3 //! Confirmation unifies the output type parameters of the trait
4 //! with the values found in the obligation, possibly yielding a
5 //! type error. See the [rustc dev guide] for more details.
8 //! https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation
9 use rustc_data_structures::stack::ensure_sufficient_stack;
10 use rustc_hir::lang_items::LangItem;
11 use rustc_index::bit_set::GrowableBitSet;
12 use rustc_infer::infer::InferOk;
13 use rustc_infer::infer::LateBoundRegionConversionTime::HigherRankedType;
14 use rustc_middle::ty::{
15 self, Binder, GenericArg, GenericArgKind, GenericParamDefKind, InternalSubsts, SubstsRef,
16 ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt,
18 use rustc_session::config::TraitSolver;
19 use rustc_span::def_id::DefId;
21 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
22 use crate::traits::util::{self, closure_trait_ref_and_return_type, predicate_for_trait_def};
23 use crate::traits::vtable::{
24 count_own_vtable_entries, prepare_vtable_segments, vtable_trait_first_method_offset,
28 BuiltinDerivedObligation, ImplDerivedObligation, ImplDerivedObligationCause, ImplSource,
29 ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
30 ImplSourceConstDestructData, ImplSourceFnPointerData, ImplSourceFutureData,
31 ImplSourceGeneratorData, ImplSourceObjectData, ImplSourceTraitAliasData,
32 ImplSourceTraitUpcastingData, ImplSourceUserDefinedData, Normalized, ObjectCastObligation,
33 Obligation, ObligationCause, OutputTypeParameterMismatch, PredicateObligation, Selection,
34 SelectionError, TraitNotObjectSafe, TraitObligation, Unimplemented,
37 use super::BuiltinImplConditions;
38 use super::SelectionCandidate::{self, *};
39 use super::SelectionContext;
42 use std::ops::ControlFlow;
44 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
45 #[instrument(level = "debug", skip(self))]
46 pub(super) fn confirm_candidate(
48 obligation: &TraitObligation<'tcx>,
49 candidate: SelectionCandidate<'tcx>,
50 ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
51 let mut impl_src = match candidate {
52 BuiltinCandidate { has_nested } => {
53 let data = self.confirm_builtin_candidate(obligation, has_nested);
54 ImplSource::Builtin(data)
57 TransmutabilityCandidate => {
58 let data = self.confirm_transmutability_candidate(obligation)?;
59 ImplSource::Builtin(data)
62 ParamCandidate(param) => {
64 self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
65 ImplSource::Param(obligations, param.skip_binder().constness)
68 ImplCandidate(impl_def_id) => {
69 ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id))
72 AutoImplCandidate => {
73 let data = self.confirm_auto_impl_candidate(obligation);
74 ImplSource::AutoImpl(data)
77 ProjectionCandidate(idx, constness) => {
78 let obligations = self.confirm_projection_candidate(obligation, idx)?;
79 ImplSource::Param(obligations, constness)
82 ObjectCandidate(idx) => {
83 let data = self.confirm_object_candidate(obligation, idx)?;
84 ImplSource::Object(data)
87 ClosureCandidate { .. } => {
88 let vtable_closure = self.confirm_closure_candidate(obligation)?;
89 ImplSource::Closure(vtable_closure)
92 GeneratorCandidate => {
93 let vtable_generator = self.confirm_generator_candidate(obligation)?;
94 ImplSource::Generator(vtable_generator)
98 let vtable_future = self.confirm_future_candidate(obligation)?;
99 ImplSource::Future(vtable_future)
102 FnPointerCandidate { is_const } => {
103 let data = self.confirm_fn_pointer_candidate(obligation, is_const)?;
104 ImplSource::FnPointer(data)
107 TraitAliasCandidate => {
108 let data = self.confirm_trait_alias_candidate(obligation);
109 ImplSource::TraitAlias(data)
112 BuiltinObjectCandidate => {
113 // This indicates something like `Trait + Send: Send`. In this case, we know that
114 // this holds because that's what the object type is telling us, and there's really
115 // no additional obligations to prove and no types in particular to unify, etc.
116 ImplSource::Param(Vec::new(), ty::BoundConstness::NotConst)
119 BuiltinUnsizeCandidate => {
120 let data = self.confirm_builtin_unsize_candidate(obligation)?;
121 ImplSource::Builtin(data)
124 TraitUpcastingUnsizeCandidate(idx) => {
125 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
126 ImplSource::TraitUpcasting(data)
129 ConstDestructCandidate(def_id) => {
130 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
131 ImplSource::ConstDestruct(data)
135 if !obligation.predicate.is_const_if_const() {
136 // normalize nested predicates according to parent predicate's constness.
137 impl_src = impl_src.map(|mut o| {
138 o.predicate = o.predicate.without_const(self.tcx());
146 fn confirm_projection_candidate(
148 obligation: &TraitObligation<'tcx>,
150 ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
151 let tcx = self.tcx();
153 let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
154 let placeholder_trait_predicate =
155 self.infcx.replace_bound_vars_with_placeholders(trait_predicate).trait_ref;
156 let placeholder_self_ty = placeholder_trait_predicate.self_ty();
157 let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
158 let (def_id, substs) = match *placeholder_self_ty.kind() {
159 ty::Alias(_, ty::AliasTy { def_id, substs, .. }) => (def_id, substs),
160 _ => bug!("projection candidate for unexpected type: {:?}", placeholder_self_ty),
163 let candidate_predicate =
164 tcx.bound_item_bounds(def_id).map_bound(|i| i[idx]).subst(tcx, substs);
165 let candidate = candidate_predicate
166 .to_opt_poly_trait_pred()
167 .expect("projection candidate is not a trait predicate")
168 .map_bound(|t| t.trait_ref);
169 let mut obligations = Vec::new();
170 let candidate = normalize_with_depth_to(
172 obligation.param_env,
173 obligation.cause.clone(),
174 obligation.recursion_depth + 1,
179 obligations.extend(self.infcx.commit_if_ok(|_| {
181 .at(&obligation.cause, obligation.param_env)
182 .sup(placeholder_trait_predicate, candidate)
183 .map(|InferOk { obligations, .. }| obligations)
184 .map_err(|_| Unimplemented)
187 if let ty::Alias(ty::Projection, ..) = placeholder_self_ty.kind() {
188 let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates;
189 debug!(?predicates, "projection predicates");
190 for predicate in predicates {
191 let normalized = normalize_with_depth_to(
193 obligation.param_env,
194 obligation.cause.clone(),
195 obligation.recursion_depth + 1,
199 obligations.push(Obligation::with_depth(
201 obligation.cause.clone(),
202 obligation.recursion_depth + 1,
203 obligation.param_env,
212 fn confirm_param_candidate(
214 obligation: &TraitObligation<'tcx>,
215 param: ty::PolyTraitRef<'tcx>,
216 ) -> Vec<PredicateObligation<'tcx>> {
217 debug!(?obligation, ?param, "confirm_param_candidate");
219 // During evaluation, we already checked that this
220 // where-clause trait-ref could be unified with the obligation
221 // trait-ref. Repeat that unification now without any
222 // transactional boundary; it should not fail.
223 match self.match_where_clause_trait_ref(obligation, param) {
224 Ok(obligations) => obligations,
227 "Where clause `{:?}` was applicable to `{:?}` but now is not",
235 fn confirm_builtin_candidate(
237 obligation: &TraitObligation<'tcx>,
239 ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
240 debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
242 let lang_items = self.tcx().lang_items();
243 let obligations = if has_nested {
244 let trait_def = obligation.predicate.def_id();
245 let conditions = if Some(trait_def) == lang_items.sized_trait() {
246 self.sized_conditions(obligation)
247 } else if Some(trait_def) == lang_items.copy_trait() {
248 self.copy_clone_conditions(obligation)
249 } else if Some(trait_def) == lang_items.clone_trait() {
250 self.copy_clone_conditions(obligation)
252 bug!("unexpected builtin trait {:?}", trait_def)
254 let BuiltinImplConditions::Where(nested) = conditions else {
255 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
258 let cause = obligation.derived_cause(BuiltinDerivedObligation);
259 ensure_sufficient_stack(|| {
260 self.collect_predicates_for_types(
261 obligation.param_env,
263 obligation.recursion_depth + 1,
272 debug!(?obligations);
274 ImplSourceBuiltinData { nested: obligations }
277 fn confirm_transmutability_candidate(
279 obligation: &TraitObligation<'tcx>,
280 ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
281 debug!(?obligation, "confirm_transmutability_candidate");
283 let predicate = obligation.predicate;
285 let type_at = |i| predicate.map_bound(|p| p.trait_ref.substs.type_at(i));
286 let const_at = |i| predicate.skip_binder().trait_ref.substs.const_at(i);
288 let src_and_dst = predicate.map_bound(|p| rustc_transmute::Types {
289 dst: p.trait_ref.substs.type_at(0),
290 src: p.trait_ref.substs.type_at(1),
293 let scope = type_at(2).skip_binder();
296 rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3)) else {
297 return Err(Unimplemented);
300 let cause = obligation.cause.clone();
302 let mut transmute_env = rustc_transmute::TransmuteTypeEnv::new(self.infcx);
304 let maybe_transmutable = transmute_env.is_transmutable(cause, src_and_dst, scope, assume);
306 use rustc_transmute::Answer;
308 match maybe_transmutable {
309 Answer::Yes => Ok(ImplSourceBuiltinData { nested: vec![] }),
310 _ => Err(Unimplemented),
314 /// This handles the case where an `auto trait Foo` impl is being used.
315 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
317 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
318 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
319 fn confirm_auto_impl_candidate(
321 obligation: &TraitObligation<'tcx>,
322 ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
323 debug!(?obligation, "confirm_auto_impl_candidate");
325 let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
326 let types = self.constituent_types_for_ty(self_ty);
327 self.vtable_auto_impl(obligation, obligation.predicate.def_id(), types)
330 /// See `confirm_auto_impl_candidate`.
333 obligation: &TraitObligation<'tcx>,
335 nested: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
336 ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
337 debug!(?nested, "vtable_auto_impl");
338 ensure_sufficient_stack(|| {
339 let cause = obligation.derived_cause(BuiltinDerivedObligation);
341 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
342 let trait_ref = self.infcx.replace_bound_vars_with_placeholders(poly_trait_ref);
343 let trait_obligations: Vec<PredicateObligation<'_>> = self.impl_or_trait_obligations(
345 obligation.recursion_depth + 1,
346 obligation.param_env,
349 obligation.predicate,
352 let mut obligations = self.collect_predicates_for_types(
353 obligation.param_env,
355 obligation.recursion_depth + 1,
360 // Adds the predicates from the trait. Note that this contains a `Self: Trait`
361 // predicate as usual. It won't have any effect since auto traits are coinductive.
362 obligations.extend(trait_obligations);
364 debug!(?obligations, "vtable_auto_impl");
366 ImplSourceAutoImplData { trait_def_id, nested: obligations }
370 fn confirm_impl_candidate(
372 obligation: &TraitObligation<'tcx>,
374 ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
375 debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");
377 // First, create the substitutions by matching the impl again,
378 // this time not in a probe.
379 let substs = self.rematch_impl(impl_def_id, obligation);
380 debug!(?substs, "impl substs");
381 ensure_sufficient_stack(|| {
386 obligation.recursion_depth + 1,
387 obligation.param_env,
388 obligation.predicate,
396 substs: Normalized<'tcx, SubstsRef<'tcx>>,
397 cause: &ObligationCause<'tcx>,
398 recursion_depth: usize,
399 param_env: ty::ParamEnv<'tcx>,
400 parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
401 ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
402 debug!(?impl_def_id, ?substs, ?recursion_depth, "vtable_impl");
404 let mut impl_obligations = self.impl_or_trait_obligations(
413 debug!(?impl_obligations, "vtable_impl");
415 // Because of RFC447, the impl-trait-ref and obligations
416 // are sufficient to determine the impl substs, without
417 // relying on projections in the impl-trait-ref.
419 // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
420 impl_obligations.extend(substs.obligations);
422 ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations }
425 fn confirm_object_candidate(
427 obligation: &TraitObligation<'tcx>,
429 ) -> Result<ImplSourceObjectData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
430 let tcx = self.tcx();
431 debug!(?obligation, ?index, "confirm_object_candidate");
433 let trait_predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
434 let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
435 let obligation_trait_ref = ty::Binder::dummy(trait_predicate.trait_ref);
436 let ty::Dynamic(data, ..) = *self_ty.kind() else {
437 span_bug!(obligation.cause.span, "object candidate with non-object");
440 let object_trait_ref = data.principal().unwrap_or_else(|| {
441 span_bug!(obligation.cause.span, "object candidate with no principal")
443 let object_trait_ref = self.infcx.replace_bound_vars_with_fresh_vars(
444 obligation.cause.span,
448 let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);
450 let mut nested = vec![];
452 let mut supertraits = util::supertraits(tcx, ty::Binder::dummy(object_trait_ref));
453 let unnormalized_upcast_trait_ref =
454 supertraits.nth(index).expect("supertraits iterator no longer has as many elements");
456 let upcast_trait_ref = normalize_with_depth_to(
458 obligation.param_env,
459 obligation.cause.clone(),
460 obligation.recursion_depth + 1,
461 unnormalized_upcast_trait_ref,
465 nested.extend(self.infcx.commit_if_ok(|_| {
467 .at(&obligation.cause, obligation.param_env)
468 .sup(obligation_trait_ref, upcast_trait_ref)
469 .map(|InferOk { obligations, .. }| obligations)
470 .map_err(|_| Unimplemented)
473 // Check supertraits hold. This is so that their associated type bounds
474 // will be checked in the code below.
475 for super_trait in tcx
476 .super_predicates_of(trait_predicate.def_id())
477 .instantiate(tcx, trait_predicate.trait_ref.substs)
481 let normalized_super_trait = normalize_with_depth_to(
483 obligation.param_env,
484 obligation.cause.clone(),
485 obligation.recursion_depth + 1,
489 nested.push(obligation.with(tcx, normalized_super_trait));
492 let assoc_types: Vec<_> = tcx
493 .associated_items(trait_predicate.def_id())
494 .in_definition_order()
496 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
500 for assoc_type in assoc_types {
501 let defs: &ty::Generics = tcx.generics_of(assoc_type);
503 if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
504 tcx.sess.delay_span_bug(
505 obligation.cause.span,
506 "GATs in trait object shouldn't have been considered",
508 return Err(SelectionError::Unimplemented);
511 // This maybe belongs in wf, but that can't (doesn't) handle
512 // higher-ranked things.
513 // Prevent, e.g., `dyn Iterator<Item = str>`.
514 for bound in self.tcx().bound_item_bounds(assoc_type).transpose_iter() {
516 if defs.count() == 0 {
517 bound.subst(tcx, trait_predicate.trait_ref.substs)
519 let mut substs = smallvec::SmallVec::with_capacity(defs.count());
520 substs.extend(trait_predicate.trait_ref.substs.iter());
521 let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
522 smallvec::SmallVec::with_capacity(
523 bound.0.kind().bound_vars().len() + defs.count(),
525 bound_vars.extend(bound.0.kind().bound_vars().into_iter());
526 InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
529 GenericParamDefKind::Type { .. } => {
530 let kind = ty::BoundTyKind::Param(param.name);
531 let bound_var = ty::BoundVariableKind::Ty(kind);
532 bound_vars.push(bound_var);
536 var: ty::BoundVar::from_usize(bound_vars.len() - 1),
542 GenericParamDefKind::Lifetime => {
543 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
544 let bound_var = ty::BoundVariableKind::Region(kind);
545 bound_vars.push(bound_var);
546 tcx.mk_region(ty::ReLateBound(
549 var: ty::BoundVar::from_usize(bound_vars.len() - 1),
555 GenericParamDefKind::Const { .. } => {
556 let bound_var = ty::BoundVariableKind::Const;
557 bound_vars.push(bound_var);
559 ty::ConstKind::Bound(
561 ty::BoundVar::from_usize(bound_vars.len() - 1),
563 tcx.type_of(param.def_id),
568 let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
569 let assoc_ty_substs = tcx.intern_substs(&substs);
571 let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
573 bound.map_bound(|b| b.kind().skip_binder()).subst(tcx, assoc_ty_substs);
574 tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
576 let normalized_bound = normalize_with_depth_to(
578 obligation.param_env,
579 obligation.cause.clone(),
580 obligation.recursion_depth + 1,
584 nested.push(obligation.with(tcx, normalized_bound));
588 debug!(?nested, "object nested obligations");
590 let vtable_base = vtable_trait_first_method_offset(
592 (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
595 Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
598 fn confirm_fn_pointer_candidate(
600 obligation: &TraitObligation<'tcx>,
602 ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
604 debug!(?obligation, "confirm_fn_pointer_candidate");
606 let tcx = self.tcx();
609 .shallow_resolve(obligation.self_ty().no_bound_vars())
610 .expect("fn pointer should not capture bound vars from predicate");
611 let sig = self_ty.fn_sig(tcx);
612 let trait_ref = closure_trait_ref_and_return_type(
614 obligation.predicate.def_id(),
617 util::TupleArgumentsFlag::Yes,
619 .map_bound(|(trait_ref, _)| trait_ref);
621 let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
622 let cause = obligation.derived_cause(BuiltinDerivedObligation);
624 if obligation.is_const() && !is_const {
625 // function is a trait method
626 if let ty::FnDef(def_id, substs) = self_ty.kind() && let Some(trait_id) = tcx.trait_of_item(*def_id) {
627 let trait_ref = TraitRef::from_method(tcx, trait_id, *substs);
628 let poly_trait_pred = Binder::dummy(trait_ref).with_constness(ty::BoundConstness::ConstIfConst);
629 let obligation = Obligation::new(tcx, cause.clone(), obligation.param_env, poly_trait_pred);
630 nested.push(obligation);
634 // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
635 let output_ty = self.infcx.replace_bound_vars_with_placeholders(sig.output());
636 let output_ty = normalize_with_depth_to(
638 obligation.param_env,
640 obligation.recursion_depth,
645 ty::Binder::dummy(self.tcx().at(cause.span).mk_trait_ref(LangItem::Sized, [output_ty]));
646 nested.push(Obligation::new(self.infcx.tcx, cause, obligation.param_env, tr));
648 Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
651 fn confirm_trait_alias_candidate(
653 obligation: &TraitObligation<'tcx>,
654 ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
655 debug!(?obligation, "confirm_trait_alias_candidate");
657 let alias_def_id = obligation.predicate.def_id();
658 let predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
659 let trait_ref = predicate.trait_ref;
660 let trait_def_id = trait_ref.def_id;
661 let substs = trait_ref.substs;
663 let trait_obligations = self.impl_or_trait_obligations(
665 obligation.recursion_depth,
666 obligation.param_env,
669 obligation.predicate,
672 debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
674 ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
677 fn confirm_generator_candidate(
679 obligation: &TraitObligation<'tcx>,
680 ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
682 // Okay to skip binder because the substs on generator types never
683 // touch bound regions, they just capture the in-scope
684 // type/region parameters.
685 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
686 let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
687 bug!("closure candidate for non-closure {:?}", obligation);
690 debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
692 let gen_sig = substs.as_generator().poly_sig();
694 // NOTE: The self-type is a generator type and hence is
695 // in fact unparameterized (or at least does not reference any
696 // regions bound in the obligation).
697 let self_ty = obligation
701 .expect("unboxed closure type should not capture bound vars from the predicate");
703 let trait_ref = super::util::generator_trait_ref_and_outputs(
705 obligation.predicate.def_id(),
709 .map_bound(|(trait_ref, ..)| trait_ref);
711 let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
712 debug!(?trait_ref, ?nested, "generator candidate obligations");
714 Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
717 fn confirm_future_candidate(
719 obligation: &TraitObligation<'tcx>,
720 ) -> Result<ImplSourceFutureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
721 // Okay to skip binder because the substs on generator types never
722 // touch bound regions, they just capture the in-scope
723 // type/region parameters.
724 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
725 let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
726 bug!("closure candidate for non-closure {:?}", obligation);
729 debug!(?obligation, ?generator_def_id, ?substs, "confirm_future_candidate");
731 let gen_sig = substs.as_generator().poly_sig();
733 let trait_ref = super::util::future_trait_ref_and_outputs(
735 obligation.predicate.def_id(),
736 obligation.predicate.no_bound_vars().expect("future has no bound vars").self_ty(),
739 .map_bound(|(trait_ref, ..)| trait_ref);
741 let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
742 debug!(?trait_ref, ?nested, "future candidate obligations");
744 Ok(ImplSourceFutureData { generator_def_id, substs, nested })
747 #[instrument(skip(self), level = "debug")]
748 fn confirm_closure_candidate(
750 obligation: &TraitObligation<'tcx>,
751 ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
754 .fn_trait_kind_from_def_id(obligation.predicate.def_id())
755 .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
757 // Okay to skip binder because the substs on closure types never
758 // touch bound regions, they just capture the in-scope
759 // type/region parameters.
760 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
761 let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
762 bug!("closure candidate for non-closure {:?}", obligation);
765 let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
766 let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
768 debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
771 if self.tcx().sess.opts.unstable_opts.trait_solver != TraitSolver::Chalk {
772 nested.push(obligation.with(
774 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind)),
778 Ok(ImplSourceClosureData { closure_def_id, substs, nested })
781 /// In the case of closure types and fn pointers,
782 /// we currently treat the input type parameters on the trait as
783 /// outputs. This means that when we have a match we have only
784 /// considered the self type, so we have to go back and make sure
785 /// to relate the argument types too. This is kind of wrong, but
786 /// since we control the full set of impls, also not that wrong,
787 /// and it DOES yield better error messages (since we don't report
788 /// errors as if there is no applicable impl, but rather report
789 /// errors are about mismatched argument types.
791 /// Here is an example. Imagine we have a closure expression
792 /// and we desugared it so that the type of the expression is
793 /// `Closure`, and `Closure` expects `i32` as argument. Then it
794 /// is "as if" the compiler generated this impl:
795 /// ```ignore (illustrative)
796 /// impl Fn(i32) for Closure { ... }
798 /// Now imagine our obligation is `Closure: Fn(usize)`. So far
799 /// we have matched the self type `Closure`. At this point we'll
800 /// compare the `i32` to `usize` and generate an error.
802 /// Note that this checking occurs *after* the impl has selected,
803 /// because these output type parameters should not affect the
804 /// selection of the impl. Therefore, if there is a mismatch, we
805 /// report an error to the user.
806 #[instrument(skip(self), level = "trace")]
807 fn confirm_poly_trait_refs(
809 obligation: &TraitObligation<'tcx>,
810 expected_trait_ref: ty::PolyTraitRef<'tcx>,
811 ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
812 let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
813 // Normalize the obligation and expected trait refs together, because why not
814 let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
815 ensure_sufficient_stack(|| {
816 normalize_with_depth(
818 obligation.param_env,
819 obligation.cause.clone(),
820 obligation.recursion_depth + 1,
821 (obligation_trait_ref, expected_trait_ref),
826 .at(&obligation.cause, obligation.param_env)
827 .sup(obligation_trait_ref, expected_trait_ref)
828 .map(|InferOk { mut obligations, .. }| {
829 obligations.extend(nested);
832 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
835 fn confirm_trait_upcasting_unsize_candidate(
837 obligation: &TraitObligation<'tcx>,
839 ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
841 let tcx = self.tcx();
843 // `assemble_candidates_for_unsizing` should ensure there are no late-bound
844 // regions here. See the comment there for more details.
845 let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
846 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
847 let target = self.infcx.shallow_resolve(target);
849 debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
851 let mut nested = vec![];
852 let source_trait_ref;
853 let upcast_trait_ref;
854 match (source.kind(), target.kind()) {
855 // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
857 &ty::Dynamic(ref data_a, r_a, repr_a @ ty::Dyn),
858 &ty::Dynamic(ref data_b, r_b, ty::Dyn),
860 // See `assemble_candidates_for_unsizing` for more info.
861 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
862 let principal_a = data_a.principal().unwrap();
863 source_trait_ref = principal_a.with_self_ty(tcx, source);
864 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
865 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
866 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
867 ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
871 let iter = Some(existential_predicate)
876 .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
881 .map(ty::ExistentialPredicate::AutoTrait)
882 .map(ty::Binder::dummy),
884 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
885 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_a);
887 // Require that the traits involved in this upcast are **equal**;
888 // only the **lifetime bound** is changed.
889 let InferOk { obligations, .. } = self
891 .at(&obligation.cause, obligation.param_env)
892 .sup(target, source_trait)
893 .map_err(|_| Unimplemented)?;
894 nested.extend(obligations);
896 // Register one obligation for 'a: 'b.
897 let cause = ObligationCause::new(
898 obligation.cause.span,
899 obligation.cause.body_id,
900 ObjectCastObligation(source, target),
902 let outlives = ty::OutlivesPredicate(r_a, r_b);
903 nested.push(Obligation::with_depth(
906 obligation.recursion_depth + 1,
907 obligation.param_env,
908 obligation.predicate.rebind(outlives),
914 let vtable_segment_callback = {
915 let mut vptr_offset = 0;
918 VtblSegment::MetadataDSA => {
919 vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
921 VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
922 vptr_offset += count_own_vtable_entries(tcx, trait_ref);
923 if trait_ref == upcast_trait_ref {
925 return ControlFlow::Break(Some(vptr_offset));
927 return ControlFlow::Break(None);
936 ControlFlow::Continue(())
940 let vtable_vptr_slot =
941 prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback).unwrap();
943 Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
946 fn confirm_builtin_unsize_candidate(
948 obligation: &TraitObligation<'tcx>,
949 ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
950 let tcx = self.tcx();
952 // `assemble_candidates_for_unsizing` should ensure there are no late-bound
953 // regions here. See the comment there for more details.
954 let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
955 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
956 let target = self.infcx.shallow_resolve(target);
958 debug!(?source, ?target, "confirm_builtin_unsize_candidate");
960 let mut nested = vec![];
961 match (source.kind(), target.kind()) {
962 // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
963 (&ty::Dynamic(ref data_a, r_a, dyn_a), &ty::Dynamic(ref data_b, r_b, dyn_b))
966 // See `assemble_candidates_for_unsizing` for more info.
967 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
970 .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
975 .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
980 .map(ty::ExistentialPredicate::AutoTrait)
981 .map(ty::Binder::dummy),
983 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
984 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, dyn_a);
986 // Require that the traits involved in this upcast are **equal**;
987 // only the **lifetime bound** is changed.
988 let InferOk { obligations, .. } = self
990 .at(&obligation.cause, obligation.param_env)
991 .sup(target, source_trait)
992 .map_err(|_| Unimplemented)?;
993 nested.extend(obligations);
995 // Register one obligation for 'a: 'b.
996 let cause = ObligationCause::new(
997 obligation.cause.span,
998 obligation.cause.body_id,
999 ObjectCastObligation(source, target),
1001 let outlives = ty::OutlivesPredicate(r_a, r_b);
1002 nested.push(Obligation::with_depth(
1005 obligation.recursion_depth + 1,
1006 obligation.param_env,
1007 obligation.predicate.rebind(outlives),
1012 (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
1013 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
1014 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
1015 return Err(TraitNotObjectSafe(did));
1018 let cause = ObligationCause::new(
1019 obligation.cause.span,
1020 obligation.cause.body_id,
1021 ObjectCastObligation(source, target),
1024 let predicate_to_obligation = |predicate| {
1025 Obligation::with_depth(
1028 obligation.recursion_depth + 1,
1029 obligation.param_env,
1034 // Create obligations:
1035 // - Casting `T` to `Trait`
1036 // - For all the various builtin bounds attached to the object cast. (In other
1037 // words, if the object type is `Foo + Send`, this would create an obligation for
1038 // the `Send` check.)
1039 // - Projection predicates
1041 data.iter().map(|predicate| {
1042 predicate_to_obligation(predicate.with_self_ty(tcx, source))
1046 // We can only make objects from sized types.
1048 ty::Binder::dummy(tcx.at(cause.span).mk_trait_ref(LangItem::Sized, [source]));
1049 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
1051 // If the type is `Foo + 'a`, ensure that the type
1052 // being cast to `Foo + 'a` outlives `'a`:
1053 let outlives = ty::OutlivesPredicate(source, r);
1054 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
1057 // `[T; n]` -> `[T]`
1058 (&ty::Array(a, _), &ty::Slice(b)) => {
1059 let InferOk { obligations, .. } = self
1061 .at(&obligation.cause, obligation.param_env)
1063 .map_err(|_| Unimplemented)?;
1064 nested.extend(obligations);
1067 // `Struct<T>` -> `Struct<U>`
1068 (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
1069 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
1070 GenericArgKind::Type(ty) => match ty.kind() {
1071 ty::Param(p) => Some(p.index),
1075 // Lifetimes aren't allowed to change during unsizing.
1076 GenericArgKind::Lifetime(_) => None,
1078 GenericArgKind::Const(ct) => match ct.kind() {
1079 ty::ConstKind::Param(p) => Some(p.index),
1084 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1085 // by putting it in a query; it would only need the `DefId` as it
1086 // looks at declared field types, not anything substituted.
1088 // The last field of the structure has to exist and contain type/const parameters.
1089 let (tail_field, prefix_fields) =
1090 def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1091 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1093 let mut unsizing_params = GrowableBitSet::new_empty();
1094 for arg in tail_field_ty.0.walk() {
1095 if let Some(i) = maybe_unsizing_param_idx(arg) {
1096 unsizing_params.insert(i);
1100 // Ensure none of the other fields mention the parameters used
1102 for field in prefix_fields {
1103 for arg in tcx.type_of(field.did).walk() {
1104 if let Some(i) = maybe_unsizing_param_idx(arg) {
1105 unsizing_params.remove(i);
1110 if unsizing_params.is_empty() {
1111 return Err(Unimplemented);
1114 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
1115 // normalizing in the process, since `type_of` returns something directly from
1116 // astconv (which means it's un-normalized).
1117 let source_tail = normalize_with_depth_to(
1119 obligation.param_env,
1120 obligation.cause.clone(),
1121 obligation.recursion_depth + 1,
1122 tail_field_ty.subst(tcx, substs_a),
1125 let target_tail = normalize_with_depth_to(
1127 obligation.param_env,
1128 obligation.cause.clone(),
1129 obligation.recursion_depth + 1,
1130 tail_field_ty.subst(tcx, substs_b),
1134 // Check that the source struct with the target's
1135 // unsizing parameters is equal to the target.
1136 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1137 if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1139 let new_struct = tcx.mk_adt(def, substs);
1140 let InferOk { obligations, .. } = self
1142 .at(&obligation.cause, obligation.param_env)
1143 .eq(target, new_struct)
1144 .map_err(|_| Unimplemented)?;
1145 nested.extend(obligations);
1147 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1148 nested.push(predicate_for_trait_def(
1150 obligation.param_env,
1151 obligation.cause.clone(),
1152 obligation.predicate.def_id(),
1153 obligation.recursion_depth + 1,
1154 [source_tail, target_tail],
1158 // `(.., T)` -> `(.., U)`
1159 (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1160 assert_eq!(tys_a.len(), tys_b.len());
1162 // The last field of the tuple has to exist.
1163 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1164 let &b_last = tys_b.last().unwrap();
1166 // Check that the source tuple with the target's
1167 // last element is equal to the target.
1168 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1169 let InferOk { obligations, .. } = self
1171 .at(&obligation.cause, obligation.param_env)
1172 .eq(target, new_tuple)
1173 .map_err(|_| Unimplemented)?;
1174 nested.extend(obligations);
1176 // Construct the nested `T: Unsize<U>` predicate.
1177 nested.push(ensure_sufficient_stack(|| {
1178 predicate_for_trait_def(
1180 obligation.param_env,
1181 obligation.cause.clone(),
1182 obligation.predicate.def_id(),
1183 obligation.recursion_depth + 1,
1189 _ => bug!("source: {source}, target: {target}"),
1192 Ok(ImplSourceBuiltinData { nested })
1195 fn confirm_const_destruct_candidate(
1197 obligation: &TraitObligation<'tcx>,
1198 impl_def_id: Option<DefId>,
1199 ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1200 // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1201 if !obligation.is_const() {
1202 return Ok(ImplSourceConstDestructData { nested: vec![] });
1205 let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1207 let tcx = self.tcx();
1208 let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1210 let mut nested = vec![];
1211 let cause = obligation.derived_cause(BuiltinDerivedObligation);
1213 // If we have a custom `impl const Drop`, then
1214 // first check it like a regular impl candidate.
1215 // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1216 if let Some(impl_def_id) = impl_def_id {
1217 let mut new_obligation = obligation.clone();
1218 new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1219 trait_pred.trait_ref.def_id = drop_trait;
1222 let substs = self.rematch_impl(impl_def_id, &new_obligation);
1223 debug!(?substs, "impl substs");
1225 let cause = obligation.derived_cause(|derived| {
1226 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1229 span: obligation.cause.span,
1232 let obligations = ensure_sufficient_stack(|| {
1237 new_obligation.recursion_depth + 1,
1238 new_obligation.param_env,
1239 obligation.predicate,
1242 nested.extend(obligations.nested);
1245 // We want to confirm the ADT's fields if we have an ADT
1246 let mut stack = match *self_ty.skip_binder().kind() {
1247 ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1248 _ => vec![self_ty.skip_binder()],
1251 while let Some(nested_ty) = stack.pop() {
1252 match *nested_ty.kind() {
1253 // We know these types are trivially drop
1259 | ty::Infer(ty::IntVar(_))
1260 | ty::Infer(ty::FloatVar(_))
1267 | ty::Foreign(_) => {}
1269 // `ManuallyDrop` is trivially drop
1270 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
1272 // These types are built-in, so we can fast-track by registering
1273 // nested predicates for their constituent type(s)
1274 ty::Array(ty, _) | ty::Slice(ty) => {
1278 stack.extend(tys.iter());
1280 ty::Closure(_, substs) => {
1281 stack.push(substs.as_closure().tupled_upvars_ty());
1283 ty::Generator(_, substs, _) => {
1284 let generator = substs.as_generator();
1285 stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1287 ty::GeneratorWitness(tys) => {
1288 stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1291 // If we have a projection type, make sure to normalize it so we replace it
1292 // with a fresh infer variable
1293 ty::Alias(ty::Projection, ..) => {
1294 let predicate = normalize_with_depth_to(
1296 obligation.param_env,
1298 obligation.recursion_depth + 1,
1299 self_ty.rebind(ty::TraitPredicate {
1303 .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1304 constness: ty::BoundConstness::ConstIfConst,
1305 polarity: ty::ImplPolarity::Positive,
1310 nested.push(Obligation::with_depth(
1313 obligation.recursion_depth + 1,
1314 obligation.param_env,
1319 // If we have any other type (e.g. an ADT), just register a nested obligation
1320 // since it's either not `const Drop` (and we raise an error during selection),
1321 // or it's an ADT (and we need to check for a custom impl during selection)
1323 let predicate = self_ty.rebind(ty::TraitPredicate {
1327 .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1328 constness: ty::BoundConstness::ConstIfConst,
1329 polarity: ty::ImplPolarity::Positive,
1332 nested.push(Obligation::with_depth(
1335 obligation.recursion_depth + 1,
1336 obligation.param_env,
1343 Ok(ImplSourceConstDestructData { nested })