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::{VtableImplData, VtableClosureData, VtableGeneratorData, VtableFnPointerData};
25 use hir::def_id::DefId;
26 use infer::{InferCtxt, InferOk};
27 use infer::type_variable::TypeVariableOrigin;
28 use mir::interpret::ConstValue;
29 use mir::interpret::{GlobalId};
30 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
31 use syntax::ast::Ident;
32 use ty::subst::{Subst, Substs};
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, Hash)]
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 codegen 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)
112 Select(Selection<'tcx>),
115 enum ProjectionTyCandidateSet<'tcx> {
117 Single(ProjectionTyCandidate<'tcx>),
119 Error(SelectionError<'tcx>),
122 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
123 fn mark_ambiguous(&mut self) {
124 *self = ProjectionTyCandidateSet::Ambiguous;
127 fn mark_error(&mut self, err: SelectionError<'tcx>) {
128 *self = ProjectionTyCandidateSet::Error(err);
131 // Returns true if the push was successful, or false if the candidate
132 // was discarded -- this could be because of ambiguity, or because
133 // a higher-priority candidate is already there.
134 fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
135 use self::ProjectionTyCandidateSet::*;
136 use self::ProjectionTyCandidate::*;
138 // This wacky variable is just used to try and
139 // make code readable and avoid confusing paths.
140 // It is assigned a "value" of `()` only on those
141 // paths in which we wish to convert `*self` to
142 // ambiguous (and return false, because the candidate
143 // was not used). On other paths, it is not assigned,
144 // and hence if those paths *could* reach the code that
145 // comes after the match, this fn would not compile.
146 let convert_to_ambiguous;
150 *self = Single(candidate);
155 // Duplicates can happen inside ParamEnv. In the case, we
156 // perform a lazy deduplication.
157 if current == &candidate {
161 // Prefer where-clauses. As in select, if there are multiple
162 // candidates, we prefer where-clause candidates over impls. This
163 // may seem a bit surprising, since impls are the source of
164 // "truth" in some sense, but in fact some of the impls that SEEM
165 // applicable are not, because of nested obligations. Where
166 // clauses are the safer choice. See the comment on
167 // `select::SelectionCandidate` and #21974 for more details.
168 match (current, candidate) {
169 (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
170 (ParamEnv(..), _) => return false,
171 (_, ParamEnv(..)) => unreachable!(),
172 (_, _) => convert_to_ambiguous = (),
176 Ambiguous | Error(..) => {
181 // We only ever get here when we moved from a single candidate
183 let () = convert_to_ambiguous;
189 /// Evaluates constraints of the form:
191 /// for<...> <T as Trait>::U == V
193 /// If successful, this may result in additional obligations. Also returns
194 /// the projection cache key used to track these additional obligations.
195 pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
196 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
197 obligation: &PolyProjectionObligation<'tcx>)
198 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
199 MismatchedProjectionTypes<'tcx>>
201 debug!("poly_project_and_unify_type(obligation={:?})",
204 let infcx = selcx.infcx();
205 infcx.commit_if_ok(|snapshot| {
206 let (placeholder_predicate, placeholder_map) =
207 infcx.replace_bound_vars_with_placeholders(&obligation.predicate);
209 let skol_obligation = obligation.with(placeholder_predicate);
210 let r = match project_and_unify_type(selcx, &skol_obligation) {
212 let span = obligation.cause.span;
213 match infcx.leak_check(false, span, &placeholder_map, snapshot) {
214 Ok(()) => Ok(infcx.plug_leaks(placeholder_map, snapshot, result)),
216 debug!("poly_project_and_unify_type: leak check encountered error {:?}", e);
217 Err(MismatchedProjectionTypes { err: e })
230 /// Evaluates constraints of the form:
232 /// <T as Trait>::U == V
234 /// If successful, this may result in additional obligations.
235 fn project_and_unify_type<'cx, 'gcx, 'tcx>(
236 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
237 obligation: &ProjectionObligation<'tcx>)
238 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
239 MismatchedProjectionTypes<'tcx>>
241 debug!("project_and_unify_type(obligation={:?})",
244 let mut obligations = vec![];
246 match opt_normalize_projection_type(selcx,
247 obligation.param_env,
248 obligation.predicate.projection_ty,
249 obligation.cause.clone(),
250 obligation.recursion_depth,
253 None => return Ok(None),
256 debug!("project_and_unify_type: normalized_ty={:?} obligations={:?}",
260 let infcx = selcx.infcx();
261 match infcx.at(&obligation.cause, obligation.param_env)
262 .eq(normalized_ty, obligation.predicate.ty) {
263 Ok(InferOk { obligations: inferred_obligations, value: () }) => {
264 obligations.extend(inferred_obligations);
265 Ok(Some(obligations))
268 debug!("project_and_unify_type: equating types encountered error {:?}", err);
269 Err(MismatchedProjectionTypes { err })
274 /// Normalizes any associated type projections in `value`, replacing
275 /// them with a fully resolved type where possible. The return value
276 /// combines the normalized result and any additional obligations that
277 /// were incurred as result.
278 pub fn normalize<'a, 'b, 'gcx, 'tcx, T>(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
279 param_env: ty::ParamEnv<'tcx>,
280 cause: ObligationCause<'tcx>,
282 -> Normalized<'tcx, T>
283 where T : TypeFoldable<'tcx>
285 normalize_with_depth(selcx, param_env, cause, 0, value)
288 /// As `normalize`, but with a custom depth.
289 pub fn normalize_with_depth<'a, 'b, 'gcx, 'tcx, T>(
290 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
291 param_env: ty::ParamEnv<'tcx>,
292 cause: ObligationCause<'tcx>,
295 -> Normalized<'tcx, T>
297 where T : TypeFoldable<'tcx>
299 debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
300 let mut normalizer = AssociatedTypeNormalizer::new(selcx, param_env, cause, depth);
301 let result = normalizer.fold(value);
302 debug!("normalize_with_depth: depth={} result={:?} with {} obligations",
303 depth, result, normalizer.obligations.len());
304 debug!("normalize_with_depth: depth={} obligations={:?}",
305 depth, normalizer.obligations);
308 obligations: normalizer.obligations,
312 struct AssociatedTypeNormalizer<'a, 'b: 'a, 'gcx: 'b+'tcx, 'tcx: 'b> {
313 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
314 param_env: ty::ParamEnv<'tcx>,
315 cause: ObligationCause<'tcx>,
316 obligations: Vec<PredicateObligation<'tcx>>,
320 impl<'a, 'b, 'gcx, 'tcx> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
321 fn new(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
322 param_env: ty::ParamEnv<'tcx>,
323 cause: ObligationCause<'tcx>,
325 -> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx>
327 AssociatedTypeNormalizer {
336 fn fold<T:TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
337 let value = self.selcx.infcx().resolve_type_vars_if_possible(value);
339 if !value.has_projections() {
342 value.fold_with(self)
347 impl<'a, 'b, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
348 fn tcx<'c>(&'c self) -> TyCtxt<'c, 'gcx, 'tcx> {
352 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
353 // We don't want to normalize associated types that occur inside of region
354 // binders, because they may contain bound regions, and we can't cope with that.
358 // for<'a> fn(<T as Foo<&'a>>::A)
360 // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
361 // normalize it when we instantiate those bound regions (which
362 // should occur eventually).
364 let ty = ty.super_fold_with(self);
366 ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => { // (*)
367 // Only normalize `impl Trait` after type-checking, usually in codegen.
368 match self.param_env.reveal {
369 Reveal::UserFacing => ty,
372 let recursion_limit = *self.tcx().sess.recursion_limit.get();
373 if self.depth >= recursion_limit {
374 let obligation = Obligation::with_depth(
380 self.selcx.infcx().report_overflow_error(&obligation, true);
383 let generic_ty = self.tcx().type_of(def_id);
384 let concrete_ty = generic_ty.subst(self.tcx(), substs);
386 let folded_ty = self.fold_ty(concrete_ty);
393 ty::Projection(ref data) if !data.has_escaping_bound_vars() => { // (*)
395 // (*) This is kind of hacky -- we need to be able to
396 // handle normalization within binders because
397 // otherwise we wind up a need to normalize when doing
398 // trait matching (since you can have a trait
399 // obligation like `for<'a> T::B : Fn(&'a int)`), but
400 // we can't normalize with bound regions in scope. So
401 // far now we just ignore binders but only normalize
402 // if all bound regions are gone (and then we still
403 // have to renormalize whenever we instantiate a
404 // binder). It would be better to normalize in a
405 // binding-aware fashion.
407 let normalized_ty = normalize_projection_type(self.selcx,
412 &mut self.obligations);
413 debug!("AssociatedTypeNormalizer: depth={} normalized {:?} to {:?}, \
414 now with {} obligations",
415 self.depth, ty, normalized_ty, self.obligations.len());
423 fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
424 if let ConstValue::Unevaluated(def_id, substs) = constant.val {
425 let tcx = self.selcx.tcx().global_tcx();
426 if let Some(param_env) = self.tcx().lift_to_global(&self.param_env) {
427 if substs.needs_infer() || substs.has_placeholders() {
428 let identity_substs = Substs::identity_for_item(tcx, def_id);
429 let instance = ty::Instance::resolve(tcx, param_env, def_id, identity_substs);
430 if let Some(instance) = instance {
435 if let Ok(evaluated) = tcx.const_eval(param_env.and(cid)) {
436 let evaluated = evaluated.subst(self.tcx(), substs);
437 return self.fold_const(evaluated);
441 if let Some(substs) = self.tcx().lift_to_global(&substs) {
442 let instance = ty::Instance::resolve(tcx, param_env, def_id, substs);
443 if let Some(instance) = instance {
448 if let Ok(evaluated) = tcx.const_eval(param_env.and(cid)) {
449 return self.fold_const(evaluated)
461 pub struct Normalized<'tcx,T> {
463 pub obligations: Vec<PredicateObligation<'tcx>>,
466 pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
468 impl<'tcx,T> Normalized<'tcx,T> {
469 pub fn with<U>(self, value: U) -> Normalized<'tcx,U> {
470 Normalized { value: value, obligations: self.obligations }
474 /// The guts of `normalize`: normalize a specific projection like `<T
475 /// as Trait>::Item`. The result is always a type (and possibly
476 /// additional obligations). If ambiguity arises, which implies that
477 /// there are unresolved type variables in the projection, we will
478 /// substitute a fresh type variable `$X` and generate a new
479 /// obligation `<T as Trait>::Item == $X` for later.
480 pub fn normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
481 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
482 param_env: ty::ParamEnv<'tcx>,
483 projection_ty: ty::ProjectionTy<'tcx>,
484 cause: ObligationCause<'tcx>,
486 obligations: &mut Vec<PredicateObligation<'tcx>>)
489 opt_normalize_projection_type(selcx, param_env, projection_ty.clone(), cause.clone(), depth,
491 .unwrap_or_else(move || {
492 // if we bottom out in ambiguity, create a type variable
493 // and a deferred predicate to resolve this when more type
494 // information is available.
496 let tcx = selcx.infcx().tcx;
497 let def_id = projection_ty.item_def_id;
498 let ty_var = selcx.infcx().next_ty_var(
499 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
500 let projection = ty::Binder::dummy(ty::ProjectionPredicate {
504 let obligation = Obligation::with_depth(
505 cause, depth + 1, param_env, projection.to_predicate());
506 obligations.push(obligation);
511 /// The guts of `normalize`: normalize a specific projection like `<T
512 /// as Trait>::Item`. The result is always a type (and possibly
513 /// additional obligations). Returns `None` in the case of ambiguity,
514 /// which indicates that there are unbound type variables.
516 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
517 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
518 /// often immediately appended to another obligations vector. So now this
519 /// function takes an obligations vector and appends to it directly, which is
520 /// slightly uglier but avoids the need for an extra short-lived allocation.
521 fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
522 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
523 param_env: ty::ParamEnv<'tcx>,
524 projection_ty: ty::ProjectionTy<'tcx>,
525 cause: ObligationCause<'tcx>,
527 obligations: &mut Vec<PredicateObligation<'tcx>>)
530 let infcx = selcx.infcx();
532 let projection_ty = infcx.resolve_type_vars_if_possible(&projection_ty);
533 let cache_key = ProjectionCacheKey { ty: projection_ty };
535 debug!("opt_normalize_projection_type(\
536 projection_ty={:?}, \
541 // FIXME(#20304) For now, I am caching here, which is good, but it
542 // means we don't capture the type variables that are created in
543 // the case of ambiguity. Which means we may create a large stream
544 // of such variables. OTOH, if we move the caching up a level, we
545 // would not benefit from caching when proving `T: Trait<U=Foo>`
546 // bounds. It might be the case that we want two distinct caches,
547 // or else another kind of cache entry.
549 let cache_result = infcx.projection_cache.borrow_mut().try_start(cache_key);
552 Err(ProjectionCacheEntry::Ambiguous) => {
553 // If we found ambiguity the last time, that generally
554 // means we will continue to do so until some type in the
555 // key changes (and we know it hasn't, because we just
556 // fully resolved it). One exception though is closure
557 // types, which can transition from having a fixed kind to
558 // no kind with no visible change in the key.
560 // FIXME(#32286) refactor this so that closure type
562 debug!("opt_normalize_projection_type: \
563 found cache entry: ambiguous");
564 if !projection_ty.has_closure_types() {
568 Err(ProjectionCacheEntry::InProgress) => {
569 // If while normalized A::B, we are asked to normalize
570 // A::B, just return A::B itself. This is a conservative
571 // answer, in the sense that A::B *is* clearly equivalent
572 // to A::B, though there may be a better value we can
575 // Under lazy normalization, this can arise when
576 // bootstrapping. That is, imagine an environment with a
577 // where-clause like `A::B == u32`. Now, if we are asked
578 // to normalize `A::B`, we will want to check the
579 // where-clauses in scope. So we will try to unify `A::B`
580 // with `A::B`, which can trigger a recursive
581 // normalization. In that case, I think we will want this code:
584 // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
585 // projection_ty.substs;
586 // return Some(NormalizedTy { value: v, obligations: vec![] });
589 debug!("opt_normalize_projection_type: \
590 found cache entry: in-progress");
592 // But for now, let's classify this as an overflow:
593 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
594 let obligation = Obligation::with_depth(cause,
598 selcx.infcx().report_overflow_error(&obligation, false);
600 Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
601 // This is the hottest path in this function.
603 // If we find the value in the cache, then return it along
604 // with the obligations that went along with it. Note
605 // that, when using a fulfillment context, these
606 // obligations could in principle be ignored: they have
607 // already been registered when the cache entry was
608 // created (and hence the new ones will quickly be
609 // discarded as duplicated). But when doing trait
610 // evaluation this is not the case, and dropping the trait
611 // evaluations can causes ICEs (e.g., #43132).
612 debug!("opt_normalize_projection_type: \
613 found normalized ty `{:?}`",
616 // Once we have inferred everything we need to know, we
617 // can ignore the `obligations` from that point on.
618 if !infcx.any_unresolved_type_vars(&ty.value) {
619 infcx.projection_cache.borrow_mut().complete_normalized(cache_key, &ty);
620 // No need to extend `obligations`.
622 obligations.extend(ty.obligations);
625 obligations.push(get_paranoid_cache_value_obligation(infcx,
630 return Some(ty.value);
632 Err(ProjectionCacheEntry::Error) => {
633 debug!("opt_normalize_projection_type: \
635 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
636 obligations.extend(result.obligations);
637 return Some(result.value)
641 let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
642 match project_type(selcx, &obligation) {
643 Ok(ProjectedTy::Progress(Progress { ty: projected_ty,
644 obligations: mut projected_obligations })) => {
645 // if projection succeeded, then what we get out of this
646 // is also non-normalized (consider: it was derived from
647 // an impl, where-clause etc) and hence we must
650 debug!("opt_normalize_projection_type: \
653 projected_obligations={:?}",
656 projected_obligations);
658 let result = if projected_ty.has_projections() {
659 let mut normalizer = AssociatedTypeNormalizer::new(selcx,
663 let normalized_ty = normalizer.fold(&projected_ty);
665 debug!("opt_normalize_projection_type: \
666 normalized_ty={:?} depth={}",
670 projected_obligations.extend(normalizer.obligations);
672 value: normalized_ty,
673 obligations: projected_obligations,
678 obligations: projected_obligations,
682 let cache_value = prune_cache_value_obligations(infcx, &result);
683 infcx.projection_cache.borrow_mut().insert_ty(cache_key, cache_value);
684 obligations.extend(result.obligations);
687 Ok(ProjectedTy::NoProgress(projected_ty)) => {
688 debug!("opt_normalize_projection_type: \
689 projected_ty={:?} no progress",
691 let result = Normalized {
695 infcx.projection_cache.borrow_mut().insert_ty(cache_key, result.clone());
696 // No need to extend `obligations`.
699 Err(ProjectionTyError::TooManyCandidates) => {
700 debug!("opt_normalize_projection_type: \
701 too many candidates");
702 infcx.projection_cache.borrow_mut()
703 .ambiguous(cache_key);
706 Err(ProjectionTyError::TraitSelectionError(_)) => {
707 debug!("opt_normalize_projection_type: ERROR");
708 // if we got an error processing the `T as Trait` part,
709 // just return `ty::err` but add the obligation `T :
710 // Trait`, which when processed will cause the error to be
713 infcx.projection_cache.borrow_mut()
715 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
716 obligations.extend(result.obligations);
722 /// If there are unresolved type variables, then we need to include
723 /// any subobligations that bind them, at least until those type
724 /// variables are fully resolved.
725 fn prune_cache_value_obligations<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
726 result: &NormalizedTy<'tcx>)
727 -> NormalizedTy<'tcx> {
728 if !infcx.any_unresolved_type_vars(&result.value) {
729 return NormalizedTy { value: result.value, obligations: vec![] };
732 let mut obligations: Vec<_> =
735 .filter(|obligation| match obligation.predicate {
736 // We found a `T: Foo<X = U>` predicate, let's check
737 // if `U` references any unresolved type
738 // variables. In principle, we only care if this
739 // projection can help resolve any of the type
740 // variables found in `result.value` -- but we just
741 // check for any type variables here, for fear of
742 // indirect obligations (e.g., we project to `?0`,
743 // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
745 ty::Predicate::Projection(ref data) =>
746 infcx.any_unresolved_type_vars(&data.ty()),
748 // We are only interested in `T: Foo<X = U>` predicates, whre
749 // `U` references one of `unresolved_type_vars`. =)
755 obligations.shrink_to_fit();
757 NormalizedTy { value: result.value, obligations }
760 /// Whenever we give back a cache result for a projection like `<T as
761 /// Trait>::Item ==> X`, we *always* include the obligation to prove
762 /// that `T: Trait` (we may also include some other obligations). This
763 /// may or may not be necessary -- in principle, all the obligations
764 /// that must be proven to show that `T: Trait` were also returned
765 /// when the cache was first populated. But there are some vague concerns,
766 /// and so we take the precautionary measure of including `T: Trait` in
769 /// Concern #1. The current setup is fragile. Perhaps someone could
770 /// have failed to prove the concerns from when the cache was
771 /// populated, but also not have used a snapshot, in which case the
772 /// cache could remain populated even though `T: Trait` has not been
773 /// shown. In this case, the "other code" is at fault -- when you
774 /// project something, you are supposed to either have a snapshot or
775 /// else prove all the resulting obligations -- but it's still easy to
778 /// Concern #2. Even within the snapshot, if those original
779 /// obligations are not yet proven, then we are able to do projections
780 /// that may yet turn out to be wrong. This *may* lead to some sort
781 /// of trouble, though we don't have a concrete example of how that
782 /// can occur yet. But it seems risky at best.
783 fn get_paranoid_cache_value_obligation<'a, 'gcx, 'tcx>(
784 infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
785 param_env: ty::ParamEnv<'tcx>,
786 projection_ty: ty::ProjectionTy<'tcx>,
787 cause: ObligationCause<'tcx>,
789 -> PredicateObligation<'tcx>
791 let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
794 recursion_depth: depth,
796 predicate: trait_ref.to_predicate(),
800 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
801 /// hold. In various error cases, we cannot generate a valid
802 /// normalized projection. Therefore, we create an inference variable
803 /// return an associated obligation that, when fulfilled, will lead to
806 /// Note that we used to return `Error` here, but that was quite
807 /// dubious -- the premise was that an error would *eventually* be
808 /// reported, when the obligation was processed. But in general once
809 /// you see a `Error` you are supposed to be able to assume that an
810 /// error *has been* reported, so that you can take whatever heuristic
811 /// paths you want to take. To make things worse, it was possible for
812 /// cycles to arise, where you basically had a setup like `<MyType<$0>
813 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
814 /// Trait>::Foo> to `[type error]` would lead to an obligation of
815 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
816 /// an error for this obligation, but we legitimately should not,
817 /// because it contains `[type error]`. Yuck! (See issue #29857 for
818 /// one case where this arose.)
819 fn normalize_to_error<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
820 param_env: ty::ParamEnv<'tcx>,
821 projection_ty: ty::ProjectionTy<'tcx>,
822 cause: ObligationCause<'tcx>,
824 -> NormalizedTy<'tcx>
826 let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
827 let trait_obligation = Obligation { cause,
828 recursion_depth: depth,
830 predicate: trait_ref.to_predicate() };
831 let tcx = selcx.infcx().tcx;
832 let def_id = projection_ty.item_def_id;
833 let new_value = selcx.infcx().next_ty_var(
834 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
837 obligations: vec![trait_obligation]
841 enum ProjectedTy<'tcx> {
842 Progress(Progress<'tcx>),
843 NoProgress(Ty<'tcx>),
846 struct Progress<'tcx> {
848 obligations: Vec<PredicateObligation<'tcx>>,
851 impl<'tcx> Progress<'tcx> {
852 fn error<'a,'gcx>(tcx: TyCtxt<'a,'gcx,'tcx>) -> Self {
859 fn with_addl_obligations(mut self,
860 mut obligations: Vec<PredicateObligation<'tcx>>)
862 debug!("with_addl_obligations: self.obligations.len={} obligations.len={}",
863 self.obligations.len(), obligations.len());
865 debug!("with_addl_obligations: self.obligations={:?} obligations={:?}",
866 self.obligations, obligations);
868 self.obligations.append(&mut obligations);
873 /// Compute the result of a projection type (if we can).
876 /// - `obligation` must be fully normalized
877 fn project_type<'cx, 'gcx, 'tcx>(
878 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
879 obligation: &ProjectionTyObligation<'tcx>)
880 -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>>
882 debug!("project(obligation={:?})",
885 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
886 if obligation.recursion_depth >= recursion_limit {
887 debug!("project: overflow!");
888 return Err(ProjectionTyError::TraitSelectionError(SelectionError::Overflow));
891 let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
893 debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
895 if obligation_trait_ref.references_error() {
896 return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
899 let mut candidates = ProjectionTyCandidateSet::None;
901 // Make sure that the following procedures are kept in order. ParamEnv
902 // needs to be first because it has highest priority, and Select checks
903 // the return value of push_candidate which assumes it's ran at last.
904 assemble_candidates_from_param_env(selcx,
906 &obligation_trait_ref,
909 assemble_candidates_from_trait_def(selcx,
911 &obligation_trait_ref,
914 assemble_candidates_from_impls(selcx,
916 &obligation_trait_ref,
920 ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
921 confirm_candidate(selcx,
923 &obligation_trait_ref,
925 ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
926 selcx.tcx().mk_projection(
927 obligation.predicate.item_def_id,
928 obligation.predicate.substs))),
929 // Error occurred while trying to processing impls.
930 ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
931 // Inherent ambiguity that prevents us from even enumerating the
933 ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
938 /// The first thing we have to do is scan through the parameter
939 /// environment to see whether there are any projection predicates
940 /// there that can answer this question.
941 fn assemble_candidates_from_param_env<'cx, 'gcx, 'tcx>(
942 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
943 obligation: &ProjectionTyObligation<'tcx>,
944 obligation_trait_ref: &ty::TraitRef<'tcx>,
945 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
947 debug!("assemble_candidates_from_param_env(..)");
948 assemble_candidates_from_predicates(selcx,
950 obligation_trait_ref,
952 ProjectionTyCandidate::ParamEnv,
953 obligation.param_env.caller_bounds.iter().cloned());
956 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
957 /// that the definition of `Foo` has some clues:
961 /// type FooT : Bar<BarT=i32>
965 /// Here, for example, we could conclude that the result is `i32`.
966 fn assemble_candidates_from_trait_def<'cx, 'gcx, 'tcx>(
967 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
968 obligation: &ProjectionTyObligation<'tcx>,
969 obligation_trait_ref: &ty::TraitRef<'tcx>,
970 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
972 debug!("assemble_candidates_from_trait_def(..)");
974 let tcx = selcx.tcx();
975 // Check whether the self-type is itself a projection.
976 let (def_id, substs) = match obligation_trait_ref.self_ty().sty {
977 ty::Projection(ref data) => {
978 (data.trait_ref(tcx).def_id, data.substs)
980 ty::Opaque(def_id, substs) => (def_id, substs),
981 ty::Infer(ty::TyVar(_)) => {
982 // If the self-type is an inference variable, then it MAY wind up
983 // being a projected type, so induce an ambiguity.
984 candidate_set.mark_ambiguous();
990 // If so, extract what we know from the trait and try to come up with a good answer.
991 let trait_predicates = tcx.predicates_of(def_id);
992 let bounds = trait_predicates.instantiate(tcx, substs);
993 let bounds = elaborate_predicates(tcx, bounds.predicates);
994 assemble_candidates_from_predicates(selcx,
996 obligation_trait_ref,
998 ProjectionTyCandidate::TraitDef,
1002 fn assemble_candidates_from_predicates<'cx, 'gcx, 'tcx, I>(
1003 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1004 obligation: &ProjectionTyObligation<'tcx>,
1005 obligation_trait_ref: &ty::TraitRef<'tcx>,
1006 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
1007 ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
1009 where I: IntoIterator<Item=ty::Predicate<'tcx>>
1011 debug!("assemble_candidates_from_predicates(obligation={:?})",
1013 let infcx = selcx.infcx();
1014 for predicate in env_predicates {
1015 debug!("assemble_candidates_from_predicates: predicate={:?}",
1017 if let ty::Predicate::Projection(data) = predicate {
1018 let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
1020 let is_match = same_def_id && infcx.probe(|_| {
1021 let data_poly_trait_ref =
1022 data.to_poly_trait_ref(infcx.tcx);
1023 let obligation_poly_trait_ref =
1024 obligation_trait_ref.to_poly_trait_ref();
1025 infcx.at(&obligation.cause, obligation.param_env)
1026 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1027 .map(|InferOk { obligations: _, value: () }| {
1028 // FIXME(#32730) -- do we need to take obligations
1029 // into account in any way? At the moment, no.
1034 debug!("assemble_candidates_from_predicates: candidate={:?} \
1035 is_match={} same_def_id={}",
1036 data, is_match, same_def_id);
1039 candidate_set.push_candidate(ctor(data));
1045 fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
1046 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1047 obligation: &ProjectionTyObligation<'tcx>,
1048 obligation_trait_ref: &ty::TraitRef<'tcx>,
1049 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
1051 // If we are resolving `<T as TraitRef<...>>::Item == Type`,
1052 // start out by selecting the predicate `T as TraitRef<...>`:
1053 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1054 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1055 let _ = selcx.infcx().commit_if_ok(|_| {
1056 let vtable = match selcx.select(&trait_obligation) {
1057 Ok(Some(vtable)) => vtable,
1059 candidate_set.mark_ambiguous();
1063 debug!("assemble_candidates_from_impls: selection error {:?}", e);
1064 candidate_set.mark_error(e);
1069 let eligible = match &vtable {
1070 super::VtableClosure(_) |
1071 super::VtableGenerator(_) |
1072 super::VtableFnPointer(_) |
1073 super::VtableObject(_) |
1074 super::VtableTraitAlias(_) => {
1075 debug!("assemble_candidates_from_impls: vtable={:?}",
1079 super::VtableImpl(impl_data) => {
1080 // We have to be careful when projecting out of an
1081 // impl because of specialization. If we are not in
1082 // codegen (i.e., projection mode is not "any"), and the
1083 // impl's type is declared as default, then we disable
1084 // projection (even if the trait ref is fully
1085 // monomorphic). In the case where trait ref is not
1086 // fully monomorphic (i.e., includes type parameters),
1087 // this is because those type parameters may
1088 // ultimately be bound to types from other crates that
1089 // may have specialized impls we can't see. In the
1090 // case where the trait ref IS fully monomorphic, this
1091 // is a policy decision that we made in the RFC in
1092 // order to preserve flexibility for the crate that
1093 // defined the specializable impl to specialize later
1094 // for existing types.
1096 // In either case, we handle this by not adding a
1097 // candidate for an impl if it contains a `default`
1099 let node_item = assoc_ty_def(selcx,
1100 impl_data.impl_def_id,
1101 obligation.predicate.item_def_id);
1103 let is_default = if node_item.node.is_from_trait() {
1104 // If true, the impl inherited a `type Foo = Bar`
1105 // given in the trait, which is implicitly default.
1106 // Otherwise, the impl did not specify `type` and
1107 // neither did the trait:
1110 // trait Foo { type T; }
1111 // impl Foo for Bar { }
1114 // This is an error, but it will be
1115 // reported in `check_impl_items_against_trait`.
1116 // We accept it here but will flag it as
1117 // an error when we confirm the candidate
1118 // (which will ultimately lead to `normalize_to_error`
1120 node_item.item.defaultness.has_value()
1122 node_item.item.defaultness.is_default() ||
1123 selcx.tcx().impl_is_default(node_item.node.def_id())
1126 // Only reveal a specializable default if we're past type-checking
1127 // and the obligations is monomorphic, otherwise passes such as
1128 // transmute checking and polymorphic MIR optimizations could
1129 // get a result which isn't correct for all monomorphizations.
1132 } else if obligation.param_env.reveal == Reveal::All {
1133 debug_assert!(!poly_trait_ref.needs_infer());
1134 if !poly_trait_ref.needs_subst() {
1143 super::VtableParam(..) => {
1144 // This case tell us nothing about the value of an
1145 // associated type. Consider:
1148 // trait SomeTrait { type Foo; }
1149 // fn foo<T:SomeTrait>(...) { }
1152 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1153 // : SomeTrait` binding does not help us decide what the
1154 // type `Foo` is (at least, not more specifically than
1155 // what we already knew).
1157 // But wait, you say! What about an example like this:
1160 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1163 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1164 // resolve `T::Foo`? And of course it does, but in fact
1165 // that single predicate is desugared into two predicates
1166 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1167 // projection. And the projection where clause is handled
1168 // in `assemble_candidates_from_param_env`.
1171 super::VtableAutoImpl(..) |
1172 super::VtableBuiltin(..) => {
1173 // These traits have no associated types.
1175 obligation.cause.span,
1176 "Cannot project an associated type from `{:?}`",
1182 if candidate_set.push_candidate(ProjectionTyCandidate::Select(vtable)) {
1193 fn confirm_candidate<'cx, 'gcx, 'tcx>(
1194 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1195 obligation: &ProjectionTyObligation<'tcx>,
1196 obligation_trait_ref: &ty::TraitRef<'tcx>,
1197 candidate: ProjectionTyCandidate<'tcx>)
1200 debug!("confirm_candidate(candidate={:?}, obligation={:?})",
1205 ProjectionTyCandidate::ParamEnv(poly_projection) |
1206 ProjectionTyCandidate::TraitDef(poly_projection) => {
1207 confirm_param_env_candidate(selcx, obligation, poly_projection)
1210 ProjectionTyCandidate::Select(vtable) => {
1211 confirm_select_candidate(selcx, obligation, obligation_trait_ref, vtable)
1216 fn confirm_select_candidate<'cx, 'gcx, 'tcx>(
1217 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1218 obligation: &ProjectionTyObligation<'tcx>,
1219 obligation_trait_ref: &ty::TraitRef<'tcx>,
1220 vtable: Selection<'tcx>)
1224 super::VtableImpl(data) =>
1225 confirm_impl_candidate(selcx, obligation, data),
1226 super::VtableGenerator(data) =>
1227 confirm_generator_candidate(selcx, obligation, data),
1228 super::VtableClosure(data) =>
1229 confirm_closure_candidate(selcx, obligation, data),
1230 super::VtableFnPointer(data) =>
1231 confirm_fn_pointer_candidate(selcx, obligation, data),
1232 super::VtableObject(_) =>
1233 confirm_object_candidate(selcx, obligation, obligation_trait_ref),
1234 super::VtableAutoImpl(..) |
1235 super::VtableParam(..) |
1236 super::VtableBuiltin(..) |
1237 super::VtableTraitAlias(..) =>
1238 // we don't create Select candidates with this kind of resolution
1240 obligation.cause.span,
1241 "Cannot project an associated type from `{:?}`",
1246 fn confirm_object_candidate<'cx, 'gcx, 'tcx>(
1247 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1248 obligation: &ProjectionTyObligation<'tcx>,
1249 obligation_trait_ref: &ty::TraitRef<'tcx>)
1252 let self_ty = obligation_trait_ref.self_ty();
1253 let object_ty = selcx.infcx().shallow_resolve(self_ty);
1254 debug!("confirm_object_candidate(object_ty={:?})",
1256 let data = match object_ty.sty {
1257 ty::Dynamic(ref data, ..) => data,
1260 obligation.cause.span,
1261 "confirm_object_candidate called with non-object: {:?}",
1265 let env_predicates = data.projection_bounds().map(|p| {
1266 p.with_self_ty(selcx.tcx(), object_ty).to_predicate()
1268 let env_predicate = {
1269 let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1271 // select only those projections that are actually projecting an
1272 // item with the correct name
1273 let env_predicates = env_predicates.filter_map(|p| match p {
1274 ty::Predicate::Projection(data) =>
1275 if data.projection_def_id() == obligation.predicate.item_def_id {
1283 // select those with a relevant trait-ref
1284 let mut env_predicates = env_predicates.filter(|data| {
1285 let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1286 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1287 selcx.infcx().probe(|_|
1288 selcx.infcx().at(&obligation.cause, obligation.param_env)
1289 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1294 // select the first matching one; there really ought to be one or
1295 // else the object type is not WF, since an object type should
1296 // include all of its projections explicitly
1297 match env_predicates.next() {
1298 Some(env_predicate) => env_predicate,
1300 debug!("confirm_object_candidate: no env-predicate \
1301 found in object type `{:?}`; ill-formed",
1303 return Progress::error(selcx.tcx());
1308 confirm_param_env_candidate(selcx, obligation, env_predicate)
1311 fn confirm_generator_candidate<'cx, 'gcx, 'tcx>(
1312 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1313 obligation: &ProjectionTyObligation<'tcx>,
1314 vtable: VtableGeneratorData<'tcx, PredicateObligation<'tcx>>)
1317 let gen_sig = vtable.substs.poly_sig(vtable.generator_def_id, selcx.tcx());
1321 } = normalize_with_depth(selcx,
1322 obligation.param_env,
1323 obligation.cause.clone(),
1324 obligation.recursion_depth+1,
1327 debug!("confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1332 let tcx = selcx.tcx();
1334 let gen_def_id = tcx.lang_items().gen_trait().unwrap();
1337 tcx.generator_trait_ref_and_outputs(gen_def_id,
1338 obligation.predicate.self_ty(),
1340 .map_bound(|(trait_ref, yield_ty, return_ty)| {
1341 let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1342 let ty = if name == "Return" {
1344 } else if name == "Yield" {
1350 ty::ProjectionPredicate {
1351 projection_ty: ty::ProjectionTy {
1352 substs: trait_ref.substs,
1353 item_def_id: obligation.predicate.item_def_id,
1359 confirm_param_env_candidate(selcx, obligation, predicate)
1360 .with_addl_obligations(vtable.nested)
1361 .with_addl_obligations(obligations)
1364 fn confirm_fn_pointer_candidate<'cx, 'gcx, 'tcx>(
1365 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1366 obligation: &ProjectionTyObligation<'tcx>,
1367 fn_pointer_vtable: VtableFnPointerData<'tcx, PredicateObligation<'tcx>>)
1370 let fn_type = selcx.infcx().shallow_resolve(fn_pointer_vtable.fn_ty);
1371 let sig = fn_type.fn_sig(selcx.tcx());
1375 } = normalize_with_depth(selcx,
1376 obligation.param_env,
1377 obligation.cause.clone(),
1378 obligation.recursion_depth+1,
1381 confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1382 .with_addl_obligations(fn_pointer_vtable.nested)
1383 .with_addl_obligations(obligations)
1386 fn confirm_closure_candidate<'cx, 'gcx, 'tcx>(
1387 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1388 obligation: &ProjectionTyObligation<'tcx>,
1389 vtable: VtableClosureData<'tcx, PredicateObligation<'tcx>>)
1392 let tcx = selcx.tcx();
1393 let infcx = selcx.infcx();
1394 let closure_sig_ty = vtable.substs.closure_sig_ty(vtable.closure_def_id, tcx);
1395 let closure_sig = infcx.shallow_resolve(&closure_sig_ty).fn_sig(tcx);
1399 } = normalize_with_depth(selcx,
1400 obligation.param_env,
1401 obligation.cause.clone(),
1402 obligation.recursion_depth+1,
1405 debug!("confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1410 confirm_callable_candidate(selcx,
1413 util::TupleArgumentsFlag::No)
1414 .with_addl_obligations(vtable.nested)
1415 .with_addl_obligations(obligations)
1418 fn confirm_callable_candidate<'cx, 'gcx, 'tcx>(
1419 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1420 obligation: &ProjectionTyObligation<'tcx>,
1421 fn_sig: ty::PolyFnSig<'tcx>,
1422 flag: util::TupleArgumentsFlag)
1425 let tcx = selcx.tcx();
1427 debug!("confirm_callable_candidate({:?},{:?})",
1431 // the `Output` associated type is declared on `FnOnce`
1432 let fn_once_def_id = tcx.lang_items().fn_once_trait().unwrap();
1435 tcx.closure_trait_ref_and_return_type(fn_once_def_id,
1436 obligation.predicate.self_ty(),
1439 .map_bound(|(trait_ref, ret_type)|
1440 ty::ProjectionPredicate {
1441 projection_ty: ty::ProjectionTy::from_ref_and_name(
1444 Ident::from_str(FN_OUTPUT_NAME),
1450 confirm_param_env_candidate(selcx, obligation, predicate)
1453 fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
1454 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1455 obligation: &ProjectionTyObligation<'tcx>,
1456 poly_projection: ty::PolyProjectionPredicate<'tcx>)
1459 let infcx = selcx.infcx();
1460 let cause = obligation.cause.clone();
1461 let param_env = obligation.param_env;
1462 let trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1463 match infcx.match_poly_projection_predicate(cause, param_env, poly_projection, trait_ref) {
1464 Ok(InferOk { value: ty_match, obligations }) => {
1472 obligation.cause.span,
1473 "Failed to unify obligation `{:?}` \
1474 with poly_projection `{:?}`: {:?}",
1482 fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
1483 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1484 obligation: &ProjectionTyObligation<'tcx>,
1485 impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>)
1488 let VtableImplData { impl_def_id, substs, nested } = impl_vtable;
1490 let tcx = selcx.tcx();
1491 let param_env = obligation.param_env;
1492 let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_def_id);
1494 if !assoc_ty.item.defaultness.has_value() {
1495 // This means that the impl is missing a definition for the
1496 // associated type. This error will be reported by the type
1497 // checker method `check_impl_items_against_trait`, so here we
1498 // just return Error.
1499 debug!("confirm_impl_candidate: no associated type {:?} for {:?}",
1500 assoc_ty.item.ident,
1501 obligation.predicate);
1504 obligations: nested,
1507 let substs = translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.node);
1508 let ty = if let ty::AssociatedKind::Existential = assoc_ty.item.kind {
1509 let item_substs = Substs::identity_for_item(tcx, assoc_ty.item.def_id);
1510 tcx.mk_opaque(assoc_ty.item.def_id, item_substs)
1512 tcx.type_of(assoc_ty.item.def_id)
1515 ty: ty.subst(tcx, substs),
1516 obligations: nested,
1520 /// Locate the definition of an associated type in the specialization hierarchy,
1521 /// starting from the given impl.
1523 /// Based on the "projection mode", this lookup may in fact only examine the
1524 /// topmost impl. See the comments for `Reveal` for more details.
1525 fn assoc_ty_def<'cx, 'gcx, 'tcx>(
1526 selcx: &SelectionContext<'cx, 'gcx, 'tcx>,
1528 assoc_ty_def_id: DefId)
1529 -> specialization_graph::NodeItem<ty::AssociatedItem>
1531 let tcx = selcx.tcx();
1532 let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1533 let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1534 let trait_def = tcx.trait_def(trait_def_id);
1536 // This function may be called while we are still building the
1537 // specialization graph that is queried below (via TraidDef::ancestors()),
1538 // so, in order to avoid unnecessary infinite recursion, we manually look
1539 // for the associated item at the given impl.
1540 // If there is no such item in that impl, this function will fail with a
1541 // cycle error if the specialization graph is currently being built.
1542 let impl_node = specialization_graph::Node::Impl(impl_def_id);
1543 for item in impl_node.items(tcx) {
1544 if item.kind == ty::AssociatedKind::Type &&
1545 tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id) {
1546 return specialization_graph::NodeItem {
1547 node: specialization_graph::Node::Impl(impl_def_id),
1553 if let Some(assoc_item) = trait_def
1554 .ancestors(tcx, impl_def_id)
1555 .defs(tcx, assoc_ty_name, ty::AssociatedKind::Type, trait_def_id)
1559 // This is saying that neither the trait nor
1560 // the impl contain a definition for this
1561 // associated type. Normally this situation
1562 // could only arise through a compiler bug --
1563 // if the user wrote a bad item name, it
1564 // should have failed in astconv.
1565 bug!("No associated type `{}` for {}",
1567 tcx.item_path_str(impl_def_id))
1573 /// The projection cache. Unlike the standard caches, this can include
1574 /// infcx-dependent type variables - therefore, we have to roll the
1575 /// cache back each time we roll a snapshot back, to avoid assumptions
1576 /// on yet-unresolved inference variables. Types with placeholder
1577 /// regions also have to be removed when the respective snapshot ends.
1579 /// Because of that, projection cache entries can be "stranded" and left
1580 /// inaccessible when type variables inside the key are resolved. We make no
1581 /// attempt to recover or remove "stranded" entries, but rather let them be
1582 /// (for the lifetime of the infcx).
1584 /// Entries in the projection cache might contain inference variables
1585 /// that will be resolved by obligations on the projection cache entry - e.g.
1586 /// when a type parameter in the associated type is constrained through
1587 /// an "RFC 447" projection on the impl.
1589 /// When working with a fulfillment context, the derived obligations of each
1590 /// projection cache entry will be registered on the fulfillcx, so any users
1591 /// that can wait for a fulfillcx fixed point need not care about this. However,
1592 /// users that don't wait for a fixed point (e.g., trait evaluation) have to
1593 /// resolve the obligations themselves to make sure the projected result is
1594 /// ok and avoid issues like #43132.
1596 /// If that is done, after evaluation the obligations, it is a good idea to
1597 /// call `ProjectionCache::complete` to make sure the obligations won't be
1598 /// re-evaluated and avoid an exponential worst-case.
1600 /// FIXME: we probably also want some sort of cross-infcx cache here to
1601 /// reduce the amount of duplication. Let's see what we get with the Chalk
1604 pub struct ProjectionCache<'tcx> {
1605 map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
1608 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
1609 pub struct ProjectionCacheKey<'tcx> {
1610 ty: ty::ProjectionTy<'tcx>
1613 impl<'cx, 'gcx, 'tcx> ProjectionCacheKey<'tcx> {
1614 pub fn from_poly_projection_predicate(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1615 predicate: &ty::PolyProjectionPredicate<'tcx>)
1618 let infcx = selcx.infcx();
1619 // We don't do cross-snapshot caching of obligations with escaping regions,
1620 // so there's no cache key to use
1621 predicate.no_bound_vars()
1622 .map(|predicate| ProjectionCacheKey {
1623 // We don't attempt to match up with a specific type-variable state
1624 // from a specific call to `opt_normalize_projection_type` - if
1625 // there's no precise match, the original cache entry is "stranded"
1627 ty: infcx.resolve_type_vars_if_possible(&predicate.projection_ty)
1632 #[derive(Clone, Debug)]
1633 enum ProjectionCacheEntry<'tcx> {
1637 NormalizedTy(NormalizedTy<'tcx>),
1640 // N.B., intentionally not Clone
1641 pub struct ProjectionCacheSnapshot {
1645 impl<'tcx> ProjectionCache<'tcx> {
1646 pub fn clear(&mut self) {
1650 pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
1651 ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
1654 pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
1655 self.map.rollback_to(snapshot.snapshot);
1658 pub fn rollback_placeholder(&mut self, snapshot: &ProjectionCacheSnapshot) {
1659 self.map.partial_rollback(&snapshot.snapshot, &|k| k.ty.has_re_placeholders());
1662 pub fn commit(&mut self, snapshot: ProjectionCacheSnapshot) {
1663 self.map.commit(snapshot.snapshot);
1666 /// Try to start normalize `key`; returns an error if
1667 /// normalization already occurred (this error corresponds to a
1668 /// cache hit, so it's actually a good thing).
1669 fn try_start(&mut self, key: ProjectionCacheKey<'tcx>)
1670 -> Result<(), ProjectionCacheEntry<'tcx>> {
1671 if let Some(entry) = self.map.get(&key) {
1672 return Err(entry.clone());
1675 self.map.insert(key, ProjectionCacheEntry::InProgress);
1679 /// Indicates that `key` was normalized to `value`.
1680 fn insert_ty(&mut self, key: ProjectionCacheKey<'tcx>, value: NormalizedTy<'tcx>) {
1681 debug!("ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
1683 let fresh_key = self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
1684 assert!(!fresh_key, "never started projecting `{:?}`", key);
1687 /// Mark the relevant projection cache key as having its derived obligations
1688 /// complete, so they won't have to be re-computed (this is OK to do in a
1689 /// snapshot - if the snapshot is rolled back, the obligations will be
1690 /// marked as incomplete again).
1691 pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
1692 let ty = match self.map.get(&key) {
1693 Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
1694 debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}",
1699 // Type inference could "strand behind" old cache entries. Leave
1700 // them alone for now.
1701 debug!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}",
1707 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1713 /// A specialized version of `complete` for when the key's value is known
1714 /// to be a NormalizedTy.
1715 pub fn complete_normalized(&mut self, key: ProjectionCacheKey<'tcx>, ty: &NormalizedTy<'tcx>) {
1716 // We want to insert `ty` with no obligations. If the existing value
1717 // already has no obligations (as is common) we don't insert anything.
1718 if !ty.obligations.is_empty() {
1719 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1726 /// Indicates that trying to normalize `key` resulted in
1727 /// ambiguity. No point in trying it again then until we gain more
1728 /// type information (in which case, the "fully resolved" key will
1730 fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
1731 let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
1732 assert!(!fresh, "never started projecting `{:?}`", key);
1735 /// Indicates that trying to normalize `key` resulted in
1737 fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
1738 let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
1739 assert!(!fresh, "never started projecting `{:?}`", key);