1 //! Code for projecting associated types out of trait references.
3 use super::specialization_graph;
4 use super::translate_substs;
6 use super::MismatchedProjectionTypes;
8 use super::ObligationCause;
9 use super::PredicateObligation;
11 use super::SelectionContext;
12 use super::SelectionError;
14 ImplSourceClosureData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
15 ImplSourceGeneratorData, ImplSourcePointeeData, ImplSourceUserDefinedData,
17 use super::{Normalized, NormalizedTy, ProjectionCacheEntry, ProjectionCacheKey};
19 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
20 use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
21 use crate::traits::error_reporting::InferCtxtExt as _;
22 use rustc_data_structures::sso::SsoHashSet;
23 use rustc_data_structures::stack::ensure_sufficient_stack;
24 use rustc_errors::ErrorReported;
25 use rustc_hir::def::DefKind;
26 use rustc_hir::def_id::DefId;
27 use rustc_hir::lang_items::LangItem;
28 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
29 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
30 use rustc_middle::ty::subst::Subst;
31 use rustc_middle::ty::{self, Term, ToPredicate, Ty, TyCtxt};
32 use rustc_span::symbol::sym;
34 use std::collections::BTreeMap;
36 pub use rustc_middle::traits::Reveal;
38 pub type PolyProjectionObligation<'tcx> = Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
40 pub type ProjectionObligation<'tcx> = Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
42 pub type ProjectionTyObligation<'tcx> = Obligation<'tcx, ty::ProjectionTy<'tcx>>;
44 pub(super) struct InProgress;
46 /// When attempting to resolve `<T as TraitRef>::Name` ...
48 pub enum ProjectionError<'tcx> {
49 /// ...we found multiple sources of information and couldn't resolve the ambiguity.
52 /// ...an error occurred matching `T : TraitRef`
53 TraitSelectionError(SelectionError<'tcx>),
56 #[derive(PartialEq, Eq, Debug)]
57 enum ProjectionCandidate<'tcx> {
58 /// From a where-clause in the env or object type
59 ParamEnv(ty::PolyProjectionPredicate<'tcx>),
61 /// From the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
62 TraitDef(ty::PolyProjectionPredicate<'tcx>),
64 /// Bounds specified on an object type
65 Object(ty::PolyProjectionPredicate<'tcx>),
67 /// From an "impl" (or a "pseudo-impl" returned by select)
68 Select(Selection<'tcx>),
71 enum ProjectionCandidateSet<'tcx> {
73 Single(ProjectionCandidate<'tcx>),
75 Error(SelectionError<'tcx>),
78 impl<'tcx> ProjectionCandidateSet<'tcx> {
79 fn mark_ambiguous(&mut self) {
80 *self = ProjectionCandidateSet::Ambiguous;
83 fn mark_error(&mut self, err: SelectionError<'tcx>) {
84 *self = ProjectionCandidateSet::Error(err);
87 // Returns true if the push was successful, or false if the candidate
88 // was discarded -- this could be because of ambiguity, or because
89 // a higher-priority candidate is already there.
90 fn push_candidate(&mut self, candidate: ProjectionCandidate<'tcx>) -> bool {
91 use self::ProjectionCandidate::*;
92 use self::ProjectionCandidateSet::*;
94 // This wacky variable is just used to try and
95 // make code readable and avoid confusing paths.
96 // It is assigned a "value" of `()` only on those
97 // paths in which we wish to convert `*self` to
98 // ambiguous (and return false, because the candidate
99 // was not used). On other paths, it is not assigned,
100 // and hence if those paths *could* reach the code that
101 // comes after the match, this fn would not compile.
102 let convert_to_ambiguous;
106 *self = Single(candidate);
111 // Duplicates can happen inside ParamEnv. In the case, we
112 // perform a lazy deduplication.
113 if current == &candidate {
117 // Prefer where-clauses. As in select, if there are multiple
118 // candidates, we prefer where-clause candidates over impls. This
119 // may seem a bit surprising, since impls are the source of
120 // "truth" in some sense, but in fact some of the impls that SEEM
121 // applicable are not, because of nested obligations. Where
122 // clauses are the safer choice. See the comment on
123 // `select::SelectionCandidate` and #21974 for more details.
124 match (current, candidate) {
125 (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
126 (ParamEnv(..), _) => return false,
127 (_, ParamEnv(..)) => unreachable!(),
128 (_, _) => convert_to_ambiguous = (),
132 Ambiguous | Error(..) => {
137 // We only ever get here when we moved from a single candidate
139 let () = convert_to_ambiguous;
145 /// Evaluates constraints of the form:
147 /// for<...> <T as Trait>::U == V
149 /// If successful, this may result in additional obligations. Also returns
150 /// the projection cache key used to track these additional obligations.
154 /// - `Err(_)`: the projection can be normalized, but is not equal to the
156 /// - `Ok(Err(InProgress))`: this is called recursively while normalizing
157 /// the same projection.
158 /// - `Ok(Ok(None))`: The projection cannot be normalized due to ambiguity
159 /// (resolving some inference variables in the projection may fix this).
160 /// - `Ok(Ok(Some(obligations)))`: The projection bound holds subject to
161 /// the given obligations. If the projection cannot be normalized because
162 /// the required trait bound doesn't hold this returned with `obligations`
163 /// being a predicate that cannot be proven.
164 #[instrument(level = "debug", skip(selcx))]
165 pub(super) fn poly_project_and_unify_type<'cx, 'tcx>(
166 selcx: &mut SelectionContext<'cx, 'tcx>,
167 obligation: &PolyProjectionObligation<'tcx>,
169 Result<Option<Vec<PredicateObligation<'tcx>>>, InProgress>,
170 MismatchedProjectionTypes<'tcx>,
172 let infcx = selcx.infcx();
173 infcx.commit_if_ok(|_snapshot| {
174 let placeholder_predicate =
175 infcx.replace_bound_vars_with_placeholders(obligation.predicate);
177 let placeholder_obligation = obligation.with(placeholder_predicate);
178 let result = project_and_unify_type(selcx, &placeholder_obligation)?;
183 /// Evaluates constraints of the form:
185 /// <T as Trait>::U == V
187 /// If successful, this may result in additional obligations.
189 /// See [poly_project_and_unify_type] for an explanation of the return value.
190 fn project_and_unify_type<'cx, 'tcx>(
191 selcx: &mut SelectionContext<'cx, 'tcx>,
192 obligation: &ProjectionObligation<'tcx>,
194 Result<Option<Vec<PredicateObligation<'tcx>>>, InProgress>,
195 MismatchedProjectionTypes<'tcx>,
197 debug!(?obligation, "project_and_unify_type");
199 let mut obligations = vec![];
201 let infcx = selcx.infcx();
202 let normalized = match opt_normalize_projection_type(
204 obligation.param_env,
205 obligation.predicate.projection_ty,
206 obligation.cause.clone(),
207 obligation.recursion_depth,
211 Ok(None) => return Ok(Ok(None)),
212 Err(InProgress) => return Ok(Err(InProgress)),
214 debug!(?normalized, ?obligations, "project_and_unify_type result");
216 .at(&obligation.cause, obligation.param_env)
217 .eq(normalized, obligation.predicate.term)
219 Ok(InferOk { obligations: inferred_obligations, value: () }) => {
220 obligations.extend(inferred_obligations);
221 Ok(Ok(Some(obligations)))
224 debug!("project_and_unify_type: equating types encountered error {:?}", err);
225 Err(MismatchedProjectionTypes { err })
230 /// Normalizes any associated type projections in `value`, replacing
231 /// them with a fully resolved type where possible. The return value
232 /// combines the normalized result and any additional obligations that
233 /// were incurred as result.
234 pub fn normalize<'a, 'b, 'tcx, T>(
235 selcx: &'a mut SelectionContext<'b, 'tcx>,
236 param_env: ty::ParamEnv<'tcx>,
237 cause: ObligationCause<'tcx>,
239 ) -> Normalized<'tcx, T>
241 T: TypeFoldable<'tcx>,
243 let mut obligations = Vec::new();
244 let value = normalize_to(selcx, param_env, cause, value, &mut obligations);
245 Normalized { value, obligations }
248 pub fn normalize_to<'a, 'b, 'tcx, T>(
249 selcx: &'a mut SelectionContext<'b, 'tcx>,
250 param_env: ty::ParamEnv<'tcx>,
251 cause: ObligationCause<'tcx>,
253 obligations: &mut Vec<PredicateObligation<'tcx>>,
256 T: TypeFoldable<'tcx>,
258 normalize_with_depth_to(selcx, param_env, cause, 0, value, obligations)
261 /// As `normalize`, but with a custom depth.
262 pub fn normalize_with_depth<'a, 'b, 'tcx, T>(
263 selcx: &'a mut SelectionContext<'b, 'tcx>,
264 param_env: ty::ParamEnv<'tcx>,
265 cause: ObligationCause<'tcx>,
268 ) -> Normalized<'tcx, T>
270 T: TypeFoldable<'tcx>,
272 let mut obligations = Vec::new();
273 let value = normalize_with_depth_to(selcx, param_env, cause, depth, value, &mut obligations);
274 Normalized { value, obligations }
277 #[instrument(level = "info", skip(selcx, param_env, cause, obligations))]
278 pub fn normalize_with_depth_to<'a, 'b, 'tcx, T>(
279 selcx: &'a mut SelectionContext<'b, 'tcx>,
280 param_env: ty::ParamEnv<'tcx>,
281 cause: ObligationCause<'tcx>,
284 obligations: &mut Vec<PredicateObligation<'tcx>>,
287 T: TypeFoldable<'tcx>,
289 debug!(obligations.len = obligations.len());
290 let mut normalizer = AssocTypeNormalizer::new(selcx, param_env, cause, depth, obligations);
291 let result = ensure_sufficient_stack(|| normalizer.fold(value));
292 debug!(?result, obligations.len = normalizer.obligations.len());
293 debug!(?normalizer.obligations,);
297 pub(crate) fn needs_normalization<'tcx, T: TypeFoldable<'tcx>>(value: &T, reveal: Reveal) -> bool {
299 Reveal::UserFacing => value
300 .has_type_flags(ty::TypeFlags::HAS_TY_PROJECTION | ty::TypeFlags::HAS_CT_PROJECTION),
301 Reveal::All => value.has_type_flags(
302 ty::TypeFlags::HAS_TY_PROJECTION
303 | ty::TypeFlags::HAS_TY_OPAQUE
304 | ty::TypeFlags::HAS_CT_PROJECTION,
309 struct AssocTypeNormalizer<'a, 'b, 'tcx> {
310 selcx: &'a mut SelectionContext<'b, 'tcx>,
311 param_env: ty::ParamEnv<'tcx>,
312 cause: ObligationCause<'tcx>,
313 obligations: &'a mut Vec<PredicateObligation<'tcx>>,
315 universes: Vec<Option<ty::UniverseIndex>>,
318 impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
320 selcx: &'a mut SelectionContext<'b, 'tcx>,
321 param_env: ty::ParamEnv<'tcx>,
322 cause: ObligationCause<'tcx>,
324 obligations: &'a mut Vec<PredicateObligation<'tcx>>,
325 ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
326 AssocTypeNormalizer { selcx, param_env, cause, obligations, depth, universes: vec![] }
329 fn fold<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
330 let value = self.selcx.infcx().resolve_vars_if_possible(value);
334 !value.has_escaping_bound_vars(),
335 "Normalizing {:?} without wrapping in a `Binder`",
339 if !needs_normalization(&value, self.param_env.reveal()) {
342 value.fold_with(self)
347 impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
348 fn tcx<'c>(&'c self) -> TyCtxt<'tcx> {
352 fn fold_binder<T: TypeFoldable<'tcx>>(
354 t: ty::Binder<'tcx, T>,
355 ) -> ty::Binder<'tcx, T> {
356 self.universes.push(None);
357 let t = t.super_fold_with(self);
358 self.universes.pop();
362 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
363 if !needs_normalization(&ty, self.param_env.reveal()) {
367 // We try to be a little clever here as a performance optimization in
368 // cases where there are nested projections under binders.
371 // for<'a> fn(<T as Foo>::One<'a, Box<dyn Bar<'a, Item=<T as Foo>::Two<'a>>>>)
373 // We normalize the substs on the projection before the projecting, but
374 // if we're naive, we'll
375 // replace bound vars on inner, project inner, replace placeholders on inner,
376 // replace bound vars on outer, project outer, replace placeholders on outer
378 // However, if we're a bit more clever, we can replace the bound vars
379 // on the entire type before normalizing nested projections, meaning we
380 // replace bound vars on outer, project inner,
381 // project outer, replace placeholders on outer
383 // This is possible because the inner `'a` will already be a placeholder
384 // when we need to normalize the inner projection
386 // On the other hand, this does add a bit of complexity, since we only
387 // replace bound vars if the current type is a `Projection` and we need
388 // to make sure we don't forget to fold the substs regardless.
391 // This is really important. While we *can* handle this, this has
392 // severe performance implications for large opaque types with
393 // late-bound regions. See `issue-88862` benchmark.
394 ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => {
395 // Only normalize `impl Trait` outside of type inference, usually in codegen.
396 match self.param_env.reveal() {
397 Reveal::UserFacing => ty.super_fold_with(self),
400 let recursion_limit = self.tcx().recursion_limit();
401 if !recursion_limit.value_within_limit(self.depth) {
402 let obligation = Obligation::with_depth(
408 self.selcx.infcx().report_overflow_error(&obligation, true);
411 let substs = substs.super_fold_with(self);
412 let generic_ty = self.tcx().type_of(def_id);
413 let concrete_ty = generic_ty.subst(self.tcx(), substs);
415 let folded_ty = self.fold_ty(concrete_ty);
422 ty::Projection(data) if !data.has_escaping_bound_vars() => {
423 // This branch is *mostly* just an optimization: when we don't
424 // have escaping bound vars, we don't need to replace them with
425 // placeholders (see branch below). *Also*, we know that we can
426 // register an obligation to *later* project, since we know
427 // there won't be bound vars there.
429 let data = data.super_fold_with(self);
430 let normalized_ty = normalize_projection_type(
436 &mut self.obligations,
442 obligations.len = ?self.obligations.len(),
443 "AssocTypeNormalizer: normalized type"
445 normalized_ty.ty().unwrap()
448 ty::Projection(data) => {
449 // If there are escaping bound vars, we temporarily replace the
450 // bound vars with placeholders. Note though, that in the case
451 // that we still can't project for whatever reason (e.g. self
452 // type isn't known enough), we *can't* register an obligation
453 // and return an inference variable (since then that obligation
454 // would have bound vars and that's a can of worms). Instead,
455 // we just give up and fall back to pretending like we never tried!
457 // Note: this isn't necessarily the final approach here; we may
458 // want to figure out how to register obligations with escaping vars
459 // or handle this some other way.
461 let infcx = self.selcx.infcx();
462 let (data, mapped_regions, mapped_types, mapped_consts) =
463 BoundVarReplacer::replace_bound_vars(infcx, &mut self.universes, data);
464 let data = data.super_fold_with(self);
465 let normalized_ty = opt_normalize_projection_type(
471 &mut self.obligations,
475 .map(|term| term.ty().unwrap())
476 .map(|normalized_ty| {
477 PlaceholderReplacer::replace_placeholders(
486 .unwrap_or_else(|| ty.super_fold_with(self));
492 obligations.len = ?self.obligations.len(),
493 "AssocTypeNormalizer: normalized type"
498 _ => ty.super_fold_with(self),
502 fn fold_const(&mut self, constant: ty::Const<'tcx>) -> ty::Const<'tcx> {
503 if self.selcx.tcx().lazy_normalization() {
506 let constant = constant.super_fold_with(self);
507 constant.eval(self.selcx.tcx(), self.param_env)
512 pub struct BoundVarReplacer<'me, 'tcx> {
513 infcx: &'me InferCtxt<'me, 'tcx>,
514 // These three maps track the bound variable that were replaced by placeholders. It might be
515 // nice to remove these since we already have the `kind` in the placeholder; we really just need
516 // the `var` (but we *could* bring that into scope if we were to track them as we pass them).
517 mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
518 mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
519 mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
520 // The current depth relative to *this* folding, *not* the entire normalization. In other words,
521 // the depth of binders we've passed here.
522 current_index: ty::DebruijnIndex,
523 // The `UniverseIndex` of the binding levels above us. These are optional, since we are lazy:
524 // we don't actually create a universe until we see a bound var we have to replace.
525 universe_indices: &'me mut Vec<Option<ty::UniverseIndex>>,
528 impl<'me, 'tcx> BoundVarReplacer<'me, 'tcx> {
529 /// Returns `Some` if we *were* able to replace bound vars. If there are any bound vars that
530 /// use a binding level above `universe_indices.len()`, we fail.
531 pub fn replace_bound_vars<T: TypeFoldable<'tcx>>(
532 infcx: &'me InferCtxt<'me, 'tcx>,
533 universe_indices: &'me mut Vec<Option<ty::UniverseIndex>>,
537 BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
538 BTreeMap<ty::PlaceholderType, ty::BoundTy>,
539 BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
541 let mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion> = BTreeMap::new();
542 let mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy> = BTreeMap::new();
543 let mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar> = BTreeMap::new();
545 let mut replacer = BoundVarReplacer {
550 current_index: ty::INNERMOST,
554 let value = value.super_fold_with(&mut replacer);
556 (value, replacer.mapped_regions, replacer.mapped_types, replacer.mapped_consts)
559 fn universe_for(&mut self, debruijn: ty::DebruijnIndex) -> ty::UniverseIndex {
560 let infcx = self.infcx;
562 self.universe_indices.len() + self.current_index.as_usize() - debruijn.as_usize() - 1;
563 let universe = self.universe_indices[index].unwrap_or_else(|| {
564 for i in self.universe_indices.iter_mut().take(index + 1) {
565 *i = i.or_else(|| Some(infcx.create_next_universe()))
567 self.universe_indices[index].unwrap()
573 impl<'tcx> TypeFolder<'tcx> for BoundVarReplacer<'_, 'tcx> {
574 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
578 fn fold_binder<T: TypeFoldable<'tcx>>(
580 t: ty::Binder<'tcx, T>,
581 ) -> ty::Binder<'tcx, T> {
582 self.current_index.shift_in(1);
583 let t = t.super_fold_with(self);
584 self.current_index.shift_out(1);
588 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
590 ty::ReLateBound(debruijn, _)
591 if debruijn.as_usize() + 1
592 > self.current_index.as_usize() + self.universe_indices.len() =>
594 bug!("Bound vars outside of `self.universe_indices`");
596 ty::ReLateBound(debruijn, br) if debruijn >= self.current_index => {
597 let universe = self.universe_for(debruijn);
598 let p = ty::PlaceholderRegion { universe, name: br.kind };
599 self.mapped_regions.insert(p, br);
600 self.infcx.tcx.mk_region(ty::RePlaceholder(p))
606 fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
608 ty::Bound(debruijn, _)
609 if debruijn.as_usize() + 1
610 > self.current_index.as_usize() + self.universe_indices.len() =>
612 bug!("Bound vars outside of `self.universe_indices`");
614 ty::Bound(debruijn, bound_ty) if debruijn >= self.current_index => {
615 let universe = self.universe_for(debruijn);
616 let p = ty::PlaceholderType { universe, name: bound_ty.var };
617 self.mapped_types.insert(p, bound_ty);
618 self.infcx.tcx.mk_ty(ty::Placeholder(p))
620 _ if t.has_vars_bound_at_or_above(self.current_index) => t.super_fold_with(self),
625 fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
627 ty::ConstKind::Bound(debruijn, _)
628 if debruijn.as_usize() + 1
629 > self.current_index.as_usize() + self.universe_indices.len() =>
631 bug!("Bound vars outside of `self.universe_indices`");
633 ty::ConstKind::Bound(debruijn, bound_const) if debruijn >= self.current_index => {
634 let universe = self.universe_for(debruijn);
635 let p = ty::PlaceholderConst {
637 name: ty::BoundConst { var: bound_const, ty: ct.ty() },
639 self.mapped_consts.insert(p, bound_const);
642 .mk_const(ty::ConstS { val: ty::ConstKind::Placeholder(p), ty: ct.ty() })
644 _ if ct.has_vars_bound_at_or_above(self.current_index) => ct.super_fold_with(self),
650 // The inverse of `BoundVarReplacer`: replaces placeholders with the bound vars from which they came.
651 pub struct PlaceholderReplacer<'me, 'tcx> {
652 infcx: &'me InferCtxt<'me, 'tcx>,
653 mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
654 mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
655 mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
656 universe_indices: &'me Vec<Option<ty::UniverseIndex>>,
657 current_index: ty::DebruijnIndex,
660 impl<'me, 'tcx> PlaceholderReplacer<'me, 'tcx> {
661 pub fn replace_placeholders<T: TypeFoldable<'tcx>>(
662 infcx: &'me InferCtxt<'me, 'tcx>,
663 mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
664 mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
665 mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
666 universe_indices: &'me Vec<Option<ty::UniverseIndex>>,
669 let mut replacer = PlaceholderReplacer {
675 current_index: ty::INNERMOST,
677 value.super_fold_with(&mut replacer)
681 impl<'tcx> TypeFolder<'tcx> for PlaceholderReplacer<'_, 'tcx> {
682 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
686 fn fold_binder<T: TypeFoldable<'tcx>>(
688 t: ty::Binder<'tcx, T>,
689 ) -> ty::Binder<'tcx, T> {
690 if !t.has_placeholders() && !t.has_infer_regions() {
693 self.current_index.shift_in(1);
694 let t = t.super_fold_with(self);
695 self.current_index.shift_out(1);
699 fn fold_region(&mut self, r0: ty::Region<'tcx>) -> ty::Region<'tcx> {
705 .unwrap_region_constraints()
706 .opportunistic_resolve_region(self.infcx.tcx, r0),
711 ty::RePlaceholder(p) => {
712 let replace_var = self.mapped_regions.get(&p);
714 Some(replace_var) => {
718 .position(|u| matches!(u, Some(pu) if *pu == p.universe))
719 .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
720 let db = ty::DebruijnIndex::from_usize(
721 self.universe_indices.len() - index + self.current_index.as_usize() - 1,
723 self.tcx().mk_region(ty::ReLateBound(db, *replace_var))
731 debug!(?r0, ?r1, ?r2, "fold_region");
736 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
738 ty::Placeholder(p) => {
739 let replace_var = self.mapped_types.get(&p);
741 Some(replace_var) => {
745 .position(|u| matches!(u, Some(pu) if *pu == p.universe))
746 .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
747 let db = ty::DebruijnIndex::from_usize(
748 self.universe_indices.len() - index + self.current_index.as_usize() - 1,
750 self.tcx().mk_ty(ty::Bound(db, *replace_var))
756 _ if ty.has_placeholders() || ty.has_infer_regions() => ty.super_fold_with(self),
761 fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
762 if let ty::ConstKind::Placeholder(p) = ct.val() {
763 let replace_var = self.mapped_consts.get(&p);
765 Some(replace_var) => {
769 .position(|u| matches!(u, Some(pu) if *pu == p.universe))
770 .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
771 let db = ty::DebruijnIndex::from_usize(
772 self.universe_indices.len() - index + self.current_index.as_usize() - 1,
774 self.tcx().mk_const(ty::ConstS {
775 val: ty::ConstKind::Bound(db, *replace_var),
782 ct.super_fold_with(self)
787 /// The guts of `normalize`: normalize a specific projection like `<T
788 /// as Trait>::Item`. The result is always a type (and possibly
789 /// additional obligations). If ambiguity arises, which implies that
790 /// there are unresolved type variables in the projection, we will
791 /// substitute a fresh type variable `$X` and generate a new
792 /// obligation `<T as Trait>::Item == $X` for later.
793 pub fn normalize_projection_type<'a, 'b, 'tcx>(
794 selcx: &'a mut SelectionContext<'b, 'tcx>,
795 param_env: ty::ParamEnv<'tcx>,
796 projection_ty: ty::ProjectionTy<'tcx>,
797 cause: ObligationCause<'tcx>,
799 obligations: &mut Vec<PredicateObligation<'tcx>>,
801 opt_normalize_projection_type(
811 .unwrap_or_else(move || {
812 // if we bottom out in ambiguity, create a type variable
813 // and a deferred predicate to resolve this when more type
814 // information is available.
818 .infer_projection(param_env, projection_ty, cause, depth + 1, obligations)
823 /// The guts of `normalize`: normalize a specific projection like `<T
824 /// as Trait>::Item`. The result is always a type (and possibly
825 /// additional obligations). Returns `None` in the case of ambiguity,
826 /// which indicates that there are unbound type variables.
828 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
829 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
830 /// often immediately appended to another obligations vector. So now this
831 /// function takes an obligations vector and appends to it directly, which is
832 /// slightly uglier but avoids the need for an extra short-lived allocation.
833 #[instrument(level = "debug", skip(selcx, param_env, cause, obligations))]
834 fn opt_normalize_projection_type<'a, 'b, 'tcx>(
835 selcx: &'a mut SelectionContext<'b, 'tcx>,
836 param_env: ty::ParamEnv<'tcx>,
837 projection_ty: ty::ProjectionTy<'tcx>,
838 cause: ObligationCause<'tcx>,
840 obligations: &mut Vec<PredicateObligation<'tcx>>,
841 ) -> Result<Option<Term<'tcx>>, InProgress> {
842 let infcx = selcx.infcx();
843 // Don't use the projection cache in intercrate mode -
844 // the `infcx` may be re-used between intercrate in non-intercrate
845 // mode, which could lead to using incorrect cache results.
846 let use_cache = !selcx.is_intercrate();
848 let projection_ty = infcx.resolve_vars_if_possible(projection_ty);
849 let cache_key = ProjectionCacheKey::new(projection_ty);
851 // FIXME(#20304) For now, I am caching here, which is good, but it
852 // means we don't capture the type variables that are created in
853 // the case of ambiguity. Which means we may create a large stream
854 // of such variables. OTOH, if we move the caching up a level, we
855 // would not benefit from caching when proving `T: Trait<U=Foo>`
856 // bounds. It might be the case that we want two distinct caches,
857 // or else another kind of cache entry.
859 let cache_result = if use_cache {
860 infcx.inner.borrow_mut().projection_cache().try_start(cache_key)
865 Ok(()) => debug!("no cache"),
866 Err(ProjectionCacheEntry::Ambiguous) => {
867 // If we found ambiguity the last time, that means we will continue
868 // to do so until some type in the key changes (and we know it
869 // hasn't, because we just fully resolved it).
870 debug!("found cache entry: ambiguous");
873 Err(ProjectionCacheEntry::InProgress) => {
874 // Under lazy normalization, this can arise when
875 // bootstrapping. That is, imagine an environment with a
876 // where-clause like `A::B == u32`. Now, if we are asked
877 // to normalize `A::B`, we will want to check the
878 // where-clauses in scope. So we will try to unify `A::B`
879 // with `A::B`, which can trigger a recursive
882 debug!("found cache entry: in-progress");
884 // Cache that normalizing this projection resulted in a cycle. This
885 // should ensure that, unless this happens within a snapshot that's
886 // rolled back, fulfillment or evaluation will notice the cycle.
889 infcx.inner.borrow_mut().projection_cache().recur(cache_key);
891 return Err(InProgress);
893 Err(ProjectionCacheEntry::Recur) => {
894 debug!("recur cache");
895 return Err(InProgress);
897 Err(ProjectionCacheEntry::NormalizedTy { ty, complete: _ }) => {
898 // This is the hottest path in this function.
900 // If we find the value in the cache, then return it along
901 // with the obligations that went along with it. Note
902 // that, when using a fulfillment context, these
903 // obligations could in principle be ignored: they have
904 // already been registered when the cache entry was
905 // created (and hence the new ones will quickly be
906 // discarded as duplicated). But when doing trait
907 // evaluation this is not the case, and dropping the trait
908 // evaluations can causes ICEs (e.g., #43132).
909 debug!(?ty, "found normalized ty");
910 obligations.extend(ty.obligations);
911 return Ok(Some(ty.value));
913 Err(ProjectionCacheEntry::Error) => {
914 debug!("opt_normalize_projection_type: found error");
915 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
916 obligations.extend(result.obligations);
917 return Ok(Some(result.value.into()));
921 let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
923 match project(selcx, &obligation) {
924 Ok(Projected::Progress(Progress {
925 term: projected_term,
926 obligations: mut projected_obligations,
928 // if projection succeeded, then what we get out of this
929 // is also non-normalized (consider: it was derived from
930 // an impl, where-clause etc) and hence we must
933 let projected_term = selcx.infcx().resolve_vars_if_possible(projected_term);
935 let mut result = if projected_term.has_projections() {
936 let mut normalizer = AssocTypeNormalizer::new(
941 &mut projected_obligations,
943 let normalized_ty = normalizer.fold(projected_term);
945 Normalized { value: normalized_ty, obligations: projected_obligations }
947 Normalized { value: projected_term, obligations: projected_obligations }
950 let mut deduped: SsoHashSet<_> = Default::default();
951 result.obligations.drain_filter(|projected_obligation| {
952 if !deduped.insert(projected_obligation.clone()) {
959 infcx.inner.borrow_mut().projection_cache().insert_term(cache_key, result.clone());
961 obligations.extend(result.obligations);
962 Ok(Some(result.value.into()))
964 Ok(Projected::NoProgress(projected_ty)) => {
965 let result = Normalized { value: projected_ty, obligations: vec![] };
967 infcx.inner.borrow_mut().projection_cache().insert_term(cache_key, result.clone());
969 // No need to extend `obligations`.
970 Ok(Some(result.value))
972 Err(ProjectionError::TooManyCandidates) => {
973 debug!("opt_normalize_projection_type: too many candidates");
975 infcx.inner.borrow_mut().projection_cache().ambiguous(cache_key);
979 Err(ProjectionError::TraitSelectionError(_)) => {
980 debug!("opt_normalize_projection_type: ERROR");
981 // if we got an error processing the `T as Trait` part,
982 // just return `ty::err` but add the obligation `T :
983 // Trait`, which when processed will cause the error to be
987 infcx.inner.borrow_mut().projection_cache().error(cache_key);
989 let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
990 obligations.extend(result.obligations);
991 Ok(Some(result.value.into()))
996 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
997 /// hold. In various error cases, we cannot generate a valid
998 /// normalized projection. Therefore, we create an inference variable
999 /// return an associated obligation that, when fulfilled, will lead to
1002 /// Note that we used to return `Error` here, but that was quite
1003 /// dubious -- the premise was that an error would *eventually* be
1004 /// reported, when the obligation was processed. But in general once
1005 /// you see an `Error` you are supposed to be able to assume that an
1006 /// error *has been* reported, so that you can take whatever heuristic
1007 /// paths you want to take. To make things worse, it was possible for
1008 /// cycles to arise, where you basically had a setup like `<MyType<$0>
1009 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
1010 /// Trait>::Foo> to `[type error]` would lead to an obligation of
1011 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
1012 /// an error for this obligation, but we legitimately should not,
1013 /// because it contains `[type error]`. Yuck! (See issue #29857 for
1014 /// one case where this arose.)
1015 fn normalize_to_error<'a, 'tcx>(
1016 selcx: &mut SelectionContext<'a, 'tcx>,
1017 param_env: ty::ParamEnv<'tcx>,
1018 projection_ty: ty::ProjectionTy<'tcx>,
1019 cause: ObligationCause<'tcx>,
1021 ) -> NormalizedTy<'tcx> {
1022 let trait_ref = ty::Binder::dummy(projection_ty.trait_ref(selcx.tcx()));
1023 let trait_obligation = Obligation {
1025 recursion_depth: depth,
1027 predicate: trait_ref.without_const().to_predicate(selcx.tcx()),
1029 let tcx = selcx.infcx().tcx;
1030 let def_id = projection_ty.item_def_id;
1031 let new_value = selcx.infcx().next_ty_var(TypeVariableOrigin {
1032 kind: TypeVariableOriginKind::NormalizeProjectionType,
1033 span: tcx.def_span(def_id),
1035 Normalized { value: new_value, obligations: vec![trait_obligation] }
1038 enum Projected<'tcx> {
1039 Progress(Progress<'tcx>),
1040 NoProgress(ty::Term<'tcx>),
1043 struct Progress<'tcx> {
1044 term: ty::Term<'tcx>,
1045 obligations: Vec<PredicateObligation<'tcx>>,
1048 impl<'tcx> Progress<'tcx> {
1049 fn error(tcx: TyCtxt<'tcx>) -> Self {
1050 Progress { term: tcx.ty_error().into(), obligations: vec![] }
1053 fn with_addl_obligations(mut self, mut obligations: Vec<PredicateObligation<'tcx>>) -> Self {
1054 self.obligations.append(&mut obligations);
1059 /// Computes the result of a projection type (if we can).
1062 /// - `obligation` must be fully normalized
1063 #[tracing::instrument(level = "info", skip(selcx))]
1064 fn project<'cx, 'tcx>(
1065 selcx: &mut SelectionContext<'cx, 'tcx>,
1066 obligation: &ProjectionTyObligation<'tcx>,
1067 ) -> Result<Projected<'tcx>, ProjectionError<'tcx>> {
1068 if !selcx.tcx().recursion_limit().value_within_limit(obligation.recursion_depth) {
1069 // This should really be an immediate error, but some existing code
1070 // relies on being able to recover from this.
1071 return Err(ProjectionError::TraitSelectionError(SelectionError::Overflow));
1074 if obligation.predicate.references_error() {
1075 return Ok(Projected::Progress(Progress::error(selcx.tcx())));
1078 // If the obligation contains any inference types or consts in associated
1079 // type substs, then we don't assemble any candidates.
1080 // This isn't really correct, but otherwise we can end up in a case where
1081 // we constrain inference variables by selecting a single predicate, when
1082 // we need to stay general. See issue #91762.
1083 let (_, predicate_own_substs) = obligation.predicate.trait_ref_and_own_substs(selcx.tcx());
1084 if predicate_own_substs.iter().any(|g| g.has_infer_types_or_consts()) {
1085 return Err(ProjectionError::TooManyCandidates);
1088 let mut candidates = ProjectionCandidateSet::None;
1090 // Make sure that the following procedures are kept in order. ParamEnv
1091 // needs to be first because it has highest priority, and Select checks
1092 // the return value of push_candidate which assumes it's ran at last.
1093 assemble_candidates_from_param_env(selcx, obligation, &mut candidates);
1095 assemble_candidates_from_trait_def(selcx, obligation, &mut candidates);
1097 assemble_candidates_from_object_ty(selcx, obligation, &mut candidates);
1099 if let ProjectionCandidateSet::Single(ProjectionCandidate::Object(_)) = candidates {
1100 // Avoid normalization cycle from selection (see
1101 // `assemble_candidates_from_object_ty`).
1102 // FIXME(lazy_normalization): Lazy normalization should save us from
1103 // having to special case this.
1105 assemble_candidates_from_impls(selcx, obligation, &mut candidates);
1109 ProjectionCandidateSet::Single(candidate) => {
1110 Ok(Projected::Progress(confirm_candidate(selcx, obligation, candidate)))
1112 ProjectionCandidateSet::None => Ok(Projected::NoProgress(
1113 // FIXME(associated_const_generics): this may need to change in the future?
1114 // need to investigate whether or not this is fine.
1117 .mk_projection(obligation.predicate.item_def_id, obligation.predicate.substs)
1120 // Error occurred while trying to processing impls.
1121 ProjectionCandidateSet::Error(e) => Err(ProjectionError::TraitSelectionError(e)),
1122 // Inherent ambiguity that prevents us from even enumerating the
1124 ProjectionCandidateSet::Ambiguous => Err(ProjectionError::TooManyCandidates),
1128 /// The first thing we have to do is scan through the parameter
1129 /// environment to see whether there are any projection predicates
1130 /// there that can answer this question.
1131 fn assemble_candidates_from_param_env<'cx, 'tcx>(
1132 selcx: &mut SelectionContext<'cx, 'tcx>,
1133 obligation: &ProjectionTyObligation<'tcx>,
1134 candidate_set: &mut ProjectionCandidateSet<'tcx>,
1136 assemble_candidates_from_predicates(
1140 ProjectionCandidate::ParamEnv,
1141 obligation.param_env.caller_bounds().iter(),
1146 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
1147 /// that the definition of `Foo` has some clues:
1151 /// type FooT : Bar<BarT=i32>
1155 /// Here, for example, we could conclude that the result is `i32`.
1156 fn assemble_candidates_from_trait_def<'cx, 'tcx>(
1157 selcx: &mut SelectionContext<'cx, 'tcx>,
1158 obligation: &ProjectionTyObligation<'tcx>,
1159 candidate_set: &mut ProjectionCandidateSet<'tcx>,
1161 debug!("assemble_candidates_from_trait_def(..)");
1163 let tcx = selcx.tcx();
1164 // Check whether the self-type is itself a projection.
1165 // If so, extract what we know from the trait and try to come up with a good answer.
1166 let bounds = match *obligation.predicate.self_ty().kind() {
1167 ty::Projection(ref data) => tcx.item_bounds(data.item_def_id).subst(tcx, data.substs),
1168 ty::Opaque(def_id, substs) => tcx.item_bounds(def_id).subst(tcx, substs),
1169 ty::Infer(ty::TyVar(_)) => {
1170 // If the self-type is an inference variable, then it MAY wind up
1171 // being a projected type, so induce an ambiguity.
1172 candidate_set.mark_ambiguous();
1178 assemble_candidates_from_predicates(
1182 ProjectionCandidate::TraitDef,
1188 /// In the case of a trait object like
1189 /// `<dyn Iterator<Item = ()> as Iterator>::Item` we can use the existential
1190 /// predicate in the trait object.
1192 /// We don't go through the select candidate for these bounds to avoid cycles:
1193 /// In the above case, `dyn Iterator<Item = ()>: Iterator` would create a
1194 /// nested obligation of `<dyn Iterator<Item = ()> as Iterator>::Item: Sized`,
1195 /// this then has to be normalized without having to prove
1196 /// `dyn Iterator<Item = ()>: Iterator` again.
1197 fn assemble_candidates_from_object_ty<'cx, 'tcx>(
1198 selcx: &mut SelectionContext<'cx, 'tcx>,
1199 obligation: &ProjectionTyObligation<'tcx>,
1200 candidate_set: &mut ProjectionCandidateSet<'tcx>,
1202 debug!("assemble_candidates_from_object_ty(..)");
1204 let tcx = selcx.tcx();
1206 let self_ty = obligation.predicate.self_ty();
1207 let object_ty = selcx.infcx().shallow_resolve(self_ty);
1208 let data = match object_ty.kind() {
1209 ty::Dynamic(data, ..) => data,
1210 ty::Infer(ty::TyVar(_)) => {
1211 // If the self-type is an inference variable, then it MAY wind up
1212 // being an object type, so induce an ambiguity.
1213 candidate_set.mark_ambiguous();
1218 let env_predicates = data
1219 .projection_bounds()
1220 .filter(|bound| bound.item_def_id() == obligation.predicate.item_def_id)
1221 .map(|p| p.with_self_ty(tcx, object_ty).to_predicate(tcx));
1223 assemble_candidates_from_predicates(
1227 ProjectionCandidate::Object,
1233 #[tracing::instrument(
1235 skip(selcx, candidate_set, ctor, env_predicates, potentially_unnormalized_candidates)
1237 fn assemble_candidates_from_predicates<'cx, 'tcx>(
1238 selcx: &mut SelectionContext<'cx, 'tcx>,
1239 obligation: &ProjectionTyObligation<'tcx>,
1240 candidate_set: &mut ProjectionCandidateSet<'tcx>,
1241 ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionCandidate<'tcx>,
1242 env_predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
1243 potentially_unnormalized_candidates: bool,
1245 let infcx = selcx.infcx();
1246 for predicate in env_predicates {
1247 let bound_predicate = predicate.kind();
1248 if let ty::PredicateKind::Projection(data) = predicate.kind().skip_binder() {
1249 let data = bound_predicate.rebind(data);
1250 let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
1252 let is_match = same_def_id
1253 && infcx.probe(|_| {
1254 selcx.match_projection_projections(
1257 potentially_unnormalized_candidates,
1262 candidate_set.push_candidate(ctor(data));
1264 if potentially_unnormalized_candidates
1265 && !obligation.predicate.has_infer_types_or_consts()
1267 // HACK: Pick the first trait def candidate for a fully
1268 // inferred predicate. This is to allow duplicates that
1269 // differ only in normalization.
1277 #[tracing::instrument(level = "debug", skip(selcx, obligation, candidate_set))]
1278 fn assemble_candidates_from_impls<'cx, 'tcx>(
1279 selcx: &mut SelectionContext<'cx, 'tcx>,
1280 obligation: &ProjectionTyObligation<'tcx>,
1281 candidate_set: &mut ProjectionCandidateSet<'tcx>,
1283 // If we are resolving `<T as TraitRef<...>>::Item == Type`,
1284 // start out by selecting the predicate `T as TraitRef<...>`:
1285 let poly_trait_ref = ty::Binder::dummy(obligation.predicate.trait_ref(selcx.tcx()));
1286 let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1287 let _ = selcx.infcx().commit_if_ok(|_| {
1288 let impl_source = match selcx.select(&trait_obligation) {
1289 Ok(Some(impl_source)) => impl_source,
1291 candidate_set.mark_ambiguous();
1295 debug!(error = ?e, "selection error");
1296 candidate_set.mark_error(e);
1301 let eligible = match &impl_source {
1302 super::ImplSource::Closure(_)
1303 | super::ImplSource::Generator(_)
1304 | super::ImplSource::FnPointer(_)
1305 | super::ImplSource::TraitAlias(_) => true,
1306 super::ImplSource::UserDefined(impl_data) => {
1307 // We have to be careful when projecting out of an
1308 // impl because of specialization. If we are not in
1309 // codegen (i.e., projection mode is not "any"), and the
1310 // impl's type is declared as default, then we disable
1311 // projection (even if the trait ref is fully
1312 // monomorphic). In the case where trait ref is not
1313 // fully monomorphic (i.e., includes type parameters),
1314 // this is because those type parameters may
1315 // ultimately be bound to types from other crates that
1316 // may have specialized impls we can't see. In the
1317 // case where the trait ref IS fully monomorphic, this
1318 // is a policy decision that we made in the RFC in
1319 // order to preserve flexibility for the crate that
1320 // defined the specializable impl to specialize later
1321 // for existing types.
1323 // In either case, we handle this by not adding a
1324 // candidate for an impl if it contains a `default`
1327 // NOTE: This should be kept in sync with the similar code in
1328 // `rustc_ty_utils::instance::resolve_associated_item()`.
1330 assoc_def(selcx, impl_data.impl_def_id, obligation.predicate.item_def_id)
1331 .map_err(|ErrorReported| ())?;
1333 if node_item.is_final() {
1334 // Non-specializable items are always projectable.
1337 // Only reveal a specializable default if we're past type-checking
1338 // and the obligation is monomorphic, otherwise passes such as
1339 // transmute checking and polymorphic MIR optimizations could
1340 // get a result which isn't correct for all monomorphizations.
1341 if obligation.param_env.reveal() == Reveal::All {
1342 // NOTE(eddyb) inference variables can resolve to parameters, so
1343 // assume `poly_trait_ref` isn't monomorphic, if it contains any.
1344 let poly_trait_ref = selcx.infcx().resolve_vars_if_possible(poly_trait_ref);
1345 !poly_trait_ref.still_further_specializable()
1348 assoc_ty = ?selcx.tcx().def_path_str(node_item.item.def_id),
1349 ?obligation.predicate,
1350 "assemble_candidates_from_impls: not eligible due to default",
1356 super::ImplSource::DiscriminantKind(..) => {
1357 // While `DiscriminantKind` is automatically implemented for every type,
1358 // the concrete discriminant may not be known yet.
1360 // Any type with multiple potential discriminant types is therefore not eligible.
1361 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1363 match self_ty.kind() {
1381 | ty::GeneratorWitness(..)
1384 // Integers and floats always have `u8` as their discriminant.
1385 | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
1391 | ty::Placeholder(..)
1393 | ty::Error(_) => false,
1396 super::ImplSource::Pointee(..) => {
1397 // While `Pointee` is automatically implemented for every type,
1398 // the concrete metadata type may not be known yet.
1400 // Any type with multiple potential metadata types is therefore not eligible.
1401 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1403 let tail = selcx.tcx().struct_tail_with_normalize(self_ty, |ty| {
1404 normalize_with_depth(
1406 obligation.param_env,
1407 obligation.cause.clone(),
1408 obligation.recursion_depth + 1,
1431 | ty::GeneratorWitness(..)
1433 // If returned by `struct_tail_without_normalization` this is a unit struct
1434 // without any fields, or not a struct, and therefore is Sized.
1436 // If returned by `struct_tail_without_normalization` this is the empty tuple.
1438 // Integers and floats are always Sized, and so have unit type metadata.
1439 | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
1445 | ty::Placeholder(..)
1448 if tail.has_infer_types() {
1449 candidate_set.mark_ambiguous();
1455 super::ImplSource::Param(..) => {
1456 // This case tell us nothing about the value of an
1457 // associated type. Consider:
1460 // trait SomeTrait { type Foo; }
1461 // fn foo<T:SomeTrait>(...) { }
1464 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1465 // : SomeTrait` binding does not help us decide what the
1466 // type `Foo` is (at least, not more specifically than
1467 // what we already knew).
1469 // But wait, you say! What about an example like this:
1472 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1475 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1476 // resolve `T::Foo`? And of course it does, but in fact
1477 // that single predicate is desugared into two predicates
1478 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1479 // projection. And the projection where clause is handled
1480 // in `assemble_candidates_from_param_env`.
1483 super::ImplSource::Object(_) => {
1484 // Handled by the `Object` projection candidate. See
1485 // `assemble_candidates_from_object_ty` for an explanation of
1486 // why we special case object types.
1489 super::ImplSource::AutoImpl(..)
1490 | super::ImplSource::Builtin(..)
1491 | super::ImplSource::TraitUpcasting(_)
1492 | super::ImplSource::ConstDrop(_) => {
1493 // These traits have no associated types.
1494 selcx.tcx().sess.delay_span_bug(
1495 obligation.cause.span,
1496 &format!("Cannot project an associated type from `{:?}`", impl_source),
1503 if candidate_set.push_candidate(ProjectionCandidate::Select(impl_source)) {
1514 fn confirm_candidate<'cx, 'tcx>(
1515 selcx: &mut SelectionContext<'cx, 'tcx>,
1516 obligation: &ProjectionTyObligation<'tcx>,
1517 candidate: ProjectionCandidate<'tcx>,
1518 ) -> Progress<'tcx> {
1519 debug!(?obligation, ?candidate, "confirm_candidate");
1520 let mut progress = match candidate {
1521 ProjectionCandidate::ParamEnv(poly_projection)
1522 | ProjectionCandidate::Object(poly_projection) => {
1523 confirm_param_env_candidate(selcx, obligation, poly_projection, false)
1526 ProjectionCandidate::TraitDef(poly_projection) => {
1527 confirm_param_env_candidate(selcx, obligation, poly_projection, true)
1530 ProjectionCandidate::Select(impl_source) => {
1531 confirm_select_candidate(selcx, obligation, impl_source)
1535 // When checking for cycle during evaluation, we compare predicates with
1536 // "syntactic" equality. Since normalization generally introduces a type
1537 // with new region variables, we need to resolve them to existing variables
1538 // when possible for this to work. See `auto-trait-projection-recursion.rs`
1539 // for a case where this matters.
1540 if progress.term.has_infer_regions() {
1542 progress.term.fold_with(&mut OpportunisticRegionResolver::new(selcx.infcx()));
1547 fn confirm_select_candidate<'cx, 'tcx>(
1548 selcx: &mut SelectionContext<'cx, 'tcx>,
1549 obligation: &ProjectionTyObligation<'tcx>,
1550 impl_source: Selection<'tcx>,
1551 ) -> Progress<'tcx> {
1553 super::ImplSource::UserDefined(data) => confirm_impl_candidate(selcx, obligation, data),
1554 super::ImplSource::Generator(data) => confirm_generator_candidate(selcx, obligation, data),
1555 super::ImplSource::Closure(data) => confirm_closure_candidate(selcx, obligation, data),
1556 super::ImplSource::FnPointer(data) => confirm_fn_pointer_candidate(selcx, obligation, data),
1557 super::ImplSource::DiscriminantKind(data) => {
1558 confirm_discriminant_kind_candidate(selcx, obligation, data)
1560 super::ImplSource::Pointee(data) => confirm_pointee_candidate(selcx, obligation, data),
1561 super::ImplSource::Object(_)
1562 | super::ImplSource::AutoImpl(..)
1563 | super::ImplSource::Param(..)
1564 | super::ImplSource::Builtin(..)
1565 | super::ImplSource::TraitUpcasting(_)
1566 | super::ImplSource::TraitAlias(..)
1567 | super::ImplSource::ConstDrop(_) => {
1568 // we don't create Select candidates with this kind of resolution
1570 obligation.cause.span,
1571 "Cannot project an associated type from `{:?}`",
1578 fn confirm_generator_candidate<'cx, 'tcx>(
1579 selcx: &mut SelectionContext<'cx, 'tcx>,
1580 obligation: &ProjectionTyObligation<'tcx>,
1581 impl_source: ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>,
1582 ) -> Progress<'tcx> {
1583 let gen_sig = impl_source.substs.as_generator().poly_sig();
1584 let Normalized { value: gen_sig, obligations } = normalize_with_depth(
1586 obligation.param_env,
1587 obligation.cause.clone(),
1588 obligation.recursion_depth + 1,
1592 debug!(?obligation, ?gen_sig, ?obligations, "confirm_generator_candidate");
1594 let tcx = selcx.tcx();
1596 let gen_def_id = tcx.require_lang_item(LangItem::Generator, None);
1598 let predicate = super::util::generator_trait_ref_and_outputs(
1601 obligation.predicate.self_ty(),
1604 .map_bound(|(trait_ref, yield_ty, return_ty)| {
1605 let name = tcx.associated_item(obligation.predicate.item_def_id).name;
1606 let ty = if name == sym::Return {
1608 } else if name == sym::Yield {
1614 ty::ProjectionPredicate {
1615 projection_ty: ty::ProjectionTy {
1616 substs: trait_ref.substs,
1617 item_def_id: obligation.predicate.item_def_id,
1623 confirm_param_env_candidate(selcx, obligation, predicate, false)
1624 .with_addl_obligations(impl_source.nested)
1625 .with_addl_obligations(obligations)
1628 fn confirm_discriminant_kind_candidate<'cx, 'tcx>(
1629 selcx: &mut SelectionContext<'cx, 'tcx>,
1630 obligation: &ProjectionTyObligation<'tcx>,
1631 _: ImplSourceDiscriminantKindData,
1632 ) -> Progress<'tcx> {
1633 let tcx = selcx.tcx();
1635 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1636 // We get here from `poly_project_and_unify_type` which replaces bound vars
1637 // with placeholders
1638 debug_assert!(!self_ty.has_escaping_bound_vars());
1639 let substs = tcx.mk_substs([self_ty.into()].iter());
1641 let discriminant_def_id = tcx.require_lang_item(LangItem::Discriminant, None);
1643 let predicate = ty::ProjectionPredicate {
1644 projection_ty: ty::ProjectionTy { substs, item_def_id: discriminant_def_id },
1645 term: self_ty.discriminant_ty(tcx).into(),
1648 // We get here from `poly_project_and_unify_type` which replaces bound vars
1649 // with placeholders, so dummy is okay here.
1650 confirm_param_env_candidate(selcx, obligation, ty::Binder::dummy(predicate), false)
1653 fn confirm_pointee_candidate<'cx, 'tcx>(
1654 selcx: &mut SelectionContext<'cx, 'tcx>,
1655 obligation: &ProjectionTyObligation<'tcx>,
1656 _: ImplSourcePointeeData,
1657 ) -> Progress<'tcx> {
1658 let tcx = selcx.tcx();
1659 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1661 let mut obligations = vec![];
1662 let metadata_ty = self_ty.ptr_metadata_ty(tcx, |ty| {
1663 normalize_with_depth_to(
1665 obligation.param_env,
1666 obligation.cause.clone(),
1667 obligation.recursion_depth + 1,
1673 let substs = tcx.mk_substs([self_ty.into()].iter());
1674 let metadata_def_id = tcx.require_lang_item(LangItem::Metadata, None);
1676 let predicate = ty::ProjectionPredicate {
1677 projection_ty: ty::ProjectionTy { substs, item_def_id: metadata_def_id },
1678 term: metadata_ty.into(),
1681 confirm_param_env_candidate(selcx, obligation, ty::Binder::dummy(predicate), false)
1682 .with_addl_obligations(obligations)
1685 fn confirm_fn_pointer_candidate<'cx, 'tcx>(
1686 selcx: &mut SelectionContext<'cx, 'tcx>,
1687 obligation: &ProjectionTyObligation<'tcx>,
1688 fn_pointer_impl_source: ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>,
1689 ) -> Progress<'tcx> {
1690 let fn_type = selcx.infcx().shallow_resolve(fn_pointer_impl_source.fn_ty);
1691 let sig = fn_type.fn_sig(selcx.tcx());
1692 let Normalized { value: sig, obligations } = normalize_with_depth(
1694 obligation.param_env,
1695 obligation.cause.clone(),
1696 obligation.recursion_depth + 1,
1700 confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1701 .with_addl_obligations(fn_pointer_impl_source.nested)
1702 .with_addl_obligations(obligations)
1705 fn confirm_closure_candidate<'cx, 'tcx>(
1706 selcx: &mut SelectionContext<'cx, 'tcx>,
1707 obligation: &ProjectionTyObligation<'tcx>,
1708 impl_source: ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>,
1709 ) -> Progress<'tcx> {
1710 let closure_sig = impl_source.substs.as_closure().sig();
1711 let Normalized { value: closure_sig, obligations } = normalize_with_depth(
1713 obligation.param_env,
1714 obligation.cause.clone(),
1715 obligation.recursion_depth + 1,
1719 debug!(?obligation, ?closure_sig, ?obligations, "confirm_closure_candidate");
1721 confirm_callable_candidate(selcx, obligation, closure_sig, util::TupleArgumentsFlag::No)
1722 .with_addl_obligations(impl_source.nested)
1723 .with_addl_obligations(obligations)
1726 fn confirm_callable_candidate<'cx, 'tcx>(
1727 selcx: &mut SelectionContext<'cx, 'tcx>,
1728 obligation: &ProjectionTyObligation<'tcx>,
1729 fn_sig: ty::PolyFnSig<'tcx>,
1730 flag: util::TupleArgumentsFlag,
1731 ) -> Progress<'tcx> {
1732 let tcx = selcx.tcx();
1734 debug!(?obligation, ?fn_sig, "confirm_callable_candidate");
1736 let fn_once_def_id = tcx.require_lang_item(LangItem::FnOnce, None);
1737 let fn_once_output_def_id = tcx.require_lang_item(LangItem::FnOnceOutput, None);
1739 let predicate = super::util::closure_trait_ref_and_return_type(
1742 obligation.predicate.self_ty(),
1746 .map_bound(|(trait_ref, ret_type)| ty::ProjectionPredicate {
1747 projection_ty: ty::ProjectionTy {
1748 substs: trait_ref.substs,
1749 item_def_id: fn_once_output_def_id,
1751 term: ret_type.into(),
1754 confirm_param_env_candidate(selcx, obligation, predicate, true)
1757 fn confirm_param_env_candidate<'cx, 'tcx>(
1758 selcx: &mut SelectionContext<'cx, 'tcx>,
1759 obligation: &ProjectionTyObligation<'tcx>,
1760 poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
1761 potentially_unnormalized_candidate: bool,
1762 ) -> Progress<'tcx> {
1763 let infcx = selcx.infcx();
1764 let cause = &obligation.cause;
1765 let param_env = obligation.param_env;
1767 let (cache_entry, _) = infcx.replace_bound_vars_with_fresh_vars(
1769 LateBoundRegionConversionTime::HigherRankedType,
1773 let cache_projection = cache_entry.projection_ty;
1774 let mut nested_obligations = Vec::new();
1775 let obligation_projection = obligation.predicate;
1776 let obligation_projection = ensure_sufficient_stack(|| {
1777 normalize_with_depth_to(
1779 obligation.param_env,
1780 obligation.cause.clone(),
1781 obligation.recursion_depth + 1,
1782 obligation_projection,
1783 &mut nested_obligations,
1786 let cache_projection = if potentially_unnormalized_candidate {
1787 ensure_sufficient_stack(|| {
1788 normalize_with_depth_to(
1790 obligation.param_env,
1791 obligation.cause.clone(),
1792 obligation.recursion_depth + 1,
1794 &mut nested_obligations,
1801 debug!(?cache_projection, ?obligation_projection);
1803 match infcx.at(cause, param_env).eq(cache_projection, obligation_projection) {
1804 Ok(InferOk { value: _, obligations }) => {
1805 nested_obligations.extend(obligations);
1806 assoc_ty_own_obligations(selcx, obligation, &mut nested_obligations);
1807 // FIXME(associated_const_equality): Handle consts here as well? Maybe this progress type should just take
1809 Progress { term: cache_entry.term, obligations: nested_obligations }
1813 "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
1814 obligation, poly_cache_entry, e,
1816 debug!("confirm_param_env_candidate: {}", msg);
1817 let err = infcx.tcx.ty_error_with_message(obligation.cause.span, &msg);
1818 Progress { term: err.into(), obligations: vec![] }
1823 fn confirm_impl_candidate<'cx, 'tcx>(
1824 selcx: &mut SelectionContext<'cx, 'tcx>,
1825 obligation: &ProjectionTyObligation<'tcx>,
1826 impl_impl_source: ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>,
1827 ) -> Progress<'tcx> {
1828 let tcx = selcx.tcx();
1830 let ImplSourceUserDefinedData { impl_def_id, substs, mut nested } = impl_impl_source;
1831 let assoc_item_id = obligation.predicate.item_def_id;
1832 let trait_def_id = tcx.trait_id_of_impl(impl_def_id).unwrap();
1834 let param_env = obligation.param_env;
1835 let assoc_ty = match assoc_def(selcx, impl_def_id, assoc_item_id) {
1836 Ok(assoc_ty) => assoc_ty,
1837 Err(ErrorReported) => return Progress { term: tcx.ty_error().into(), obligations: nested },
1840 if !assoc_ty.item.defaultness.has_value() {
1841 // This means that the impl is missing a definition for the
1842 // associated type. This error will be reported by the type
1843 // checker method `check_impl_items_against_trait`, so here we
1844 // just return Error.
1846 "confirm_impl_candidate: no associated type {:?} for {:?}",
1847 assoc_ty.item.name, obligation.predicate
1849 return Progress { term: tcx.ty_error().into(), obligations: nested };
1851 // If we're trying to normalize `<Vec<u32> as X>::A<S>` using
1852 //`impl<T> X for Vec<T> { type A<Y> = Box<Y>; }`, then:
1854 // * `obligation.predicate.substs` is `[Vec<u32>, S]`
1855 // * `substs` is `[u32]`
1856 // * `substs` ends up as `[u32, S]`
1857 let substs = obligation.predicate.substs.rebase_onto(tcx, trait_def_id, substs);
1859 translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.defining_node);
1860 let ty = tcx.type_of(assoc_ty.item.def_id);
1861 let is_const = matches!(tcx.def_kind(assoc_ty.item.def_id), DefKind::AssocConst);
1862 let term: ty::Term<'tcx> = if is_const {
1863 let identity_substs =
1864 crate::traits::InternalSubsts::identity_for_item(tcx, assoc_ty.item.def_id);
1865 let did = ty::WithOptConstParam::unknown(assoc_ty.item.def_id);
1866 let val = ty::ConstKind::Unevaluated(ty::Unevaluated::new(did, identity_substs));
1867 tcx.mk_const(ty::ConstS { ty, val }).into()
1871 if substs.len() != tcx.generics_of(assoc_ty.item.def_id).count() {
1872 let err = tcx.ty_error_with_message(
1873 obligation.cause.span,
1874 "impl item and trait item have different parameter counts",
1876 Progress { term: err.into(), obligations: nested }
1878 assoc_ty_own_obligations(selcx, obligation, &mut nested);
1879 Progress { term: term.subst(tcx, substs), obligations: nested }
1883 // Get obligations corresponding to the predicates from the where-clause of the
1884 // associated type itself.
1885 // Note: `feature(generic_associated_types)` is required to write such
1886 // predicates, even for non-generic associcated types.
1887 fn assoc_ty_own_obligations<'cx, 'tcx>(
1888 selcx: &mut SelectionContext<'cx, 'tcx>,
1889 obligation: &ProjectionTyObligation<'tcx>,
1890 nested: &mut Vec<PredicateObligation<'tcx>>,
1892 let tcx = selcx.tcx();
1893 for predicate in tcx
1894 .predicates_of(obligation.predicate.item_def_id)
1895 .instantiate_own(tcx, obligation.predicate.substs)
1898 let normalized = normalize_with_depth_to(
1900 obligation.param_env,
1901 obligation.cause.clone(),
1902 obligation.recursion_depth + 1,
1906 nested.push(Obligation::with_depth(
1907 obligation.cause.clone(),
1908 obligation.recursion_depth + 1,
1909 obligation.param_env,
1915 /// Locate the definition of an associated type in the specialization hierarchy,
1916 /// starting from the given impl.
1918 /// Based on the "projection mode", this lookup may in fact only examine the
1919 /// topmost impl. See the comments for `Reveal` for more details.
1921 selcx: &SelectionContext<'_, '_>,
1923 assoc_def_id: DefId,
1924 ) -> Result<specialization_graph::LeafDef, ErrorReported> {
1925 let tcx = selcx.tcx();
1926 let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1927 let trait_def = tcx.trait_def(trait_def_id);
1929 // This function may be called while we are still building the
1930 // specialization graph that is queried below (via TraitDef::ancestors()),
1931 // so, in order to avoid unnecessary infinite recursion, we manually look
1932 // for the associated item at the given impl.
1933 // If there is no such item in that impl, this function will fail with a
1934 // cycle error if the specialization graph is currently being built.
1935 if let Some(&impl_item_id) = tcx.impl_item_implementor_ids(impl_def_id).get(&assoc_def_id) {
1936 let item = tcx.associated_item(impl_item_id);
1937 let impl_node = specialization_graph::Node::Impl(impl_def_id);
1938 return Ok(specialization_graph::LeafDef {
1940 defining_node: impl_node,
1941 finalizing_node: if item.defaultness.is_default() { None } else { Some(impl_node) },
1945 let ancestors = trait_def.ancestors(tcx, impl_def_id)?;
1946 if let Some(assoc_item) = ancestors.leaf_def(tcx, assoc_def_id) {
1949 // This is saying that neither the trait nor
1950 // the impl contain a definition for this
1951 // associated type. Normally this situation
1952 // could only arise through a compiler bug --
1953 // if the user wrote a bad item name, it
1954 // should have failed in astconv.
1956 "No associated type `{}` for {}",
1957 tcx.item_name(assoc_def_id),
1958 tcx.def_path_str(impl_def_id)
1963 crate trait ProjectionCacheKeyExt<'cx, 'tcx>: Sized {
1964 fn from_poly_projection_predicate(
1965 selcx: &mut SelectionContext<'cx, 'tcx>,
1966 predicate: ty::PolyProjectionPredicate<'tcx>,
1970 impl<'cx, 'tcx> ProjectionCacheKeyExt<'cx, 'tcx> for ProjectionCacheKey<'tcx> {
1971 fn from_poly_projection_predicate(
1972 selcx: &mut SelectionContext<'cx, 'tcx>,
1973 predicate: ty::PolyProjectionPredicate<'tcx>,
1975 let infcx = selcx.infcx();
1976 // We don't do cross-snapshot caching of obligations with escaping regions,
1977 // so there's no cache key to use
1978 predicate.no_bound_vars().map(|predicate| {
1979 ProjectionCacheKey::new(
1980 // We don't attempt to match up with a specific type-variable state
1981 // from a specific call to `opt_normalize_projection_type` - if
1982 // there's no precise match, the original cache entry is "stranded"
1984 infcx.resolve_vars_if_possible(predicate.projection_ty),