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::MismatchedProjectionTypes;
9 use super::ObligationCause;
10 use super::PredicateObligation;
12 use super::SelectionContext;
13 use super::SelectionError;
15 ImplSourceClosureData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
16 ImplSourceGeneratorData, ImplSourceUserDefinedData,
18 use super::{Normalized, NormalizedTy, ProjectionCacheEntry, ProjectionCacheKey};
20 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
21 use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
22 use crate::traits::error_reporting::InferCtxtExt;
23 use rustc_data_structures::stack::ensure_sufficient_stack;
24 use rustc_errors::ErrorReported;
25 use rustc_hir::def_id::DefId;
26 use rustc_hir::lang_items::{FnOnceTraitLangItem, GeneratorTraitLangItem};
27 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
28 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
29 use rustc_middle::ty::subst::Subst;
30 use rustc_middle::ty::util::IntTypeExt;
31 use rustc_middle::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, WithConstness};
32 use rustc_span::symbol::{sym, Ident};
33 use rustc_span::DUMMY_SP;
35 pub use rustc_middle::traits::Reveal;
37 pub type PolyProjectionObligation<'tcx> = Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
39 pub type ProjectionObligation<'tcx> = Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
41 pub type ProjectionTyObligation<'tcx> = Obligation<'tcx, ty::ProjectionTy<'tcx>>;
43 /// When attempting to resolve `<T as TraitRef>::Name` ...
45 pub enum ProjectionTyError<'tcx> {
46 /// ...we found multiple sources of information and couldn't resolve the ambiguity.
49 /// ...an error occurred matching `T : TraitRef`
50 TraitSelectionError(SelectionError<'tcx>),
53 #[derive(PartialEq, Eq, Debug)]
54 enum ProjectionTyCandidate<'tcx> {
55 // from a where-clause in the env or object type
56 ParamEnv(ty::PolyProjectionPredicate<'tcx>),
58 // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
59 TraitDef(ty::PolyProjectionPredicate<'tcx>),
61 // from a "impl" (or a "pseudo-impl" returned by select)
62 Select(Selection<'tcx>),
65 enum ProjectionTyCandidateSet<'tcx> {
67 Single(ProjectionTyCandidate<'tcx>),
69 Error(SelectionError<'tcx>),
72 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
73 fn mark_ambiguous(&mut self) {
74 *self = ProjectionTyCandidateSet::Ambiguous;
77 fn mark_error(&mut self, err: SelectionError<'tcx>) {
78 *self = ProjectionTyCandidateSet::Error(err);
81 // Returns true if the push was successful, or false if the candidate
82 // was discarded -- this could be because of ambiguity, or because
83 // a higher-priority candidate is already there.
84 fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
85 use self::ProjectionTyCandidate::*;
86 use self::ProjectionTyCandidateSet::*;
88 // This wacky variable is just used to try and
89 // make code readable and avoid confusing paths.
90 // It is assigned a "value" of `()` only on those
91 // paths in which we wish to convert `*self` to
92 // ambiguous (and return false, because the candidate
93 // was not used). On other paths, it is not assigned,
94 // and hence if those paths *could* reach the code that
95 // comes after the match, this fn would not compile.
96 let convert_to_ambiguous;
100 *self = Single(candidate);
105 // Duplicates can happen inside ParamEnv. In the case, we
106 // perform a lazy deduplication.
107 if current == &candidate {
111 // Prefer where-clauses. As in select, if there are multiple
112 // candidates, we prefer where-clause candidates over impls. This
113 // may seem a bit surprising, since impls are the source of
114 // "truth" in some sense, but in fact some of the impls that SEEM
115 // applicable are not, because of nested obligations. Where
116 // clauses are the safer choice. See the comment on
117 // `select::SelectionCandidate` and #21974 for more details.
118 match (current, candidate) {
119 (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
120 (ParamEnv(..), _) => return false,
121 (_, ParamEnv(..)) => unreachable!(),
122 (_, _) => convert_to_ambiguous = (),
126 Ambiguous | Error(..) => {
131 // We only ever get here when we moved from a single candidate
133 let () = convert_to_ambiguous;
139 /// Evaluates constraints of the form:
141 /// for<...> <T as Trait>::U == V
143 /// If successful, this may result in additional obligations. Also returns
144 /// the projection cache key used to track these additional obligations.
145 pub fn poly_project_and_unify_type<'cx, 'tcx>(
146 selcx: &mut SelectionContext<'cx, 'tcx>,
147 obligation: &PolyProjectionObligation<'tcx>,
148 ) -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>> {
149 debug!("poly_project_and_unify_type(obligation={:?})", obligation);
151 let infcx = selcx.infcx();
152 infcx.commit_if_ok(|snapshot| {
153 let (placeholder_predicate, placeholder_map) =
154 infcx.replace_bound_vars_with_placeholders(&obligation.predicate);
156 let placeholder_obligation = obligation.with(placeholder_predicate);
157 let result = project_and_unify_type(selcx, &placeholder_obligation)?;
159 .leak_check(false, &placeholder_map, snapshot)
160 .map_err(|err| MismatchedProjectionTypes { err })?;
165 /// Evaluates constraints of the form:
167 /// <T as Trait>::U == V
169 /// If successful, this may result in additional obligations.
170 fn project_and_unify_type<'cx, 'tcx>(
171 selcx: &mut SelectionContext<'cx, 'tcx>,
172 obligation: &ProjectionObligation<'tcx>,
173 ) -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>> {
174 debug!("project_and_unify_type(obligation={:?})", obligation);
176 let mut obligations = vec![];
177 let normalized_ty = match opt_normalize_projection_type(
179 obligation.param_env,
180 obligation.predicate.projection_ty,
181 obligation.cause.clone(),
182 obligation.recursion_depth,
186 None => return Ok(None),
190 "project_and_unify_type: normalized_ty={:?} obligations={:?}",
191 normalized_ty, obligations
194 let infcx = selcx.infcx();
196 .at(&obligation.cause, obligation.param_env)
197 .eq(normalized_ty, obligation.predicate.ty)
199 Ok(InferOk { obligations: inferred_obligations, value: () }) => {
200 obligations.extend(inferred_obligations);
201 Ok(Some(obligations))
204 debug!("project_and_unify_type: equating types encountered error {:?}", err);
205 Err(MismatchedProjectionTypes { err })
210 /// Normalizes any associated type projections in `value`, replacing
211 /// them with a fully resolved type where possible. The return value
212 /// combines the normalized result and any additional obligations that
213 /// were incurred as result.
214 pub fn normalize<'a, 'b, 'tcx, T>(
215 selcx: &'a mut SelectionContext<'b, 'tcx>,
216 param_env: ty::ParamEnv<'tcx>,
217 cause: ObligationCause<'tcx>,
219 ) -> Normalized<'tcx, T>
221 T: TypeFoldable<'tcx>,
223 let mut obligations = Vec::new();
224 let value = normalize_to(selcx, param_env, cause, value, &mut obligations);
225 Normalized { value, obligations }
228 pub fn normalize_to<'a, 'b, 'tcx, T>(
229 selcx: &'a mut SelectionContext<'b, 'tcx>,
230 param_env: ty::ParamEnv<'tcx>,
231 cause: ObligationCause<'tcx>,
233 obligations: &mut Vec<PredicateObligation<'tcx>>,
236 T: TypeFoldable<'tcx>,
238 normalize_with_depth_to(selcx, param_env, cause, 0, value, obligations)
241 /// As `normalize`, but with a custom depth.
242 pub fn normalize_with_depth<'a, 'b, 'tcx, T>(
243 selcx: &'a mut SelectionContext<'b, 'tcx>,
244 param_env: ty::ParamEnv<'tcx>,
245 cause: ObligationCause<'tcx>,
248 ) -> Normalized<'tcx, T>
250 T: TypeFoldable<'tcx>,
252 let mut obligations = Vec::new();
253 let value = normalize_with_depth_to(selcx, param_env, cause, depth, value, &mut obligations);
254 Normalized { value, obligations }
257 pub fn normalize_with_depth_to<'a, 'b, 'tcx, T>(
258 selcx: &'a mut SelectionContext<'b, 'tcx>,
259 param_env: ty::ParamEnv<'tcx>,
260 cause: ObligationCause<'tcx>,
263 obligations: &mut Vec<PredicateObligation<'tcx>>,
266 T: TypeFoldable<'tcx>,
268 debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
269 let mut normalizer = AssocTypeNormalizer::new(selcx, param_env, cause, depth, obligations);
270 let result = ensure_sufficient_stack(|| normalizer.fold(value));
272 "normalize_with_depth: depth={} result={:?} with {} obligations",
275 normalizer.obligations.len()
277 debug!("normalize_with_depth: depth={} obligations={:?}", depth, normalizer.obligations);
281 struct AssocTypeNormalizer<'a, 'b, 'tcx> {
282 selcx: &'a mut SelectionContext<'b, 'tcx>,
283 param_env: ty::ParamEnv<'tcx>,
284 cause: ObligationCause<'tcx>,
285 obligations: &'a mut Vec<PredicateObligation<'tcx>>,
289 impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
291 selcx: &'a mut SelectionContext<'b, 'tcx>,
292 param_env: ty::ParamEnv<'tcx>,
293 cause: ObligationCause<'tcx>,
295 obligations: &'a mut Vec<PredicateObligation<'tcx>>,
296 ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
297 AssocTypeNormalizer { selcx, param_env, cause, obligations, depth }
300 fn fold<T: TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
301 let value = self.selcx.infcx().resolve_vars_if_possible(value);
303 if !value.has_projections() { value } else { value.fold_with(self) }
307 impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
308 fn tcx<'c>(&'c self) -> TyCtxt<'tcx> {
312 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
313 if !ty.has_projections() {
316 // We don't want to normalize associated types that occur inside of region
317 // binders, because they may contain bound regions, and we can't cope with that.
321 // for<'a> fn(<T as Foo<&'a>>::A)
323 // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
324 // normalize it when we instantiate those bound regions (which
325 // should occur eventually).
327 let ty = ty.super_fold_with(self);
329 ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => {
331 // Only normalize `impl Trait` after type-checking, usually in codegen.
332 match self.param_env.reveal {
333 Reveal::UserFacing => ty,
336 let recursion_limit = self.tcx().sess.recursion_limit();
337 if !recursion_limit.value_within_limit(self.depth) {
338 let obligation = Obligation::with_depth(
344 self.selcx.infcx().report_overflow_error(&obligation, true);
347 let generic_ty = self.tcx().type_of(def_id);
348 let concrete_ty = generic_ty.subst(self.tcx(), substs);
350 let folded_ty = self.fold_ty(concrete_ty);
357 ty::Projection(ref data) if !data.has_escaping_bound_vars() => {
360 // (*) This is kind of hacky -- we need to be able to
361 // handle normalization within binders because
362 // otherwise we wind up a need to normalize when doing
363 // trait matching (since you can have a trait
364 // obligation like `for<'a> T::B: Fn(&'a i32)`), but
365 // we can't normalize with bound regions in scope. So
366 // far now we just ignore binders but only normalize
367 // if all bound regions are gone (and then we still
368 // have to renormalize whenever we instantiate a
369 // binder). It would be better to normalize in a
370 // binding-aware fashion.
372 let normalized_ty = normalize_projection_type(
378 &mut self.obligations,
381 "AssocTypeNormalizer: depth={} normalized {:?} to {:?}, \
382 now with {} obligations",
386 self.obligations.len()
395 fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
396 if self.selcx.tcx().lazy_normalization() {
399 let constant = constant.super_fold_with(self);
400 constant.eval(self.selcx.tcx(), self.param_env)
405 /// The guts of `normalize`: normalize a specific projection like `<T
406 /// as Trait>::Item`. The result is always a type (and possibly
407 /// additional obligations). If ambiguity arises, which implies that
408 /// there are unresolved type variables in the projection, we will
409 /// substitute a fresh type variable `$X` and generate a new
410 /// obligation `<T as Trait>::Item == $X` for later.
411 pub fn normalize_projection_type<'a, 'b, 'tcx>(
412 selcx: &'a mut SelectionContext<'b, 'tcx>,
413 param_env: ty::ParamEnv<'tcx>,
414 projection_ty: ty::ProjectionTy<'tcx>,
415 cause: ObligationCause<'tcx>,
417 obligations: &mut Vec<PredicateObligation<'tcx>>,
419 opt_normalize_projection_type(
427 .unwrap_or_else(move || {
428 // if we bottom out in ambiguity, create a type variable
429 // and a deferred predicate to resolve this when more type
430 // information is available.
432 let tcx = selcx.infcx().tcx;
433 let def_id = projection_ty.item_def_id;
434 let ty_var = selcx.infcx().next_ty_var(TypeVariableOrigin {
435 kind: TypeVariableOriginKind::NormalizeProjectionType,
436 span: tcx.def_span(def_id),
438 let projection = ty::Binder::dummy(ty::ProjectionPredicate { projection_ty, ty: ty_var });
440 Obligation::with_depth(cause, depth + 1, param_env, projection.to_predicate(tcx));
441 obligations.push(obligation);
446 /// The guts of `normalize`: normalize a specific projection like `<T
447 /// as Trait>::Item`. The result is always a type (and possibly
448 /// additional obligations). Returns `None` in the case of ambiguity,
449 /// which indicates that there are unbound type variables.
451 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
452 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
453 /// often immediately appended to another obligations vector. So now this
454 /// function takes an obligations vector and appends to it directly, which is
455 /// slightly uglier but avoids the need for an extra short-lived allocation.
456 fn opt_normalize_projection_type<'a, 'b, 'tcx>(
457 selcx: &'a mut SelectionContext<'b, 'tcx>,
458 param_env: ty::ParamEnv<'tcx>,
459 projection_ty: ty::ProjectionTy<'tcx>,
460 cause: ObligationCause<'tcx>,
462 obligations: &mut Vec<PredicateObligation<'tcx>>,
463 ) -> Option<Ty<'tcx>> {
464 let infcx = selcx.infcx();
466 let projection_ty = infcx.resolve_vars_if_possible(&projection_ty);
467 let cache_key = ProjectionCacheKey::new(projection_ty);
470 "opt_normalize_projection_type(\
471 projection_ty={:?}, \
476 // FIXME(#20304) For now, I am caching here, which is good, but it
477 // means we don't capture the type variables that are created in
478 // the case of ambiguity. Which means we may create a large stream
479 // of such variables. OTOH, if we move the caching up a level, we
480 // would not benefit from caching when proving `T: Trait<U=Foo>`
481 // bounds. It might be the case that we want two distinct caches,
482 // or else another kind of cache entry.
484 let cache_result = infcx.inner.borrow_mut().projection_cache().try_start(cache_key);
487 Err(ProjectionCacheEntry::Ambiguous) => {
488 // If we found ambiguity the last time, that means we will continue
489 // to do so until some type in the key changes (and we know it
490 // hasn't, because we just fully resolved it).
492 "opt_normalize_projection_type: \
493 found cache entry: ambiguous"
497 Err(ProjectionCacheEntry::InProgress) => {
498 // If while normalized A::B, we are asked to normalize
499 // A::B, just return A::B itself. This is a conservative
500 // answer, in the sense that A::B *is* clearly equivalent
501 // to A::B, though there may be a better value we can
504 // Under lazy normalization, this can arise when
505 // bootstrapping. That is, imagine an environment with a
506 // where-clause like `A::B == u32`. Now, if we are asked
507 // to normalize `A::B`, we will want to check the
508 // where-clauses in scope. So we will try to unify `A::B`
509 // with `A::B`, which can trigger a recursive
510 // normalization. In that case, I think we will want this code:
513 // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
514 // projection_ty.substs;
515 // return Some(NormalizedTy { value: v, obligations: vec![] });
519 "opt_normalize_projection_type: \
520 found cache entry: in-progress"
523 // But for now, let's classify this as an overflow:
524 let recursion_limit = selcx.tcx().sess.recursion_limit();
526 Obligation::with_depth(cause, recursion_limit.0, param_env, projection_ty);
527 selcx.infcx().report_overflow_error(&obligation, false);
529 Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
530 // This is the hottest path in this function.
532 // If we find the value in the cache, then return it along
533 // with the obligations that went along with it. Note
534 // that, when using a fulfillment context, these
535 // obligations could in principle be ignored: they have
536 // already been registered when the cache entry was
537 // created (and hence the new ones will quickly be
538 // discarded as duplicated). But when doing trait
539 // evaluation this is not the case, and dropping the trait
540 // evaluations can causes ICEs (e.g., #43132).
542 "opt_normalize_projection_type: \
543 found normalized ty `{:?}`",
547 // Once we have inferred everything we need to know, we
548 // can ignore the `obligations` from that point on.
549 if infcx.unresolved_type_vars(&ty.value).is_none() {
550 infcx.inner.borrow_mut().projection_cache().complete_normalized(cache_key, &ty);
551 // No need to extend `obligations`.
553 obligations.extend(ty.obligations);
556 obligations.push(get_paranoid_cache_value_obligation(
563 return Some(ty.value);
565 Err(ProjectionCacheEntry::Error) => {
567 "opt_normalize_projection_type: \
570 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
571 obligations.extend(result.obligations);
572 return Some(result.value);
576 let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
577 match project_type(selcx, &obligation) {
578 Ok(ProjectedTy::Progress(Progress {
580 obligations: mut projected_obligations,
582 // if projection succeeded, then what we get out of this
583 // is also non-normalized (consider: it was derived from
584 // an impl, where-clause etc) and hence we must
588 "opt_normalize_projection_type: \
591 projected_obligations={:?}",
592 projected_ty, depth, projected_obligations
595 let result = if projected_ty.has_projections() {
596 let mut normalizer = AssocTypeNormalizer::new(
601 &mut projected_obligations,
603 let normalized_ty = normalizer.fold(&projected_ty);
606 "opt_normalize_projection_type: \
607 normalized_ty={:?} depth={}",
611 Normalized { value: normalized_ty, obligations: projected_obligations }
613 Normalized { value: projected_ty, obligations: projected_obligations }
616 let cache_value = prune_cache_value_obligations(infcx, &result);
617 infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, cache_value);
618 obligations.extend(result.obligations);
621 Ok(ProjectedTy::NoProgress(projected_ty)) => {
623 "opt_normalize_projection_type: \
624 projected_ty={:?} no progress",
627 let result = Normalized { value: projected_ty, obligations: vec![] };
628 infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, result.clone());
629 // No need to extend `obligations`.
632 Err(ProjectionTyError::TooManyCandidates) => {
634 "opt_normalize_projection_type: \
637 infcx.inner.borrow_mut().projection_cache().ambiguous(cache_key);
640 Err(ProjectionTyError::TraitSelectionError(_)) => {
641 debug!("opt_normalize_projection_type: ERROR");
642 // if we got an error processing the `T as Trait` part,
643 // just return `ty::err` but add the obligation `T :
644 // Trait`, which when processed will cause the error to be
647 infcx.inner.borrow_mut().projection_cache().error(cache_key);
648 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
649 obligations.extend(result.obligations);
655 /// If there are unresolved type variables, then we need to include
656 /// any subobligations that bind them, at least until those type
657 /// variables are fully resolved.
658 fn prune_cache_value_obligations<'a, 'tcx>(
659 infcx: &'a InferCtxt<'a, 'tcx>,
660 result: &NormalizedTy<'tcx>,
661 ) -> NormalizedTy<'tcx> {
662 if infcx.unresolved_type_vars(&result.value).is_none() {
663 return NormalizedTy { value: result.value, obligations: vec![] };
666 let mut obligations: Vec<_> = result
669 .filter(|obligation| match obligation.predicate.kind() {
670 // We found a `T: Foo<X = U>` predicate, let's check
671 // if `U` references any unresolved type
672 // variables. In principle, we only care if this
673 // projection can help resolve any of the type
674 // variables found in `result.value` -- but we just
675 // check for any type variables here, for fear of
676 // indirect obligations (e.g., we project to `?0`,
677 // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
679 ty::PredicateKind::Projection(ref data) => {
680 infcx.unresolved_type_vars(&data.ty()).is_some()
683 // We are only interested in `T: Foo<X = U>` predicates, whre
684 // `U` references one of `unresolved_type_vars`. =)
690 obligations.shrink_to_fit();
692 NormalizedTy { value: result.value, obligations }
695 /// Whenever we give back a cache result for a projection like `<T as
696 /// Trait>::Item ==> X`, we *always* include the obligation to prove
697 /// that `T: Trait` (we may also include some other obligations). This
698 /// may or may not be necessary -- in principle, all the obligations
699 /// that must be proven to show that `T: Trait` were also returned
700 /// when the cache was first populated. But there are some vague concerns,
701 /// and so we take the precautionary measure of including `T: Trait` in
704 /// Concern #1. The current setup is fragile. Perhaps someone could
705 /// have failed to prove the concerns from when the cache was
706 /// populated, but also not have used a snapshot, in which case the
707 /// cache could remain populated even though `T: Trait` has not been
708 /// shown. In this case, the "other code" is at fault -- when you
709 /// project something, you are supposed to either have a snapshot or
710 /// else prove all the resulting obligations -- but it's still easy to
713 /// Concern #2. Even within the snapshot, if those original
714 /// obligations are not yet proven, then we are able to do projections
715 /// that may yet turn out to be wrong. This *may* lead to some sort
716 /// of trouble, though we don't have a concrete example of how that
717 /// can occur yet. But it seems risky at best.
718 fn get_paranoid_cache_value_obligation<'a, 'tcx>(
719 infcx: &'a InferCtxt<'a, 'tcx>,
720 param_env: ty::ParamEnv<'tcx>,
721 projection_ty: ty::ProjectionTy<'tcx>,
722 cause: ObligationCause<'tcx>,
724 ) -> PredicateObligation<'tcx> {
725 let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
728 recursion_depth: depth,
730 predicate: trait_ref.without_const().to_predicate(infcx.tcx),
734 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
735 /// hold. In various error cases, we cannot generate a valid
736 /// normalized projection. Therefore, we create an inference variable
737 /// return an associated obligation that, when fulfilled, will lead to
740 /// Note that we used to return `Error` here, but that was quite
741 /// dubious -- the premise was that an error would *eventually* be
742 /// reported, when the obligation was processed. But in general once
743 /// you see a `Error` you are supposed to be able to assume that an
744 /// error *has been* reported, so that you can take whatever heuristic
745 /// paths you want to take. To make things worse, it was possible for
746 /// cycles to arise, where you basically had a setup like `<MyType<$0>
747 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
748 /// Trait>::Foo> to `[type error]` would lead to an obligation of
749 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
750 /// an error for this obligation, but we legitimately should not,
751 /// because it contains `[type error]`. Yuck! (See issue #29857 for
752 /// one case where this arose.)
753 fn normalize_to_error<'a, 'tcx>(
754 selcx: &mut SelectionContext<'a, 'tcx>,
755 param_env: ty::ParamEnv<'tcx>,
756 projection_ty: ty::ProjectionTy<'tcx>,
757 cause: ObligationCause<'tcx>,
759 ) -> NormalizedTy<'tcx> {
760 let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
761 let trait_obligation = Obligation {
763 recursion_depth: depth,
765 predicate: trait_ref.without_const().to_predicate(selcx.tcx()),
767 let tcx = selcx.infcx().tcx;
768 let def_id = projection_ty.item_def_id;
769 let new_value = selcx.infcx().next_ty_var(TypeVariableOrigin {
770 kind: TypeVariableOriginKind::NormalizeProjectionType,
771 span: tcx.def_span(def_id),
773 Normalized { value: new_value, obligations: vec![trait_obligation] }
776 enum ProjectedTy<'tcx> {
777 Progress(Progress<'tcx>),
778 NoProgress(Ty<'tcx>),
781 struct Progress<'tcx> {
783 obligations: Vec<PredicateObligation<'tcx>>,
786 impl<'tcx> Progress<'tcx> {
787 fn error(tcx: TyCtxt<'tcx>) -> Self {
788 Progress { ty: tcx.ty_error(), obligations: vec![] }
791 fn with_addl_obligations(mut self, mut obligations: Vec<PredicateObligation<'tcx>>) -> Self {
793 "with_addl_obligations: self.obligations.len={} obligations.len={}",
794 self.obligations.len(),
799 "with_addl_obligations: self.obligations={:?} obligations={:?}",
800 self.obligations, obligations
803 self.obligations.append(&mut obligations);
808 /// Computes the result of a projection type (if we can).
811 /// - `obligation` must be fully normalized
812 fn project_type<'cx, 'tcx>(
813 selcx: &mut SelectionContext<'cx, 'tcx>,
814 obligation: &ProjectionTyObligation<'tcx>,
815 ) -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>> {
816 debug!("project(obligation={:?})", obligation);
818 if !selcx.tcx().sess.recursion_limit().value_within_limit(obligation.recursion_depth) {
819 debug!("project: overflow!");
820 return Err(ProjectionTyError::TraitSelectionError(SelectionError::Overflow));
823 let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
825 debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
827 if obligation_trait_ref.references_error() {
828 return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
831 let mut candidates = ProjectionTyCandidateSet::None;
833 // Make sure that the following procedures are kept in order. ParamEnv
834 // needs to be first because it has highest priority, and Select checks
835 // the return value of push_candidate which assumes it's ran at last.
836 assemble_candidates_from_param_env(selcx, obligation, &obligation_trait_ref, &mut candidates);
838 assemble_candidates_from_trait_def(selcx, obligation, &obligation_trait_ref, &mut candidates);
840 assemble_candidates_from_impls(selcx, obligation, &obligation_trait_ref, &mut candidates);
843 ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
844 confirm_candidate(selcx, obligation, &obligation_trait_ref, candidate),
846 ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
849 .mk_projection(obligation.predicate.item_def_id, obligation.predicate.substs),
851 // Error occurred while trying to processing impls.
852 ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
853 // Inherent ambiguity that prevents us from even enumerating the
855 ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
859 /// The first thing we have to do is scan through the parameter
860 /// environment to see whether there are any projection predicates
861 /// there that can answer this question.
862 fn assemble_candidates_from_param_env<'cx, 'tcx>(
863 selcx: &mut SelectionContext<'cx, 'tcx>,
864 obligation: &ProjectionTyObligation<'tcx>,
865 obligation_trait_ref: &ty::TraitRef<'tcx>,
866 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
868 debug!("assemble_candidates_from_param_env(..)");
869 assemble_candidates_from_predicates(
872 obligation_trait_ref,
874 ProjectionTyCandidate::ParamEnv,
875 obligation.param_env.caller_bounds.iter(),
879 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
880 /// that the definition of `Foo` has some clues:
884 /// type FooT : Bar<BarT=i32>
888 /// Here, for example, we could conclude that the result is `i32`.
889 fn assemble_candidates_from_trait_def<'cx, 'tcx>(
890 selcx: &mut SelectionContext<'cx, 'tcx>,
891 obligation: &ProjectionTyObligation<'tcx>,
892 obligation_trait_ref: &ty::TraitRef<'tcx>,
893 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
895 debug!("assemble_candidates_from_trait_def(..)");
897 let tcx = selcx.tcx();
898 // Check whether the self-type is itself a projection.
899 // If so, extract what we know from the trait and try to come up with a good answer.
900 let bounds = match obligation_trait_ref.self_ty().kind {
901 ty::Projection(ref data) => {
902 tcx.projection_predicates(data.item_def_id).subst(tcx, data.substs)
904 ty::Opaque(def_id, substs) => tcx.projection_predicates(def_id).subst(tcx, substs),
905 ty::Infer(ty::TyVar(_)) => {
906 // If the self-type is an inference variable, then it MAY wind up
907 // being a projected type, so induce an ambiguity.
908 candidate_set.mark_ambiguous();
914 assemble_candidates_from_predicates(
917 obligation_trait_ref,
919 ProjectionTyCandidate::TraitDef,
924 fn assemble_candidates_from_predicates<'cx, 'tcx>(
925 selcx: &mut SelectionContext<'cx, 'tcx>,
926 obligation: &ProjectionTyObligation<'tcx>,
927 obligation_trait_ref: &ty::TraitRef<'tcx>,
928 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
929 ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
930 env_predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
932 debug!("assemble_candidates_from_predicates(obligation={:?})", obligation);
933 let infcx = selcx.infcx();
934 for predicate in env_predicates {
935 debug!("assemble_candidates_from_predicates: predicate={:?}", predicate);
936 if let &ty::PredicateKind::Projection(data) = predicate.kind() {
937 let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
939 let is_match = same_def_id
941 let data_poly_trait_ref = data.to_poly_trait_ref(infcx.tcx);
942 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
944 .at(&obligation.cause, obligation.param_env)
945 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
946 .map(|InferOk { obligations: _, value: () }| {
947 // FIXME(#32730) -- do we need to take obligations
948 // into account in any way? At the moment, no.
954 "assemble_candidates_from_predicates: candidate={:?} \
955 is_match={} same_def_id={}",
956 data, is_match, same_def_id
960 candidate_set.push_candidate(ctor(data));
966 fn assemble_candidates_from_impls<'cx, 'tcx>(
967 selcx: &mut SelectionContext<'cx, 'tcx>,
968 obligation: &ProjectionTyObligation<'tcx>,
969 obligation_trait_ref: &ty::TraitRef<'tcx>,
970 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
972 // If we are resolving `<T as TraitRef<...>>::Item == Type`,
973 // start out by selecting the predicate `T as TraitRef<...>`:
974 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
975 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
976 let _ = selcx.infcx().commit_if_ok(|_| {
977 let impl_source = match selcx.select(&trait_obligation) {
978 Ok(Some(impl_source)) => impl_source,
980 candidate_set.mark_ambiguous();
984 debug!("assemble_candidates_from_impls: selection error {:?}", e);
985 candidate_set.mark_error(e);
990 let eligible = match &impl_source {
991 super::ImplSourceClosure(_)
992 | super::ImplSourceGenerator(_)
993 | super::ImplSourceFnPointer(_)
994 | super::ImplSourceObject(_)
995 | super::ImplSourceTraitAlias(_) => {
996 debug!("assemble_candidates_from_impls: impl_source={:?}", impl_source);
999 super::ImplSourceUserDefined(impl_data) => {
1000 // We have to be careful when projecting out of an
1001 // impl because of specialization. If we are not in
1002 // codegen (i.e., projection mode is not "any"), and the
1003 // impl's type is declared as default, then we disable
1004 // projection (even if the trait ref is fully
1005 // monomorphic). In the case where trait ref is not
1006 // fully monomorphic (i.e., includes type parameters),
1007 // this is because those type parameters may
1008 // ultimately be bound to types from other crates that
1009 // may have specialized impls we can't see. In the
1010 // case where the trait ref IS fully monomorphic, this
1011 // is a policy decision that we made in the RFC in
1012 // order to preserve flexibility for the crate that
1013 // defined the specializable impl to specialize later
1014 // for existing types.
1016 // In either case, we handle this by not adding a
1017 // candidate for an impl if it contains a `default`
1020 // NOTE: This should be kept in sync with the similar code in
1021 // `rustc_ty::instance::resolve_associated_item()`.
1023 assoc_ty_def(selcx, impl_data.impl_def_id, obligation.predicate.item_def_id)
1024 .map_err(|ErrorReported| ())?;
1026 if node_item.is_final() {
1027 // Non-specializable items are always projectable.
1030 // Only reveal a specializable default if we're past type-checking
1031 // and the obligation is monomorphic, otherwise passes such as
1032 // transmute checking and polymorphic MIR optimizations could
1033 // get a result which isn't correct for all monomorphizations.
1034 if obligation.param_env.reveal == Reveal::All {
1035 // NOTE(eddyb) inference variables can resolve to parameters, so
1036 // assume `poly_trait_ref` isn't monomorphic, if it contains any.
1037 let poly_trait_ref =
1038 selcx.infcx().resolve_vars_if_possible(&poly_trait_ref);
1039 !poly_trait_ref.still_further_specializable()
1042 "assemble_candidates_from_impls: not eligible due to default: \
1043 assoc_ty={} predicate={}",
1044 selcx.tcx().def_path_str(node_item.item.def_id),
1045 obligation.predicate,
1051 super::ImplSourceDiscriminantKind(..) => {
1052 // While `DiscriminantKind` is automatically implemented for every type,
1053 // the concrete discriminant may not be known yet.
1055 // Any type with multiple potential discriminant types is therefore not eligible.
1056 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1058 match self_ty.kind {
1076 | ty::GeneratorWitness(..)
1079 // Integers and floats always have `u8` as their discriminant.
1080 | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
1086 | ty::Placeholder(..)
1088 | ty::Error(_) => false,
1091 super::ImplSourceParam(..) => {
1092 // This case tell us nothing about the value of an
1093 // associated type. Consider:
1096 // trait SomeTrait { type Foo; }
1097 // fn foo<T:SomeTrait>(...) { }
1100 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1101 // : SomeTrait` binding does not help us decide what the
1102 // type `Foo` is (at least, not more specifically than
1103 // what we already knew).
1105 // But wait, you say! What about an example like this:
1108 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1111 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1112 // resolve `T::Foo`? And of course it does, but in fact
1113 // that single predicate is desugared into two predicates
1114 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1115 // projection. And the projection where clause is handled
1116 // in `assemble_candidates_from_param_env`.
1119 super::ImplSourceAutoImpl(..) | super::ImplSourceBuiltin(..) => {
1120 // These traits have no associated types.
1122 obligation.cause.span,
1123 "Cannot project an associated type from `{:?}`",
1130 if candidate_set.push_candidate(ProjectionTyCandidate::Select(impl_source)) {
1141 fn confirm_candidate<'cx, 'tcx>(
1142 selcx: &mut SelectionContext<'cx, 'tcx>,
1143 obligation: &ProjectionTyObligation<'tcx>,
1144 obligation_trait_ref: &ty::TraitRef<'tcx>,
1145 candidate: ProjectionTyCandidate<'tcx>,
1146 ) -> Progress<'tcx> {
1147 debug!("confirm_candidate(candidate={:?}, obligation={:?})", candidate, obligation);
1149 let mut progress = match candidate {
1150 ProjectionTyCandidate::ParamEnv(poly_projection)
1151 | ProjectionTyCandidate::TraitDef(poly_projection) => {
1152 confirm_param_env_candidate(selcx, obligation, poly_projection)
1155 ProjectionTyCandidate::Select(impl_source) => {
1156 confirm_select_candidate(selcx, obligation, obligation_trait_ref, impl_source)
1159 // When checking for cycle during evaluation, we compare predicates with
1160 // "syntactic" equality. Since normalization generally introduces a type
1161 // with new region variables, we need to resolve them to existing variables
1162 // when possible for this to work. See `auto-trait-projection-recursion.rs`
1163 // for a case where this matters.
1164 if progress.ty.has_infer_regions() {
1165 progress.ty = OpportunisticRegionResolver::new(selcx.infcx()).fold_ty(progress.ty);
1170 fn confirm_select_candidate<'cx, 'tcx>(
1171 selcx: &mut SelectionContext<'cx, 'tcx>,
1172 obligation: &ProjectionTyObligation<'tcx>,
1173 obligation_trait_ref: &ty::TraitRef<'tcx>,
1174 impl_source: Selection<'tcx>,
1175 ) -> Progress<'tcx> {
1177 super::ImplSourceUserDefined(data) => confirm_impl_candidate(selcx, obligation, data),
1178 super::ImplSourceGenerator(data) => confirm_generator_candidate(selcx, obligation, data),
1179 super::ImplSourceClosure(data) => confirm_closure_candidate(selcx, obligation, data),
1180 super::ImplSourceFnPointer(data) => confirm_fn_pointer_candidate(selcx, obligation, data),
1181 super::ImplSourceDiscriminantKind(data) => {
1182 confirm_discriminant_kind_candidate(selcx, obligation, data)
1184 super::ImplSourceObject(_) => {
1185 confirm_object_candidate(selcx, obligation, obligation_trait_ref)
1187 super::ImplSourceAutoImpl(..)
1188 | super::ImplSourceParam(..)
1189 | super::ImplSourceBuiltin(..)
1190 | super::ImplSourceTraitAlias(..) =>
1191 // we don't create Select candidates with this kind of resolution
1194 obligation.cause.span,
1195 "Cannot project an associated type from `{:?}`",
1202 fn confirm_object_candidate<'cx, 'tcx>(
1203 selcx: &mut SelectionContext<'cx, 'tcx>,
1204 obligation: &ProjectionTyObligation<'tcx>,
1205 obligation_trait_ref: &ty::TraitRef<'tcx>,
1206 ) -> Progress<'tcx> {
1207 let self_ty = obligation_trait_ref.self_ty();
1208 let object_ty = selcx.infcx().shallow_resolve(self_ty);
1209 debug!("confirm_object_candidate(object_ty={:?})", object_ty);
1210 let data = match object_ty.kind {
1211 ty::Dynamic(ref data, ..) => data,
1213 obligation.cause.span,
1214 "confirm_object_candidate called with non-object: {:?}",
1218 let env_predicates = data
1219 .projection_bounds()
1220 .map(|p| p.with_self_ty(selcx.tcx(), object_ty).to_predicate(selcx.tcx()));
1221 let env_predicate = {
1222 let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1224 // select only those projections that are actually projecting an
1225 // item with the correct name
1226 let env_predicates = env_predicates.filter_map(|o| match o.predicate.kind() {
1227 &ty::PredicateKind::Projection(data)
1228 if data.projection_def_id() == obligation.predicate.item_def_id =>
1235 // select those with a relevant trait-ref
1236 let mut env_predicates = env_predicates.filter(|data| {
1237 let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1238 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1239 selcx.infcx().probe(|_| {
1242 .at(&obligation.cause, obligation.param_env)
1243 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1248 // select the first matching one; there really ought to be one or
1249 // else the object type is not WF, since an object type should
1250 // include all of its projections explicitly
1251 match env_predicates.next() {
1252 Some(env_predicate) => env_predicate,
1255 "confirm_object_candidate: no env-predicate \
1256 found in object type `{:?}`; ill-formed",
1259 return Progress::error(selcx.tcx());
1264 confirm_param_env_candidate(selcx, obligation, env_predicate)
1267 fn confirm_generator_candidate<'cx, 'tcx>(
1268 selcx: &mut SelectionContext<'cx, 'tcx>,
1269 obligation: &ProjectionTyObligation<'tcx>,
1270 impl_source: ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>,
1271 ) -> Progress<'tcx> {
1272 let gen_sig = impl_source.substs.as_generator().poly_sig();
1273 let Normalized { value: gen_sig, obligations } = normalize_with_depth(
1275 obligation.param_env,
1276 obligation.cause.clone(),
1277 obligation.recursion_depth + 1,
1282 "confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1283 obligation, gen_sig, obligations
1286 let tcx = selcx.tcx();
1288 let gen_def_id = tcx.require_lang_item(GeneratorTraitLangItem, None);
1290 let predicate = super::util::generator_trait_ref_and_outputs(
1293 obligation.predicate.self_ty(),
1296 .map_bound(|(trait_ref, yield_ty, return_ty)| {
1297 let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1298 let ty = if name == sym::Return {
1300 } else if name == sym::Yield {
1306 ty::ProjectionPredicate {
1307 projection_ty: ty::ProjectionTy {
1308 substs: trait_ref.substs,
1309 item_def_id: obligation.predicate.item_def_id,
1315 confirm_param_env_candidate(selcx, obligation, predicate)
1316 .with_addl_obligations(impl_source.nested)
1317 .with_addl_obligations(obligations)
1320 fn confirm_discriminant_kind_candidate<'cx, 'tcx>(
1321 selcx: &mut SelectionContext<'cx, 'tcx>,
1322 obligation: &ProjectionTyObligation<'tcx>,
1323 _: ImplSourceDiscriminantKindData,
1324 ) -> Progress<'tcx> {
1325 let tcx = selcx.tcx();
1327 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1328 let substs = tcx.mk_substs([self_ty.into()].iter());
1330 let assoc_items = tcx.associated_items(tcx.lang_items().discriminant_kind_trait().unwrap());
1331 // FIXME: emit an error if the trait definition is wrong
1332 let discriminant_def_id = assoc_items.in_definition_order().next().unwrap().def_id;
1334 let discriminant_ty = match self_ty.kind {
1335 // Use the discriminant type for enums.
1336 ty::Adt(adt, _) if adt.is_enum() => adt.repr.discr_type().to_ty(tcx),
1337 // Default to `i32` for generators.
1338 ty::Generator(..) => tcx.types.i32,
1339 // Use `u8` for all other types.
1343 let predicate = ty::ProjectionPredicate {
1344 projection_ty: ty::ProjectionTy { substs, item_def_id: discriminant_def_id },
1345 ty: discriminant_ty,
1348 confirm_param_env_candidate(selcx, obligation, ty::Binder::bind(predicate))
1351 fn confirm_fn_pointer_candidate<'cx, 'tcx>(
1352 selcx: &mut SelectionContext<'cx, 'tcx>,
1353 obligation: &ProjectionTyObligation<'tcx>,
1354 fn_pointer_impl_source: ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>,
1355 ) -> Progress<'tcx> {
1356 let fn_type = selcx.infcx().shallow_resolve(fn_pointer_impl_source.fn_ty);
1357 let sig = fn_type.fn_sig(selcx.tcx());
1358 let Normalized { value: sig, obligations } = normalize_with_depth(
1360 obligation.param_env,
1361 obligation.cause.clone(),
1362 obligation.recursion_depth + 1,
1366 confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1367 .with_addl_obligations(fn_pointer_impl_source.nested)
1368 .with_addl_obligations(obligations)
1371 fn confirm_closure_candidate<'cx, 'tcx>(
1372 selcx: &mut SelectionContext<'cx, 'tcx>,
1373 obligation: &ProjectionTyObligation<'tcx>,
1374 impl_source: ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>,
1375 ) -> Progress<'tcx> {
1376 let closure_sig = impl_source.substs.as_closure().sig();
1377 let Normalized { value: closure_sig, obligations } = normalize_with_depth(
1379 obligation.param_env,
1380 obligation.cause.clone(),
1381 obligation.recursion_depth + 1,
1386 "confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1387 obligation, closure_sig, obligations
1390 confirm_callable_candidate(selcx, obligation, closure_sig, util::TupleArgumentsFlag::No)
1391 .with_addl_obligations(impl_source.nested)
1392 .with_addl_obligations(obligations)
1395 fn confirm_callable_candidate<'cx, 'tcx>(
1396 selcx: &mut SelectionContext<'cx, 'tcx>,
1397 obligation: &ProjectionTyObligation<'tcx>,
1398 fn_sig: ty::PolyFnSig<'tcx>,
1399 flag: util::TupleArgumentsFlag,
1400 ) -> Progress<'tcx> {
1401 let tcx = selcx.tcx();
1403 debug!("confirm_callable_candidate({:?},{:?})", obligation, fn_sig);
1405 // the `Output` associated type is declared on `FnOnce`
1406 let fn_once_def_id = tcx.require_lang_item(FnOnceTraitLangItem, None);
1408 let predicate = super::util::closure_trait_ref_and_return_type(
1411 obligation.predicate.self_ty(),
1415 .map_bound(|(trait_ref, ret_type)| ty::ProjectionPredicate {
1416 projection_ty: ty::ProjectionTy::from_ref_and_name(
1419 Ident::with_dummy_span(rustc_hir::FN_OUTPUT_NAME),
1424 confirm_param_env_candidate(selcx, obligation, predicate)
1427 fn confirm_param_env_candidate<'cx, 'tcx>(
1428 selcx: &mut SelectionContext<'cx, 'tcx>,
1429 obligation: &ProjectionTyObligation<'tcx>,
1430 poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
1431 ) -> Progress<'tcx> {
1432 let infcx = selcx.infcx();
1433 let cause = &obligation.cause;
1434 let param_env = obligation.param_env;
1436 let (cache_entry, _) = infcx.replace_bound_vars_with_fresh_vars(
1438 LateBoundRegionConversionTime::HigherRankedType,
1442 let cache_trait_ref = cache_entry.projection_ty.trait_ref(infcx.tcx);
1443 let obligation_trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1444 match infcx.at(cause, param_env).eq(cache_trait_ref, obligation_trait_ref) {
1445 Ok(InferOk { value: _, obligations }) => Progress { ty: cache_entry.ty, obligations },
1448 "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
1449 obligation, poly_cache_entry, e,
1451 debug!("confirm_param_env_candidate: {}", msg);
1452 let err = infcx.tcx.ty_error_with_message(obligation.cause.span, &msg);
1453 Progress { ty: err, obligations: vec![] }
1458 fn confirm_impl_candidate<'cx, 'tcx>(
1459 selcx: &mut SelectionContext<'cx, 'tcx>,
1460 obligation: &ProjectionTyObligation<'tcx>,
1461 impl_impl_source: ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>,
1462 ) -> Progress<'tcx> {
1463 let tcx = selcx.tcx();
1465 let ImplSourceUserDefinedData { impl_def_id, substs, nested } = impl_impl_source;
1466 let assoc_item_id = obligation.predicate.item_def_id;
1467 let trait_def_id = tcx.trait_id_of_impl(impl_def_id).unwrap();
1469 let param_env = obligation.param_env;
1470 let assoc_ty = match assoc_ty_def(selcx, impl_def_id, assoc_item_id) {
1471 Ok(assoc_ty) => assoc_ty,
1472 Err(ErrorReported) => return Progress { ty: tcx.ty_error(), obligations: nested },
1475 if !assoc_ty.item.defaultness.has_value() {
1476 // This means that the impl is missing a definition for the
1477 // associated type. This error will be reported by the type
1478 // checker method `check_impl_items_against_trait`, so here we
1479 // just return Error.
1481 "confirm_impl_candidate: no associated type {:?} for {:?}",
1482 assoc_ty.item.ident, obligation.predicate
1484 return Progress { ty: tcx.ty_error(), obligations: nested };
1486 // If we're trying to normalize `<Vec<u32> as X>::A<S>` using
1487 //`impl<T> X for Vec<T> { type A<Y> = Box<Y>; }`, then:
1489 // * `obligation.predicate.substs` is `[Vec<u32>, S]`
1490 // * `substs` is `[u32]`
1491 // * `substs` ends up as `[u32, S]`
1492 let substs = obligation.predicate.substs.rebase_onto(tcx, trait_def_id, substs);
1494 translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.defining_node);
1495 let ty = tcx.type_of(assoc_ty.item.def_id);
1496 if substs.len() != tcx.generics_of(assoc_ty.item.def_id).count() {
1497 let err = tcx.ty_error_with_message(
1499 "impl item and trait item have different parameter counts",
1501 Progress { ty: err, obligations: nested }
1503 Progress { ty: ty.subst(tcx, substs), obligations: nested }
1507 /// Locate the definition of an associated type in the specialization hierarchy,
1508 /// starting from the given impl.
1510 /// Based on the "projection mode", this lookup may in fact only examine the
1511 /// topmost impl. See the comments for `Reveal` for more details.
1513 selcx: &SelectionContext<'_, '_>,
1515 assoc_ty_def_id: DefId,
1516 ) -> Result<specialization_graph::LeafDef, ErrorReported> {
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 TraitDef::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 matches!(item.kind, ty::AssocKind::Type)
1531 && tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id)
1533 return Ok(specialization_graph::LeafDef {
1535 defining_node: impl_node,
1536 finalizing_node: if item.defaultness.is_default() { None } else { Some(impl_node) },
1541 let ancestors = trait_def.ancestors(tcx, impl_def_id)?;
1542 if let Some(assoc_item) = ancestors.leaf_def(tcx, assoc_ty_name, ty::AssocKind::Type) {
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 {}", assoc_ty_name, tcx.def_path_str(impl_def_id))
1555 crate trait ProjectionCacheKeyExt<'tcx>: Sized {
1556 fn from_poly_projection_predicate(
1557 selcx: &mut SelectionContext<'cx, 'tcx>,
1558 predicate: ty::PolyProjectionPredicate<'tcx>,
1562 impl<'tcx> ProjectionCacheKeyExt<'tcx> for ProjectionCacheKey<'tcx> {
1563 fn from_poly_projection_predicate(
1564 selcx: &mut SelectionContext<'cx, 'tcx>,
1565 predicate: ty::PolyProjectionPredicate<'tcx>,
1567 let infcx = selcx.infcx();
1568 // We don't do cross-snapshot caching of obligations with escaping regions,
1569 // so there's no cache key to use
1570 predicate.no_bound_vars().map(|predicate| {
1571 ProjectionCacheKey::new(
1572 // We don't attempt to match up with a specific type-variable state
1573 // from a specific call to `opt_normalize_projection_type` - if
1574 // there's no precise match, the original cache entry is "stranded"
1576 infcx.resolve_vars_if_possible(&predicate.projection_ty),