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)]
41 /// At type-checking time, we refuse to project any associated
42 /// type that is marked `default`. Non-`default` ("final") types
43 /// are always projected. This is necessary in general for
44 /// soundness of specialization. However, we *could* allow
45 /// projections in fully-monomorphic cases. We choose not to,
46 /// because we prefer for `default type` to force the type
47 /// definition to be treated abstractly by any consumers of the
48 /// impl. Concretely, that means that the following example will
56 /// impl<T> Assoc for T {
57 /// default type Output = bool;
61 /// let <() as Assoc>::Output = true;
65 /// At trans time, all monomorphic projections will succeed.
66 /// Also, `impl Trait` is normalized to the concrete type,
67 /// which has to be already collected by type-checking.
69 /// NOTE: As `impl Trait`'s concrete type should *never*
70 /// be observable directly by the user, `Reveal::All`
71 /// should not be used by checks which may expose
72 /// type equality or type contents to the user.
73 /// There are some exceptions, e.g. around OIBITS and
74 /// transmute-checking, which expose some details, but
75 /// not the whole concrete type of the `impl Trait`.
79 pub type PolyProjectionObligation<'tcx> =
80 Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
82 pub type ProjectionObligation<'tcx> =
83 Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
85 pub type ProjectionTyObligation<'tcx> =
86 Obligation<'tcx, ty::ProjectionTy<'tcx>>;
88 /// When attempting to resolve `<T as TraitRef>::Name` ...
90 pub enum ProjectionTyError<'tcx> {
91 /// ...we found multiple sources of information and couldn't resolve the ambiguity.
94 /// ...an error occurred matching `T : TraitRef`
95 TraitSelectionError(SelectionError<'tcx>),
99 pub struct MismatchedProjectionTypes<'tcx> {
100 pub err: ty::error::TypeError<'tcx>
103 #[derive(PartialEq, Eq, Debug)]
104 enum ProjectionTyCandidate<'tcx> {
105 // from a where-clause in the env or object type
106 ParamEnv(ty::PolyProjectionPredicate<'tcx>),
108 // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
109 TraitDef(ty::PolyProjectionPredicate<'tcx>),
111 // from a "impl" (or a "pseudo-impl" returned by select)
115 struct ProjectionTyCandidateSet<'tcx> {
116 vec: Vec<ProjectionTyCandidate<'tcx>>,
120 /// Evaluates constraints of the form:
122 /// for<...> <T as Trait>::U == V
124 /// If successful, this may result in additional obligations.
125 pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
126 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
127 obligation: &PolyProjectionObligation<'tcx>)
128 -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>>
130 debug!("poly_project_and_unify_type(obligation={:?})",
133 let infcx = selcx.infcx();
134 infcx.commit_if_ok(|snapshot| {
135 let (skol_predicate, skol_map) =
136 infcx.skolemize_late_bound_regions(&obligation.predicate, snapshot);
138 let skol_obligation = obligation.with(skol_predicate);
139 let r = match project_and_unify_type(selcx, &skol_obligation) {
141 let span = obligation.cause.span;
142 match infcx.leak_check(false, span, &skol_map, snapshot) {
143 Ok(()) => Ok(infcx.plug_leaks(skol_map, snapshot, result)),
144 Err(e) => Err(MismatchedProjectionTypes { err: e }),
156 /// Evaluates constraints of the form:
158 /// <T as Trait>::U == V
160 /// If successful, this may result in additional obligations.
161 fn project_and_unify_type<'cx, 'gcx, 'tcx>(
162 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
163 obligation: &ProjectionObligation<'tcx>)
164 -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>>
166 debug!("project_and_unify_type(obligation={:?})",
169 let Normalized { value: normalized_ty, mut obligations } =
170 match opt_normalize_projection_type(selcx,
171 obligation.predicate.projection_ty.clone(),
172 obligation.cause.clone(),
173 obligation.recursion_depth) {
175 None => return Ok(None),
178 debug!("project_and_unify_type: normalized_ty={:?} obligations={:?}",
182 let infcx = selcx.infcx();
183 match infcx.eq_types(true, &obligation.cause, normalized_ty, obligation.predicate.ty) {
184 Ok(InferOk { obligations: inferred_obligations, value: () }) => {
185 obligations.extend(inferred_obligations);
186 Ok(Some(obligations))
188 Err(err) => Err(MismatchedProjectionTypes { err: err }),
192 /// Normalizes any associated type projections in `value`, replacing
193 /// them with a fully resolved type where possible. The return value
194 /// combines the normalized result and any additional obligations that
195 /// were incurred as result.
196 pub fn normalize<'a, 'b, 'gcx, 'tcx, T>(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
197 cause: ObligationCause<'tcx>,
199 -> Normalized<'tcx, T>
200 where T : TypeFoldable<'tcx>
202 normalize_with_depth(selcx, cause, 0, value)
205 /// As `normalize`, but with a custom depth.
206 pub fn normalize_with_depth<'a, 'b, 'gcx, 'tcx, T>(
207 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
208 cause: ObligationCause<'tcx>,
211 -> Normalized<'tcx, T>
213 where T : TypeFoldable<'tcx>
215 debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
216 let mut normalizer = AssociatedTypeNormalizer::new(selcx, cause, depth);
217 let result = normalizer.fold(value);
218 debug!("normalize_with_depth: depth={} result={:?} with {} obligations",
219 depth, result, normalizer.obligations.len());
220 debug!("normalize_with_depth: depth={} obligations={:?}",
221 depth, normalizer.obligations);
224 obligations: normalizer.obligations,
228 struct AssociatedTypeNormalizer<'a, 'b: 'a, 'gcx: 'b+'tcx, 'tcx: 'b> {
229 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
230 cause: ObligationCause<'tcx>,
231 obligations: Vec<PredicateObligation<'tcx>>,
235 impl<'a, 'b, 'gcx, 'tcx> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
236 fn new(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
237 cause: ObligationCause<'tcx>,
239 -> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx>
241 AssociatedTypeNormalizer {
249 fn fold<T:TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
250 let value = self.selcx.infcx().resolve_type_vars_if_possible(value);
252 if !value.has_projection_types() {
255 value.fold_with(self)
260 impl<'a, 'b, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
261 fn tcx<'c>(&'c self) -> TyCtxt<'c, 'gcx, 'tcx> {
265 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
266 // We don't want to normalize associated types that occur inside of region
267 // binders, because they may contain bound regions, and we can't cope with that.
271 // for<'a> fn(<T as Foo<&'a>>::A)
273 // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
274 // normalize it when we instantiate those bound regions (which
275 // should occur eventually).
277 let ty = ty.super_fold_with(self);
279 ty::TyAnon(def_id, substs) if !substs.has_escaping_regions() => { // (*)
280 // Only normalize `impl Trait` after type-checking, usually in trans.
281 if self.selcx.projection_mode() == Reveal::All {
282 let generic_ty = self.tcx().item_type(def_id);
283 let concrete_ty = generic_ty.subst(self.tcx(), substs);
284 self.fold_ty(concrete_ty)
290 ty::TyProjection(ref data) if !data.has_escaping_regions() => { // (*)
292 // (*) This is kind of hacky -- we need to be able to
293 // handle normalization within binders because
294 // otherwise we wind up a need to normalize when doing
295 // trait matching (since you can have a trait
296 // obligation like `for<'a> T::B : Fn(&'a int)`), but
297 // we can't normalize with bound regions in scope. So
298 // far now we just ignore binders but only normalize
299 // if all bound regions are gone (and then we still
300 // have to renormalize whenever we instantiate a
301 // binder). It would be better to normalize in a
302 // binding-aware fashion.
304 let Normalized { value: normalized_ty, obligations } =
305 normalize_projection_type(self.selcx,
309 debug!("AssociatedTypeNormalizer: depth={} normalized {:?} to {:?} \
310 with {} add'l obligations",
311 self.depth, ty, normalized_ty, obligations.len());
312 self.obligations.extend(obligations);
324 pub struct Normalized<'tcx,T> {
326 pub obligations: Vec<PredicateObligation<'tcx>>,
329 pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
331 impl<'tcx,T> Normalized<'tcx,T> {
332 pub fn with<U>(self, value: U) -> Normalized<'tcx,U> {
333 Normalized { value: value, obligations: self.obligations }
337 /// The guts of `normalize`: normalize a specific projection like `<T
338 /// as Trait>::Item`. The result is always a type (and possibly
339 /// additional obligations). If ambiguity arises, which implies that
340 /// there are unresolved type variables in the projection, we will
341 /// substitute a fresh type variable `$X` and generate a new
342 /// obligation `<T as Trait>::Item == $X` for later.
343 pub fn normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
344 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
345 projection_ty: ty::ProjectionTy<'tcx>,
346 cause: ObligationCause<'tcx>,
348 -> NormalizedTy<'tcx>
350 opt_normalize_projection_type(selcx, projection_ty.clone(), cause.clone(), depth)
351 .unwrap_or_else(move || {
352 // if we bottom out in ambiguity, create a type variable
353 // and a deferred predicate to resolve this when more type
354 // information is available.
356 let tcx = selcx.infcx().tcx;
357 let def_id = tcx.associated_items(projection_ty.trait_ref.def_id).find(|i|
358 i.name == projection_ty.item_name && i.kind == ty::AssociatedKind::Type
359 ).map(|i| i.def_id).unwrap();
360 let ty_var = selcx.infcx().next_ty_var(
361 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
362 let projection = ty::Binder(ty::ProjectionPredicate {
363 projection_ty: projection_ty,
366 let obligation = Obligation::with_depth(
367 cause, depth + 1, projection.to_predicate());
370 obligations: vec![obligation]
375 /// The guts of `normalize`: normalize a specific projection like `<T
376 /// as Trait>::Item`. The result is always a type (and possibly
377 /// additional obligations). Returns `None` in the case of ambiguity,
378 /// which indicates that there are unbound type variables.
379 fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
380 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
381 projection_ty: ty::ProjectionTy<'tcx>,
382 cause: ObligationCause<'tcx>,
384 -> Option<NormalizedTy<'tcx>>
386 let infcx = selcx.infcx();
388 let projection_ty = infcx.resolve_type_vars_if_possible(&projection_ty);
390 debug!("opt_normalize_projection_type(\
391 projection_ty={:?}, \
396 // FIXME(#20304) For now, I am caching here, which is good, but it
397 // means we don't capture the type variables that are created in
398 // the case of ambiguity. Which means we may create a large stream
399 // of such variables. OTOH, if we move the caching up a level, we
400 // would not benefit from caching when proving `T: Trait<U=Foo>`
401 // bounds. It might be the case that we want two distinct caches,
402 // or else another kind of cache entry.
404 match infcx.projection_cache.borrow_mut().try_start(projection_ty) {
406 Err(ProjectionCacheEntry::Ambiguous) => {
407 // If we found ambiguity the last time, that generally
408 // means we will continue to do so until some type in the
409 // key changes (and we know it hasn't, because we just
410 // fully resolved it). One exception though is closure
411 // types, which can transition from having a fixed kind to
412 // no kind with no visible change in the key.
414 // FIXME(#32286) refactor this so that closure type
416 debug!("opt_normalize_projection_type: \
417 found cache entry: ambiguous");
418 if !projection_ty.has_closure_types() {
422 Err(ProjectionCacheEntry::InProgress) => {
423 // If while normalized A::B, we are asked to normalize
424 // A::B, just return A::B itself. This is a conservative
425 // answer, in the sense that A::B *is* clearly equivalent
426 // to A::B, though there may be a better value we can
429 // Under lazy normalization, this can arise when
430 // bootstrapping. That is, imagine an environment with a
431 // where-clause like `A::B == u32`. Now, if we are asked
432 // to normalize `A::B`, we will want to check the
433 // where-clauses in scope. So we will try to unify `A::B`
434 // with `A::B`, which can trigger a recursive
435 // normalization. In that case, I think we will want this code:
438 // let ty = selcx.tcx().mk_projection(projection_ty.trait_ref,
439 // projection_ty.item_name);
440 // return Some(NormalizedTy { value: v, obligations: vec![] });
443 debug!("opt_normalize_projection_type: \
444 found cache entry: in-progress");
446 // But for now, let's classify this as an overflow:
447 let recursion_limit = selcx.tcx().sess.recursion_limit.get();
448 let obligation = Obligation::with_depth(cause.clone(),
451 selcx.infcx().report_overflow_error(&obligation, false);
453 Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
454 // If we find the value in the cache, then the obligations
455 // have already been returned from the previous entry (and
456 // should therefore have been honored).
457 debug!("opt_normalize_projection_type: \
458 found normalized ty `{:?}`",
460 return Some(NormalizedTy { value: ty, obligations: vec![] });
462 Err(ProjectionCacheEntry::Error) => {
463 debug!("opt_normalize_projection_type: \
465 return Some(normalize_to_error(selcx, projection_ty, cause, depth));
469 let obligation = Obligation::with_depth(cause.clone(), depth, projection_ty.clone());
470 match project_type(selcx, &obligation) {
471 Ok(ProjectedTy::Progress(Progress { ty: projected_ty,
474 // if projection succeeded, then what we get out of this
475 // is also non-normalized (consider: it was derived from
476 // an impl, where-clause etc) and hence we must
479 debug!("opt_normalize_projection_type: \
489 let result = if projected_ty.has_projection_types() {
490 let mut normalizer = AssociatedTypeNormalizer::new(selcx, cause, depth+1);
491 let normalized_ty = normalizer.fold(&projected_ty);
493 debug!("opt_normalize_projection_type: \
494 normalized_ty={:?} depth={}",
498 obligations.extend(normalizer.obligations);
500 value: normalized_ty,
501 obligations: obligations,
506 obligations: obligations,
509 infcx.projection_cache.borrow_mut()
510 .complete(projection_ty, &result, cacheable);
513 Ok(ProjectedTy::NoProgress(projected_ty)) => {
514 debug!("opt_normalize_projection_type: \
515 projected_ty={:?} no progress",
517 let result = Normalized {
521 infcx.projection_cache.borrow_mut()
522 .complete(projection_ty, &result, true);
525 Err(ProjectionTyError::TooManyCandidates) => {
526 debug!("opt_normalize_projection_type: \
527 too many candidates");
528 infcx.projection_cache.borrow_mut()
529 .ambiguous(projection_ty);
532 Err(ProjectionTyError::TraitSelectionError(_)) => {
533 debug!("opt_normalize_projection_type: ERROR");
534 // if we got an error processing the `T as Trait` part,
535 // just return `ty::err` but add the obligation `T :
536 // Trait`, which when processed will cause the error to be
539 infcx.projection_cache.borrow_mut()
540 .error(projection_ty);
541 Some(normalize_to_error(selcx, projection_ty, cause, depth))
546 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
547 /// hold. In various error cases, we cannot generate a valid
548 /// normalized projection. Therefore, we create an inference variable
549 /// return an associated obligation that, when fulfilled, will lead to
552 /// Note that we used to return `TyError` here, but that was quite
553 /// dubious -- the premise was that an error would *eventually* be
554 /// reported, when the obligation was processed. But in general once
555 /// you see a `TyError` you are supposed to be able to assume that an
556 /// error *has been* reported, so that you can take whatever heuristic
557 /// paths you want to take. To make things worse, it was possible for
558 /// cycles to arise, where you basically had a setup like `<MyType<$0>
559 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
560 /// Trait>::Foo> to `[type error]` would lead to an obligation of
561 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
562 /// an error for this obligation, but we legitimately should not,
563 /// because it contains `[type error]`. Yuck! (See issue #29857 for
564 /// one case where this arose.)
565 fn normalize_to_error<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
566 projection_ty: ty::ProjectionTy<'tcx>,
567 cause: ObligationCause<'tcx>,
569 -> NormalizedTy<'tcx>
571 let trait_ref = projection_ty.trait_ref.to_poly_trait_ref();
572 let trait_obligation = Obligation { cause: cause,
573 recursion_depth: depth,
574 predicate: trait_ref.to_predicate() };
575 let tcx = selcx.infcx().tcx;
576 let def_id = tcx.associated_items(projection_ty.trait_ref.def_id).find(|i|
577 i.name == projection_ty.item_name && i.kind == ty::AssociatedKind::Type
578 ).map(|i| i.def_id).unwrap();
579 let new_value = selcx.infcx().next_ty_var(
580 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
583 obligations: vec![trait_obligation]
587 enum ProjectedTy<'tcx> {
588 Progress(Progress<'tcx>),
589 NoProgress(Ty<'tcx>),
592 struct Progress<'tcx> {
594 obligations: Vec<PredicateObligation<'tcx>>,
598 impl<'tcx> Progress<'tcx> {
599 fn error<'a,'gcx>(tcx: TyCtxt<'a,'gcx,'tcx>) -> Self {
607 fn with_addl_obligations(mut self,
608 mut obligations: Vec<PredicateObligation<'tcx>>)
610 debug!("with_addl_obligations: self.obligations.len={} obligations.len={}",
611 self.obligations.len(), obligations.len());
613 debug!("with_addl_obligations: self.obligations={:?} obligations={:?}",
614 self.obligations, obligations);
616 self.obligations.append(&mut obligations);
621 /// Compute the result of a projection type (if we can).
624 /// - `obligation` must be fully normalized
625 fn project_type<'cx, 'gcx, 'tcx>(
626 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
627 obligation: &ProjectionTyObligation<'tcx>)
628 -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>>
630 debug!("project(obligation={:?})",
633 let recursion_limit = selcx.tcx().sess.recursion_limit.get();
634 if obligation.recursion_depth >= recursion_limit {
635 debug!("project: overflow!");
636 selcx.infcx().report_overflow_error(&obligation, true);
639 let obligation_trait_ref = &obligation.predicate.trait_ref;
641 debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
643 if obligation_trait_ref.references_error() {
644 return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
647 let mut candidates = ProjectionTyCandidateSet {
652 assemble_candidates_from_param_env(selcx,
654 &obligation_trait_ref,
657 assemble_candidates_from_trait_def(selcx,
659 &obligation_trait_ref,
662 if let Err(e) = assemble_candidates_from_impls(selcx,
664 &obligation_trait_ref,
666 return Err(ProjectionTyError::TraitSelectionError(e));
669 debug!("{} candidates, ambiguous={}",
670 candidates.vec.len(),
671 candidates.ambiguous);
673 // Inherent ambiguity that prevents us from even enumerating the
675 if candidates.ambiguous {
676 return Err(ProjectionTyError::TooManyCandidates);
681 // Note: `candidates.vec` seems to be on the critical path of the
682 // compiler. Replacing it with an hash set was also tried, which would
683 // render the following dedup unnecessary. It led to cleaner code but
684 // prolonged compiling time of `librustc` from 5m30s to 6m in one test, or
685 // ~9% performance lost.
686 if candidates.vec.len() > 1 {
688 while i < candidates.vec.len() {
689 let has_dup = (0..i).any(|j| candidates.vec[i] == candidates.vec[j]);
691 candidates.vec.swap_remove(i);
698 // Prefer where-clauses. As in select, if there are multiple
699 // candidates, we prefer where-clause candidates over impls. This
700 // may seem a bit surprising, since impls are the source of
701 // "truth" in some sense, but in fact some of the impls that SEEM
702 // applicable are not, because of nested obligations. Where
703 // clauses are the safer choice. See the comment on
704 // `select::SelectionCandidate` and #21974 for more details.
705 if candidates.vec.len() > 1 {
706 debug!("retaining param-env candidates only from {:?}", candidates.vec);
707 candidates.vec.retain(|c| match *c {
708 ProjectionTyCandidate::ParamEnv(..) => true,
709 ProjectionTyCandidate::TraitDef(..) |
710 ProjectionTyCandidate::Select => false,
712 debug!("resulting candidate set: {:?}", candidates.vec);
713 if candidates.vec.len() != 1 {
714 return Err(ProjectionTyError::TooManyCandidates);
718 assert!(candidates.vec.len() <= 1);
720 match candidates.vec.pop() {
722 Ok(ProjectedTy::Progress(
723 confirm_candidate(selcx,
725 &obligation_trait_ref,
729 Ok(ProjectedTy::NoProgress(
730 selcx.tcx().mk_projection(
731 obligation.predicate.trait_ref.clone(),
732 obligation.predicate.item_name)))
737 /// The first thing we have to do is scan through the parameter
738 /// environment to see whether there are any projection predicates
739 /// there that can answer this question.
740 fn assemble_candidates_from_param_env<'cx, 'gcx, 'tcx>(
741 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
742 obligation: &ProjectionTyObligation<'tcx>,
743 obligation_trait_ref: &ty::TraitRef<'tcx>,
744 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
746 debug!("assemble_candidates_from_param_env(..)");
747 let env_predicates = selcx.param_env().caller_bounds.iter().cloned();
748 assemble_candidates_from_predicates(selcx,
750 obligation_trait_ref,
752 ProjectionTyCandidate::ParamEnv,
756 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
757 /// that the definition of `Foo` has some clues:
761 /// type FooT : Bar<BarT=i32>
765 /// Here, for example, we could conclude that the result is `i32`.
766 fn assemble_candidates_from_trait_def<'cx, 'gcx, 'tcx>(
767 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
768 obligation: &ProjectionTyObligation<'tcx>,
769 obligation_trait_ref: &ty::TraitRef<'tcx>,
770 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
772 debug!("assemble_candidates_from_trait_def(..)");
774 // Check whether the self-type is itself a projection.
775 let (def_id, substs) = match obligation_trait_ref.self_ty().sty {
776 ty::TyProjection(ref data) => {
777 (data.trait_ref.def_id, data.trait_ref.substs)
779 ty::TyAnon(def_id, substs) => (def_id, substs),
780 ty::TyInfer(ty::TyVar(_)) => {
781 // If the self-type is an inference variable, then it MAY wind up
782 // being a projected type, so induce an ambiguity.
783 candidate_set.ambiguous = true;
789 // If so, extract what we know from the trait and try to come up with a good answer.
790 let trait_predicates = selcx.tcx().item_predicates(def_id);
791 let bounds = trait_predicates.instantiate(selcx.tcx(), substs);
792 let bounds = elaborate_predicates(selcx.tcx(), bounds.predicates);
793 assemble_candidates_from_predicates(selcx,
795 obligation_trait_ref,
797 ProjectionTyCandidate::TraitDef,
801 fn assemble_candidates_from_predicates<'cx, 'gcx, 'tcx, I>(
802 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
803 obligation: &ProjectionTyObligation<'tcx>,
804 obligation_trait_ref: &ty::TraitRef<'tcx>,
805 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
806 ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
808 where I: Iterator<Item=ty::Predicate<'tcx>>
810 debug!("assemble_candidates_from_predicates(obligation={:?})",
812 let infcx = selcx.infcx();
813 for predicate in env_predicates {
814 debug!("assemble_candidates_from_predicates: predicate={:?}",
817 ty::Predicate::Projection(ref data) => {
818 let same_name = data.item_name() == obligation.predicate.item_name;
820 let is_match = same_name && infcx.probe(|_| {
821 let data_poly_trait_ref =
822 data.to_poly_trait_ref();
823 let obligation_poly_trait_ref =
824 obligation_trait_ref.to_poly_trait_ref();
825 infcx.sub_poly_trait_refs(false,
826 obligation.cause.clone(),
828 obligation_poly_trait_ref)
829 .map(|InferOk { obligations: _, value: () }| {
830 // FIXME(#32730) -- do we need to take obligations
831 // into account in any way? At the moment, no.
836 debug!("assemble_candidates_from_predicates: candidate={:?} \
837 is_match={} same_name={}",
838 data, is_match, same_name);
841 candidate_set.vec.push(ctor(data.clone()));
849 fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
850 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
851 obligation: &ProjectionTyObligation<'tcx>,
852 obligation_trait_ref: &ty::TraitRef<'tcx>,
853 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
854 -> Result<(), SelectionError<'tcx>>
856 // If we are resolving `<T as TraitRef<...>>::Item == Type`,
857 // start out by selecting the predicate `T as TraitRef<...>`:
858 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
859 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
860 selcx.infcx().probe(|_| {
861 let vtable = match selcx.select(&trait_obligation) {
862 Ok(Some(vtable)) => vtable,
864 candidate_set.ambiguous = true;
868 debug!("assemble_candidates_from_impls: selection error {:?}",
875 super::VtableClosure(_) |
876 super::VtableFnPointer(_) |
877 super::VtableObject(_) => {
878 debug!("assemble_candidates_from_impls: vtable={:?}",
881 candidate_set.vec.push(ProjectionTyCandidate::Select);
883 super::VtableImpl(ref impl_data) => {
884 // We have to be careful when projecting out of an
885 // impl because of specialization. If we are not in
886 // trans (i.e., projection mode is not "any"), and the
887 // impl's type is declared as default, then we disable
888 // projection (even if the trait ref is fully
889 // monomorphic). In the case where trait ref is not
890 // fully monomorphic (i.e., includes type parameters),
891 // this is because those type parameters may
892 // ultimately be bound to types from other crates that
893 // may have specialized impls we can't see. In the
894 // case where the trait ref IS fully monomorphic, this
895 // is a policy decision that we made in the RFC in
896 // order to preserve flexibility for the crate that
897 // defined the specializable impl to specialize later
898 // for existing types.
900 // In either case, we handle this by not adding a
901 // candidate for an impl if it contains a `default`
903 let opt_node_item = assoc_ty_def(selcx,
904 impl_data.impl_def_id,
905 obligation.predicate.item_name);
906 let new_candidate = if let Some(node_item) = opt_node_item {
907 let is_default = if node_item.node.is_from_trait() {
908 // If true, the impl inherited a `type Foo = Bar`
909 // given in the trait, which is implicitly default.
910 // Otherwise, the impl did not specify `type` and
911 // neither did the trait:
914 // trait Foo { type T; }
915 // impl Foo for Bar { }
918 // This is an error, but it will be
919 // reported in `check_impl_items_against_trait`.
920 // We accept it here but will flag it as
921 // an error when we confirm the candidate
922 // (which will ultimately lead to `normalize_to_error`
924 node_item.item.defaultness.has_value()
926 node_item.item.defaultness.is_default()
929 // Only reveal a specializable default if we're past type-checking
930 // and the obligations is monomorphic, otherwise passes such as
931 // transmute checking and polymorphic MIR optimizations could
932 // get a result which isn't correct for all monomorphizations.
934 Some(ProjectionTyCandidate::Select)
935 } else if selcx.projection_mode() == Reveal::All {
936 assert!(!poly_trait_ref.needs_infer());
937 if !poly_trait_ref.needs_subst() {
938 Some(ProjectionTyCandidate::Select)
946 // This is saying that neither the trait nor
947 // the impl contain a definition for this
948 // associated type. Normally this situation
949 // could only arise through a compiler bug --
950 // if the user wrote a bad item name, it
951 // should have failed in astconv. **However**,
952 // at coherence-checking time, we only look at
953 // the topmost impl (we don't even consider
954 // the trait itself) for the definition -- and
955 // so in that case it may be that the trait
956 // *DOES* have a declaration, but we don't see
957 // it, and we end up in this branch.
959 // This is kind of tricky to handle actually.
960 // For now, we just unconditionally ICE,
961 // because otherwise, examples like the
962 // following will succeed:
969 // impl<T> Assoc for T {
970 // default type Output = bool;
973 // impl Assoc for u8 {}
974 // impl Assoc for u16 {}
977 // impl Foo for <u8 as Assoc>::Output {}
978 // impl Foo for <u16 as Assoc>::Output {}
983 // The essential problem here is that the
984 // projection fails, leaving two unnormalized
985 // types, which appear not to unify -- so the
986 // overlap check succeeds, when it should
988 span_bug!(obligation.cause.span,
989 "Tried to project an inherited associated type during \
990 coherence checking, which is currently not supported.");
992 candidate_set.vec.extend(new_candidate);
994 super::VtableParam(..) => {
995 // This case tell us nothing about the value of an
996 // associated type. Consider:
999 // trait SomeTrait { type Foo; }
1000 // fn foo<T:SomeTrait>(...) { }
1003 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1004 // : SomeTrait` binding does not help us decide what the
1005 // type `Foo` is (at least, not more specifically than
1006 // what we already knew).
1008 // But wait, you say! What about an example like this:
1011 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1014 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1015 // resolve `T::Foo`? And of course it does, but in fact
1016 // that single predicate is desugared into two predicates
1017 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1018 // projection. And the projection where clause is handled
1019 // in `assemble_candidates_from_param_env`.
1021 super::VtableDefaultImpl(..) |
1022 super::VtableBuiltin(..) => {
1023 // These traits have no associated types.
1025 obligation.cause.span,
1026 "Cannot project an associated type from `{:?}`",
1035 fn confirm_candidate<'cx, 'gcx, 'tcx>(
1036 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1037 obligation: &ProjectionTyObligation<'tcx>,
1038 obligation_trait_ref: &ty::TraitRef<'tcx>,
1039 candidate: ProjectionTyCandidate<'tcx>)
1042 debug!("confirm_candidate(candidate={:?}, obligation={:?})",
1047 ProjectionTyCandidate::ParamEnv(poly_projection) |
1048 ProjectionTyCandidate::TraitDef(poly_projection) => {
1049 confirm_param_env_candidate(selcx, obligation, poly_projection)
1052 ProjectionTyCandidate::Select => {
1053 confirm_select_candidate(selcx, obligation, obligation_trait_ref)
1058 fn confirm_select_candidate<'cx, 'gcx, 'tcx>(
1059 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1060 obligation: &ProjectionTyObligation<'tcx>,
1061 obligation_trait_ref: &ty::TraitRef<'tcx>)
1064 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1065 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1066 let vtable = match selcx.select(&trait_obligation) {
1067 Ok(Some(vtable)) => vtable,
1070 obligation.cause.span,
1071 "Failed to select `{:?}`",
1077 super::VtableImpl(data) =>
1078 confirm_impl_candidate(selcx, obligation, data),
1079 super::VtableClosure(data) =>
1080 confirm_closure_candidate(selcx, obligation, data),
1081 super::VtableFnPointer(data) =>
1082 confirm_fn_pointer_candidate(selcx, obligation, data),
1083 super::VtableObject(_) =>
1084 confirm_object_candidate(selcx, obligation, obligation_trait_ref),
1085 super::VtableDefaultImpl(..) |
1086 super::VtableParam(..) |
1087 super::VtableBuiltin(..) =>
1088 // we don't create Select candidates with this kind of resolution
1090 obligation.cause.span,
1091 "Cannot project an associated type from `{:?}`",
1096 fn confirm_object_candidate<'cx, 'gcx, 'tcx>(
1097 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1098 obligation: &ProjectionTyObligation<'tcx>,
1099 obligation_trait_ref: &ty::TraitRef<'tcx>)
1102 let self_ty = obligation_trait_ref.self_ty();
1103 let object_ty = selcx.infcx().shallow_resolve(self_ty);
1104 debug!("confirm_object_candidate(object_ty={:?})",
1106 let data = match object_ty.sty {
1107 ty::TyDynamic(ref data, ..) => data,
1110 obligation.cause.span,
1111 "confirm_object_candidate called with non-object: {:?}",
1115 let env_predicates = data.projection_bounds().map(|p| {
1116 p.with_self_ty(selcx.tcx(), object_ty).to_predicate()
1118 let env_predicate = {
1119 let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1121 // select only those projections that are actually projecting an
1122 // item with the correct name
1123 let env_predicates = env_predicates.filter_map(|p| match p {
1124 ty::Predicate::Projection(data) =>
1125 if data.item_name() == obligation.predicate.item_name {
1133 // select those with a relevant trait-ref
1134 let mut env_predicates = env_predicates.filter(|data| {
1135 let data_poly_trait_ref = data.to_poly_trait_ref();
1136 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1137 selcx.infcx().probe(|_| {
1138 selcx.infcx().sub_poly_trait_refs(false,
1139 obligation.cause.clone(),
1140 data_poly_trait_ref,
1141 obligation_poly_trait_ref).is_ok()
1145 // select the first matching one; there really ought to be one or
1146 // else the object type is not WF, since an object type should
1147 // include all of its projections explicitly
1148 match env_predicates.next() {
1149 Some(env_predicate) => env_predicate,
1151 debug!("confirm_object_candidate: no env-predicate \
1152 found in object type `{:?}`; ill-formed",
1154 return Progress::error(selcx.tcx());
1159 confirm_param_env_candidate(selcx, obligation, env_predicate)
1162 fn confirm_fn_pointer_candidate<'cx, 'gcx, 'tcx>(
1163 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1164 obligation: &ProjectionTyObligation<'tcx>,
1165 fn_pointer_vtable: VtableFnPointerData<'tcx, PredicateObligation<'tcx>>)
1168 let fn_type = selcx.infcx().shallow_resolve(fn_pointer_vtable.fn_ty);
1169 let sig = fn_type.fn_sig();
1170 confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1171 .with_addl_obligations(fn_pointer_vtable.nested)
1174 fn confirm_closure_candidate<'cx, 'gcx, 'tcx>(
1175 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1176 obligation: &ProjectionTyObligation<'tcx>,
1177 vtable: VtableClosureData<'tcx, PredicateObligation<'tcx>>)
1180 let closure_typer = selcx.closure_typer();
1181 let closure_type = closure_typer.closure_type(vtable.closure_def_id)
1182 .subst(selcx.tcx(), vtable.substs.substs);
1184 value: closure_type,
1186 } = normalize_with_depth(selcx,
1187 obligation.cause.clone(),
1188 obligation.recursion_depth+1,
1191 debug!("confirm_closure_candidate: obligation={:?},closure_type={:?},obligations={:?}",
1196 confirm_callable_candidate(selcx,
1199 util::TupleArgumentsFlag::No)
1200 .with_addl_obligations(vtable.nested)
1201 .with_addl_obligations(obligations)
1204 fn confirm_callable_candidate<'cx, 'gcx, 'tcx>(
1205 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1206 obligation: &ProjectionTyObligation<'tcx>,
1207 fn_sig: ty::PolyFnSig<'tcx>,
1208 flag: util::TupleArgumentsFlag)
1211 let tcx = selcx.tcx();
1213 debug!("confirm_callable_candidate({:?},{:?})",
1217 // the `Output` associated type is declared on `FnOnce`
1218 let fn_once_def_id = tcx.lang_items.fn_once_trait().unwrap();
1220 // Note: we unwrap the binder here but re-create it below (1)
1221 let ty::Binder((trait_ref, ret_type)) =
1222 tcx.closure_trait_ref_and_return_type(fn_once_def_id,
1223 obligation.predicate.trait_ref.self_ty(),
1227 let predicate = ty::Binder(ty::ProjectionPredicate { // (1) recreate binder here
1228 projection_ty: ty::ProjectionTy {
1229 trait_ref: trait_ref,
1230 item_name: Symbol::intern(FN_OUTPUT_NAME),
1235 confirm_param_env_candidate(selcx, obligation, predicate)
1238 fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
1239 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1240 obligation: &ProjectionTyObligation<'tcx>,
1241 poly_projection: ty::PolyProjectionPredicate<'tcx>)
1244 let infcx = selcx.infcx();
1245 let cause = obligation.cause.clone();
1246 let trait_ref = obligation.predicate.trait_ref;
1247 match infcx.match_poly_projection_predicate(cause, poly_projection, trait_ref) {
1248 Ok(InferOk { value: ty_match, obligations }) => {
1251 obligations: obligations,
1252 cacheable: ty_match.unconstrained_regions.is_empty(),
1257 obligation.cause.span,
1258 "Failed to unify obligation `{:?}` \
1259 with poly_projection `{:?}`: {:?}",
1267 fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
1268 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1269 obligation: &ProjectionTyObligation<'tcx>,
1270 impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>)
1273 let VtableImplData { substs, nested, impl_def_id } = impl_vtable;
1275 let tcx = selcx.tcx();
1276 let trait_ref = obligation.predicate.trait_ref;
1277 let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_name);
1280 Some(node_item) => {
1281 let ty = if !node_item.item.defaultness.has_value() {
1282 // This means that the impl is missing a definition for the
1283 // associated type. This error will be reported by the type
1284 // checker method `check_impl_items_against_trait`, so here we
1285 // just return TyError.
1286 debug!("confirm_impl_candidate: no associated type {:?} for {:?}",
1287 node_item.item.name,
1288 obligation.predicate.trait_ref);
1291 tcx.item_type(node_item.item.def_id)
1293 let substs = translate_substs(selcx.infcx(), impl_def_id, substs, node_item.node);
1295 ty: ty.subst(tcx, substs),
1296 obligations: nested,
1301 span_bug!(obligation.cause.span,
1302 "No associated type for {:?}",
1308 /// Locate the definition of an associated type in the specialization hierarchy,
1309 /// starting from the given impl.
1311 /// Based on the "projection mode", this lookup may in fact only examine the
1312 /// topmost impl. See the comments for `Reveal` for more details.
1313 fn assoc_ty_def<'cx, 'gcx, 'tcx>(
1314 selcx: &SelectionContext<'cx, 'gcx, 'tcx>,
1316 assoc_ty_name: ast::Name)
1317 -> Option<specialization_graph::NodeItem<ty::AssociatedItem>>
1319 let trait_def_id = selcx.tcx().impl_trait_ref(impl_def_id).unwrap().def_id;
1320 let trait_def = selcx.tcx().lookup_trait_def(trait_def_id);
1322 if !trait_def.is_complete(selcx.tcx()) {
1323 let impl_node = specialization_graph::Node::Impl(impl_def_id);
1324 for item in impl_node.items(selcx.tcx()) {
1325 if item.kind == ty::AssociatedKind::Type && item.name == assoc_ty_name {
1326 return Some(specialization_graph::NodeItem {
1327 node: specialization_graph::Node::Impl(impl_def_id),
1335 .ancestors(impl_def_id)
1336 .defs(selcx.tcx(), assoc_ty_name, ty::AssociatedKind::Type)
1343 pub struct ProjectionCache<'tcx> {
1344 map: SnapshotMap<ty::ProjectionTy<'tcx>, ProjectionCacheEntry<'tcx>>,
1347 #[derive(Clone, Debug)]
1348 enum ProjectionCacheEntry<'tcx> {
1352 NormalizedTy(Ty<'tcx>),
1355 // NB: intentionally not Clone
1356 pub struct ProjectionCacheSnapshot {
1360 impl<'tcx> ProjectionCache<'tcx> {
1361 pub fn new() -> Self {
1363 map: SnapshotMap::new()
1367 pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
1368 ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
1371 pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
1372 self.map.rollback_to(snapshot.snapshot);
1375 pub fn rollback_skolemized(&mut self, snapshot: &ProjectionCacheSnapshot) {
1376 self.map.partial_rollback(&snapshot.snapshot, &|k| k.has_re_skol());
1379 pub fn commit(&mut self, snapshot: ProjectionCacheSnapshot) {
1380 self.map.commit(snapshot.snapshot);
1383 /// Try to start normalize `key`; returns an error if
1384 /// normalization already occured (this error corresponds to a
1385 /// cache hit, so it's actually a good thing).
1386 fn try_start(&mut self, key: ty::ProjectionTy<'tcx>)
1387 -> Result<(), ProjectionCacheEntry<'tcx>> {
1388 if let Some(entry) = self.map.get(&key) {
1389 return Err(entry.clone());
1392 self.map.insert(key, ProjectionCacheEntry::InProgress);
1396 /// Indicates that `key` was normalized to `value`. If `cacheable` is false,
1397 /// then this result is sadly not cacheable.
1398 fn complete(&mut self,
1399 key: ty::ProjectionTy<'tcx>,
1400 value: &NormalizedTy<'tcx>,
1402 let fresh_key = if cacheable {
1403 debug!("ProjectionCacheEntry::complete: adding cache entry: key={:?}, value={:?}",
1405 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value.value))
1407 debug!("ProjectionCacheEntry::complete: cannot cache: key={:?}, value={:?}",
1409 !self.map.remove(key)
1412 assert!(!fresh_key, "never started projecting `{:?}`", key);
1415 /// Indicates that trying to normalize `key` resulted in
1416 /// ambiguity. No point in trying it again then until we gain more
1417 /// type information (in which case, the "fully resolved" key will
1419 fn ambiguous(&mut self, key: ty::ProjectionTy<'tcx>) {
1420 let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
1421 assert!(!fresh, "never started projecting `{:?}`", key);
1424 /// Indicates that trying to normalize `key` resulted in
1426 fn error(&mut self, key: ty::ProjectionTy<'tcx>) {
1427 let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
1428 assert!(!fresh, "never started projecting `{:?}`", key);