1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Code for projecting associated types out of trait references.
13 use super::elaborate_predicates;
14 use super::specialization_graph;
15 use super::translate_substs;
16 use super::Obligation;
17 use super::ObligationCause;
18 use super::PredicateObligation;
20 use super::SelectionContext;
21 use super::SelectionError;
22 use super::VtableClosureData;
23 use super::VtableGeneratorData;
24 use super::VtableFnPointerData;
25 use super::VtableImplData;
28 use hir::def_id::DefId;
29 use infer::{InferCtxt, InferOk};
30 use infer::type_variable::TypeVariableOrigin;
31 use mir::interpret::ConstValue;
32 use mir::interpret::{GlobalId};
33 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
34 use syntax::ast::Ident;
35 use ty::subst::{Subst, Substs};
36 use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt};
37 use ty::fold::{TypeFoldable, TypeFolder};
38 use util::common::FN_OUTPUT_NAME;
40 /// Depending on the stage of compilation, we want projection to be
41 /// more or less conservative.
42 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
44 /// At type-checking time, we refuse to project any associated
45 /// type that is marked `default`. Non-`default` ("final") types
46 /// are always projected. This is necessary in general for
47 /// soundness of specialization. However, we *could* allow
48 /// projections in fully-monomorphic cases. We choose not to,
49 /// because we prefer for `default type` to force the type
50 /// definition to be treated abstractly by any consumers of the
51 /// impl. Concretely, that means that the following example will
59 /// impl<T> Assoc for T {
60 /// default type Output = bool;
64 /// let <() as Assoc>::Output = true;
68 /// At codegen time, all monomorphic projections will succeed.
69 /// Also, `impl Trait` is normalized to the concrete type,
70 /// which has to be already collected by type-checking.
72 /// NOTE: As `impl Trait`'s concrete type should *never*
73 /// be observable directly by the user, `Reveal::All`
74 /// should not be used by checks which may expose
75 /// type equality or type contents to the user.
76 /// There are some exceptions, e.g. around OIBITS and
77 /// transmute-checking, which expose some details, but
78 /// not the whole concrete type of the `impl Trait`.
82 pub type PolyProjectionObligation<'tcx> =
83 Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
85 pub type ProjectionObligation<'tcx> =
86 Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
88 pub type ProjectionTyObligation<'tcx> =
89 Obligation<'tcx, ty::ProjectionTy<'tcx>>;
91 /// When attempting to resolve `<T as TraitRef>::Name` ...
93 pub enum ProjectionTyError<'tcx> {
94 /// ...we found multiple sources of information and couldn't resolve the ambiguity.
97 /// ...an error occurred matching `T : TraitRef`
98 TraitSelectionError(SelectionError<'tcx>),
102 pub struct MismatchedProjectionTypes<'tcx> {
103 pub err: ty::error::TypeError<'tcx>
106 #[derive(PartialEq, Eq, Debug)]
107 enum ProjectionTyCandidate<'tcx> {
108 // from a where-clause in the env or object type
109 ParamEnv(ty::PolyProjectionPredicate<'tcx>),
111 // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
112 TraitDef(ty::PolyProjectionPredicate<'tcx>),
114 // from a "impl" (or a "pseudo-impl" returned by select)
115 Select(Selection<'tcx>),
118 enum ProjectionTyCandidateSet<'tcx> {
120 Single(ProjectionTyCandidate<'tcx>),
122 Error(SelectionError<'tcx>),
125 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
126 fn mark_ambiguous(&mut self) {
127 *self = ProjectionTyCandidateSet::Ambiguous;
130 fn mark_error(&mut self, err: SelectionError<'tcx>) {
131 *self = ProjectionTyCandidateSet::Error(err);
134 // Returns true if the push was successful, or false if the candidate
135 // was discarded -- this could be because of ambiguity, or because
136 // a higher-priority candidate is already there.
137 fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
138 use self::ProjectionTyCandidateSet::*;
139 use self::ProjectionTyCandidate::*;
141 // This wacky variable is just used to try and
142 // make code readable and avoid confusing paths.
143 // It is assigned a "value" of `()` only on those
144 // paths in which we wish to convert `*self` to
145 // ambiguous (and return false, because the candidate
146 // was not used). On other paths, it is not assigned,
147 // and hence if those paths *could* reach the code that
148 // comes after the match, this fn would not compile.
149 let convert_to_ambiguous;
153 *self = Single(candidate);
158 // Duplicates can happen inside ParamEnv. In the case, we
159 // perform a lazy deduplication.
160 if current == &candidate {
164 // Prefer where-clauses. As in select, if there are multiple
165 // candidates, we prefer where-clause candidates over impls. This
166 // may seem a bit surprising, since impls are the source of
167 // "truth" in some sense, but in fact some of the impls that SEEM
168 // applicable are not, because of nested obligations. Where
169 // clauses are the safer choice. See the comment on
170 // `select::SelectionCandidate` and #21974 for more details.
171 match (current, candidate) {
172 (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
173 (ParamEnv(..), _) => return false,
174 (_, ParamEnv(..)) => unreachable!(),
175 (_, _) => convert_to_ambiguous = (),
179 Ambiguous | Error(..) => {
184 // We only ever get here when we moved from a single candidate
186 let () = convert_to_ambiguous;
192 /// Evaluates constraints of the form:
194 /// for<...> <T as Trait>::U == V
196 /// If successful, this may result in additional obligations. Also returns
197 /// the projection cache key used to track these additional obligations.
198 pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
199 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
200 obligation: &PolyProjectionObligation<'tcx>)
201 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
202 MismatchedProjectionTypes<'tcx>>
204 debug!("poly_project_and_unify_type(obligation={:?})",
207 let infcx = selcx.infcx();
208 infcx.commit_if_ok(|snapshot| {
209 let (placeholder_predicate, placeholder_map) =
210 infcx.replace_late_bound_regions_with_placeholders(&obligation.predicate);
212 let skol_obligation = obligation.with(placeholder_predicate);
213 let r = match project_and_unify_type(selcx, &skol_obligation) {
215 let span = obligation.cause.span;
216 match infcx.leak_check(false, span, &placeholder_map, snapshot) {
217 Ok(()) => Ok(infcx.plug_leaks(placeholder_map, snapshot, result)),
219 debug!("poly_project_and_unify_type: leak check encountered error {:?}", e);
220 Err(MismatchedProjectionTypes { err: e })
233 /// Evaluates constraints of the form:
235 /// <T as Trait>::U == V
237 /// If successful, this may result in additional obligations.
238 fn project_and_unify_type<'cx, 'gcx, 'tcx>(
239 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
240 obligation: &ProjectionObligation<'tcx>)
241 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
242 MismatchedProjectionTypes<'tcx>>
244 debug!("project_and_unify_type(obligation={:?})",
247 let mut obligations = vec![];
249 match opt_normalize_projection_type(selcx,
250 obligation.param_env,
251 obligation.predicate.projection_ty,
252 obligation.cause.clone(),
253 obligation.recursion_depth,
256 None => return Ok(None),
259 debug!("project_and_unify_type: normalized_ty={:?} obligations={:?}",
263 let infcx = selcx.infcx();
264 match infcx.at(&obligation.cause, obligation.param_env)
265 .eq(normalized_ty, obligation.predicate.ty) {
266 Ok(InferOk { obligations: inferred_obligations, value: () }) => {
267 obligations.extend(inferred_obligations);
268 Ok(Some(obligations))
271 debug!("project_and_unify_type: equating types encountered error {:?}", err);
272 Err(MismatchedProjectionTypes { err: err })
277 /// Normalizes any associated type projections in `value`, replacing
278 /// them with a fully resolved type where possible. The return value
279 /// combines the normalized result and any additional obligations that
280 /// were incurred as result.
281 pub fn normalize<'a, 'b, 'gcx, 'tcx, T>(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
282 param_env: ty::ParamEnv<'tcx>,
283 cause: ObligationCause<'tcx>,
285 -> Normalized<'tcx, T>
286 where T : TypeFoldable<'tcx>
288 normalize_with_depth(selcx, param_env, cause, 0, value)
291 /// As `normalize`, but with a custom depth.
292 pub fn normalize_with_depth<'a, 'b, 'gcx, 'tcx, T>(
293 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
294 param_env: ty::ParamEnv<'tcx>,
295 cause: ObligationCause<'tcx>,
298 -> Normalized<'tcx, T>
300 where T : TypeFoldable<'tcx>
302 debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
303 let mut normalizer = AssociatedTypeNormalizer::new(selcx, param_env, cause, depth);
304 let result = normalizer.fold(value);
305 debug!("normalize_with_depth: depth={} result={:?} with {} obligations",
306 depth, result, normalizer.obligations.len());
307 debug!("normalize_with_depth: depth={} obligations={:?}",
308 depth, normalizer.obligations);
311 obligations: normalizer.obligations,
315 struct AssociatedTypeNormalizer<'a, 'b: 'a, 'gcx: 'b+'tcx, 'tcx: 'b> {
316 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
317 param_env: ty::ParamEnv<'tcx>,
318 cause: ObligationCause<'tcx>,
319 obligations: Vec<PredicateObligation<'tcx>>,
323 impl<'a, 'b, 'gcx, 'tcx> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
324 fn new(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
325 param_env: ty::ParamEnv<'tcx>,
326 cause: ObligationCause<'tcx>,
328 -> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx>
330 AssociatedTypeNormalizer {
339 fn fold<T:TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
340 let value = self.selcx.infcx().resolve_type_vars_if_possible(value);
342 if !value.has_projections() {
345 value.fold_with(self)
350 impl<'a, 'b, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
351 fn tcx<'c>(&'c self) -> TyCtxt<'c, 'gcx, 'tcx> {
355 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
356 // We don't want to normalize associated types that occur inside of region
357 // binders, because they may contain bound regions, and we can't cope with that.
361 // for<'a> fn(<T as Foo<&'a>>::A)
363 // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
364 // normalize it when we instantiate those bound regions (which
365 // should occur eventually).
367 let ty = ty.super_fold_with(self);
369 ty::Opaque(def_id, substs) if !substs.has_escaping_regions() => { // (*)
370 // Only normalize `impl Trait` after type-checking, usually in codegen.
371 match self.param_env.reveal {
372 Reveal::UserFacing => ty,
375 let recursion_limit = *self.tcx().sess.recursion_limit.get();
376 if self.depth >= recursion_limit {
377 let obligation = Obligation::with_depth(
383 self.selcx.infcx().report_overflow_error(&obligation, true);
386 let generic_ty = self.tcx().type_of(def_id);
387 let concrete_ty = generic_ty.subst(self.tcx(), substs);
389 let folded_ty = self.fold_ty(concrete_ty);
396 ty::Projection(ref data) if !data.has_escaping_regions() => { // (*)
398 // (*) This is kind of hacky -- we need to be able to
399 // handle normalization within binders because
400 // otherwise we wind up a need to normalize when doing
401 // trait matching (since you can have a trait
402 // obligation like `for<'a> T::B : Fn(&'a int)`), but
403 // we can't normalize with bound regions in scope. So
404 // far now we just ignore binders but only normalize
405 // if all bound regions are gone (and then we still
406 // have to renormalize whenever we instantiate a
407 // binder). It would be better to normalize in a
408 // binding-aware fashion.
410 let normalized_ty = normalize_projection_type(self.selcx,
415 &mut self.obligations);
416 debug!("AssociatedTypeNormalizer: depth={} normalized {:?} to {:?}, \
417 now with {} obligations",
418 self.depth, ty, normalized_ty, self.obligations.len());
426 fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
427 if let ConstValue::Unevaluated(def_id, substs) = constant.val {
428 let tcx = self.selcx.tcx().global_tcx();
429 if let Some(param_env) = self.tcx().lift_to_global(&self.param_env) {
430 if substs.needs_infer() || substs.has_skol() {
431 let identity_substs = Substs::identity_for_item(tcx, def_id);
432 let instance = ty::Instance::resolve(tcx, param_env, def_id, identity_substs);
433 if let Some(instance) = instance {
438 if let Ok(evaluated) = tcx.const_eval(param_env.and(cid)) {
439 let evaluated = evaluated.subst(self.tcx(), substs);
440 return self.fold_const(evaluated);
444 if let Some(substs) = self.tcx().lift_to_global(&substs) {
445 let instance = ty::Instance::resolve(tcx, param_env, def_id, substs);
446 if let Some(instance) = instance {
451 if let Ok(evaluated) = tcx.const_eval(param_env.and(cid)) {
452 return self.fold_const(evaluated)
464 pub struct Normalized<'tcx,T> {
466 pub obligations: Vec<PredicateObligation<'tcx>>,
469 pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
471 impl<'tcx,T> Normalized<'tcx,T> {
472 pub fn with<U>(self, value: U) -> Normalized<'tcx,U> {
473 Normalized { value: value, obligations: self.obligations }
477 /// The guts of `normalize`: normalize a specific projection like `<T
478 /// as Trait>::Item`. The result is always a type (and possibly
479 /// additional obligations). If ambiguity arises, which implies that
480 /// there are unresolved type variables in the projection, we will
481 /// substitute a fresh type variable `$X` and generate a new
482 /// obligation `<T as Trait>::Item == $X` for later.
483 pub fn normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
484 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
485 param_env: ty::ParamEnv<'tcx>,
486 projection_ty: ty::ProjectionTy<'tcx>,
487 cause: ObligationCause<'tcx>,
489 obligations: &mut Vec<PredicateObligation<'tcx>>)
492 opt_normalize_projection_type(selcx, param_env, projection_ty.clone(), cause.clone(), depth,
494 .unwrap_or_else(move || {
495 // if we bottom out in ambiguity, create a type variable
496 // and a deferred predicate to resolve this when more type
497 // information is available.
499 let tcx = selcx.infcx().tcx;
500 let def_id = projection_ty.item_def_id;
501 let ty_var = selcx.infcx().next_ty_var(
502 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
503 let projection = ty::Binder::dummy(ty::ProjectionPredicate {
507 let obligation = Obligation::with_depth(
508 cause, depth + 1, param_env, projection.to_predicate());
509 obligations.push(obligation);
514 /// The guts of `normalize`: normalize a specific projection like `<T
515 /// as Trait>::Item`. The result is always a type (and possibly
516 /// additional obligations). Returns `None` in the case of ambiguity,
517 /// which indicates that there are unbound type variables.
519 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
520 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
521 /// often immediately appended to another obligations vector. So now this
522 /// function takes an obligations vector and appends to it directly, which is
523 /// slightly uglier but avoids the need for an extra short-lived allocation.
524 fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
525 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
526 param_env: ty::ParamEnv<'tcx>,
527 projection_ty: ty::ProjectionTy<'tcx>,
528 cause: ObligationCause<'tcx>,
530 obligations: &mut Vec<PredicateObligation<'tcx>>)
533 let infcx = selcx.infcx();
535 let projection_ty = infcx.resolve_type_vars_if_possible(&projection_ty);
536 let cache_key = ProjectionCacheKey { ty: projection_ty };
538 debug!("opt_normalize_projection_type(\
539 projection_ty={:?}, \
544 // FIXME(#20304) For now, I am caching here, which is good, but it
545 // means we don't capture the type variables that are created in
546 // the case of ambiguity. Which means we may create a large stream
547 // of such variables. OTOH, if we move the caching up a level, we
548 // would not benefit from caching when proving `T: Trait<U=Foo>`
549 // bounds. It might be the case that we want two distinct caches,
550 // or else another kind of cache entry.
552 let cache_result = infcx.projection_cache.borrow_mut().try_start(cache_key);
555 Err(ProjectionCacheEntry::Ambiguous) => {
556 // If we found ambiguity the last time, that generally
557 // means we will continue to do so until some type in the
558 // key changes (and we know it hasn't, because we just
559 // fully resolved it). One exception though is closure
560 // types, which can transition from having a fixed kind to
561 // no kind with no visible change in the key.
563 // FIXME(#32286) refactor this so that closure type
565 debug!("opt_normalize_projection_type: \
566 found cache entry: ambiguous");
567 if !projection_ty.has_closure_types() {
571 Err(ProjectionCacheEntry::InProgress) => {
572 // If while normalized A::B, we are asked to normalize
573 // A::B, just return A::B itself. This is a conservative
574 // answer, in the sense that A::B *is* clearly equivalent
575 // to A::B, though there may be a better value we can
578 // Under lazy normalization, this can arise when
579 // bootstrapping. That is, imagine an environment with a
580 // where-clause like `A::B == u32`. Now, if we are asked
581 // to normalize `A::B`, we will want to check the
582 // where-clauses in scope. So we will try to unify `A::B`
583 // with `A::B`, which can trigger a recursive
584 // normalization. In that case, I think we will want this code:
587 // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
588 // projection_ty.substs;
589 // return Some(NormalizedTy { value: v, obligations: vec![] });
592 debug!("opt_normalize_projection_type: \
593 found cache entry: in-progress");
595 // But for now, let's classify this as an overflow:
596 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
597 let obligation = Obligation::with_depth(cause.clone(),
601 selcx.infcx().report_overflow_error(&obligation, false);
603 Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
604 // This is the hottest path in this function.
606 // If we find the value in the cache, then return it along
607 // with the obligations that went along with it. Note
608 // that, when using a fulfillment context, these
609 // obligations could in principle be ignored: they have
610 // already been registered when the cache entry was
611 // created (and hence the new ones will quickly be
612 // discarded as duplicated). But when doing trait
613 // evaluation this is not the case, and dropping the trait
614 // evaluations can causes ICEs (e.g. #43132).
615 debug!("opt_normalize_projection_type: \
616 found normalized ty `{:?}`",
619 // Once we have inferred everything we need to know, we
620 // can ignore the `obligations` from that point on.
621 if !infcx.any_unresolved_type_vars(&ty.value) {
622 infcx.projection_cache.borrow_mut().complete_normalized(cache_key, &ty);
623 // No need to extend `obligations`.
625 obligations.extend(ty.obligations);
628 obligations.push(get_paranoid_cache_value_obligation(infcx,
633 return Some(ty.value);
635 Err(ProjectionCacheEntry::Error) => {
636 debug!("opt_normalize_projection_type: \
638 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
639 obligations.extend(result.obligations);
640 return Some(result.value)
644 let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
645 match project_type(selcx, &obligation) {
646 Ok(ProjectedTy::Progress(Progress { ty: projected_ty,
647 obligations: mut projected_obligations })) => {
648 // if projection succeeded, then what we get out of this
649 // is also non-normalized (consider: it was derived from
650 // an impl, where-clause etc) and hence we must
653 debug!("opt_normalize_projection_type: \
656 projected_obligations={:?}",
659 projected_obligations);
661 let result = if projected_ty.has_projections() {
662 let mut normalizer = AssociatedTypeNormalizer::new(selcx,
666 let normalized_ty = normalizer.fold(&projected_ty);
668 debug!("opt_normalize_projection_type: \
669 normalized_ty={:?} depth={}",
673 projected_obligations.extend(normalizer.obligations);
675 value: normalized_ty,
676 obligations: projected_obligations,
681 obligations: projected_obligations,
685 let cache_value = prune_cache_value_obligations(infcx, &result);
686 infcx.projection_cache.borrow_mut().insert_ty(cache_key, cache_value);
687 obligations.extend(result.obligations);
690 Ok(ProjectedTy::NoProgress(projected_ty)) => {
691 debug!("opt_normalize_projection_type: \
692 projected_ty={:?} no progress",
694 let result = Normalized {
698 infcx.projection_cache.borrow_mut().insert_ty(cache_key, result.clone());
699 // No need to extend `obligations`.
702 Err(ProjectionTyError::TooManyCandidates) => {
703 debug!("opt_normalize_projection_type: \
704 too many candidates");
705 infcx.projection_cache.borrow_mut()
706 .ambiguous(cache_key);
709 Err(ProjectionTyError::TraitSelectionError(_)) => {
710 debug!("opt_normalize_projection_type: ERROR");
711 // if we got an error processing the `T as Trait` part,
712 // just return `ty::err` but add the obligation `T :
713 // Trait`, which when processed will cause the error to be
716 infcx.projection_cache.borrow_mut()
718 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
719 obligations.extend(result.obligations);
725 /// If there are unresolved type variables, then we need to include
726 /// any subobligations that bind them, at least until those type
727 /// variables are fully resolved.
728 fn prune_cache_value_obligations<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
729 result: &NormalizedTy<'tcx>)
730 -> NormalizedTy<'tcx> {
731 if !infcx.any_unresolved_type_vars(&result.value) {
732 return NormalizedTy { value: result.value, obligations: vec![] };
735 let mut obligations: Vec<_> =
738 .filter(|obligation| match obligation.predicate {
739 // We found a `T: Foo<X = U>` predicate, let's check
740 // if `U` references any unresolved type
741 // variables. In principle, we only care if this
742 // projection can help resolve any of the type
743 // variables found in `result.value` -- but we just
744 // check for any type variables here, for fear of
745 // indirect obligations (e.g., we project to `?0`,
746 // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
748 ty::Predicate::Projection(ref data) =>
749 infcx.any_unresolved_type_vars(&data.ty()),
751 // We are only interested in `T: Foo<X = U>` predicates, whre
752 // `U` references one of `unresolved_type_vars`. =)
758 obligations.shrink_to_fit();
760 NormalizedTy { value: result.value, obligations }
763 /// Whenever we give back a cache result for a projection like `<T as
764 /// Trait>::Item ==> X`, we *always* include the obligation to prove
765 /// that `T: Trait` (we may also include some other obligations). This
766 /// may or may not be necessary -- in principle, all the obligations
767 /// that must be proven to show that `T: Trait` were also returned
768 /// when the cache was first populated. But there are some vague concerns,
769 /// and so we take the precautionary measure of including `T: Trait` in
772 /// Concern #1. The current setup is fragile. Perhaps someone could
773 /// have failed to prove the concerns from when the cache was
774 /// populated, but also not have used a snapshot, in which case the
775 /// cache could remain populated even though `T: Trait` has not been
776 /// shown. In this case, the "other code" is at fault -- when you
777 /// project something, you are supposed to either have a snapshot or
778 /// else prove all the resulting obligations -- but it's still easy to
781 /// Concern #2. Even within the snapshot, if those original
782 /// obligations are not yet proven, then we are able to do projections
783 /// that may yet turn out to be wrong. This *may* lead to some sort
784 /// of trouble, though we don't have a concrete example of how that
785 /// can occur yet. But it seems risky at best.
786 fn get_paranoid_cache_value_obligation<'a, 'gcx, 'tcx>(
787 infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
788 param_env: ty::ParamEnv<'tcx>,
789 projection_ty: ty::ProjectionTy<'tcx>,
790 cause: ObligationCause<'tcx>,
792 -> PredicateObligation<'tcx>
794 let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
797 recursion_depth: depth,
799 predicate: trait_ref.to_predicate(),
803 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
804 /// hold. In various error cases, we cannot generate a valid
805 /// normalized projection. Therefore, we create an inference variable
806 /// return an associated obligation that, when fulfilled, will lead to
809 /// Note that we used to return `Error` here, but that was quite
810 /// dubious -- the premise was that an error would *eventually* be
811 /// reported, when the obligation was processed. But in general once
812 /// you see a `Error` you are supposed to be able to assume that an
813 /// error *has been* reported, so that you can take whatever heuristic
814 /// paths you want to take. To make things worse, it was possible for
815 /// cycles to arise, where you basically had a setup like `<MyType<$0>
816 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
817 /// Trait>::Foo> to `[type error]` would lead to an obligation of
818 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
819 /// an error for this obligation, but we legitimately should not,
820 /// because it contains `[type error]`. Yuck! (See issue #29857 for
821 /// one case where this arose.)
822 fn normalize_to_error<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
823 param_env: ty::ParamEnv<'tcx>,
824 projection_ty: ty::ProjectionTy<'tcx>,
825 cause: ObligationCause<'tcx>,
827 -> NormalizedTy<'tcx>
829 let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
830 let trait_obligation = Obligation { cause,
831 recursion_depth: depth,
833 predicate: trait_ref.to_predicate() };
834 let tcx = selcx.infcx().tcx;
835 let def_id = projection_ty.item_def_id;
836 let new_value = selcx.infcx().next_ty_var(
837 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
840 obligations: vec![trait_obligation]
844 enum ProjectedTy<'tcx> {
845 Progress(Progress<'tcx>),
846 NoProgress(Ty<'tcx>),
849 struct Progress<'tcx> {
851 obligations: Vec<PredicateObligation<'tcx>>,
854 impl<'tcx> Progress<'tcx> {
855 fn error<'a,'gcx>(tcx: TyCtxt<'a,'gcx,'tcx>) -> Self {
862 fn with_addl_obligations(mut self,
863 mut obligations: Vec<PredicateObligation<'tcx>>)
865 debug!("with_addl_obligations: self.obligations.len={} obligations.len={}",
866 self.obligations.len(), obligations.len());
868 debug!("with_addl_obligations: self.obligations={:?} obligations={:?}",
869 self.obligations, obligations);
871 self.obligations.append(&mut obligations);
876 /// Compute the result of a projection type (if we can).
879 /// - `obligation` must be fully normalized
880 fn project_type<'cx, 'gcx, 'tcx>(
881 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
882 obligation: &ProjectionTyObligation<'tcx>)
883 -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>>
885 debug!("project(obligation={:?})",
888 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
889 if obligation.recursion_depth >= recursion_limit {
890 debug!("project: overflow!");
891 selcx.infcx().report_overflow_error(&obligation, true);
894 let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
896 debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
898 if obligation_trait_ref.references_error() {
899 return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
902 let mut candidates = ProjectionTyCandidateSet::None;
904 // Make sure that the following procedures are kept in order. ParamEnv
905 // needs to be first because it has highest priority, and Select checks
906 // the return value of push_candidate which assumes it's ran at last.
907 assemble_candidates_from_param_env(selcx,
909 &obligation_trait_ref,
912 assemble_candidates_from_trait_def(selcx,
914 &obligation_trait_ref,
917 assemble_candidates_from_impls(selcx,
919 &obligation_trait_ref,
923 ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
924 confirm_candidate(selcx,
926 &obligation_trait_ref,
928 ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
929 selcx.tcx().mk_projection(
930 obligation.predicate.item_def_id,
931 obligation.predicate.substs))),
932 // Error occurred while trying to processing impls.
933 ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
934 // Inherent ambiguity that prevents us from even enumerating the
936 ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
941 /// The first thing we have to do is scan through the parameter
942 /// environment to see whether there are any projection predicates
943 /// there that can answer this question.
944 fn assemble_candidates_from_param_env<'cx, 'gcx, 'tcx>(
945 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
946 obligation: &ProjectionTyObligation<'tcx>,
947 obligation_trait_ref: &ty::TraitRef<'tcx>,
948 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
950 debug!("assemble_candidates_from_param_env(..)");
951 assemble_candidates_from_predicates(selcx,
953 obligation_trait_ref,
955 ProjectionTyCandidate::ParamEnv,
956 obligation.param_env.caller_bounds.iter().cloned());
959 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
960 /// that the definition of `Foo` has some clues:
964 /// type FooT : Bar<BarT=i32>
968 /// Here, for example, we could conclude that the result is `i32`.
969 fn assemble_candidates_from_trait_def<'cx, 'gcx, 'tcx>(
970 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
971 obligation: &ProjectionTyObligation<'tcx>,
972 obligation_trait_ref: &ty::TraitRef<'tcx>,
973 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
975 debug!("assemble_candidates_from_trait_def(..)");
977 let tcx = selcx.tcx();
978 // Check whether the self-type is itself a projection.
979 let (def_id, substs) = match obligation_trait_ref.self_ty().sty {
980 ty::Projection(ref data) => {
981 (data.trait_ref(tcx).def_id, data.substs)
983 ty::Opaque(def_id, substs) => (def_id, substs),
984 ty::Infer(ty::TyVar(_)) => {
985 // If the self-type is an inference variable, then it MAY wind up
986 // being a projected type, so induce an ambiguity.
987 candidate_set.mark_ambiguous();
993 // If so, extract what we know from the trait and try to come up with a good answer.
994 let trait_predicates = tcx.predicates_of(def_id);
995 let bounds = trait_predicates.instantiate(tcx, substs);
996 let bounds = elaborate_predicates(tcx, bounds.predicates);
997 assemble_candidates_from_predicates(selcx,
999 obligation_trait_ref,
1001 ProjectionTyCandidate::TraitDef,
1005 fn assemble_candidates_from_predicates<'cx, 'gcx, 'tcx, I>(
1006 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1007 obligation: &ProjectionTyObligation<'tcx>,
1008 obligation_trait_ref: &ty::TraitRef<'tcx>,
1009 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
1010 ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
1012 where I: IntoIterator<Item=ty::Predicate<'tcx>>
1014 debug!("assemble_candidates_from_predicates(obligation={:?})",
1016 let infcx = selcx.infcx();
1017 for predicate in env_predicates {
1018 debug!("assemble_candidates_from_predicates: predicate={:?}",
1020 if let ty::Predicate::Projection(data) = predicate {
1021 let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
1023 let is_match = same_def_id && infcx.probe(|_| {
1024 let data_poly_trait_ref =
1025 data.to_poly_trait_ref(infcx.tcx);
1026 let obligation_poly_trait_ref =
1027 obligation_trait_ref.to_poly_trait_ref();
1028 infcx.at(&obligation.cause, obligation.param_env)
1029 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1030 .map(|InferOk { obligations: _, value: () }| {
1031 // FIXME(#32730) -- do we need to take obligations
1032 // into account in any way? At the moment, no.
1037 debug!("assemble_candidates_from_predicates: candidate={:?} \
1038 is_match={} same_def_id={}",
1039 data, is_match, same_def_id);
1042 candidate_set.push_candidate(ctor(data));
1048 fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
1049 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1050 obligation: &ProjectionTyObligation<'tcx>,
1051 obligation_trait_ref: &ty::TraitRef<'tcx>,
1052 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
1054 // If we are resolving `<T as TraitRef<...>>::Item == Type`,
1055 // start out by selecting the predicate `T as TraitRef<...>`:
1056 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1057 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1058 let _ = selcx.infcx().commit_if_ok(|_| {
1059 let vtable = match selcx.select(&trait_obligation) {
1060 Ok(Some(vtable)) => vtable,
1062 candidate_set.mark_ambiguous();
1066 debug!("assemble_candidates_from_impls: selection error {:?}", e);
1067 candidate_set.mark_error(e);
1072 let eligible = match &vtable {
1073 super::VtableClosure(_) |
1074 super::VtableGenerator(_) |
1075 super::VtableFnPointer(_) |
1076 super::VtableObject(_) => {
1077 debug!("assemble_candidates_from_impls: vtable={:?}",
1081 super::VtableImpl(impl_data) => {
1082 // We have to be careful when projecting out of an
1083 // impl because of specialization. If we are not in
1084 // codegen (i.e., projection mode is not "any"), and the
1085 // impl's type is declared as default, then we disable
1086 // projection (even if the trait ref is fully
1087 // monomorphic). In the case where trait ref is not
1088 // fully monomorphic (i.e., includes type parameters),
1089 // this is because those type parameters may
1090 // ultimately be bound to types from other crates that
1091 // may have specialized impls we can't see. In the
1092 // case where the trait ref IS fully monomorphic, this
1093 // is a policy decision that we made in the RFC in
1094 // order to preserve flexibility for the crate that
1095 // defined the specializable impl to specialize later
1096 // for existing types.
1098 // In either case, we handle this by not adding a
1099 // candidate for an impl if it contains a `default`
1101 let node_item = assoc_ty_def(selcx,
1102 impl_data.impl_def_id,
1103 obligation.predicate.item_def_id);
1105 let is_default = if node_item.node.is_from_trait() {
1106 // If true, the impl inherited a `type Foo = Bar`
1107 // given in the trait, which is implicitly default.
1108 // Otherwise, the impl did not specify `type` and
1109 // neither did the trait:
1112 // trait Foo { type T; }
1113 // impl Foo for Bar { }
1116 // This is an error, but it will be
1117 // reported in `check_impl_items_against_trait`.
1118 // We accept it here but will flag it as
1119 // an error when we confirm the candidate
1120 // (which will ultimately lead to `normalize_to_error`
1122 node_item.item.defaultness.has_value()
1124 node_item.item.defaultness.is_default() ||
1125 selcx.tcx().impl_is_default(node_item.node.def_id())
1128 // Only reveal a specializable default if we're past type-checking
1129 // and the obligations is monomorphic, otherwise passes such as
1130 // transmute checking and polymorphic MIR optimizations could
1131 // get a result which isn't correct for all monomorphizations.
1134 } else if obligation.param_env.reveal == Reveal::All {
1135 debug_assert!(!poly_trait_ref.needs_infer());
1136 if !poly_trait_ref.needs_subst() {
1145 super::VtableParam(..) => {
1146 // This case tell us nothing about the value of an
1147 // associated type. Consider:
1150 // trait SomeTrait { type Foo; }
1151 // fn foo<T:SomeTrait>(...) { }
1154 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1155 // : SomeTrait` binding does not help us decide what the
1156 // type `Foo` is (at least, not more specifically than
1157 // what we already knew).
1159 // But wait, you say! What about an example like this:
1162 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1165 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1166 // resolve `T::Foo`? And of course it does, but in fact
1167 // that single predicate is desugared into two predicates
1168 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1169 // projection. And the projection where clause is handled
1170 // in `assemble_candidates_from_param_env`.
1173 super::VtableAutoImpl(..) |
1174 super::VtableBuiltin(..) => {
1175 // These traits have no associated types.
1177 obligation.cause.span,
1178 "Cannot project an associated type from `{:?}`",
1184 if candidate_set.push_candidate(ProjectionTyCandidate::Select(vtable)) {
1195 fn confirm_candidate<'cx, 'gcx, 'tcx>(
1196 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1197 obligation: &ProjectionTyObligation<'tcx>,
1198 obligation_trait_ref: &ty::TraitRef<'tcx>,
1199 candidate: ProjectionTyCandidate<'tcx>)
1202 debug!("confirm_candidate(candidate={:?}, obligation={:?})",
1207 ProjectionTyCandidate::ParamEnv(poly_projection) |
1208 ProjectionTyCandidate::TraitDef(poly_projection) => {
1209 confirm_param_env_candidate(selcx, obligation, poly_projection)
1212 ProjectionTyCandidate::Select(vtable) => {
1213 confirm_select_candidate(selcx, obligation, obligation_trait_ref, vtable)
1218 fn confirm_select_candidate<'cx, 'gcx, 'tcx>(
1219 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1220 obligation: &ProjectionTyObligation<'tcx>,
1221 obligation_trait_ref: &ty::TraitRef<'tcx>,
1222 vtable: Selection<'tcx>)
1226 super::VtableImpl(data) =>
1227 confirm_impl_candidate(selcx, obligation, data),
1228 super::VtableGenerator(data) =>
1229 confirm_generator_candidate(selcx, obligation, data),
1230 super::VtableClosure(data) =>
1231 confirm_closure_candidate(selcx, obligation, data),
1232 super::VtableFnPointer(data) =>
1233 confirm_fn_pointer_candidate(selcx, obligation, data),
1234 super::VtableObject(_) =>
1235 confirm_object_candidate(selcx, obligation, obligation_trait_ref),
1236 super::VtableAutoImpl(..) |
1237 super::VtableParam(..) |
1238 super::VtableBuiltin(..) =>
1239 // we don't create Select candidates with this kind of resolution
1241 obligation.cause.span,
1242 "Cannot project an associated type from `{:?}`",
1247 fn confirm_object_candidate<'cx, 'gcx, 'tcx>(
1248 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1249 obligation: &ProjectionTyObligation<'tcx>,
1250 obligation_trait_ref: &ty::TraitRef<'tcx>)
1253 let self_ty = obligation_trait_ref.self_ty();
1254 let object_ty = selcx.infcx().shallow_resolve(self_ty);
1255 debug!("confirm_object_candidate(object_ty={:?})",
1257 let data = match object_ty.sty {
1258 ty::Dynamic(ref data, ..) => data,
1261 obligation.cause.span,
1262 "confirm_object_candidate called with non-object: {:?}",
1266 let env_predicates = data.projection_bounds().map(|p| {
1267 p.with_self_ty(selcx.tcx(), object_ty).to_predicate()
1269 let env_predicate = {
1270 let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1272 // select only those projections that are actually projecting an
1273 // item with the correct name
1274 let env_predicates = env_predicates.filter_map(|p| match p {
1275 ty::Predicate::Projection(data) =>
1276 if data.projection_def_id() == obligation.predicate.item_def_id {
1284 // select those with a relevant trait-ref
1285 let mut env_predicates = env_predicates.filter(|data| {
1286 let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1287 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1288 selcx.infcx().probe(|_|
1289 selcx.infcx().at(&obligation.cause, obligation.param_env)
1290 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1295 // select the first matching one; there really ought to be one or
1296 // else the object type is not WF, since an object type should
1297 // include all of its projections explicitly
1298 match env_predicates.next() {
1299 Some(env_predicate) => env_predicate,
1301 debug!("confirm_object_candidate: no env-predicate \
1302 found in object type `{:?}`; ill-formed",
1304 return Progress::error(selcx.tcx());
1309 confirm_param_env_candidate(selcx, obligation, env_predicate)
1312 fn confirm_generator_candidate<'cx, 'gcx, 'tcx>(
1313 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1314 obligation: &ProjectionTyObligation<'tcx>,
1315 vtable: VtableGeneratorData<'tcx, PredicateObligation<'tcx>>)
1318 let gen_sig = vtable.substs.poly_sig(vtable.generator_def_id, selcx.tcx());
1322 } = normalize_with_depth(selcx,
1323 obligation.param_env,
1324 obligation.cause.clone(),
1325 obligation.recursion_depth+1,
1328 debug!("confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1333 let tcx = selcx.tcx();
1335 let gen_def_id = tcx.lang_items().gen_trait().unwrap();
1338 tcx.generator_trait_ref_and_outputs(gen_def_id,
1339 obligation.predicate.self_ty(),
1341 .map_bound(|(trait_ref, yield_ty, return_ty)| {
1342 let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1343 let ty = if name == "Return" {
1345 } else if name == "Yield" {
1351 ty::ProjectionPredicate {
1352 projection_ty: ty::ProjectionTy {
1353 substs: trait_ref.substs,
1354 item_def_id: obligation.predicate.item_def_id,
1360 confirm_param_env_candidate(selcx, obligation, predicate)
1361 .with_addl_obligations(vtable.nested)
1362 .with_addl_obligations(obligations)
1365 fn confirm_fn_pointer_candidate<'cx, 'gcx, 'tcx>(
1366 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1367 obligation: &ProjectionTyObligation<'tcx>,
1368 fn_pointer_vtable: VtableFnPointerData<'tcx, PredicateObligation<'tcx>>)
1371 let fn_type = selcx.infcx().shallow_resolve(fn_pointer_vtable.fn_ty);
1372 let sig = fn_type.fn_sig(selcx.tcx());
1376 } = normalize_with_depth(selcx,
1377 obligation.param_env,
1378 obligation.cause.clone(),
1379 obligation.recursion_depth+1,
1382 confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1383 .with_addl_obligations(fn_pointer_vtable.nested)
1384 .with_addl_obligations(obligations)
1387 fn confirm_closure_candidate<'cx, 'gcx, 'tcx>(
1388 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1389 obligation: &ProjectionTyObligation<'tcx>,
1390 vtable: VtableClosureData<'tcx, PredicateObligation<'tcx>>)
1393 let tcx = selcx.tcx();
1394 let infcx = selcx.infcx();
1395 let closure_sig_ty = vtable.substs.closure_sig_ty(vtable.closure_def_id, tcx);
1396 let closure_sig = infcx.shallow_resolve(&closure_sig_ty).fn_sig(tcx);
1400 } = normalize_with_depth(selcx,
1401 obligation.param_env,
1402 obligation.cause.clone(),
1403 obligation.recursion_depth+1,
1406 debug!("confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1411 confirm_callable_candidate(selcx,
1414 util::TupleArgumentsFlag::No)
1415 .with_addl_obligations(vtable.nested)
1416 .with_addl_obligations(obligations)
1419 fn confirm_callable_candidate<'cx, 'gcx, 'tcx>(
1420 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1421 obligation: &ProjectionTyObligation<'tcx>,
1422 fn_sig: ty::PolyFnSig<'tcx>,
1423 flag: util::TupleArgumentsFlag)
1426 let tcx = selcx.tcx();
1428 debug!("confirm_callable_candidate({:?},{:?})",
1432 // the `Output` associated type is declared on `FnOnce`
1433 let fn_once_def_id = tcx.lang_items().fn_once_trait().unwrap();
1436 tcx.closure_trait_ref_and_return_type(fn_once_def_id,
1437 obligation.predicate.self_ty(),
1440 .map_bound(|(trait_ref, ret_type)|
1441 ty::ProjectionPredicate {
1442 projection_ty: ty::ProjectionTy::from_ref_and_name(
1445 Ident::from_str(FN_OUTPUT_NAME),
1451 confirm_param_env_candidate(selcx, obligation, predicate)
1454 fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
1455 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1456 obligation: &ProjectionTyObligation<'tcx>,
1457 poly_projection: ty::PolyProjectionPredicate<'tcx>)
1460 let infcx = selcx.infcx();
1461 let cause = obligation.cause.clone();
1462 let param_env = obligation.param_env;
1463 let trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1464 match infcx.match_poly_projection_predicate(cause, param_env, poly_projection, trait_ref) {
1465 Ok(InferOk { value: ty_match, obligations }) => {
1473 obligation.cause.span,
1474 "Failed to unify obligation `{:?}` \
1475 with poly_projection `{:?}`: {:?}",
1483 fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
1484 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1485 obligation: &ProjectionTyObligation<'tcx>,
1486 impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>)
1489 let VtableImplData { substs, nested, impl_def_id } = impl_vtable;
1491 let tcx = selcx.tcx();
1492 let param_env = obligation.param_env;
1493 let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_def_id);
1495 if !assoc_ty.item.defaultness.has_value() {
1496 // This means that the impl is missing a definition for the
1497 // associated type. This error will be reported by the type
1498 // checker method `check_impl_items_against_trait`, so here we
1499 // just return Error.
1500 debug!("confirm_impl_candidate: no associated type {:?} for {:?}",
1501 assoc_ty.item.ident,
1502 obligation.predicate);
1505 obligations: nested,
1508 let substs = translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.node);
1509 let ty = if let ty::AssociatedKind::Existential = assoc_ty.item.kind {
1510 let item_substs = Substs::identity_for_item(tcx, assoc_ty.item.def_id);
1511 tcx.mk_opaque(assoc_ty.item.def_id, item_substs)
1513 tcx.type_of(assoc_ty.item.def_id)
1516 ty: ty.subst(tcx, substs),
1517 obligations: nested,
1521 /// Locate the definition of an associated type in the specialization hierarchy,
1522 /// starting from the given impl.
1524 /// Based on the "projection mode", this lookup may in fact only examine the
1525 /// topmost impl. See the comments for `Reveal` for more details.
1526 fn assoc_ty_def<'cx, 'gcx, 'tcx>(
1527 selcx: &SelectionContext<'cx, 'gcx, 'tcx>,
1529 assoc_ty_def_id: DefId)
1530 -> specialization_graph::NodeItem<ty::AssociatedItem>
1532 let tcx = selcx.tcx();
1533 let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1534 let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1535 let trait_def = tcx.trait_def(trait_def_id);
1537 // This function may be called while we are still building the
1538 // specialization graph that is queried below (via TraidDef::ancestors()),
1539 // so, in order to avoid unnecessary infinite recursion, we manually look
1540 // for the associated item at the given impl.
1541 // If there is no such item in that impl, this function will fail with a
1542 // cycle error if the specialization graph is currently being built.
1543 let impl_node = specialization_graph::Node::Impl(impl_def_id);
1544 for item in impl_node.items(tcx) {
1545 if item.kind == ty::AssociatedKind::Type &&
1546 tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id) {
1547 return specialization_graph::NodeItem {
1548 node: specialization_graph::Node::Impl(impl_def_id),
1554 if let Some(assoc_item) = trait_def
1555 .ancestors(tcx, impl_def_id)
1556 .defs(tcx, assoc_ty_name, ty::AssociatedKind::Type, trait_def_id)
1560 // This is saying that neither the trait nor
1561 // the impl contain a definition for this
1562 // associated type. Normally this situation
1563 // could only arise through a compiler bug --
1564 // if the user wrote a bad item name, it
1565 // should have failed in astconv.
1566 bug!("No associated type `{}` for {}",
1568 tcx.item_path_str(impl_def_id))
1574 /// The projection cache. Unlike the standard caches, this can include
1575 /// infcx-dependent type variables - therefore, we have to roll the
1576 /// cache back each time we roll a snapshot back, to avoid assumptions
1577 /// on yet-unresolved inference variables. Types with placeholder
1578 /// regions also have to be removed when the respective snapshot ends.
1580 /// Because of that, projection cache entries can be "stranded" and left
1581 /// inaccessible when type variables inside the key are resolved. We make no
1582 /// attempt to recover or remove "stranded" entries, but rather let them be
1583 /// (for the lifetime of the infcx).
1585 /// Entries in the projection cache might contain inference variables
1586 /// that will be resolved by obligations on the projection cache entry - e.g.
1587 /// when a type parameter in the associated type is constrained through
1588 /// an "RFC 447" projection on the impl.
1590 /// When working with a fulfillment context, the derived obligations of each
1591 /// projection cache entry will be registered on the fulfillcx, so any users
1592 /// that can wait for a fulfillcx fixed point need not care about this. However,
1593 /// users that don't wait for a fixed point (e.g. trait evaluation) have to
1594 /// resolve the obligations themselves to make sure the projected result is
1595 /// ok and avoid issues like #43132.
1597 /// If that is done, after evaluation the obligations, it is a good idea to
1598 /// call `ProjectionCache::complete` to make sure the obligations won't be
1599 /// re-evaluated and avoid an exponential worst-case.
1601 /// FIXME: we probably also want some sort of cross-infcx cache here to
1602 /// reduce the amount of duplication. Let's see what we get with the Chalk
1605 pub struct ProjectionCache<'tcx> {
1606 map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
1609 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
1610 pub struct ProjectionCacheKey<'tcx> {
1611 ty: ty::ProjectionTy<'tcx>
1614 impl<'cx, 'gcx, 'tcx> ProjectionCacheKey<'tcx> {
1615 pub fn from_poly_projection_predicate(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1616 predicate: &ty::PolyProjectionPredicate<'tcx>)
1619 let infcx = selcx.infcx();
1620 // We don't do cross-snapshot caching of obligations with escaping regions,
1621 // so there's no cache key to use
1622 predicate.no_late_bound_regions()
1623 .map(|predicate| ProjectionCacheKey {
1624 // We don't attempt to match up with a specific type-variable state
1625 // from a specific call to `opt_normalize_projection_type` - if
1626 // there's no precise match, the original cache entry is "stranded"
1628 ty: infcx.resolve_type_vars_if_possible(&predicate.projection_ty)
1633 #[derive(Clone, Debug)]
1634 enum ProjectionCacheEntry<'tcx> {
1638 NormalizedTy(NormalizedTy<'tcx>),
1641 // NB: intentionally not Clone
1642 pub struct ProjectionCacheSnapshot {
1646 impl<'tcx> ProjectionCache<'tcx> {
1647 pub fn clear(&mut self) {
1651 pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
1652 ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
1655 pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
1656 self.map.rollback_to(&snapshot.snapshot);
1659 pub fn rollback_placeholder(&mut self, snapshot: &ProjectionCacheSnapshot) {
1660 self.map.partial_rollback(&snapshot.snapshot, &|k| k.ty.has_re_skol());
1663 pub fn commit(&mut self, snapshot: &ProjectionCacheSnapshot) {
1664 self.map.commit(&snapshot.snapshot);
1667 /// Try to start normalize `key`; returns an error if
1668 /// normalization already occurred (this error corresponds to a
1669 /// cache hit, so it's actually a good thing).
1670 fn try_start(&mut self, key: ProjectionCacheKey<'tcx>)
1671 -> Result<(), ProjectionCacheEntry<'tcx>> {
1672 if let Some(entry) = self.map.get(&key) {
1673 return Err(entry.clone());
1676 self.map.insert(key, ProjectionCacheEntry::InProgress);
1680 /// Indicates that `key` was normalized to `value`.
1681 fn insert_ty(&mut self, key: ProjectionCacheKey<'tcx>, value: NormalizedTy<'tcx>) {
1682 debug!("ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
1684 let fresh_key = self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
1685 assert!(!fresh_key, "never started projecting `{:?}`", key);
1688 /// Mark the relevant projection cache key as having its derived obligations
1689 /// complete, so they won't have to be re-computed (this is OK to do in a
1690 /// snapshot - if the snapshot is rolled back, the obligations will be
1691 /// marked as incomplete again).
1692 pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
1693 let ty = match self.map.get(&key) {
1694 Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
1695 debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}",
1700 // Type inference could "strand behind" old cache entries. Leave
1701 // them alone for now.
1702 debug!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}",
1708 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1714 /// A specialized version of `complete` for when the key's value is known
1715 /// to be a NormalizedTy.
1716 pub fn complete_normalized(&mut self, key: ProjectionCacheKey<'tcx>, ty: &NormalizedTy<'tcx>) {
1717 // We want to insert `ty` with no obligations. If the existing value
1718 // already has no obligations (as is common) we can use `insert_noop`
1719 // to do a minimal amount of work -- the HashMap insertion is skipped,
1720 // and minimal changes are made to the undo log.
1721 if ty.obligations.is_empty() {
1722 self.map.insert_noop();
1724 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1731 /// Indicates that trying to normalize `key` resulted in
1732 /// ambiguity. No point in trying it again then until we gain more
1733 /// type information (in which case, the "fully resolved" key will
1735 fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
1736 let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
1737 assert!(!fresh, "never started projecting `{:?}`", key);
1740 /// Indicates that trying to normalize `key` resulted in
1742 fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
1743 let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
1744 assert!(!fresh, "never started projecting `{:?}`", key);