // "truth" in some sense, but in fact some of the impls that SEEM
732 // applicable are not, because of nested obligations. Where
733 // clauses are the safer choice. See the comment on
734 // `select::SelectionCandidate` and #21974 for more details.
735 if candidates.vec.len() > 1 {
736 debug!("retaining param-env candidates only from {:?}", candidates.vec);
737 candidates.vec.retain(|c| match *c {
738 ProjectionTyCandidate::ParamEnv(..) => true,
739 ProjectionTyCandidate::TraitDef(..) |
740 ProjectionTyCandidate::Select => false,
742 debug!("resulting candidate set: {:?}", candidates.vec);
743 if candidates.vec.len() != 1 {
744 return Err(ProjectionTyError::TooManyCandidates);
748 assert!(candidates.vec.len() <= 1);
750 match candidates.vec.pop() {
752 Ok(ProjectedTy::Progress(
753 confirm_candidate(selcx,
755 &obligation_trait_ref,
759 Ok(ProjectedTy::NoProgress(
760 selcx.tcx().mk_projection(
761 obligation.predicate.trait_ref.clone(),
762 obligation.predicate.item_name)))
767 /// The first thing we have to do is scan through the parameter
768 /// environment to see whether there are any projection predicates
769 /// there that can answer this question.
770 fn assemble_candidates_from_param_env<'cx, 'gcx, 'tcx>(
771 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
772 obligation: &ProjectionTyObligation<'tcx>,
773 obligation_trait_ref: &ty::TraitRef<'tcx>,
774 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
776 debug!("assemble_candidates_from_param_env(..)");
777 let env_predicates = selcx.param_env().caller_bounds.iter().cloned();
778 assemble_candidates_from_predicates(selcx,
780 obligation_trait_ref,
782 ProjectionTyCandidate::ParamEnv,
786 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
787 /// that the definition of `Foo` has some clues:
791 /// type FooT : Bar<BarT=i32>
795 /// Here, for example, we could conclude that the result is `i32`.
796 fn assemble_candidates_from_trait_def<'cx, 'gcx, 'tcx>(
797 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
798 obligation: &ProjectionTyObligation<'tcx>,
799 obligation_trait_ref: &ty::TraitRef<'tcx>,
800 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
802 debug!("assemble_candidates_from_trait_def(..)");
804 // Check whether the self-type is itself a projection.
805 let (def_id, substs) = match obligation_trait_ref.self_ty().sty {
806 ty::TyProjection(ref data) => {
807 (data.trait_ref.def_id, data.trait_ref.substs)
809 ty::TyAnon(def_id, substs) => (def_id, substs),
810 ty::TyInfer(ty::TyVar(_)) => {
811 // If the self-type is an inference variable, then it MAY wind up
812 // being a projected type, so induce an ambiguity.
813 candidate_set.ambiguous = true;
819 // If so, extract what we know from the trait and try to come up with a good answer.
820 let trait_predicates = selcx.tcx().item_predicates(def_id);
821 let bounds = trait_predicates.instantiate(selcx.tcx(), substs);
822 let bounds = elaborate_predicates(selcx.tcx(), bounds.predicates);
823 assemble_candidates_from_predicates(selcx,
825 obligation_trait_ref,
827 ProjectionTyCandidate::TraitDef,
831 fn assemble_candidates_from_predicates<'cx, 'gcx, 'tcx, I>(
832 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
833 obligation: &ProjectionTyObligation<'tcx>,
834 obligation_trait_ref: &ty::TraitRef<'tcx>,
835 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
836 ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
838 where I: Iterator<Item=ty::Predicate<'tcx>>
840 debug!("assemble_candidates_from_predicates(obligation={:?})",
842 let infcx = selcx.infcx();
843 for predicate in env_predicates {
844 debug!("assemble_candidates_from_predicates: predicate={:?}",
847 ty::Predicate::Projection(ref data) => {
848 let same_name = data.item_name() == obligation.predicate.item_name;
850 let is_match = same_name && infcx.probe(|_| {
851 let data_poly_trait_ref =
852 data.to_poly_trait_ref();
853 let obligation_poly_trait_ref =
854 obligation_trait_ref.to_poly_trait_ref();
855 infcx.sub_poly_trait_refs(false,
856 obligation.cause.clone(),
858 obligation_poly_trait_ref)
859 .map(|InferOk { obligations: _, value: () }| {
860 // FIXME(#32730) -- do we need to take obligations
861 // into account in any way? At the moment, no.
866 debug!("assemble_candidates_from_predicates: candidate={:?} \
867 is_match={} same_name={}",
868 data, is_match, same_name);
871 candidate_set.vec.push(ctor(data.clone()));
879 fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
880 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
881 obligation: &ProjectionTyObligation<'tcx>,
882 obligation_trait_ref: &ty::TraitRef<'tcx>,
883 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
884 -> Result<(), SelectionError<'tcx>>
886 // If we are resolving `<T as TraitRef<...>>::Item == Type`,
887 // start out by selecting the predicate `T as TraitRef<...>`:
888 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
889 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
890 selcx.infcx().probe(|_| {
891 let vtable = match selcx.select(&trait_obligation) {
892 Ok(Some(vtable)) => vtable,
894 candidate_set.ambiguous = true;
898 debug!("assemble_candidates_from_impls: selection error {:?}",
905 super::VtableClosure(_) |
906 super::VtableFnPointer(_) |
907 super::VtableObject(_) => {
908 debug!("assemble_candidates_from_impls: vtable={:?}",
911 candidate_set.vec.push(ProjectionTyCandidate::Select);
913 super::VtableImpl(ref impl_data) => {
914 // We have to be careful when projecting out of an
915 // impl because of specialization. If we are not in
916 // trans (i.e., projection mode is not "any"), and the
917 // impl's type is declared as default, then we disable
918 // projection (even if the trait ref is fully
919 // monomorphic). In the case where trait ref is not
920 // fully monomorphic (i.e., includes type parameters),
921 // this is because those type parameters may
922 // ultimately be bound to types from other crates that
923 // may have specialized impls we can't see. In the
924 // case where the trait ref IS fully monomorphic, this
925 // is a policy decision that we made in the RFC in
926 // order to preserve flexibility for the crate that
927 // defined the specializable impl to specialize later
928 // for existing types.
930 // In either case, we handle this by not adding a
931 // candidate for an impl if it contains a `default`
933 let opt_node_item = assoc_ty_def(selcx,
934 impl_data.impl_def_id,
935 obligation.predicate.item_name);
936 let new_candidate = if let Some(node_item) = opt_node_item {
937 let is_default = if node_item.node.is_from_trait() {
938 // If true, the impl inherited a `type Foo = Bar`
939 // given in the trait, which is implicitly default.
940 // Otherwise, the impl did not specify `type` and
941 // neither did the trait:
944 // trait Foo { type T; }
945 // impl Foo for Bar { }
948 // This is an error, but it will be
949 // reported in `check_impl_items_against_trait`.
950 // We accept it here but will flag it as
951 // an error when we confirm the candidate
952 // (which will ultimately lead to `normalize_to_error`
954 node_item.item.defaultness.has_value()
956 node_item.item.defaultness.is_default()
959 // Only reveal a specializable default if we're past type-checking
960 // and the obligations is monomorphic, otherwise passes such as
961 // transmute checking and polymorphic MIR optimizations could
962 // get a result which isn't correct for all monomorphizations.
964 Some(ProjectionTyCandidate::Select)
965 } else if selcx.projection_mode() == Reveal::All {
966 assert!(!poly_trait_ref.needs_infer());
967 if !poly_trait_ref.needs_subst() {
968 Some(ProjectionTyCandidate::Select)
976 // This is saying that neither the trait nor
977 // the impl contain a definition for this
978 // associated type. Normally this situation
979 // could only arise through a compiler bug --
980 // if the user wrote a bad item name, it
981 // should have failed in astconv. **However**,
982 // at coherence-checking time, we only look at
983 // the topmost impl (we don't even consider
984 // the trait itself) for the definition -- and
985 // so in that case it may be that the trait
986 // *DOES* have a declaration, but we don't see
987 // it, and we end up in this branch.
989 // This is kind of tricky to handle actually.
990 // For now, we just unconditionally ICE,
991 // because otherwise, examples like the
992 // following will succeed:
999 // impl<T> Assoc for T {
1000 // default type Output = bool;
1003 // impl Assoc for u8 {}
1004 // impl Assoc for u16 {}
1007 // impl Foo for <u8 as Assoc>::Output {}
1008 // impl Foo for <u16 as Assoc>::Output {}
1013 // The essential problem here is that the
1014 // projection fails, leaving two unnormalized
1015 // types, which appear not to unify -- so the
1016 // overlap check succeeds, when it should
1018 span_bug!(obligation.cause.span,
1019 "Tried to project an inherited associated type during \
1020 coherence checking, which is currently not supported.");
1022 candidate_set.vec.extend(new_candidate);
1024 super::VtableParam(..) => {
1025 // This case tell us nothing about the value of an
1026 // associated type. Consider:
1029 // trait SomeTrait { type Foo; }
1030 // fn foo<T:SomeTrait>(...) { }
1033 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1034 // : SomeTrait` binding does not help us decide what the
1035 // type `Foo` is (at least, not more specifically than
1036 // what we already knew).
1038 // But wait, you say! What about an example like this:
1041 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1044 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1045 // resolve `T::Foo`? And of course it does, but in fact
1046 // that single predicate is desugared into two predicates
1047 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1048 // projection. And the projection where clause is handled
1049 // in `assemble_candidates_from_param_env`.
1051 super::VtableDefaultImpl(..) |
1052 super::VtableBuiltin(..) => {
1053 // These traits have no associated types.
1055 obligation.cause.span,
1056 "Cannot project an associated type from `{:?}`",
1065 fn confirm_candidate<'cx, 'gcx, 'tcx>(
1066 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1067 obligation: &ProjectionTyObligation<'tcx>,
1068 obligation_trait_ref: &ty::TraitRef<'tcx>,
1069 candidate: ProjectionTyCandidate<'tcx>)
1072 debug!("confirm_candidate(candidate={:?}, obligation={:?})",
1077 ProjectionTyCandidate::ParamEnv(poly_projection) |
1078 ProjectionTyCandidate::TraitDef(poly_projection) => {
1079 confirm_param_env_candidate(selcx, obligation, poly_projection)
1082 ProjectionTyCandidate::Select => {
1083 confirm_select_candidate(selcx, obligation, obligation_trait_ref)
1088 fn confirm_select_candidate<'cx, 'gcx, 'tcx>(
1089 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1090 obligation: &ProjectionTyObligation<'tcx>,
1091 obligation_trait_ref: &ty::TraitRef<'tcx>)
1094 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1095 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1096 let vtable = match selcx.select(&trait_obligation) {
1097 Ok(Some(vtable)) => vtable,
1100 obligation.cause.span,
1101 "Failed to select `{:?}`",
1107 super::VtableImpl(data) =>
1108 confirm_impl_candidate(selcx, obligation, data),
1109 super::VtableClosure(data) =>
1110 confirm_closure_candidate(selcx, obligation, data),
1111 super::VtableFnPointer(data) =>
1112 confirm_fn_pointer_candidate(selcx, obligation, data),
1113 super::VtableObject(_) =>
1114 confirm_object_candidate(selcx, obligation, obligation_trait_ref),
1115 super::VtableDefaultImpl(..) |
1116 super::VtableParam(..) |
1117 super::VtableBuiltin(..) =>
1118 // we don't create Select candidates with this kind of resolution
1120 obligation.cause.span,
1121 "Cannot project an associated type from `{:?}`",
1126 fn confirm_object_candidate<'cx, 'gcx, 'tcx>(
1127 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1128 obligation: &ProjectionTyObligation<'tcx>,
1129 obligation_trait_ref: &ty::TraitRef<'tcx>)
1132 let self_ty = obligation_trait_ref.self_ty();
1133 let object_ty = selcx.infcx().shallow_resolve(self_ty);
1134 debug!("confirm_object_candidate(object_ty={:?})",
1136 let data = match object_ty.sty {
1137 ty::TyDynamic(ref data, ..) => data,
1140 obligation.cause.span,
1141 "confirm_object_candidate called with non-object: {:?}",
1145 let env_predicates = data.projection_bounds().map(|p| {
1146 p.with_self_ty(selcx.tcx(), object_ty).to_predicate()
1148 let env_predicate = {
1149 let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1151 // select only those projections that are actually projecting an
1152 // item with the correct name
1153 let env_predicates = env_predicates.filter_map(|p| match p {
1154 ty::Predicate::Projection(data) =>
1155 if data.item_name() == obligation.predicate.item_name {
1163 // select those with a relevant trait-ref
1164 let mut env_predicates = env_predicates.filter(|data| {
1165 let data_poly_trait_ref = data.to_poly_trait_ref();
1166 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1167 selcx.infcx().probe(|_| {
1168 selcx.infcx().sub_poly_trait_refs(false,
1169 obligation.cause.clone(),
1170 data_poly_trait_ref,
1171 obligation_poly_trait_ref).is_ok()
1175 // select the first matching one; there really ought to be one or
1176 // else the object type is not WF, since an object type should
1177 // include all of its projections explicitly
1178 match env_predicates.next() {
1179 Some(env_predicate) => env_predicate,
1181 debug!("confirm_object_candidate: no env-predicate \
1182 found in object type `{:?}`; ill-formed",
1184 return Progress::error(selcx.tcx());
1189 confirm_param_env_candidate(selcx, obligation, env_predicate)
1192 fn confirm_fn_pointer_candidate<'cx, 'gcx, 'tcx>(
1193 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1194 obligation: &ProjectionTyObligation<'tcx>,
1195 fn_pointer_vtable: VtableFnPointerData<'tcx, PredicateObligation<'tcx>>)
1198 let fn_type = selcx.infcx().shallow_resolve(fn_pointer_vtable.fn_ty);
1199 let sig = fn_type.fn_sig();
1200 confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1201 .with_addl_obligations(fn_pointer_vtable.nested)
1204 fn confirm_closure_candidate<'cx, 'gcx, 'tcx>(
1205 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1206 obligation: &ProjectionTyObligation<'tcx>,
1207 vtable: VtableClosureData<'tcx, PredicateObligation<'tcx>>)
1210 let closure_typer = selcx.closure_typer();
1211 let closure_type = closure_typer.closure_type(vtable.closure_def_id, vtable.substs);
1213 value: closure_type,
1215 } = normalize_with_depth(selcx,
1216 obligation.cause.clone(),
1217 obligation.recursion_depth+1,
1220 debug!("confirm_closure_candidate: obligation={:?},closure_type={:?},obligations={:?}",
1225 confirm_callable_candidate(selcx,
1228 util::TupleArgumentsFlag::No)
1229 .with_addl_obligations(vtable.nested)
1230 .with_addl_obligations(obligations)
1233 fn confirm_callable_candidate<'cx, 'gcx, 'tcx>(
1234 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1235 obligation: &ProjectionTyObligation<'tcx>,
1236 fn_sig: &ty::PolyFnSig<'tcx>,
1237 flag: util::TupleArgumentsFlag)
1240 let tcx = selcx.tcx();
1242 debug!("confirm_callable_candidate({:?},{:?})",
1246 // the `Output` associated type is declared on `FnOnce`
1247 let fn_once_def_id = tcx.lang_items.fn_once_trait().unwrap();
1249 // Note: we unwrap the binder here but re-create it below (1)
1250 let ty::Binder((trait_ref, ret_type)) =
1251 tcx.closure_trait_ref_and_return_type(fn_once_def_id,
1252 obligation.predicate.trait_ref.self_ty(),
1256 let predicate = ty::Binder(ty::ProjectionPredicate { // (1) recreate binder here
1257 projection_ty: ty::ProjectionTy {
1258 trait_ref: trait_ref,
1259 item_name: Symbol::intern(FN_OUTPUT_NAME),
1264 confirm_param_env_candidate(selcx, obligation, predicate)
1267 fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
1268 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1269 obligation: &ProjectionTyObligation<'tcx>,
1270 poly_projection: ty::PolyProjectionPredicate<'tcx>)
1273 let infcx = selcx.infcx();
1274 let cause = obligation.cause.clone();
1275 let trait_ref = obligation.predicate.trait_ref;
1276 match infcx.match_poly_projection_predicate(cause, poly_projection, trait_ref) {
1277 Ok(InferOk { value: ty_match, obligations }) => {
1280 obligations: obligations,
1281 cacheable: ty_match.unconstrained_regions.is_empty(),
1286 obligation.cause.span,
1287 "Failed to unify obligation `{:?}` \
1288 with poly_projection `{:?}`: {:?}",
1296 fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
1297 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1298 obligation: &ProjectionTyObligation<'tcx>,
1299 impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>)
1302 let VtableImplData { substs, nested, impl_def_id } = impl_vtable;
1304 let tcx = selcx.tcx();
1305 let trait_ref = obligation.predicate.trait_ref;
1306 let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_name);
1309 Some(node_item) => {
1310 let ty = if !node_item.item.defaultness.has_value() {
1311 // This means that the impl is missing a definition for the
1312 // associated type. This error will be reported by the type
1313 // checker method `check_impl_items_against_trait`, so here we
1314 // just return TyError.
1315 debug!("confirm_impl_candidate: no associated type {:?} for {:?}",
1316 node_item.item.name,
1317 obligation.predicate.trait_ref);
1320 tcx.item_type(node_item.item.def_id)
1322 let substs = translate_substs(selcx.infcx(), impl_def_id, substs, node_item.node);
1324 ty: ty.subst(tcx, substs),
1325 obligations: nested,
1330 span_bug!(obligation.cause.span,
1331 "No associated type for {:?}",
1337 /// Locate the definition of an associated type in the specialization hierarchy,
1338 /// starting from the given impl.
1340 /// Based on the "projection mode", this lookup may in fact only examine the
1341 /// topmost impl. See the comments for `Reveal` for more details.
1342 fn assoc_ty_def<'cx, 'gcx, 'tcx>(
1343 selcx: &SelectionContext<'cx, 'gcx, 'tcx>,
1345 assoc_ty_name: ast::Name)
1346 -> Option<specialization_graph::NodeItem<ty::AssociatedItem>>
1348 let trait_def_id = selcx.tcx().impl_trait_ref(impl_def_id).unwrap().def_id;
1350 if selcx.projection_mode() == Reveal::ExactMatch {
1351 let impl_node = specialization_graph::Node::Impl(impl_def_id);
1352 for item in impl_node.items(selcx.tcx()) {
1353 if item.kind == ty::AssociatedKind::Type && item.name == assoc_ty_name {
1354 return Some(specialization_graph::NodeItem {
1355 node: specialization_graph::Node::Impl(impl_def_id),
1362 selcx.tcx().lookup_trait_def(trait_def_id)
1363 .ancestors(impl_def_id)
1364 .defs(selcx.tcx(), assoc_ty_name, ty::AssociatedKind::Type)
1371 pub struct ProjectionCache<'tcx> {
1372 map: SnapshotMap<ty::ProjectionTy<'tcx>, ProjectionCacheEntry<'tcx>>,
1375 #[derive(Clone, Debug)]
1376 enum ProjectionCacheEntry<'tcx> {
1380 NormalizedTy(Ty<'tcx>),
1383 // NB: intentionally not Clone
1384 pub struct ProjectionCacheSnapshot {
1388 impl<'tcx> ProjectionCache<'tcx> {
1389 pub fn new() -> Self {
1391 map: SnapshotMap::new()
1395 pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
1396 ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
1399 pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
1400 self.map.rollback_to(snapshot.snapshot);
1403 pub fn rollback_skolemized(&mut self, snapshot: &ProjectionCacheSnapshot) {
1404 self.map.partial_rollback(&snapshot.snapshot, &|k| k.has_re_skol());
1407 pub fn commit(&mut self, snapshot: ProjectionCacheSnapshot) {
1408 self.map.commit(snapshot.snapshot);
1411 /// Try to start normalize `key`; returns an error if
1412 /// normalization already occured (this error corresponds to a
1413 /// cache hit, so it's actually a good thing).
1414 fn try_start(&mut self, key: ty::ProjectionTy<'tcx>)
1415 -> Result<(), ProjectionCacheEntry<'tcx>> {
1416 if let Some(entry) = self.map.get(&key) {
1417 return Err(entry.clone());
1420 self.map.insert(key, ProjectionCacheEntry::InProgress);
1424 /// Indicates that `key` was normalized to `value`. If `cacheable` is false,
1425 /// then this result is sadly not cacheable.
1426 fn complete(&mut self,
1427 key: ty::ProjectionTy<'tcx>,
1428 value: &NormalizedTy<'tcx>,
1430 let fresh_key = if cacheable {
1431 debug!("ProjectionCacheEntry::complete: adding cache entry: key={:?}, value={:?}",
1433 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value.value))
1435 debug!("ProjectionCacheEntry::complete: cannot cache: key={:?}, value={:?}",
1437 !self.map.remove(key)
1440 assert!(!fresh_key, "never started projecting `{:?}`", key);
1443 /// Indicates that trying to normalize `key` resulted in
1444 /// ambiguity. No point in trying it again then until we gain more
1445 /// type information (in which case, the "fully resolved" key will
1447 fn ambiguous(&mut self, key: ty::ProjectionTy<'tcx>) {
1448 let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
1449 assert!(!fresh, "never started projecting `{:?}`", key);
1452 /// Indicates that trying to normalize `key` resulted in
1454 fn error(&mut self, key: ty::ProjectionTy<'tcx>) {
1455 let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
1456 assert!(!fresh, "never started projecting `{:?}`", key);