1 //! Code for projecting associated types out of trait references.
3 use super::elaborate_predicates;
4 use super::specialization_graph;
5 use super::translate_substs;
7 use super::ObligationCause;
8 use super::PredicateObligation;
10 use super::SelectionContext;
11 use super::SelectionError;
12 use super::{VtableImplData, VtableClosureData, VtableGeneratorData, VtableFnPointerData};
15 use crate::hir::def_id::DefId;
16 use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
17 use crate::infer::type_variable::TypeVariableOrigin;
18 use crate::mir::interpret::{GlobalId, ConstValue};
19 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
20 use rustc_macros::HashStable;
21 use syntax::ast::Ident;
22 use crate::ty::subst::{Subst, InternalSubsts};
23 use crate::ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt};
24 use crate::ty::fold::{TypeFoldable, TypeFolder};
25 use crate::util::common::FN_OUTPUT_NAME;
27 /// Depending on the stage of compilation, we want projection to be
28 /// more or less conservative.
29 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, HashStable)]
31 /// At type-checking time, we refuse to project any associated
32 /// type that is marked `default`. Non-`default` ("final") types
33 /// are always projected. This is necessary in general for
34 /// soundness of specialization. However, we *could* allow
35 /// projections in fully-monomorphic cases. We choose not to,
36 /// because we prefer for `default type` to force the type
37 /// definition to be treated abstractly by any consumers of the
38 /// impl. Concretely, that means that the following example will
46 /// impl<T> Assoc for T {
47 /// default type Output = bool;
51 /// let <() as Assoc>::Output = true;
55 /// At codegen time, all monomorphic projections will succeed.
56 /// Also, `impl Trait` is normalized to the concrete type,
57 /// which has to be already collected by type-checking.
59 /// NOTE: as `impl Trait`'s concrete type should *never*
60 /// be observable directly by the user, `Reveal::All`
61 /// should not be used by checks which may expose
62 /// type equality or type contents to the user.
63 /// There are some exceptions, e.g., around OIBITS and
64 /// transmute-checking, which expose some details, but
65 /// not the whole concrete type of the `impl Trait`.
69 pub type PolyProjectionObligation<'tcx> =
70 Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
72 pub type ProjectionObligation<'tcx> =
73 Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
75 pub type ProjectionTyObligation<'tcx> =
76 Obligation<'tcx, ty::ProjectionTy<'tcx>>;
78 /// When attempting to resolve `<T as TraitRef>::Name` ...
80 pub enum ProjectionTyError<'tcx> {
81 /// ...we found multiple sources of information and couldn't resolve the ambiguity.
84 /// ...an error occurred matching `T : TraitRef`
85 TraitSelectionError(SelectionError<'tcx>),
89 pub struct MismatchedProjectionTypes<'tcx> {
90 pub err: ty::error::TypeError<'tcx>
93 #[derive(PartialEq, Eq, Debug)]
94 enum ProjectionTyCandidate<'tcx> {
95 // from a where-clause in the env or object type
96 ParamEnv(ty::PolyProjectionPredicate<'tcx>),
98 // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
99 TraitDef(ty::PolyProjectionPredicate<'tcx>),
101 // from a "impl" (or a "pseudo-impl" returned by select)
102 Select(Selection<'tcx>),
105 enum ProjectionTyCandidateSet<'tcx> {
107 Single(ProjectionTyCandidate<'tcx>),
109 Error(SelectionError<'tcx>),
112 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
113 fn mark_ambiguous(&mut self) {
114 *self = ProjectionTyCandidateSet::Ambiguous;
117 fn mark_error(&mut self, err: SelectionError<'tcx>) {
118 *self = ProjectionTyCandidateSet::Error(err);
121 // Returns true if the push was successful, or false if the candidate
122 // was discarded -- this could be because of ambiguity, or because
123 // a higher-priority candidate is already there.
124 fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
125 use self::ProjectionTyCandidateSet::*;
126 use self::ProjectionTyCandidate::*;
128 // This wacky variable is just used to try and
129 // make code readable and avoid confusing paths.
130 // It is assigned a "value" of `()` only on those
131 // paths in which we wish to convert `*self` to
132 // ambiguous (and return false, because the candidate
133 // was not used). On other paths, it is not assigned,
134 // and hence if those paths *could* reach the code that
135 // comes after the match, this fn would not compile.
136 let convert_to_ambiguous;
140 *self = Single(candidate);
145 // Duplicates can happen inside ParamEnv. In the case, we
146 // perform a lazy deduplication.
147 if current == &candidate {
151 // Prefer where-clauses. As in select, if there are multiple
152 // candidates, we prefer where-clause candidates over impls. This
153 // may seem a bit surprising, since impls are the source of
154 // "truth" in some sense, but in fact some of the impls that SEEM
155 // applicable are not, because of nested obligations. Where
156 // clauses are the safer choice. See the comment on
157 // `select::SelectionCandidate` and #21974 for more details.
158 match (current, candidate) {
159 (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
160 (ParamEnv(..), _) => return false,
161 (_, ParamEnv(..)) => unreachable!(),
162 (_, _) => convert_to_ambiguous = (),
166 Ambiguous | Error(..) => {
171 // We only ever get here when we moved from a single candidate
173 let () = convert_to_ambiguous;
179 /// Evaluates constraints of the form:
181 /// for<...> <T as Trait>::U == V
183 /// If successful, this may result in additional obligations. Also returns
184 /// the projection cache key used to track these additional obligations.
185 pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
186 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
187 obligation: &PolyProjectionObligation<'tcx>)
188 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
189 MismatchedProjectionTypes<'tcx>>
191 debug!("poly_project_and_unify_type(obligation={:?})",
194 let infcx = selcx.infcx();
195 infcx.commit_if_ok(|snapshot| {
196 let (placeholder_predicate, placeholder_map) =
197 infcx.replace_bound_vars_with_placeholders(&obligation.predicate);
199 let placeholder_obligation = obligation.with(placeholder_predicate);
200 let result = project_and_unify_type(selcx, &placeholder_obligation)?;
201 infcx.leak_check(false, &placeholder_map, snapshot)
202 .map_err(|err| MismatchedProjectionTypes { err })?;
207 /// Evaluates constraints of the form:
209 /// <T as Trait>::U == V
211 /// If successful, this may result in additional obligations.
212 fn project_and_unify_type<'cx, 'gcx, 'tcx>(
213 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
214 obligation: &ProjectionObligation<'tcx>)
215 -> Result<Option<Vec<PredicateObligation<'tcx>>>,
216 MismatchedProjectionTypes<'tcx>>
218 debug!("project_and_unify_type(obligation={:?})",
221 let mut obligations = vec![];
223 match opt_normalize_projection_type(selcx,
224 obligation.param_env,
225 obligation.predicate.projection_ty,
226 obligation.cause.clone(),
227 obligation.recursion_depth,
230 None => return Ok(None),
233 debug!("project_and_unify_type: normalized_ty={:?} obligations={:?}",
237 let infcx = selcx.infcx();
238 match infcx.at(&obligation.cause, obligation.param_env)
239 .eq(normalized_ty, obligation.predicate.ty) {
240 Ok(InferOk { obligations: inferred_obligations, value: () }) => {
241 obligations.extend(inferred_obligations);
242 Ok(Some(obligations))
245 debug!("project_and_unify_type: equating types encountered error {:?}", err);
246 Err(MismatchedProjectionTypes { err })
251 /// Normalizes any associated type projections in `value`, replacing
252 /// them with a fully resolved type where possible. The return value
253 /// combines the normalized result and any additional obligations that
254 /// were incurred as result.
255 pub fn normalize<'a, 'b, 'gcx, 'tcx, T>(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
256 param_env: ty::ParamEnv<'tcx>,
257 cause: ObligationCause<'tcx>,
259 -> Normalized<'tcx, T>
260 where T : TypeFoldable<'tcx>
262 normalize_with_depth(selcx, param_env, cause, 0, value)
265 /// As `normalize`, but with a custom depth.
266 pub fn normalize_with_depth<'a, 'b, 'gcx, 'tcx, T>(
267 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
268 param_env: ty::ParamEnv<'tcx>,
269 cause: ObligationCause<'tcx>,
272 -> Normalized<'tcx, T>
274 where T : TypeFoldable<'tcx>
276 debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
277 let mut normalizer = AssociatedTypeNormalizer::new(selcx, param_env, cause, depth);
278 let result = normalizer.fold(value);
279 debug!("normalize_with_depth: depth={} result={:?} with {} obligations",
280 depth, result, normalizer.obligations.len());
281 debug!("normalize_with_depth: depth={} obligations={:?}",
282 depth, normalizer.obligations);
285 obligations: normalizer.obligations,
289 struct AssociatedTypeNormalizer<'a, 'b: 'a, 'gcx: 'b+'tcx, 'tcx: 'b> {
290 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
291 param_env: ty::ParamEnv<'tcx>,
292 cause: ObligationCause<'tcx>,
293 obligations: Vec<PredicateObligation<'tcx>>,
297 impl<'a, 'b, 'gcx, 'tcx> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
298 fn new(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
299 param_env: ty::ParamEnv<'tcx>,
300 cause: ObligationCause<'tcx>,
302 -> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx>
304 AssociatedTypeNormalizer {
313 fn fold<T:TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
314 let value = self.selcx.infcx().resolve_type_vars_if_possible(value);
316 if !value.has_projections() {
319 value.fold_with(self)
324 impl<'a, 'b, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
325 fn tcx<'c>(&'c self) -> TyCtxt<'c, 'gcx, 'tcx> {
329 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
330 // We don't want to normalize associated types that occur inside of region
331 // binders, because they may contain bound regions, and we can't cope with that.
335 // for<'a> fn(<T as Foo<&'a>>::A)
337 // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
338 // normalize it when we instantiate those bound regions (which
339 // should occur eventually).
341 let ty = ty.super_fold_with(self);
343 ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => { // (*)
344 // Only normalize `impl Trait` after type-checking, usually in codegen.
345 match self.param_env.reveal {
346 Reveal::UserFacing => ty,
349 let recursion_limit = *self.tcx().sess.recursion_limit.get();
350 if self.depth >= recursion_limit {
351 let obligation = Obligation::with_depth(
357 self.selcx.infcx().report_overflow_error(&obligation, true);
360 let generic_ty = self.tcx().type_of(def_id);
361 let concrete_ty = generic_ty.subst(self.tcx(), substs);
363 let folded_ty = self.fold_ty(concrete_ty);
370 ty::Projection(ref data) if !data.has_escaping_bound_vars() => { // (*)
372 // (*) This is kind of hacky -- we need to be able to
373 // handle normalization within binders because
374 // otherwise we wind up a need to normalize when doing
375 // trait matching (since you can have a trait
376 // obligation like `for<'a> T::B : Fn(&'a int)`), but
377 // we can't normalize with bound regions in scope. So
378 // far now we just ignore binders but only normalize
379 // if all bound regions are gone (and then we still
380 // have to renormalize whenever we instantiate a
381 // binder). It would be better to normalize in a
382 // binding-aware fashion.
384 let normalized_ty = normalize_projection_type(self.selcx,
389 &mut self.obligations);
390 debug!("AssociatedTypeNormalizer: depth={} normalized {:?} to {:?}, \
391 now with {} obligations",
392 self.depth, ty, normalized_ty, self.obligations.len());
400 fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
401 if let ConstValue::Unevaluated(def_id, substs) = constant.val {
402 let tcx = self.selcx.tcx().global_tcx();
403 if let Some(param_env) = self.tcx().lift_to_global(&self.param_env) {
404 if substs.needs_infer() || substs.has_placeholders() {
405 let identity_substs = InternalSubsts::identity_for_item(tcx, def_id);
406 let instance = ty::Instance::resolve(tcx, param_env, def_id, identity_substs);
407 if let Some(instance) = instance {
412 if let Ok(evaluated) = tcx.const_eval(param_env.and(cid)) {
413 let substs = tcx.lift_to_global(&substs).unwrap();
414 let evaluated = tcx.mk_const(evaluated);
415 let evaluated = evaluated.subst(tcx, substs);
420 if let Some(substs) = self.tcx().lift_to_global(&substs) {
421 let instance = ty::Instance::resolve(tcx, param_env, def_id, substs);
422 if let Some(instance) = instance {
427 if let Ok(evaluated) = tcx.const_eval(param_env.and(cid)) {
428 return tcx.mk_const(evaluated);
440 pub struct Normalized<'tcx,T> {
442 pub obligations: Vec<PredicateObligation<'tcx>>,
445 pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
447 impl<'tcx,T> Normalized<'tcx,T> {
448 pub fn with<U>(self, value: U) -> Normalized<'tcx,U> {
449 Normalized { value: value, obligations: self.obligations }
453 /// The guts of `normalize`: normalize a specific projection like `<T
454 /// as Trait>::Item`. The result is always a type (and possibly
455 /// additional obligations). If ambiguity arises, which implies that
456 /// there are unresolved type variables in the projection, we will
457 /// substitute a fresh type variable `$X` and generate a new
458 /// obligation `<T as Trait>::Item == $X` for later.
459 pub fn normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
460 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
461 param_env: ty::ParamEnv<'tcx>,
462 projection_ty: ty::ProjectionTy<'tcx>,
463 cause: ObligationCause<'tcx>,
465 obligations: &mut Vec<PredicateObligation<'tcx>>)
468 opt_normalize_projection_type(selcx, param_env, projection_ty.clone(), cause.clone(), depth,
470 .unwrap_or_else(move || {
471 // if we bottom out in ambiguity, create a type variable
472 // and a deferred predicate to resolve this when more type
473 // information is available.
475 let tcx = selcx.infcx().tcx;
476 let def_id = projection_ty.item_def_id;
477 let ty_var = selcx.infcx().next_ty_var(
478 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
479 let projection = ty::Binder::dummy(ty::ProjectionPredicate {
483 let obligation = Obligation::with_depth(
484 cause, depth + 1, param_env, projection.to_predicate());
485 obligations.push(obligation);
490 /// The guts of `normalize`: normalize a specific projection like `<T
491 /// as Trait>::Item`. The result is always a type (and possibly
492 /// additional obligations). Returns `None` in the case of ambiguity,
493 /// which indicates that there are unbound type variables.
495 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
496 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
497 /// often immediately appended to another obligations vector. So now this
498 /// function takes an obligations vector and appends to it directly, which is
499 /// slightly uglier but avoids the need for an extra short-lived allocation.
500 fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
501 selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
502 param_env: ty::ParamEnv<'tcx>,
503 projection_ty: ty::ProjectionTy<'tcx>,
504 cause: ObligationCause<'tcx>,
506 obligations: &mut Vec<PredicateObligation<'tcx>>)
509 let infcx = selcx.infcx();
511 let projection_ty = infcx.resolve_type_vars_if_possible(&projection_ty);
512 let cache_key = ProjectionCacheKey { ty: projection_ty };
514 debug!("opt_normalize_projection_type(\
515 projection_ty={:?}, \
520 // FIXME(#20304) For now, I am caching here, which is good, but it
521 // means we don't capture the type variables that are created in
522 // the case of ambiguity. Which means we may create a large stream
523 // of such variables. OTOH, if we move the caching up a level, we
524 // would not benefit from caching when proving `T: Trait<U=Foo>`
525 // bounds. It might be the case that we want two distinct caches,
526 // or else another kind of cache entry.
528 let cache_result = infcx.projection_cache.borrow_mut().try_start(cache_key);
531 Err(ProjectionCacheEntry::Ambiguous) => {
532 // If we found ambiguity the last time, that generally
533 // means we will continue to do so until some type in the
534 // key changes (and we know it hasn't, because we just
535 // fully resolved it). One exception though is closure
536 // types, which can transition from having a fixed kind to
537 // no kind with no visible change in the key.
539 // FIXME(#32286) refactor this so that closure type
541 debug!("opt_normalize_projection_type: \
542 found cache entry: ambiguous");
543 if !projection_ty.has_closure_types() {
547 Err(ProjectionCacheEntry::InProgress) => {
548 // If while normalized A::B, we are asked to normalize
549 // A::B, just return A::B itself. This is a conservative
550 // answer, in the sense that A::B *is* clearly equivalent
551 // to A::B, though there may be a better value we can
554 // Under lazy normalization, this can arise when
555 // bootstrapping. That is, imagine an environment with a
556 // where-clause like `A::B == u32`. Now, if we are asked
557 // to normalize `A::B`, we will want to check the
558 // where-clauses in scope. So we will try to unify `A::B`
559 // with `A::B`, which can trigger a recursive
560 // normalization. In that case, I think we will want this code:
563 // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
564 // projection_ty.substs;
565 // return Some(NormalizedTy { value: v, obligations: vec![] });
568 debug!("opt_normalize_projection_type: \
569 found cache entry: in-progress");
571 // But for now, let's classify this as an overflow:
572 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
573 let obligation = Obligation::with_depth(cause,
577 selcx.infcx().report_overflow_error(&obligation, false);
579 Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
580 // This is the hottest path in this function.
582 // If we find the value in the cache, then return it along
583 // with the obligations that went along with it. Note
584 // that, when using a fulfillment context, these
585 // obligations could in principle be ignored: they have
586 // already been registered when the cache entry was
587 // created (and hence the new ones will quickly be
588 // discarded as duplicated). But when doing trait
589 // evaluation this is not the case, and dropping the trait
590 // evaluations can causes ICEs (e.g., #43132).
591 debug!("opt_normalize_projection_type: \
592 found normalized ty `{:?}`",
595 // Once we have inferred everything we need to know, we
596 // can ignore the `obligations` from that point on.
597 if infcx.unresolved_type_vars(&ty.value).is_none() {
598 infcx.projection_cache.borrow_mut().complete_normalized(cache_key, &ty);
599 // No need to extend `obligations`.
601 obligations.extend(ty.obligations);
604 obligations.push(get_paranoid_cache_value_obligation(infcx,
609 return Some(ty.value);
611 Err(ProjectionCacheEntry::Error) => {
612 debug!("opt_normalize_projection_type: \
614 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
615 obligations.extend(result.obligations);
616 return Some(result.value)
620 let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
621 match project_type(selcx, &obligation) {
622 Ok(ProjectedTy::Progress(Progress { ty: projected_ty,
623 obligations: mut projected_obligations })) => {
624 // if projection succeeded, then what we get out of this
625 // is also non-normalized (consider: it was derived from
626 // an impl, where-clause etc) and hence we must
629 debug!("opt_normalize_projection_type: \
632 projected_obligations={:?}",
635 projected_obligations);
637 let result = if projected_ty.has_projections() {
638 let mut normalizer = AssociatedTypeNormalizer::new(selcx,
642 let normalized_ty = normalizer.fold(&projected_ty);
644 debug!("opt_normalize_projection_type: \
645 normalized_ty={:?} depth={}",
649 projected_obligations.extend(normalizer.obligations);
651 value: normalized_ty,
652 obligations: projected_obligations,
657 obligations: projected_obligations,
661 let cache_value = prune_cache_value_obligations(infcx, &result);
662 infcx.projection_cache.borrow_mut().insert_ty(cache_key, cache_value);
663 obligations.extend(result.obligations);
666 Ok(ProjectedTy::NoProgress(projected_ty)) => {
667 debug!("opt_normalize_projection_type: \
668 projected_ty={:?} no progress",
670 let result = Normalized {
674 infcx.projection_cache.borrow_mut().insert_ty(cache_key, result.clone());
675 // No need to extend `obligations`.
678 Err(ProjectionTyError::TooManyCandidates) => {
679 debug!("opt_normalize_projection_type: \
680 too many candidates");
681 infcx.projection_cache.borrow_mut()
682 .ambiguous(cache_key);
685 Err(ProjectionTyError::TraitSelectionError(_)) => {
686 debug!("opt_normalize_projection_type: ERROR");
687 // if we got an error processing the `T as Trait` part,
688 // just return `ty::err` but add the obligation `T :
689 // Trait`, which when processed will cause the error to be
692 infcx.projection_cache.borrow_mut()
694 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
695 obligations.extend(result.obligations);
701 /// If there are unresolved type variables, then we need to include
702 /// any subobligations that bind them, at least until those type
703 /// variables are fully resolved.
704 fn prune_cache_value_obligations<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
705 result: &NormalizedTy<'tcx>)
706 -> NormalizedTy<'tcx> {
707 if infcx.unresolved_type_vars(&result.value).is_none() {
708 return NormalizedTy { value: result.value, obligations: vec![] };
711 let mut obligations: Vec<_> =
714 .filter(|obligation| match obligation.predicate {
715 // We found a `T: Foo<X = U>` predicate, let's check
716 // if `U` references any unresolved type
717 // variables. In principle, we only care if this
718 // projection can help resolve any of the type
719 // variables found in `result.value` -- but we just
720 // check for any type variables here, for fear of
721 // indirect obligations (e.g., we project to `?0`,
722 // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
724 ty::Predicate::Projection(ref data) =>
725 infcx.unresolved_type_vars(&data.ty()).is_some(),
727 // We are only interested in `T: Foo<X = U>` predicates, whre
728 // `U` references one of `unresolved_type_vars`. =)
734 obligations.shrink_to_fit();
736 NormalizedTy { value: result.value, obligations }
739 /// Whenever we give back a cache result for a projection like `<T as
740 /// Trait>::Item ==> X`, we *always* include the obligation to prove
741 /// that `T: Trait` (we may also include some other obligations). This
742 /// may or may not be necessary -- in principle, all the obligations
743 /// that must be proven to show that `T: Trait` were also returned
744 /// when the cache was first populated. But there are some vague concerns,
745 /// and so we take the precautionary measure of including `T: Trait` in
748 /// Concern #1. The current setup is fragile. Perhaps someone could
749 /// have failed to prove the concerns from when the cache was
750 /// populated, but also not have used a snapshot, in which case the
751 /// cache could remain populated even though `T: Trait` has not been
752 /// shown. In this case, the "other code" is at fault -- when you
753 /// project something, you are supposed to either have a snapshot or
754 /// else prove all the resulting obligations -- but it's still easy to
757 /// Concern #2. Even within the snapshot, if those original
758 /// obligations are not yet proven, then we are able to do projections
759 /// that may yet turn out to be wrong. This *may* lead to some sort
760 /// of trouble, though we don't have a concrete example of how that
761 /// can occur yet. But it seems risky at best.
762 fn get_paranoid_cache_value_obligation<'a, 'gcx, 'tcx>(
763 infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
764 param_env: ty::ParamEnv<'tcx>,
765 projection_ty: ty::ProjectionTy<'tcx>,
766 cause: ObligationCause<'tcx>,
768 -> PredicateObligation<'tcx>
770 let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
773 recursion_depth: depth,
775 predicate: trait_ref.to_predicate(),
779 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
780 /// hold. In various error cases, we cannot generate a valid
781 /// normalized projection. Therefore, we create an inference variable
782 /// return an associated obligation that, when fulfilled, will lead to
785 /// Note that we used to return `Error` here, but that was quite
786 /// dubious -- the premise was that an error would *eventually* be
787 /// reported, when the obligation was processed. But in general once
788 /// you see a `Error` you are supposed to be able to assume that an
789 /// error *has been* reported, so that you can take whatever heuristic
790 /// paths you want to take. To make things worse, it was possible for
791 /// cycles to arise, where you basically had a setup like `<MyType<$0>
792 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
793 /// Trait>::Foo> to `[type error]` would lead to an obligation of
794 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
795 /// an error for this obligation, but we legitimately should not,
796 /// because it contains `[type error]`. Yuck! (See issue #29857 for
797 /// one case where this arose.)
798 fn normalize_to_error<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
799 param_env: ty::ParamEnv<'tcx>,
800 projection_ty: ty::ProjectionTy<'tcx>,
801 cause: ObligationCause<'tcx>,
803 -> NormalizedTy<'tcx>
805 let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
806 let trait_obligation = Obligation { cause,
807 recursion_depth: depth,
809 predicate: trait_ref.to_predicate() };
810 let tcx = selcx.infcx().tcx;
811 let def_id = projection_ty.item_def_id;
812 let new_value = selcx.infcx().next_ty_var(
813 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
816 obligations: vec![trait_obligation]
820 enum ProjectedTy<'tcx> {
821 Progress(Progress<'tcx>),
822 NoProgress(Ty<'tcx>),
825 struct Progress<'tcx> {
827 obligations: Vec<PredicateObligation<'tcx>>,
830 impl<'tcx> Progress<'tcx> {
831 fn error<'a,'gcx>(tcx: TyCtxt<'a,'gcx,'tcx>) -> Self {
838 fn with_addl_obligations(mut self,
839 mut obligations: Vec<PredicateObligation<'tcx>>)
841 debug!("with_addl_obligations: self.obligations.len={} obligations.len={}",
842 self.obligations.len(), obligations.len());
844 debug!("with_addl_obligations: self.obligations={:?} obligations={:?}",
845 self.obligations, obligations);
847 self.obligations.append(&mut obligations);
852 /// Computes the result of a projection type (if we can).
855 /// - `obligation` must be fully normalized
856 fn project_type<'cx, 'gcx, 'tcx>(
857 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
858 obligation: &ProjectionTyObligation<'tcx>)
859 -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>>
861 debug!("project(obligation={:?})",
864 let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
865 if obligation.recursion_depth >= recursion_limit {
866 debug!("project: overflow!");
867 return Err(ProjectionTyError::TraitSelectionError(SelectionError::Overflow));
870 let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
872 debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
874 if obligation_trait_ref.references_error() {
875 return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
878 let mut candidates = ProjectionTyCandidateSet::None;
880 // Make sure that the following procedures are kept in order. ParamEnv
881 // needs to be first because it has highest priority, and Select checks
882 // the return value of push_candidate which assumes it's ran at last.
883 assemble_candidates_from_param_env(selcx,
885 &obligation_trait_ref,
888 assemble_candidates_from_trait_def(selcx,
890 &obligation_trait_ref,
893 assemble_candidates_from_impls(selcx,
895 &obligation_trait_ref,
899 ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
900 confirm_candidate(selcx,
902 &obligation_trait_ref,
904 ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
905 selcx.tcx().mk_projection(
906 obligation.predicate.item_def_id,
907 obligation.predicate.substs))),
908 // Error occurred while trying to processing impls.
909 ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
910 // Inherent ambiguity that prevents us from even enumerating the
912 ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
917 /// The first thing we have to do is scan through the parameter
918 /// environment to see whether there are any projection predicates
919 /// there that can answer this question.
920 fn assemble_candidates_from_param_env<'cx, 'gcx, 'tcx>(
921 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
922 obligation: &ProjectionTyObligation<'tcx>,
923 obligation_trait_ref: &ty::TraitRef<'tcx>,
924 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
926 debug!("assemble_candidates_from_param_env(..)");
927 assemble_candidates_from_predicates(selcx,
929 obligation_trait_ref,
931 ProjectionTyCandidate::ParamEnv,
932 obligation.param_env.caller_bounds.iter().cloned());
935 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
936 /// that the definition of `Foo` has some clues:
940 /// type FooT : Bar<BarT=i32>
944 /// Here, for example, we could conclude that the result is `i32`.
945 fn assemble_candidates_from_trait_def<'cx, 'gcx, 'tcx>(
946 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
947 obligation: &ProjectionTyObligation<'tcx>,
948 obligation_trait_ref: &ty::TraitRef<'tcx>,
949 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
951 debug!("assemble_candidates_from_trait_def(..)");
953 let tcx = selcx.tcx();
954 // Check whether the self-type is itself a projection.
955 let (def_id, substs) = match obligation_trait_ref.self_ty().sty {
956 ty::Projection(ref data) => {
957 (data.trait_ref(tcx).def_id, data.substs)
959 ty::Opaque(def_id, substs) => (def_id, substs),
960 ty::Infer(ty::TyVar(_)) => {
961 // If the self-type is an inference variable, then it MAY wind up
962 // being a projected type, so induce an ambiguity.
963 candidate_set.mark_ambiguous();
969 // If so, extract what we know from the trait and try to come up with a good answer.
970 let trait_predicates = tcx.predicates_of(def_id);
971 let bounds = trait_predicates.instantiate(tcx, substs);
972 let bounds = elaborate_predicates(tcx, bounds.predicates);
973 assemble_candidates_from_predicates(selcx,
975 obligation_trait_ref,
977 ProjectionTyCandidate::TraitDef,
981 fn assemble_candidates_from_predicates<'cx, 'gcx, 'tcx, I>(
982 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
983 obligation: &ProjectionTyObligation<'tcx>,
984 obligation_trait_ref: &ty::TraitRef<'tcx>,
985 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
986 ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
988 where I: IntoIterator<Item=ty::Predicate<'tcx>>
990 debug!("assemble_candidates_from_predicates(obligation={:?})",
992 let infcx = selcx.infcx();
993 for predicate in env_predicates {
994 debug!("assemble_candidates_from_predicates: predicate={:?}",
996 if let ty::Predicate::Projection(data) = predicate {
997 let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
999 let is_match = same_def_id && infcx.probe(|_| {
1000 let data_poly_trait_ref =
1001 data.to_poly_trait_ref(infcx.tcx);
1002 let obligation_poly_trait_ref =
1003 obligation_trait_ref.to_poly_trait_ref();
1004 infcx.at(&obligation.cause, obligation.param_env)
1005 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1006 .map(|InferOk { obligations: _, value: () }| {
1007 // FIXME(#32730) -- do we need to take obligations
1008 // into account in any way? At the moment, no.
1013 debug!("assemble_candidates_from_predicates: candidate={:?} \
1014 is_match={} same_def_id={}",
1015 data, is_match, same_def_id);
1018 candidate_set.push_candidate(ctor(data));
1024 fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
1025 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1026 obligation: &ProjectionTyObligation<'tcx>,
1027 obligation_trait_ref: &ty::TraitRef<'tcx>,
1028 candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
1030 // If we are resolving `<T as TraitRef<...>>::Item == Type`,
1031 // start out by selecting the predicate `T as TraitRef<...>`:
1032 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1033 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1034 let _ = selcx.infcx().commit_if_ok(|_| {
1035 let vtable = match selcx.select(&trait_obligation) {
1036 Ok(Some(vtable)) => vtable,
1038 candidate_set.mark_ambiguous();
1042 debug!("assemble_candidates_from_impls: selection error {:?}", e);
1043 candidate_set.mark_error(e);
1048 let eligible = match &vtable {
1049 super::VtableClosure(_) |
1050 super::VtableGenerator(_) |
1051 super::VtableFnPointer(_) |
1052 super::VtableObject(_) |
1053 super::VtableTraitAlias(_) => {
1054 debug!("assemble_candidates_from_impls: vtable={:?}",
1058 super::VtableImpl(impl_data) => {
1059 // We have to be careful when projecting out of an
1060 // impl because of specialization. If we are not in
1061 // codegen (i.e., projection mode is not "any"), and the
1062 // impl's type is declared as default, then we disable
1063 // projection (even if the trait ref is fully
1064 // monomorphic). In the case where trait ref is not
1065 // fully monomorphic (i.e., includes type parameters),
1066 // this is because those type parameters may
1067 // ultimately be bound to types from other crates that
1068 // may have specialized impls we can't see. In the
1069 // case where the trait ref IS fully monomorphic, this
1070 // is a policy decision that we made in the RFC in
1071 // order to preserve flexibility for the crate that
1072 // defined the specializable impl to specialize later
1073 // for existing types.
1075 // In either case, we handle this by not adding a
1076 // candidate for an impl if it contains a `default`
1078 let node_item = assoc_ty_def(selcx,
1079 impl_data.impl_def_id,
1080 obligation.predicate.item_def_id);
1082 let is_default = if node_item.node.is_from_trait() {
1083 // If true, the impl inherited a `type Foo = Bar`
1084 // given in the trait, which is implicitly default.
1085 // Otherwise, the impl did not specify `type` and
1086 // neither did the trait:
1089 // trait Foo { type T; }
1090 // impl Foo for Bar { }
1093 // This is an error, but it will be
1094 // reported in `check_impl_items_against_trait`.
1095 // We accept it here but will flag it as
1096 // an error when we confirm the candidate
1097 // (which will ultimately lead to `normalize_to_error`
1099 node_item.item.defaultness.has_value()
1101 node_item.item.defaultness.is_default() ||
1102 selcx.tcx().impl_is_default(node_item.node.def_id())
1105 // Only reveal a specializable default if we're past type-checking
1106 // and the obligations is monomorphic, otherwise passes such as
1107 // transmute checking and polymorphic MIR optimizations could
1108 // get a result which isn't correct for all monomorphizations.
1111 } else if obligation.param_env.reveal == Reveal::All {
1112 debug_assert!(!poly_trait_ref.needs_infer());
1113 if !poly_trait_ref.needs_subst() {
1122 super::VtableParam(..) => {
1123 // This case tell us nothing about the value of an
1124 // associated type. Consider:
1127 // trait SomeTrait { type Foo; }
1128 // fn foo<T:SomeTrait>(...) { }
1131 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1132 // : SomeTrait` binding does not help us decide what the
1133 // type `Foo` is (at least, not more specifically than
1134 // what we already knew).
1136 // But wait, you say! What about an example like this:
1139 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1142 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1143 // resolve `T::Foo`? And of course it does, but in fact
1144 // that single predicate is desugared into two predicates
1145 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1146 // projection. And the projection where clause is handled
1147 // in `assemble_candidates_from_param_env`.
1150 super::VtableAutoImpl(..) |
1151 super::VtableBuiltin(..) => {
1152 // These traits have no associated types.
1154 obligation.cause.span,
1155 "Cannot project an associated type from `{:?}`",
1161 if candidate_set.push_candidate(ProjectionTyCandidate::Select(vtable)) {
1172 fn confirm_candidate<'cx, 'gcx, 'tcx>(
1173 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1174 obligation: &ProjectionTyObligation<'tcx>,
1175 obligation_trait_ref: &ty::TraitRef<'tcx>,
1176 candidate: ProjectionTyCandidate<'tcx>)
1179 debug!("confirm_candidate(candidate={:?}, obligation={:?})",
1184 ProjectionTyCandidate::ParamEnv(poly_projection) |
1185 ProjectionTyCandidate::TraitDef(poly_projection) => {
1186 confirm_param_env_candidate(selcx, obligation, poly_projection)
1189 ProjectionTyCandidate::Select(vtable) => {
1190 confirm_select_candidate(selcx, obligation, obligation_trait_ref, vtable)
1195 fn confirm_select_candidate<'cx, 'gcx, 'tcx>(
1196 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1197 obligation: &ProjectionTyObligation<'tcx>,
1198 obligation_trait_ref: &ty::TraitRef<'tcx>,
1199 vtable: Selection<'tcx>)
1203 super::VtableImpl(data) =>
1204 confirm_impl_candidate(selcx, obligation, data),
1205 super::VtableGenerator(data) =>
1206 confirm_generator_candidate(selcx, obligation, data),
1207 super::VtableClosure(data) =>
1208 confirm_closure_candidate(selcx, obligation, data),
1209 super::VtableFnPointer(data) =>
1210 confirm_fn_pointer_candidate(selcx, obligation, data),
1211 super::VtableObject(_) =>
1212 confirm_object_candidate(selcx, obligation, obligation_trait_ref),
1213 super::VtableAutoImpl(..) |
1214 super::VtableParam(..) |
1215 super::VtableBuiltin(..) |
1216 super::VtableTraitAlias(..) =>
1217 // we don't create Select candidates with this kind of resolution
1219 obligation.cause.span,
1220 "Cannot project an associated type from `{:?}`",
1225 fn confirm_object_candidate<'cx, 'gcx, 'tcx>(
1226 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1227 obligation: &ProjectionTyObligation<'tcx>,
1228 obligation_trait_ref: &ty::TraitRef<'tcx>)
1231 let self_ty = obligation_trait_ref.self_ty();
1232 let object_ty = selcx.infcx().shallow_resolve(self_ty);
1233 debug!("confirm_object_candidate(object_ty={:?})",
1235 let data = match object_ty.sty {
1236 ty::Dynamic(ref data, ..) => data,
1239 obligation.cause.span,
1240 "confirm_object_candidate called with non-object: {:?}",
1244 let env_predicates = data.projection_bounds().map(|p| {
1245 p.with_self_ty(selcx.tcx(), object_ty).to_predicate()
1247 let env_predicate = {
1248 let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1250 // select only those projections that are actually projecting an
1251 // item with the correct name
1252 let env_predicates = env_predicates.filter_map(|p| match p {
1253 ty::Predicate::Projection(data) =>
1254 if data.projection_def_id() == obligation.predicate.item_def_id {
1262 // select those with a relevant trait-ref
1263 let mut env_predicates = env_predicates.filter(|data| {
1264 let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1265 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1266 selcx.infcx().probe(|_|
1267 selcx.infcx().at(&obligation.cause, obligation.param_env)
1268 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1273 // select the first matching one; there really ought to be one or
1274 // else the object type is not WF, since an object type should
1275 // include all of its projections explicitly
1276 match env_predicates.next() {
1277 Some(env_predicate) => env_predicate,
1279 debug!("confirm_object_candidate: no env-predicate \
1280 found in object type `{:?}`; ill-formed",
1282 return Progress::error(selcx.tcx());
1287 confirm_param_env_candidate(selcx, obligation, env_predicate)
1290 fn confirm_generator_candidate<'cx, 'gcx, 'tcx>(
1291 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1292 obligation: &ProjectionTyObligation<'tcx>,
1293 vtable: VtableGeneratorData<'tcx, PredicateObligation<'tcx>>)
1296 let gen_sig = vtable.substs.poly_sig(vtable.generator_def_id, selcx.tcx());
1300 } = normalize_with_depth(selcx,
1301 obligation.param_env,
1302 obligation.cause.clone(),
1303 obligation.recursion_depth+1,
1306 debug!("confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1311 let tcx = selcx.tcx();
1313 let gen_def_id = tcx.lang_items().gen_trait().unwrap();
1316 tcx.generator_trait_ref_and_outputs(gen_def_id,
1317 obligation.predicate.self_ty(),
1319 .map_bound(|(trait_ref, yield_ty, return_ty)| {
1320 let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1321 let ty = if name == "Return" {
1323 } else if name == "Yield" {
1329 ty::ProjectionPredicate {
1330 projection_ty: ty::ProjectionTy {
1331 substs: trait_ref.substs,
1332 item_def_id: obligation.predicate.item_def_id,
1338 confirm_param_env_candidate(selcx, obligation, predicate)
1339 .with_addl_obligations(vtable.nested)
1340 .with_addl_obligations(obligations)
1343 fn confirm_fn_pointer_candidate<'cx, 'gcx, 'tcx>(
1344 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1345 obligation: &ProjectionTyObligation<'tcx>,
1346 fn_pointer_vtable: VtableFnPointerData<'tcx, PredicateObligation<'tcx>>)
1349 let fn_type = selcx.infcx().shallow_resolve(fn_pointer_vtable.fn_ty);
1350 let sig = fn_type.fn_sig(selcx.tcx());
1354 } = normalize_with_depth(selcx,
1355 obligation.param_env,
1356 obligation.cause.clone(),
1357 obligation.recursion_depth+1,
1360 confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1361 .with_addl_obligations(fn_pointer_vtable.nested)
1362 .with_addl_obligations(obligations)
1365 fn confirm_closure_candidate<'cx, 'gcx, 'tcx>(
1366 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1367 obligation: &ProjectionTyObligation<'tcx>,
1368 vtable: VtableClosureData<'tcx, PredicateObligation<'tcx>>)
1371 let tcx = selcx.tcx();
1372 let infcx = selcx.infcx();
1373 let closure_sig_ty = vtable.substs.closure_sig_ty(vtable.closure_def_id, tcx);
1374 let closure_sig = infcx.shallow_resolve(closure_sig_ty).fn_sig(tcx);
1378 } = normalize_with_depth(selcx,
1379 obligation.param_env,
1380 obligation.cause.clone(),
1381 obligation.recursion_depth+1,
1384 debug!("confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1389 confirm_callable_candidate(selcx,
1392 util::TupleArgumentsFlag::No)
1393 .with_addl_obligations(vtable.nested)
1394 .with_addl_obligations(obligations)
1397 fn confirm_callable_candidate<'cx, 'gcx, 'tcx>(
1398 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1399 obligation: &ProjectionTyObligation<'tcx>,
1400 fn_sig: ty::PolyFnSig<'tcx>,
1401 flag: util::TupleArgumentsFlag)
1404 let tcx = selcx.tcx();
1406 debug!("confirm_callable_candidate({:?},{:?})",
1410 // the `Output` associated type is declared on `FnOnce`
1411 let fn_once_def_id = tcx.lang_items().fn_once_trait().unwrap();
1414 tcx.closure_trait_ref_and_return_type(fn_once_def_id,
1415 obligation.predicate.self_ty(),
1418 .map_bound(|(trait_ref, ret_type)|
1419 ty::ProjectionPredicate {
1420 projection_ty: ty::ProjectionTy::from_ref_and_name(
1423 Ident::from_str(FN_OUTPUT_NAME),
1429 confirm_param_env_candidate(selcx, obligation, predicate)
1432 fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
1433 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1434 obligation: &ProjectionTyObligation<'tcx>,
1435 poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
1436 ) -> Progress<'tcx> {
1437 let infcx = selcx.infcx();
1438 let cause = &obligation.cause;
1439 let param_env = obligation.param_env;
1441 let (cache_entry, _) =
1442 infcx.replace_bound_vars_with_fresh_vars(
1444 LateBoundRegionConversionTime::HigherRankedType,
1447 let cache_trait_ref = cache_entry.projection_ty.trait_ref(infcx.tcx);
1448 let obligation_trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1449 match infcx.at(cause, param_env).eq(cache_trait_ref, obligation_trait_ref) {
1450 Ok(InferOk { value: _, obligations }) => {
1458 obligation.cause.span,
1459 "Failed to unify obligation `{:?}` \
1460 with poly_projection `{:?}`: {:?}",
1468 fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
1469 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1470 obligation: &ProjectionTyObligation<'tcx>,
1471 impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>)
1474 let VtableImplData { impl_def_id, substs, nested } = impl_vtable;
1476 let tcx = selcx.tcx();
1477 let param_env = obligation.param_env;
1478 let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_def_id);
1480 if !assoc_ty.item.defaultness.has_value() {
1481 // This means that the impl is missing a definition for the
1482 // associated type. This error will be reported by the type
1483 // checker method `check_impl_items_against_trait`, so here we
1484 // just return Error.
1485 debug!("confirm_impl_candidate: no associated type {:?} for {:?}",
1486 assoc_ty.item.ident,
1487 obligation.predicate);
1490 obligations: nested,
1493 let substs = translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.node);
1494 let ty = if let ty::AssociatedKind::Existential = assoc_ty.item.kind {
1495 let item_substs = InternalSubsts::identity_for_item(tcx, assoc_ty.item.def_id);
1496 tcx.mk_opaque(assoc_ty.item.def_id, item_substs)
1498 tcx.type_of(assoc_ty.item.def_id)
1501 ty: ty.subst(tcx, substs),
1502 obligations: nested,
1506 /// Locate the definition of an associated type in the specialization hierarchy,
1507 /// starting from the given impl.
1509 /// Based on the "projection mode", this lookup may in fact only examine the
1510 /// topmost impl. See the comments for `Reveal` for more details.
1511 fn assoc_ty_def<'cx, 'gcx, 'tcx>(
1512 selcx: &SelectionContext<'cx, 'gcx, 'tcx>,
1514 assoc_ty_def_id: DefId)
1515 -> specialization_graph::NodeItem<ty::AssociatedItem>
1517 let tcx = selcx.tcx();
1518 let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1519 let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1520 let trait_def = tcx.trait_def(trait_def_id);
1522 // This function may be called while we are still building the
1523 // specialization graph that is queried below (via TraidDef::ancestors()),
1524 // so, in order to avoid unnecessary infinite recursion, we manually look
1525 // for the associated item at the given impl.
1526 // If there is no such item in that impl, this function will fail with a
1527 // cycle error if the specialization graph is currently being built.
1528 let impl_node = specialization_graph::Node::Impl(impl_def_id);
1529 for item in impl_node.items(tcx) {
1530 if item.kind == ty::AssociatedKind::Type &&
1531 tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id) {
1532 return specialization_graph::NodeItem {
1533 node: specialization_graph::Node::Impl(impl_def_id),
1539 if let Some(assoc_item) = trait_def
1540 .ancestors(tcx, impl_def_id)
1541 .defs(tcx, assoc_ty_name, ty::AssociatedKind::Type, trait_def_id)
1545 // This is saying that neither the trait nor
1546 // the impl contain a definition for this
1547 // associated type. Normally this situation
1548 // could only arise through a compiler bug --
1549 // if the user wrote a bad item name, it
1550 // should have failed in astconv.
1551 bug!("No associated type `{}` for {}",
1553 tcx.def_path_str(impl_def_id))
1559 /// The projection cache. Unlike the standard caches, this can include
1560 /// infcx-dependent type variables, therefore we have to roll the
1561 /// cache back each time we roll a snapshot back, to avoid assumptions
1562 /// on yet-unresolved inference variables. Types with placeholder
1563 /// regions also have to be removed when the respective snapshot ends.
1565 /// Because of that, projection cache entries can be "stranded" and left
1566 /// inaccessible when type variables inside the key are resolved. We make no
1567 /// attempt to recover or remove "stranded" entries, but rather let them be
1568 /// (for the lifetime of the infcx).
1570 /// Entries in the projection cache might contain inference variables
1571 /// that will be resolved by obligations on the projection cache entry (e.g.,
1572 /// when a type parameter in the associated type is constrained through
1573 /// an "RFC 447" projection on the impl).
1575 /// When working with a fulfillment context, the derived obligations of each
1576 /// projection cache entry will be registered on the fulfillcx, so any users
1577 /// that can wait for a fulfillcx fixed point need not care about this. However,
1578 /// users that don't wait for a fixed point (e.g., trait evaluation) have to
1579 /// resolve the obligations themselves to make sure the projected result is
1580 /// ok and avoid issues like #43132.
1582 /// If that is done, after evaluation the obligations, it is a good idea to
1583 /// call `ProjectionCache::complete` to make sure the obligations won't be
1584 /// re-evaluated and avoid an exponential worst-case.
1586 // FIXME: we probably also want some sort of cross-infcx cache here to
1587 // reduce the amount of duplication. Let's see what we get with the Chalk reforms.
1589 pub struct ProjectionCache<'tcx> {
1590 map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
1593 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
1594 pub struct ProjectionCacheKey<'tcx> {
1595 ty: ty::ProjectionTy<'tcx>
1598 impl<'cx, 'gcx, 'tcx> ProjectionCacheKey<'tcx> {
1599 pub fn from_poly_projection_predicate(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1600 predicate: &ty::PolyProjectionPredicate<'tcx>)
1603 let infcx = selcx.infcx();
1604 // We don't do cross-snapshot caching of obligations with escaping regions,
1605 // so there's no cache key to use
1606 predicate.no_bound_vars()
1607 .map(|predicate| ProjectionCacheKey {
1608 // We don't attempt to match up with a specific type-variable state
1609 // from a specific call to `opt_normalize_projection_type` - if
1610 // there's no precise match, the original cache entry is "stranded"
1612 ty: infcx.resolve_type_vars_if_possible(&predicate.projection_ty)
1617 #[derive(Clone, Debug)]
1618 enum ProjectionCacheEntry<'tcx> {
1622 NormalizedTy(NormalizedTy<'tcx>),
1625 // N.B., intentionally not Clone
1626 pub struct ProjectionCacheSnapshot {
1630 impl<'tcx> ProjectionCache<'tcx> {
1631 pub fn clear(&mut self) {
1635 pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
1636 ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
1639 pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
1640 self.map.rollback_to(snapshot.snapshot);
1643 pub fn rollback_placeholder(&mut self, snapshot: &ProjectionCacheSnapshot) {
1644 self.map.partial_rollback(&snapshot.snapshot, &|k| k.ty.has_re_placeholders());
1647 pub fn commit(&mut self, snapshot: ProjectionCacheSnapshot) {
1648 self.map.commit(snapshot.snapshot);
1651 /// Try to start normalize `key`; returns an error if
1652 /// normalization already occurred (this error corresponds to a
1653 /// cache hit, so it's actually a good thing).
1654 fn try_start(&mut self, key: ProjectionCacheKey<'tcx>)
1655 -> Result<(), ProjectionCacheEntry<'tcx>> {
1656 if let Some(entry) = self.map.get(&key) {
1657 return Err(entry.clone());
1660 self.map.insert(key, ProjectionCacheEntry::InProgress);
1664 /// Indicates that `key` was normalized to `value`.
1665 fn insert_ty(&mut self, key: ProjectionCacheKey<'tcx>, value: NormalizedTy<'tcx>) {
1666 debug!("ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
1668 let fresh_key = self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
1669 assert!(!fresh_key, "never started projecting `{:?}`", key);
1672 /// Mark the relevant projection cache key as having its derived obligations
1673 /// complete, so they won't have to be re-computed (this is OK to do in a
1674 /// snapshot - if the snapshot is rolled back, the obligations will be
1675 /// marked as incomplete again).
1676 pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
1677 let ty = match self.map.get(&key) {
1678 Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
1679 debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}",
1684 // Type inference could "strand behind" old cache entries. Leave
1685 // them alone for now.
1686 debug!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}",
1692 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1698 /// A specialized version of `complete` for when the key's value is known
1699 /// to be a NormalizedTy.
1700 pub fn complete_normalized(&mut self, key: ProjectionCacheKey<'tcx>, ty: &NormalizedTy<'tcx>) {
1701 // We want to insert `ty` with no obligations. If the existing value
1702 // already has no obligations (as is common) we don't insert anything.
1703 if !ty.obligations.is_empty() {
1704 self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1711 /// Indicates that trying to normalize `key` resulted in
1712 /// ambiguity. No point in trying it again then until we gain more
1713 /// type information (in which case, the "fully resolved" key will
1715 fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
1716 let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
1717 assert!(!fresh, "never started projecting `{:?}`", key);
1720 /// Indicates that trying to normalize `key` resulted in
1722 fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
1723 let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
1724 assert!(!fresh, "never started projecting `{:?}`", key);