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::{FnOnceOutputLangItem, 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;
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, _) =
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)?;
162 /// Evaluates constraints of the form:
164 /// <T as Trait>::U == V
166 /// If successful, this may result in additional obligations.
167 fn project_and_unify_type<'cx, 'tcx>(
168 selcx: &mut SelectionContext<'cx, 'tcx>,
169 obligation: &ProjectionObligation<'tcx>,
170 ) -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>> {
171 debug!("project_and_unify_type(obligation={:?})", obligation);
173 let mut obligations = vec![];
174 let normalized_ty = match opt_normalize_projection_type(
176 obligation.param_env,
177 obligation.predicate.projection_ty,
178 obligation.cause.clone(),
179 obligation.recursion_depth,
183 None => return Ok(None),
187 "project_and_unify_type: normalized_ty={:?} obligations={:?}",
188 normalized_ty, obligations
191 let infcx = selcx.infcx();
193 .at(&obligation.cause, obligation.param_env)
194 .eq(normalized_ty, obligation.predicate.ty)
196 Ok(InferOk { obligations: inferred_obligations, value: () }) => {
197 obligations.extend(inferred_obligations);
198 Ok(Some(obligations))
201 debug!("project_and_unify_type: equating types encountered error {:?}", err);
202 Err(MismatchedProjectionTypes { err })
207 /// Normalizes any associated type projections in `value`, replacing
208 /// them with a fully resolved type where possible. The return value
209 /// combines the normalized result and any additional obligations that
210 /// were incurred as result.
211 pub fn normalize<'a, 'b, 'tcx, T>(
212 selcx: &'a mut SelectionContext<'b, 'tcx>,
213 param_env: ty::ParamEnv<'tcx>,
214 cause: ObligationCause<'tcx>,
216 ) -> Normalized<'tcx, T>
218 T: TypeFoldable<'tcx>,
220 let mut obligations = Vec::new();
221 let value = normalize_to(selcx, param_env, cause, value, &mut obligations);
222 Normalized { value, obligations }
225 pub fn normalize_to<'a, 'b, 'tcx, T>(
226 selcx: &'a mut SelectionContext<'b, 'tcx>,
227 param_env: ty::ParamEnv<'tcx>,
228 cause: ObligationCause<'tcx>,
230 obligations: &mut Vec<PredicateObligation<'tcx>>,
233 T: TypeFoldable<'tcx>,
235 normalize_with_depth_to(selcx, param_env, cause, 0, value, obligations)
238 /// As `normalize`, but with a custom depth.
239 pub fn normalize_with_depth<'a, 'b, 'tcx, T>(
240 selcx: &'a mut SelectionContext<'b, 'tcx>,
241 param_env: ty::ParamEnv<'tcx>,
242 cause: ObligationCause<'tcx>,
245 ) -> Normalized<'tcx, T>
247 T: TypeFoldable<'tcx>,
249 let mut obligations = Vec::new();
250 let value = normalize_with_depth_to(selcx, param_env, cause, depth, value, &mut obligations);
251 Normalized { value, obligations }
254 pub fn normalize_with_depth_to<'a, 'b, 'tcx, T>(
255 selcx: &'a mut SelectionContext<'b, 'tcx>,
256 param_env: ty::ParamEnv<'tcx>,
257 cause: ObligationCause<'tcx>,
260 obligations: &mut Vec<PredicateObligation<'tcx>>,
263 T: TypeFoldable<'tcx>,
265 debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
266 let mut normalizer = AssocTypeNormalizer::new(selcx, param_env, cause, depth, obligations);
267 let result = ensure_sufficient_stack(|| normalizer.fold(value));
269 "normalize_with_depth: depth={} result={:?} with {} obligations",
272 normalizer.obligations.len()
274 debug!("normalize_with_depth: depth={} obligations={:?}", depth, normalizer.obligations);
278 struct AssocTypeNormalizer<'a, 'b, 'tcx> {
279 selcx: &'a mut SelectionContext<'b, 'tcx>,
280 param_env: ty::ParamEnv<'tcx>,
281 cause: ObligationCause<'tcx>,
282 obligations: &'a mut Vec<PredicateObligation<'tcx>>,
286 impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
288 selcx: &'a mut SelectionContext<'b, 'tcx>,
289 param_env: ty::ParamEnv<'tcx>,
290 cause: ObligationCause<'tcx>,
292 obligations: &'a mut Vec<PredicateObligation<'tcx>>,
293 ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
294 AssocTypeNormalizer { selcx, param_env, cause, obligations, depth }
297 fn fold<T: TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
298 let value = self.selcx.infcx().resolve_vars_if_possible(value);
300 if !value.has_projections() { value } else { value.fold_with(self) }
304 impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
305 fn tcx<'c>(&'c self) -> TyCtxt<'tcx> {
309 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
310 if !ty.has_projections() {
313 // We don't want to normalize associated types that occur inside of region
314 // binders, because they may contain bound regions, and we can't cope with that.
318 // for<'a> fn(<T as Foo<&'a>>::A)
320 // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
321 // normalize it when we instantiate those bound regions (which
322 // should occur eventually).
324 let ty = ty.super_fold_with(self);
326 ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => {
328 // Only normalize `impl Trait` after type-checking, usually in codegen.
329 match self.param_env.reveal() {
330 Reveal::UserFacing => ty,
333 let recursion_limit = self.tcx().sess.recursion_limit();
334 if !recursion_limit.value_within_limit(self.depth) {
335 let obligation = Obligation::with_depth(
341 self.selcx.infcx().report_overflow_error(&obligation, true);
344 let generic_ty = self.tcx().type_of(def_id);
345 let concrete_ty = generic_ty.subst(self.tcx(), substs);
347 let folded_ty = self.fold_ty(concrete_ty);
354 ty::Projection(ref data) if !data.has_escaping_bound_vars() => {
357 // (*) This is kind of hacky -- we need to be able to
358 // handle normalization within binders because
359 // otherwise we wind up a need to normalize when doing
360 // trait matching (since you can have a trait
361 // obligation like `for<'a> T::B: Fn(&'a i32)`), but
362 // we can't normalize with bound regions in scope. So
363 // far now we just ignore binders but only normalize
364 // if all bound regions are gone (and then we still
365 // have to renormalize whenever we instantiate a
366 // binder). It would be better to normalize in a
367 // binding-aware fashion.
369 let normalized_ty = normalize_projection_type(
375 &mut self.obligations,
378 "AssocTypeNormalizer: depth={} normalized {:?} to {:?}, \
379 now with {} obligations",
383 self.obligations.len()
392 fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
393 if self.selcx.tcx().lazy_normalization() {
396 let constant = constant.super_fold_with(self);
397 constant.eval(self.selcx.tcx(), self.param_env)
402 /// The guts of `normalize`: normalize a specific projection like `<T
403 /// as Trait>::Item`. The result is always a type (and possibly
404 /// additional obligations). If ambiguity arises, which implies that
405 /// there are unresolved type variables in the projection, we will
406 /// substitute a fresh type variable `$X` and generate a new
407 /// obligation `<T as Trait>::Item == $X` for later.
408 pub fn normalize_projection_type<'a, 'b, 'tcx>(
409 selcx: &'a mut SelectionContext<'b, 'tcx>,
410 param_env: ty::ParamEnv<'tcx>,
411 projection_ty: ty::ProjectionTy<'tcx>,
412 cause: ObligationCause<'tcx>,
414 obligations: &mut Vec<PredicateObligation<'tcx>>,
416 opt_normalize_projection_type(
424 .unwrap_or_else(move || {
425 // if we bottom out in ambiguity, create a type variable
426 // and a deferred predicate to resolve this when more type
427 // information is available.
429 let tcx = selcx.infcx().tcx;
430 let def_id = projection_ty.item_def_id;
431 let ty_var = selcx.infcx().next_ty_var(TypeVariableOrigin {
432 kind: TypeVariableOriginKind::NormalizeProjectionType,
433 span: tcx.def_span(def_id),
435 let projection = ty::Binder::dummy(ty::ProjectionPredicate { projection_ty, ty: ty_var });
437 Obligation::with_depth(cause, depth + 1, param_env, projection.to_predicate(tcx));
438 obligations.push(obligation);
443 /// The guts of `normalize`: normalize a specific projection like `<T
444 /// as Trait>::Item`. The result is always a type (and possibly
445 /// additional obligations). Returns `None` in the case of ambiguity,
446 /// which indicates that there are unbound type variables.
448 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
449 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
450 /// often immediately appended to another obligations vector. So now this
451 /// function takes an obligations vector and appends to it directly, which is
452 /// slightly uglier but avoids the need for an extra short-lived allocation.
453 fn opt_normalize_projection_type<'a, 'b, 'tcx>(
454 selcx: &'a mut SelectionContext<'b, 'tcx>,
455 param_env: ty::ParamEnv<'tcx>,
456 projection_ty: ty::ProjectionTy<'tcx>,
457 cause: ObligationCause<'tcx>,
459 obligations: &mut Vec<PredicateObligation<'tcx>>,
460 ) -> Option<Ty<'tcx>> {
461 let infcx = selcx.infcx();
463 let projection_ty = infcx.resolve_vars_if_possible(&projection_ty);
464 let cache_key = ProjectionCacheKey::new(projection_ty);
467 "opt_normalize_projection_type(\
468 projection_ty={:?}, \
473 // FIXME(#20304) For now, I am caching here, which is good, but it
474 // means we don't capture the type variables that are created in
475 // the case of ambiguity. Which means we may create a large stream
476 // of such variables. OTOH, if we move the caching up a level, we
477 // would not benefit from caching when proving `T: Trait<U=Foo>`
478 // bounds. It might be the case that we want two distinct caches,
479 // or else another kind of cache entry.
481 let cache_result = infcx.inner.borrow_mut().projection_cache().try_start(cache_key);
484 Err(ProjectionCacheEntry::Ambiguous) => {
485 // If we found ambiguity the last time, that means we will continue
486 // to do so until some type in the key changes (and we know it
487 // hasn't, because we just fully resolved it).
489 "opt_normalize_projection_type: \
490 found cache entry: ambiguous"
494 Err(ProjectionCacheEntry::InProgress) => {
495 // If while normalized A::B, we are asked to normalize
496 // A::B, just return A::B itself. This is a conservative
497 // answer, in the sense that A::B *is* clearly equivalent
498 // to A::B, though there may be a better value we can
501 // Under lazy normalization, this can arise when
502 // bootstrapping. That is, imagine an environment with a
503 // where-clause like `A::B == u32`. Now, if we are asked
504 // to normalize `A::B`, we will want to check the
505 // where-clauses in scope. So we will try to unify `A::B`
506 // with `A::B`, which can trigger a recursive
507 // normalization. In that case, I think we will want this code:
510 // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
511 // projection_ty.substs;
512 // return Some(NormalizedTy { value: v, obligations: vec![] });
516 "opt_normalize_projection_type: \
517 found cache entry: in-progress"
520 // But for now, let's classify this as an overflow:
521 let recursion_limit = selcx.tcx().sess.recursion_limit();
523 Obligation::with_depth(cause, recursion_limit.0, param_env, projection_ty);
524 selcx.infcx().report_overflow_error(&obligation, false);
526 Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
527 // This is the hottest path in this function.
529 // If we find the value in the cache, then return it along
530 // with the obligations that went along with it. Note
531 // that, when using a fulfillment context, these
532 // obligations could in principle be ignored: they have
533 // already been registered when the cache entry was
534 // created (and hence the new ones will quickly be
535 // discarded as duplicated). But when doing trait
536 // evaluation this is not the case, and dropping the trait
537 // evaluations can causes ICEs (e.g., #43132).
539 "opt_normalize_projection_type: \
540 found normalized ty `{:?}`",
544 // Once we have inferred everything we need to know, we
545 // can ignore the `obligations` from that point on.
546 if infcx.unresolved_type_vars(&ty.value).is_none() {
547 infcx.inner.borrow_mut().projection_cache().complete_normalized(cache_key, &ty);
548 // No need to extend `obligations`.
550 obligations.extend(ty.obligations);
553 obligations.push(get_paranoid_cache_value_obligation(
560 return Some(ty.value);
562 Err(ProjectionCacheEntry::Error) => {
564 "opt_normalize_projection_type: \
567 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
568 obligations.extend(result.obligations);
569 return Some(result.value);
573 let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
574 match project_type(selcx, &obligation) {
575 Ok(ProjectedTy::Progress(Progress {
577 obligations: mut projected_obligations,
579 // if projection succeeded, then what we get out of this
580 // is also non-normalized (consider: it was derived from
581 // an impl, where-clause etc) and hence we must
585 "opt_normalize_projection_type: \
588 projected_obligations={:?}",
589 projected_ty, depth, projected_obligations
592 let result = if projected_ty.has_projections() {
593 let mut normalizer = AssocTypeNormalizer::new(
598 &mut projected_obligations,
600 let normalized_ty = normalizer.fold(&projected_ty);
603 "opt_normalize_projection_type: \
604 normalized_ty={:?} depth={}",
608 Normalized { value: normalized_ty, obligations: projected_obligations }
610 Normalized { value: projected_ty, obligations: projected_obligations }
613 let cache_value = prune_cache_value_obligations(infcx, &result);
614 infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, cache_value);
615 obligations.extend(result.obligations);
618 Ok(ProjectedTy::NoProgress(projected_ty)) => {
620 "opt_normalize_projection_type: \
621 projected_ty={:?} no progress",
624 let result = Normalized { value: projected_ty, obligations: vec![] };
625 infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, result.clone());
626 // No need to extend `obligations`.
629 Err(ProjectionTyError::TooManyCandidates) => {
631 "opt_normalize_projection_type: \
634 infcx.inner.borrow_mut().projection_cache().ambiguous(cache_key);
637 Err(ProjectionTyError::TraitSelectionError(_)) => {
638 debug!("opt_normalize_projection_type: ERROR");
639 // if we got an error processing the `T as Trait` part,
640 // just return `ty::err` but add the obligation `T :
641 // Trait`, which when processed will cause the error to be
644 infcx.inner.borrow_mut().projection_cache().error(cache_key);
645 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
646 obligations.extend(result.obligations);
652 /// If there are unresolved type variables, then we need to include
653 /// any subobligations that bind them, at least until those type
654 /// variables are fully resolved.
655 fn prune_cache_value_obligations<'a, 'tcx>(
656 infcx: &'a InferCtxt<'a, 'tcx>,
657 result: &NormalizedTy<'tcx>,
658 ) -> NormalizedTy<'tcx> {
659 if infcx.unresolved_type_vars(&result.value).is_none() {
660 return NormalizedTy { value: result.value, obligations: vec![] };
663 let mut obligations: Vec<_> = result
666 .filter(|obligation| match obligation.predicate.kind() {
667 // We found a `T: Foo<X = U>` predicate, let's check
668 // if `U` references any unresolved type
669 // variables. In principle, we only care if this
670 // projection can help resolve any of the type
671 // variables found in `result.value` -- but we just
672 // check for any type variables here, for fear of
673 // indirect obligations (e.g., we project to `?0`,
674 // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
676 ty::PredicateKind::Projection(ref data) => {
677 infcx.unresolved_type_vars(&data.ty()).is_some()
680 // We are only interested in `T: Foo<X = U>` predicates, whre
681 // `U` references one of `unresolved_type_vars`. =)
687 obligations.shrink_to_fit();
689 NormalizedTy { value: result.value, obligations }
692 /// Whenever we give back a cache result for a projection like `<T as
693 /// Trait>::Item ==> X`, we *always* include the obligation to prove
694 /// that `T: Trait` (we may also include some other obligations). This
695 /// may or may not be necessary -- in principle, all the obligations
696 /// that must be proven to show that `T: Trait` were also returned
697 /// when the cache was first populated. But there are some vague concerns,
698 /// and so we take the precautionary measure of including `T: Trait` in
701 /// Concern #1. The current setup is fragile. Perhaps someone could
702 /// have failed to prove the concerns from when the cache was
703 /// populated, but also not have used a snapshot, in which case the
704 /// cache could remain populated even though `T: Trait` has not been
705 /// shown. In this case, the "other code" is at fault -- when you
706 /// project something, you are supposed to either have a snapshot or
707 /// else prove all the resulting obligations -- but it's still easy to
710 /// Concern #2. Even within the snapshot, if those original
711 /// obligations are not yet proven, then we are able to do projections
712 /// that may yet turn out to be wrong. This *may* lead to some sort
713 /// of trouble, though we don't have a concrete example of how that
714 /// can occur yet. But it seems risky at best.
715 fn get_paranoid_cache_value_obligation<'a, 'tcx>(
716 infcx: &'a InferCtxt<'a, 'tcx>,
717 param_env: ty::ParamEnv<'tcx>,
718 projection_ty: ty::ProjectionTy<'tcx>,
719 cause: ObligationCause<'tcx>,
721 ) -> PredicateObligation<'tcx> {
722 let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
725 recursion_depth: depth,
727 predicate: trait_ref.without_const().to_predicate(infcx.tcx),
731 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
732 /// hold. In various error cases, we cannot generate a valid
733 /// normalized projection. Therefore, we create an inference variable
734 /// return an associated obligation that, when fulfilled, will lead to
737 /// Note that we used to return `Error` here, but that was quite
738 /// dubious -- the premise was that an error would *eventually* be
739 /// reported, when the obligation was processed. But in general once
740 /// you see a `Error` you are supposed to be able to assume that an
741 /// error *has been* reported, so that you can take whatever heuristic
742 /// paths you want to take. To make things worse, it was possible for
743 /// cycles to arise, where you basically had a setup like `<MyType<$0>
744 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
745 /// Trait>::Foo> to `[type error]` would lead to an obligation of
746 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
747 /// an error for this obligation, but we legitimately should not,
748 /// because it contains `[type error]`. Yuck! (See issue #29857 for
749 /// one case where this arose.)
750 fn normalize_to_error<'a, 'tcx>(
751 selcx: &mut SelectionContext<'a, 'tcx>,
752 param_env: ty::ParamEnv<'tcx>,
753 projection_ty: ty::ProjectionTy<'tcx>,
754 cause: ObligationCause<'tcx>,
756 ) -> NormalizedTy<'tcx> {
757 let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
758 let trait_obligation = Obligation {
760 recursion_depth: depth,
762 predicate: trait_ref.without_const().to_predicate(selcx.tcx()),
764 let tcx = selcx.infcx().tcx;
765 let def_id = projection_ty.item_def_id;
766 let new_value = selcx.infcx().next_ty_var(TypeVariableOrigin {
767 kind: TypeVariableOriginKind::NormalizeProjectionType,
768 span: tcx.def_span(def_id),
770 Normalized { value: new_value, obligations: vec![trait_obligation] }
773 enum ProjectedTy<'tcx> {
774 Progress(Progress<'tcx>),
775 NoProgress(Ty<'tcx>),
778 struct Progress<'tcx> {
780 obligations: Vec<PredicateObligation<'tcx>>,
783 impl<'tcx> Progress<'tcx> {
784 fn error(tcx: TyCtxt<'tcx>) -> Self {
785 Progress { ty: tcx.ty_error(), obligations: vec![] }
788 fn with_addl_obligations(mut self, mut obligations: Vec<PredicateObligation<'tcx>>) -> Self {
790 "with_addl_obligations: self.obligations.len={} obligations.len={}",
791 self.obligations.len(),
796 "with_addl_obligations: self.obligations={:?} obligations={:?}",
797 self.obligations, obligations
800 self.obligations.append(&mut obligations);
805 /// Computes the result of a projection type (if we can).
808 /// - `obligation` must be fully normalized
809 fn project_type<'cx, 'tcx>(
810 selcx: &mut SelectionContext<'cx, 'tcx>,
811 obligation: &ProjectionTyObligation<'tcx>,
812 ) -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>> {
813 debug!("project(obligation={:?})", obligation);
815 if !selcx.tcx().sess.recursion_limit().value_within_limit(obligation.recursion_depth) {
816 debug!("project: overflow!");
817 return Err(ProjectionTyError::TraitSelectionError(SelectionError::Overflow));
820 let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
822 debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
824 if obligation_trait_ref.references_error() {
825 return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
828 let mut candidates = ProjectionTyCandidateSet::None;
830 // Make sure that the following procedures are kept in order. ParamEnv
831 // needs to be first because it has highest priority, and Select checks
832 // the return value of push_candidate which assumes it's ran at last.
833 assemble_candidates_from_param_env(selcx, obligation, &obligation_trait_ref, &mut candidates);
835 assemble_candidates_from_trait_def(selcx, obligation, &obligation_trait_ref, &mut candidates);
837 assemble_candidates_from_impls(selcx, obligation, &obligation_trait_ref, &mut candidates);
840 ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
841 confirm_candidate(selcx, obligation, &obligation_trait_ref, candidate),
843 ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
846 .mk_projection(obligation.predicate.item_def_id, obligation.predicate.substs),
848 // Error occurred while trying to processing impls.
849 ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
850 // Inherent ambiguity that prevents us from even enumerating the
852 ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
856 /// The first thing we have to do is scan through the parameter
857 /// environment to see whether there are any projection predicates
858 /// there that can answer this question.
859 fn assemble_candidates_from_param_env<'cx, 'tcx>(
860 selcx: &mut SelectionContext<'cx, 'tcx>,
861 obligation: &ProjectionTyObligation<'tcx>,
862 obligation_trait_ref: &ty::TraitRef<'tcx>,
863 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
865 debug!("assemble_candidates_from_param_env(..)");
866 assemble_candidates_from_predicates(
869 obligation_trait_ref,
871 ProjectionTyCandidate::ParamEnv,
872 obligation.param_env.caller_bounds().iter(),
876 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
877 /// that the definition of `Foo` has some clues:
881 /// type FooT : Bar<BarT=i32>
885 /// Here, for example, we could conclude that the result is `i32`.
886 fn assemble_candidates_from_trait_def<'cx, 'tcx>(
887 selcx: &mut SelectionContext<'cx, 'tcx>,
888 obligation: &ProjectionTyObligation<'tcx>,
889 obligation_trait_ref: &ty::TraitRef<'tcx>,
890 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
892 debug!("assemble_candidates_from_trait_def(..)");
894 let tcx = selcx.tcx();
895 // Check whether the self-type is itself a projection.
896 // If so, extract what we know from the trait and try to come up with a good answer.
897 let bounds = match obligation_trait_ref.self_ty().kind {
898 ty::Projection(ref data) => {
899 tcx.projection_predicates(data.item_def_id).subst(tcx, data.substs)
901 ty::Opaque(def_id, substs) => tcx.projection_predicates(def_id).subst(tcx, substs),
902 ty::Infer(ty::TyVar(_)) => {
903 // If the self-type is an inference variable, then it MAY wind up
904 // being a projected type, so induce an ambiguity.
905 candidate_set.mark_ambiguous();
911 assemble_candidates_from_predicates(
914 obligation_trait_ref,
916 ProjectionTyCandidate::TraitDef,
921 fn assemble_candidates_from_predicates<'cx, 'tcx>(
922 selcx: &mut SelectionContext<'cx, 'tcx>,
923 obligation: &ProjectionTyObligation<'tcx>,
924 obligation_trait_ref: &ty::TraitRef<'tcx>,
925 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
926 ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
927 env_predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
929 debug!("assemble_candidates_from_predicates(obligation={:?})", obligation);
930 let infcx = selcx.infcx();
931 for predicate in env_predicates {
932 debug!("assemble_candidates_from_predicates: predicate={:?}", predicate);
933 if let &ty::PredicateKind::Projection(data) = predicate.kind() {
934 let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
936 let is_match = same_def_id
938 let data_poly_trait_ref = data.to_poly_trait_ref(infcx.tcx);
939 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
941 .at(&obligation.cause, obligation.param_env)
942 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
943 .map(|InferOk { obligations: _, value: () }| {
944 // FIXME(#32730) -- do we need to take obligations
945 // into account in any way? At the moment, no.
951 "assemble_candidates_from_predicates: candidate={:?} \
952 is_match={} same_def_id={}",
953 data, is_match, same_def_id
957 candidate_set.push_candidate(ctor(data));
963 fn assemble_candidates_from_impls<'cx, 'tcx>(
964 selcx: &mut SelectionContext<'cx, 'tcx>,
965 obligation: &ProjectionTyObligation<'tcx>,
966 obligation_trait_ref: &ty::TraitRef<'tcx>,
967 candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
969 // If we are resolving `<T as TraitRef<...>>::Item == Type`,
970 // start out by selecting the predicate `T as TraitRef<...>`:
971 let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
972 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
973 let _ = selcx.infcx().commit_if_ok(|_| {
974 let impl_source = match selcx.select(&trait_obligation) {
975 Ok(Some(impl_source)) => impl_source,
977 candidate_set.mark_ambiguous();
981 debug!("assemble_candidates_from_impls: selection error {:?}", e);
982 candidate_set.mark_error(e);
987 let eligible = match &impl_source {
988 super::ImplSourceClosure(_)
989 | super::ImplSourceGenerator(_)
990 | super::ImplSourceFnPointer(_)
991 | super::ImplSourceObject(_)
992 | super::ImplSourceTraitAlias(_) => {
993 debug!("assemble_candidates_from_impls: impl_source={:?}", impl_source);
996 super::ImplSourceUserDefined(impl_data) => {
997 // We have to be careful when projecting out of an
998 // impl because of specialization. If we are not in
999 // codegen (i.e., projection mode is not "any"), and the
1000 // impl's type is declared as default, then we disable
1001 // projection (even if the trait ref is fully
1002 // monomorphic). In the case where trait ref is not
1003 // fully monomorphic (i.e., includes type parameters),
1004 // this is because those type parameters may
1005 // ultimately be bound to types from other crates that
1006 // may have specialized impls we can't see. In the
1007 // case where the trait ref IS fully monomorphic, this
1008 // is a policy decision that we made in the RFC in
1009 // order to preserve flexibility for the crate that
1010 // defined the specializable impl to specialize later
1011 // for existing types.
1013 // In either case, we handle this by not adding a
1014 // candidate for an impl if it contains a `default`
1017 // NOTE: This should be kept in sync with the similar code in
1018 // `rustc_ty::instance::resolve_associated_item()`.
1020 assoc_ty_def(selcx, impl_data.impl_def_id, obligation.predicate.item_def_id)
1021 .map_err(|ErrorReported| ())?;
1023 if node_item.is_final() {
1024 // Non-specializable items are always projectable.
1027 // Only reveal a specializable default if we're past type-checking
1028 // and the obligation is monomorphic, otherwise passes such as
1029 // transmute checking and polymorphic MIR optimizations could
1030 // get a result which isn't correct for all monomorphizations.
1031 if obligation.param_env.reveal() == Reveal::All {
1032 // NOTE(eddyb) inference variables can resolve to parameters, so
1033 // assume `poly_trait_ref` isn't monomorphic, if it contains any.
1034 let poly_trait_ref =
1035 selcx.infcx().resolve_vars_if_possible(&poly_trait_ref);
1036 !poly_trait_ref.still_further_specializable()
1039 "assemble_candidates_from_impls: not eligible due to default: \
1040 assoc_ty={} predicate={}",
1041 selcx.tcx().def_path_str(node_item.item.def_id),
1042 obligation.predicate,
1048 super::ImplSourceDiscriminantKind(..) => {
1049 // While `DiscriminantKind` is automatically implemented for every type,
1050 // the concrete discriminant may not be known yet.
1052 // Any type with multiple potential discriminant types is therefore not eligible.
1053 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1055 match self_ty.kind {
1073 | ty::GeneratorWitness(..)
1076 // Integers and floats always have `u8` as their discriminant.
1077 | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
1083 | ty::Placeholder(..)
1085 | ty::Error(_) => false,
1088 super::ImplSourceParam(..) => {
1089 // This case tell us nothing about the value of an
1090 // associated type. Consider:
1093 // trait SomeTrait { type Foo; }
1094 // fn foo<T:SomeTrait>(...) { }
1097 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1098 // : SomeTrait` binding does not help us decide what the
1099 // type `Foo` is (at least, not more specifically than
1100 // what we already knew).
1102 // But wait, you say! What about an example like this:
1105 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1108 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1109 // resolve `T::Foo`? And of course it does, but in fact
1110 // that single predicate is desugared into two predicates
1111 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1112 // projection. And the projection where clause is handled
1113 // in `assemble_candidates_from_param_env`.
1116 super::ImplSourceAutoImpl(..) | super::ImplSourceBuiltin(..) => {
1117 // These traits have no associated types.
1119 obligation.cause.span,
1120 "Cannot project an associated type from `{:?}`",
1127 if candidate_set.push_candidate(ProjectionTyCandidate::Select(impl_source)) {
1138 fn confirm_candidate<'cx, 'tcx>(
1139 selcx: &mut SelectionContext<'cx, 'tcx>,
1140 obligation: &ProjectionTyObligation<'tcx>,
1141 obligation_trait_ref: &ty::TraitRef<'tcx>,
1142 candidate: ProjectionTyCandidate<'tcx>,
1143 ) -> Progress<'tcx> {
1144 debug!("confirm_candidate(candidate={:?}, obligation={:?})", candidate, obligation);
1146 let mut progress = match candidate {
1147 ProjectionTyCandidate::ParamEnv(poly_projection)
1148 | ProjectionTyCandidate::TraitDef(poly_projection) => {
1149 confirm_param_env_candidate(selcx, obligation, poly_projection)
1152 ProjectionTyCandidate::Select(impl_source) => {
1153 confirm_select_candidate(selcx, obligation, obligation_trait_ref, impl_source)
1156 // When checking for cycle during evaluation, we compare predicates with
1157 // "syntactic" equality. Since normalization generally introduces a type
1158 // with new region variables, we need to resolve them to existing variables
1159 // when possible for this to work. See `auto-trait-projection-recursion.rs`
1160 // for a case where this matters.
1161 if progress.ty.has_infer_regions() {
1162 progress.ty = OpportunisticRegionResolver::new(selcx.infcx()).fold_ty(progress.ty);
1167 fn confirm_select_candidate<'cx, 'tcx>(
1168 selcx: &mut SelectionContext<'cx, 'tcx>,
1169 obligation: &ProjectionTyObligation<'tcx>,
1170 obligation_trait_ref: &ty::TraitRef<'tcx>,
1171 impl_source: Selection<'tcx>,
1172 ) -> Progress<'tcx> {
1174 super::ImplSourceUserDefined(data) => confirm_impl_candidate(selcx, obligation, data),
1175 super::ImplSourceGenerator(data) => confirm_generator_candidate(selcx, obligation, data),
1176 super::ImplSourceClosure(data) => confirm_closure_candidate(selcx, obligation, data),
1177 super::ImplSourceFnPointer(data) => confirm_fn_pointer_candidate(selcx, obligation, data),
1178 super::ImplSourceDiscriminantKind(data) => {
1179 confirm_discriminant_kind_candidate(selcx, obligation, data)
1181 super::ImplSourceObject(_) => {
1182 confirm_object_candidate(selcx, obligation, obligation_trait_ref)
1184 super::ImplSourceAutoImpl(..)
1185 | super::ImplSourceParam(..)
1186 | super::ImplSourceBuiltin(..)
1187 | super::ImplSourceTraitAlias(..) =>
1188 // we don't create Select candidates with this kind of resolution
1191 obligation.cause.span,
1192 "Cannot project an associated type from `{:?}`",
1199 fn confirm_object_candidate<'cx, 'tcx>(
1200 selcx: &mut SelectionContext<'cx, 'tcx>,
1201 obligation: &ProjectionTyObligation<'tcx>,
1202 obligation_trait_ref: &ty::TraitRef<'tcx>,
1203 ) -> Progress<'tcx> {
1204 let self_ty = obligation_trait_ref.self_ty();
1205 let object_ty = selcx.infcx().shallow_resolve(self_ty);
1206 debug!("confirm_object_candidate(object_ty={:?})", object_ty);
1207 let data = match object_ty.kind {
1208 ty::Dynamic(ref data, ..) => data,
1210 obligation.cause.span,
1211 "confirm_object_candidate called with non-object: {:?}",
1215 let env_predicates = data
1216 .projection_bounds()
1217 .map(|p| p.with_self_ty(selcx.tcx(), object_ty).to_predicate(selcx.tcx()));
1218 let env_predicate = {
1219 let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1221 // select only those projections that are actually projecting an
1222 // item with the correct name
1223 let env_predicates = env_predicates.filter_map(|o| match o.predicate.kind() {
1224 &ty::PredicateKind::Projection(data)
1225 if data.projection_def_id() == obligation.predicate.item_def_id =>
1232 // select those with a relevant trait-ref
1233 let mut env_predicates = env_predicates.filter(|data| {
1234 let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1235 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1236 selcx.infcx().probe(|_| {
1239 .at(&obligation.cause, obligation.param_env)
1240 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1245 // select the first matching one; there really ought to be one or
1246 // else the object type is not WF, since an object type should
1247 // include all of its projections explicitly
1248 match env_predicates.next() {
1249 Some(env_predicate) => env_predicate,
1252 "confirm_object_candidate: no env-predicate \
1253 found in object type `{:?}`; ill-formed",
1256 return Progress::error(selcx.tcx());
1261 confirm_param_env_candidate(selcx, obligation, env_predicate)
1264 fn confirm_generator_candidate<'cx, 'tcx>(
1265 selcx: &mut SelectionContext<'cx, 'tcx>,
1266 obligation: &ProjectionTyObligation<'tcx>,
1267 impl_source: ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>,
1268 ) -> Progress<'tcx> {
1269 let gen_sig = impl_source.substs.as_generator().poly_sig();
1270 let Normalized { value: gen_sig, obligations } = normalize_with_depth(
1272 obligation.param_env,
1273 obligation.cause.clone(),
1274 obligation.recursion_depth + 1,
1279 "confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1280 obligation, gen_sig, obligations
1283 let tcx = selcx.tcx();
1285 let gen_def_id = tcx.require_lang_item(GeneratorTraitLangItem, None);
1287 let predicate = super::util::generator_trait_ref_and_outputs(
1290 obligation.predicate.self_ty(),
1293 .map_bound(|(trait_ref, yield_ty, return_ty)| {
1294 let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1295 let ty = if name == sym::Return {
1297 } else if name == sym::Yield {
1303 ty::ProjectionPredicate {
1304 projection_ty: ty::ProjectionTy {
1305 substs: trait_ref.substs,
1306 item_def_id: obligation.predicate.item_def_id,
1312 confirm_param_env_candidate(selcx, obligation, predicate)
1313 .with_addl_obligations(impl_source.nested)
1314 .with_addl_obligations(obligations)
1317 fn confirm_discriminant_kind_candidate<'cx, 'tcx>(
1318 selcx: &mut SelectionContext<'cx, 'tcx>,
1319 obligation: &ProjectionTyObligation<'tcx>,
1320 _: ImplSourceDiscriminantKindData,
1321 ) -> Progress<'tcx> {
1322 let tcx = selcx.tcx();
1324 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1325 let substs = tcx.mk_substs([self_ty.into()].iter());
1327 let assoc_items = tcx.associated_items(tcx.lang_items().discriminant_kind_trait().unwrap());
1328 // FIXME: emit an error if the trait definition is wrong
1329 let discriminant_def_id = assoc_items.in_definition_order().next().unwrap().def_id;
1331 let discriminant_ty = match self_ty.kind {
1332 // Use the discriminant type for enums.
1333 ty::Adt(adt, _) if adt.is_enum() => adt.repr.discr_type().to_ty(tcx),
1334 // Default to `i32` for generators.
1335 ty::Generator(..) => tcx.types.i32,
1336 // Use `u8` for all other types.
1340 let predicate = ty::ProjectionPredicate {
1341 projection_ty: ty::ProjectionTy { substs, item_def_id: discriminant_def_id },
1342 ty: discriminant_ty,
1345 confirm_param_env_candidate(selcx, obligation, ty::Binder::bind(predicate))
1348 fn confirm_fn_pointer_candidate<'cx, 'tcx>(
1349 selcx: &mut SelectionContext<'cx, 'tcx>,
1350 obligation: &ProjectionTyObligation<'tcx>,
1351 fn_pointer_impl_source: ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>,
1352 ) -> Progress<'tcx> {
1353 let fn_type = selcx.infcx().shallow_resolve(fn_pointer_impl_source.fn_ty);
1354 let sig = fn_type.fn_sig(selcx.tcx());
1355 let Normalized { value: sig, obligations } = normalize_with_depth(
1357 obligation.param_env,
1358 obligation.cause.clone(),
1359 obligation.recursion_depth + 1,
1363 confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1364 .with_addl_obligations(fn_pointer_impl_source.nested)
1365 .with_addl_obligations(obligations)
1368 fn confirm_closure_candidate<'cx, 'tcx>(
1369 selcx: &mut SelectionContext<'cx, 'tcx>,
1370 obligation: &ProjectionTyObligation<'tcx>,
1371 impl_source: ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>,
1372 ) -> Progress<'tcx> {
1373 let closure_sig = impl_source.substs.as_closure().sig();
1374 let Normalized { value: closure_sig, obligations } = normalize_with_depth(
1376 obligation.param_env,
1377 obligation.cause.clone(),
1378 obligation.recursion_depth + 1,
1383 "confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1384 obligation, closure_sig, obligations
1387 confirm_callable_candidate(selcx, obligation, closure_sig, util::TupleArgumentsFlag::No)
1388 .with_addl_obligations(impl_source.nested)
1389 .with_addl_obligations(obligations)
1392 fn confirm_callable_candidate<'cx, 'tcx>(
1393 selcx: &mut SelectionContext<'cx, 'tcx>,
1394 obligation: &ProjectionTyObligation<'tcx>,
1395 fn_sig: ty::PolyFnSig<'tcx>,
1396 flag: util::TupleArgumentsFlag,
1397 ) -> Progress<'tcx> {
1398 let tcx = selcx.tcx();
1400 debug!("confirm_callable_candidate({:?},{:?})", obligation, fn_sig);
1402 let fn_once_def_id = tcx.require_lang_item(FnOnceTraitLangItem, None);
1403 let fn_once_output_def_id = tcx.require_lang_item(FnOnceOutputLangItem, None);
1405 let predicate = super::util::closure_trait_ref_and_return_type(
1408 obligation.predicate.self_ty(),
1412 .map_bound(|(trait_ref, ret_type)| ty::ProjectionPredicate {
1413 projection_ty: ty::ProjectionTy {
1414 substs: trait_ref.substs,
1415 item_def_id: fn_once_output_def_id,
1420 confirm_param_env_candidate(selcx, obligation, predicate)
1423 fn confirm_param_env_candidate<'cx, 'tcx>(
1424 selcx: &mut SelectionContext<'cx, 'tcx>,
1425 obligation: &ProjectionTyObligation<'tcx>,
1426 poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
1427 ) -> Progress<'tcx> {
1428 let infcx = selcx.infcx();
1429 let cause = &obligation.cause;
1430 let param_env = obligation.param_env;
1432 let (cache_entry, _) = infcx.replace_bound_vars_with_fresh_vars(
1434 LateBoundRegionConversionTime::HigherRankedType,
1438 let cache_trait_ref = cache_entry.projection_ty.trait_ref(infcx.tcx);
1439 let obligation_trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1440 match infcx.at(cause, param_env).eq(cache_trait_ref, obligation_trait_ref) {
1441 Ok(InferOk { value: _, obligations }) => Progress { ty: cache_entry.ty, obligations },
1444 "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
1445 obligation, poly_cache_entry, e,
1447 debug!("confirm_param_env_candidate: {}", msg);
1448 let err = infcx.tcx.ty_error_with_message(obligation.cause.span, &msg);
1449 Progress { ty: err, obligations: vec![] }
1454 fn confirm_impl_candidate<'cx, 'tcx>(
1455 selcx: &mut SelectionContext<'cx, 'tcx>,
1456 obligation: &ProjectionTyObligation<'tcx>,
1457 impl_impl_source: ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>,
1458 ) -> Progress<'tcx> {
1459 let tcx = selcx.tcx();
1461 let ImplSourceUserDefinedData { impl_def_id, substs, nested } = impl_impl_source;
1462 let assoc_item_id = obligation.predicate.item_def_id;
1463 let trait_def_id = tcx.trait_id_of_impl(impl_def_id).unwrap();
1465 let param_env = obligation.param_env;
1466 let assoc_ty = match assoc_ty_def(selcx, impl_def_id, assoc_item_id) {
1467 Ok(assoc_ty) => assoc_ty,
1468 Err(ErrorReported) => return Progress { ty: tcx.ty_error(), obligations: nested },
1471 if !assoc_ty.item.defaultness.has_value() {
1472 // This means that the impl is missing a definition for the
1473 // associated type. This error will be reported by the type
1474 // checker method `check_impl_items_against_trait`, so here we
1475 // just return Error.
1477 "confirm_impl_candidate: no associated type {:?} for {:?}",
1478 assoc_ty.item.ident, obligation.predicate
1480 return Progress { ty: tcx.ty_error(), obligations: nested };
1482 // If we're trying to normalize `<Vec<u32> as X>::A<S>` using
1483 //`impl<T> X for Vec<T> { type A<Y> = Box<Y>; }`, then:
1485 // * `obligation.predicate.substs` is `[Vec<u32>, S]`
1486 // * `substs` is `[u32]`
1487 // * `substs` ends up as `[u32, S]`
1488 let substs = obligation.predicate.substs.rebase_onto(tcx, trait_def_id, substs);
1490 translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.defining_node);
1491 let ty = tcx.type_of(assoc_ty.item.def_id);
1492 if substs.len() != tcx.generics_of(assoc_ty.item.def_id).count() {
1493 let err = tcx.ty_error_with_message(
1495 "impl item and trait item have different parameter counts",
1497 Progress { ty: err, obligations: nested }
1499 Progress { ty: ty.subst(tcx, substs), obligations: nested }
1503 /// Locate the definition of an associated type in the specialization hierarchy,
1504 /// starting from the given impl.
1506 /// Based on the "projection mode", this lookup may in fact only examine the
1507 /// topmost impl. See the comments for `Reveal` for more details.
1509 selcx: &SelectionContext<'_, '_>,
1511 assoc_ty_def_id: DefId,
1512 ) -> Result<specialization_graph::LeafDef, ErrorReported> {
1513 let tcx = selcx.tcx();
1514 let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1515 let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1516 let trait_def = tcx.trait_def(trait_def_id);
1518 // This function may be called while we are still building the
1519 // specialization graph that is queried below (via TraitDef::ancestors()),
1520 // so, in order to avoid unnecessary infinite recursion, we manually look
1521 // for the associated item at the given impl.
1522 // If there is no such item in that impl, this function will fail with a
1523 // cycle error if the specialization graph is currently being built.
1524 let impl_node = specialization_graph::Node::Impl(impl_def_id);
1525 for item in impl_node.items(tcx) {
1526 if matches!(item.kind, ty::AssocKind::Type)
1527 && tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id)
1529 return Ok(specialization_graph::LeafDef {
1531 defining_node: impl_node,
1532 finalizing_node: if item.defaultness.is_default() { None } else { Some(impl_node) },
1537 let ancestors = trait_def.ancestors(tcx, impl_def_id)?;
1538 if let Some(assoc_item) = ancestors.leaf_def(tcx, assoc_ty_name, ty::AssocKind::Type) {
1541 // This is saying that neither the trait nor
1542 // the impl contain a definition for this
1543 // associated type. Normally this situation
1544 // could only arise through a compiler bug --
1545 // if the user wrote a bad item name, it
1546 // should have failed in astconv.
1547 bug!("No associated type `{}` for {}", assoc_ty_name, tcx.def_path_str(impl_def_id))
1551 crate trait ProjectionCacheKeyExt<'tcx>: Sized {
1552 fn from_poly_projection_predicate(
1553 selcx: &mut SelectionContext<'cx, 'tcx>,
1554 predicate: ty::PolyProjectionPredicate<'tcx>,
1558 impl<'tcx> ProjectionCacheKeyExt<'tcx> for ProjectionCacheKey<'tcx> {
1559 fn from_poly_projection_predicate(
1560 selcx: &mut SelectionContext<'cx, 'tcx>,
1561 predicate: ty::PolyProjectionPredicate<'tcx>,
1563 let infcx = selcx.infcx();
1564 // We don't do cross-snapshot caching of obligations with escaping regions,
1565 // so there's no cache key to use
1566 predicate.no_bound_vars().map(|predicate| {
1567 ProjectionCacheKey::new(
1568 // We don't attempt to match up with a specific type-variable state
1569 // from a specific call to `opt_normalize_projection_type` - if
1570 // there's no precise match, the original cache entry is "stranded"
1572 infcx.resolve_vars_if_possible(&predicate.projection_ty),