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 = tcx.item_bounds(def_id).map_bound(|i| i[idx]).subst(tcx, substs);
164 let candidate = candidate_predicate
165 .to_opt_poly_trait_pred()
166 .expect("projection candidate is not a trait predicate")
167 .map_bound(|t| t.trait_ref);
168 let mut obligations = Vec::new();
169 let candidate = normalize_with_depth_to(
171 obligation.param_env,
172 obligation.cause.clone(),
173 obligation.recursion_depth + 1,
178 obligations.extend(self.infcx.commit_if_ok(|_| {
180 .at(&obligation.cause, obligation.param_env)
181 .sup(placeholder_trait_predicate, candidate)
182 .map(|InferOk { obligations, .. }| obligations)
183 .map_err(|_| Unimplemented)
186 if let ty::Alias(ty::Projection, ..) = placeholder_self_ty.kind() {
187 let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs);
188 for (predicate, _) in predicates {
189 let normalized = normalize_with_depth_to(
191 obligation.param_env,
192 obligation.cause.clone(),
193 obligation.recursion_depth + 1,
197 obligations.push(Obligation::with_depth(
199 obligation.cause.clone(),
200 obligation.recursion_depth + 1,
201 obligation.param_env,
210 fn confirm_param_candidate(
212 obligation: &TraitObligation<'tcx>,
213 param: ty::PolyTraitRef<'tcx>,
214 ) -> Vec<PredicateObligation<'tcx>> {
215 debug!(?obligation, ?param, "confirm_param_candidate");
217 // During evaluation, we already checked that this
218 // where-clause trait-ref could be unified with the obligation
219 // trait-ref. Repeat that unification now without any
220 // transactional boundary; it should not fail.
221 match self.match_where_clause_trait_ref(obligation, param) {
222 Ok(obligations) => obligations,
225 "Where clause `{:?}` was applicable to `{:?}` but now is not",
233 fn confirm_builtin_candidate(
235 obligation: &TraitObligation<'tcx>,
237 ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
238 debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
240 let lang_items = self.tcx().lang_items();
241 let obligations = if has_nested {
242 let trait_def = obligation.predicate.def_id();
243 let conditions = if Some(trait_def) == lang_items.sized_trait() {
244 self.sized_conditions(obligation)
245 } else if Some(trait_def) == lang_items.copy_trait() {
246 self.copy_clone_conditions(obligation)
247 } else if Some(trait_def) == lang_items.clone_trait() {
248 self.copy_clone_conditions(obligation)
250 bug!("unexpected builtin trait {:?}", trait_def)
252 let BuiltinImplConditions::Where(nested) = conditions else {
253 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
256 let cause = obligation.derived_cause(BuiltinDerivedObligation);
257 ensure_sufficient_stack(|| {
258 self.collect_predicates_for_types(
259 obligation.param_env,
261 obligation.recursion_depth + 1,
270 debug!(?obligations);
272 ImplSourceBuiltinData { nested: obligations }
275 fn confirm_transmutability_candidate(
277 obligation: &TraitObligation<'tcx>,
278 ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
279 debug!(?obligation, "confirm_transmutability_candidate");
281 let predicate = obligation.predicate;
283 let type_at = |i| predicate.map_bound(|p| p.trait_ref.substs.type_at(i));
284 let const_at = |i| predicate.skip_binder().trait_ref.substs.const_at(i);
286 let src_and_dst = predicate.map_bound(|p| rustc_transmute::Types {
287 dst: p.trait_ref.substs.type_at(0),
288 src: p.trait_ref.substs.type_at(1),
291 let scope = type_at(2).skip_binder();
294 rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3)) else {
295 return Err(Unimplemented);
298 let cause = obligation.cause.clone();
300 let mut transmute_env = rustc_transmute::TransmuteTypeEnv::new(self.infcx);
302 let maybe_transmutable = transmute_env.is_transmutable(cause, src_and_dst, scope, assume);
304 use rustc_transmute::Answer;
306 match maybe_transmutable {
307 Answer::Yes => Ok(ImplSourceBuiltinData { nested: vec![] }),
308 _ => Err(Unimplemented),
312 /// This handles the case where an `auto trait Foo` impl is being used.
313 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
315 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
316 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
317 fn confirm_auto_impl_candidate(
319 obligation: &TraitObligation<'tcx>,
320 ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
321 debug!(?obligation, "confirm_auto_impl_candidate");
323 let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
324 let types = self.constituent_types_for_ty(self_ty);
325 self.vtable_auto_impl(obligation, obligation.predicate.def_id(), types)
328 /// See `confirm_auto_impl_candidate`.
331 obligation: &TraitObligation<'tcx>,
333 nested: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
334 ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
335 debug!(?nested, "vtable_auto_impl");
336 ensure_sufficient_stack(|| {
337 let cause = obligation.derived_cause(BuiltinDerivedObligation);
339 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
340 let trait_ref = self.infcx.replace_bound_vars_with_placeholders(poly_trait_ref);
341 let trait_obligations: Vec<PredicateObligation<'_>> = self.impl_or_trait_obligations(
343 obligation.recursion_depth + 1,
344 obligation.param_env,
347 obligation.predicate,
350 let mut obligations = self.collect_predicates_for_types(
351 obligation.param_env,
353 obligation.recursion_depth + 1,
358 // Adds the predicates from the trait. Note that this contains a `Self: Trait`
359 // predicate as usual. It won't have any effect since auto traits are coinductive.
360 obligations.extend(trait_obligations);
362 debug!(?obligations, "vtable_auto_impl");
364 ImplSourceAutoImplData { trait_def_id, nested: obligations }
368 fn confirm_impl_candidate(
370 obligation: &TraitObligation<'tcx>,
372 ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
373 debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");
375 // First, create the substitutions by matching the impl again,
376 // this time not in a probe.
377 let substs = self.rematch_impl(impl_def_id, obligation);
378 debug!(?substs, "impl substs");
379 ensure_sufficient_stack(|| {
384 obligation.recursion_depth + 1,
385 obligation.param_env,
386 obligation.predicate,
394 substs: Normalized<'tcx, SubstsRef<'tcx>>,
395 cause: &ObligationCause<'tcx>,
396 recursion_depth: usize,
397 param_env: ty::ParamEnv<'tcx>,
398 parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
399 ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
400 debug!(?impl_def_id, ?substs, ?recursion_depth, "vtable_impl");
402 let mut impl_obligations = self.impl_or_trait_obligations(
411 debug!(?impl_obligations, "vtable_impl");
413 // Because of RFC447, the impl-trait-ref and obligations
414 // are sufficient to determine the impl substs, without
415 // relying on projections in the impl-trait-ref.
417 // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
418 impl_obligations.extend(substs.obligations);
420 ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations }
423 fn confirm_object_candidate(
425 obligation: &TraitObligation<'tcx>,
427 ) -> Result<ImplSourceObjectData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
428 let tcx = self.tcx();
429 debug!(?obligation, ?index, "confirm_object_candidate");
431 let trait_predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
432 let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
433 let obligation_trait_ref = ty::Binder::dummy(trait_predicate.trait_ref);
434 let ty::Dynamic(data, ..) = *self_ty.kind() else {
435 span_bug!(obligation.cause.span, "object candidate with non-object");
438 let object_trait_ref = data.principal().unwrap_or_else(|| {
439 span_bug!(obligation.cause.span, "object candidate with no principal")
441 let object_trait_ref = self.infcx.replace_bound_vars_with_fresh_vars(
442 obligation.cause.span,
446 let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);
448 let mut nested = vec![];
450 let mut supertraits = util::supertraits(tcx, ty::Binder::dummy(object_trait_ref));
451 let unnormalized_upcast_trait_ref =
452 supertraits.nth(index).expect("supertraits iterator no longer has as many elements");
454 let upcast_trait_ref = normalize_with_depth_to(
456 obligation.param_env,
457 obligation.cause.clone(),
458 obligation.recursion_depth + 1,
459 unnormalized_upcast_trait_ref,
463 nested.extend(self.infcx.commit_if_ok(|_| {
465 .at(&obligation.cause, obligation.param_env)
466 .sup(obligation_trait_ref, upcast_trait_ref)
467 .map(|InferOk { obligations, .. }| obligations)
468 .map_err(|_| Unimplemented)
471 // Check supertraits hold. This is so that their associated type bounds
472 // will be checked in the code below.
473 for super_trait in tcx
474 .super_predicates_of(trait_predicate.def_id())
475 .instantiate(tcx, trait_predicate.trait_ref.substs)
479 let normalized_super_trait = normalize_with_depth_to(
481 obligation.param_env,
482 obligation.cause.clone(),
483 obligation.recursion_depth + 1,
487 nested.push(obligation.with(tcx, normalized_super_trait));
490 let assoc_types: Vec<_> = tcx
491 .associated_items(trait_predicate.def_id())
492 .in_definition_order()
494 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
498 for assoc_type in assoc_types {
499 let defs: &ty::Generics = tcx.generics_of(assoc_type);
501 if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
502 tcx.sess.delay_span_bug(
503 obligation.cause.span,
504 "GATs in trait object shouldn't have been considered",
506 return Err(SelectionError::Unimplemented);
509 // This maybe belongs in wf, but that can't (doesn't) handle
510 // higher-ranked things.
511 // Prevent, e.g., `dyn Iterator<Item = str>`.
512 for bound in self.tcx().item_bounds(assoc_type).transpose_iter() {
514 if defs.count() == 0 {
515 bound.subst(tcx, trait_predicate.trait_ref.substs)
517 let mut substs = smallvec::SmallVec::with_capacity(defs.count());
518 substs.extend(trait_predicate.trait_ref.substs.iter());
519 let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
520 smallvec::SmallVec::with_capacity(
521 bound.0.kind().bound_vars().len() + defs.count(),
523 bound_vars.extend(bound.0.kind().bound_vars().into_iter());
524 InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
527 GenericParamDefKind::Type { .. } => {
528 let kind = ty::BoundTyKind::Param(param.name);
529 let bound_var = ty::BoundVariableKind::Ty(kind);
530 bound_vars.push(bound_var);
534 var: ty::BoundVar::from_usize(bound_vars.len() - 1),
540 GenericParamDefKind::Lifetime => {
541 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
542 let bound_var = ty::BoundVariableKind::Region(kind);
543 bound_vars.push(bound_var);
544 tcx.mk_region(ty::ReLateBound(
547 var: ty::BoundVar::from_usize(bound_vars.len() - 1),
553 GenericParamDefKind::Const { .. } => {
554 let bound_var = ty::BoundVariableKind::Const;
555 bound_vars.push(bound_var);
557 ty::ConstKind::Bound(
559 ty::BoundVar::from_usize(bound_vars.len() - 1),
561 tcx.type_of(param.def_id),
566 let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
567 let assoc_ty_substs = tcx.intern_substs(&substs);
569 let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
571 bound.map_bound(|b| b.kind().skip_binder()).subst(tcx, assoc_ty_substs);
572 tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
574 let normalized_bound = normalize_with_depth_to(
576 obligation.param_env,
577 obligation.cause.clone(),
578 obligation.recursion_depth + 1,
582 nested.push(obligation.with(tcx, normalized_bound));
586 debug!(?nested, "object nested obligations");
588 let vtable_base = vtable_trait_first_method_offset(
590 (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
593 Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
596 fn confirm_fn_pointer_candidate(
598 obligation: &TraitObligation<'tcx>,
600 ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
602 debug!(?obligation, "confirm_fn_pointer_candidate");
604 let tcx = self.tcx();
607 .shallow_resolve(obligation.self_ty().no_bound_vars())
608 .expect("fn pointer should not capture bound vars from predicate");
609 let sig = self_ty.fn_sig(tcx);
610 let trait_ref = closure_trait_ref_and_return_type(
612 obligation.predicate.def_id(),
615 util::TupleArgumentsFlag::Yes,
617 .map_bound(|(trait_ref, _)| trait_ref);
619 let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
620 let cause = obligation.derived_cause(BuiltinDerivedObligation);
622 if obligation.is_const() && !is_const {
623 // function is a trait method
624 if let ty::FnDef(def_id, substs) = self_ty.kind() && let Some(trait_id) = tcx.trait_of_item(*def_id) {
625 let trait_ref = TraitRef::from_method(tcx, trait_id, *substs);
626 let poly_trait_pred = Binder::dummy(trait_ref).with_constness(ty::BoundConstness::ConstIfConst);
627 let obligation = Obligation::new(tcx, cause.clone(), obligation.param_env, poly_trait_pred);
628 nested.push(obligation);
632 // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
633 let output_ty = self.infcx.replace_bound_vars_with_placeholders(sig.output());
634 let output_ty = normalize_with_depth_to(
636 obligation.param_env,
638 obligation.recursion_depth,
643 ty::Binder::dummy(self.tcx().at(cause.span).mk_trait_ref(LangItem::Sized, [output_ty]));
644 nested.push(Obligation::new(self.infcx.tcx, cause, obligation.param_env, tr));
646 Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
649 fn confirm_trait_alias_candidate(
651 obligation: &TraitObligation<'tcx>,
652 ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
653 debug!(?obligation, "confirm_trait_alias_candidate");
655 let alias_def_id = obligation.predicate.def_id();
656 let predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
657 let trait_ref = predicate.trait_ref;
658 let trait_def_id = trait_ref.def_id;
659 let substs = trait_ref.substs;
661 let trait_obligations = self.impl_or_trait_obligations(
663 obligation.recursion_depth,
664 obligation.param_env,
667 obligation.predicate,
670 debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
672 ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
675 fn confirm_generator_candidate(
677 obligation: &TraitObligation<'tcx>,
678 ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
680 // Okay to skip binder because the substs on generator types never
681 // touch bound regions, they just capture the in-scope
682 // type/region parameters.
683 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
684 let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
685 bug!("closure candidate for non-closure {:?}", obligation);
688 debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
690 let gen_sig = substs.as_generator().poly_sig();
692 // NOTE: The self-type is a generator type and hence is
693 // in fact unparameterized (or at least does not reference any
694 // regions bound in the obligation).
695 let self_ty = obligation
699 .expect("unboxed closure type should not capture bound vars from the predicate");
701 let trait_ref = super::util::generator_trait_ref_and_outputs(
703 obligation.predicate.def_id(),
707 .map_bound(|(trait_ref, ..)| trait_ref);
709 let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
710 debug!(?trait_ref, ?nested, "generator candidate obligations");
712 Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
715 fn confirm_future_candidate(
717 obligation: &TraitObligation<'tcx>,
718 ) -> Result<ImplSourceFutureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
719 // Okay to skip binder because the substs on generator types never
720 // touch bound regions, they just capture the in-scope
721 // type/region parameters.
722 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
723 let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
724 bug!("closure candidate for non-closure {:?}", obligation);
727 debug!(?obligation, ?generator_def_id, ?substs, "confirm_future_candidate");
729 let gen_sig = substs.as_generator().poly_sig();
731 let trait_ref = super::util::future_trait_ref_and_outputs(
733 obligation.predicate.def_id(),
734 obligation.predicate.no_bound_vars().expect("future has no bound vars").self_ty(),
737 .map_bound(|(trait_ref, ..)| trait_ref);
739 let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
740 debug!(?trait_ref, ?nested, "future candidate obligations");
742 Ok(ImplSourceFutureData { generator_def_id, substs, nested })
745 #[instrument(skip(self), level = "debug")]
746 fn confirm_closure_candidate(
748 obligation: &TraitObligation<'tcx>,
749 ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
752 .fn_trait_kind_from_def_id(obligation.predicate.def_id())
753 .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
755 // Okay to skip binder because the substs on closure types never
756 // touch bound regions, they just capture the in-scope
757 // type/region parameters.
758 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
759 let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
760 bug!("closure candidate for non-closure {:?}", obligation);
763 let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
764 let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
766 debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
769 if self.tcx().sess.opts.unstable_opts.trait_solver != TraitSolver::Chalk {
770 nested.push(obligation.with(
772 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind)),
776 Ok(ImplSourceClosureData { closure_def_id, substs, nested })
779 /// In the case of closure types and fn pointers,
780 /// we currently treat the input type parameters on the trait as
781 /// outputs. This means that when we have a match we have only
782 /// considered the self type, so we have to go back and make sure
783 /// to relate the argument types too. This is kind of wrong, but
784 /// since we control the full set of impls, also not that wrong,
785 /// and it DOES yield better error messages (since we don't report
786 /// errors as if there is no applicable impl, but rather report
787 /// errors are about mismatched argument types.
789 /// Here is an example. Imagine we have a closure expression
790 /// and we desugared it so that the type of the expression is
791 /// `Closure`, and `Closure` expects `i32` as argument. Then it
792 /// is "as if" the compiler generated this impl:
793 /// ```ignore (illustrative)
794 /// impl Fn(i32) for Closure { ... }
796 /// Now imagine our obligation is `Closure: Fn(usize)`. So far
797 /// we have matched the self type `Closure`. At this point we'll
798 /// compare the `i32` to `usize` and generate an error.
800 /// Note that this checking occurs *after* the impl has selected,
801 /// because these output type parameters should not affect the
802 /// selection of the impl. Therefore, if there is a mismatch, we
803 /// report an error to the user.
804 #[instrument(skip(self), level = "trace")]
805 fn confirm_poly_trait_refs(
807 obligation: &TraitObligation<'tcx>,
808 expected_trait_ref: ty::PolyTraitRef<'tcx>,
809 ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
810 let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
811 // Normalize the obligation and expected trait refs together, because why not
812 let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
813 ensure_sufficient_stack(|| {
814 normalize_with_depth(
816 obligation.param_env,
817 obligation.cause.clone(),
818 obligation.recursion_depth + 1,
819 (obligation_trait_ref, expected_trait_ref),
824 .at(&obligation.cause, obligation.param_env)
825 .sup(obligation_trait_ref, expected_trait_ref)
826 .map(|InferOk { mut obligations, .. }| {
827 obligations.extend(nested);
830 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
833 fn confirm_trait_upcasting_unsize_candidate(
835 obligation: &TraitObligation<'tcx>,
837 ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
839 let tcx = self.tcx();
841 // `assemble_candidates_for_unsizing` should ensure there are no late-bound
842 // regions here. See the comment there for more details.
843 let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
844 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
845 let target = self.infcx.shallow_resolve(target);
847 debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
849 let mut nested = vec![];
850 let source_trait_ref;
851 let upcast_trait_ref;
852 match (source.kind(), target.kind()) {
853 // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
855 &ty::Dynamic(ref data_a, r_a, repr_a @ ty::Dyn),
856 &ty::Dynamic(ref data_b, r_b, ty::Dyn),
858 // See `assemble_candidates_for_unsizing` for more info.
859 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
860 let principal_a = data_a.principal().unwrap();
861 source_trait_ref = principal_a.with_self_ty(tcx, source);
862 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
863 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
864 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
865 ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
869 let iter = Some(existential_predicate)
874 .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
879 .map(ty::ExistentialPredicate::AutoTrait)
880 .map(ty::Binder::dummy),
882 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
883 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_a);
885 // Require that the traits involved in this upcast are **equal**;
886 // only the **lifetime bound** is changed.
887 let InferOk { obligations, .. } = self
889 .at(&obligation.cause, obligation.param_env)
890 .sup(target, source_trait)
891 .map_err(|_| Unimplemented)?;
892 nested.extend(obligations);
894 // Register one obligation for 'a: 'b.
895 let cause = ObligationCause::new(
896 obligation.cause.span,
897 obligation.cause.body_id,
898 ObjectCastObligation(source, target),
900 let outlives = ty::OutlivesPredicate(r_a, r_b);
901 nested.push(Obligation::with_depth(
904 obligation.recursion_depth + 1,
905 obligation.param_env,
906 obligation.predicate.rebind(outlives),
912 let vtable_segment_callback = {
913 let mut vptr_offset = 0;
916 VtblSegment::MetadataDSA => {
917 vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
919 VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
920 vptr_offset += count_own_vtable_entries(tcx, trait_ref);
921 if trait_ref == upcast_trait_ref {
923 return ControlFlow::Break(Some(vptr_offset));
925 return ControlFlow::Break(None);
934 ControlFlow::Continue(())
938 let vtable_vptr_slot =
939 prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback).unwrap();
941 Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
944 fn confirm_builtin_unsize_candidate(
946 obligation: &TraitObligation<'tcx>,
947 ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
948 let tcx = self.tcx();
950 // `assemble_candidates_for_unsizing` should ensure there are no late-bound
951 // regions here. See the comment there for more details.
952 let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
953 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
954 let target = self.infcx.shallow_resolve(target);
956 debug!(?source, ?target, "confirm_builtin_unsize_candidate");
958 let mut nested = vec![];
959 match (source.kind(), target.kind()) {
960 // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
961 (&ty::Dynamic(ref data_a, r_a, dyn_a), &ty::Dynamic(ref data_b, r_b, dyn_b))
964 // See `assemble_candidates_for_unsizing` for more info.
965 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
968 .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
973 .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
978 .map(ty::ExistentialPredicate::AutoTrait)
979 .map(ty::Binder::dummy),
981 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
982 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, dyn_a);
984 // Require that the traits involved in this upcast are **equal**;
985 // only the **lifetime bound** is changed.
986 let InferOk { obligations, .. } = self
988 .at(&obligation.cause, obligation.param_env)
989 .sup(target, source_trait)
990 .map_err(|_| Unimplemented)?;
991 nested.extend(obligations);
993 // Register one obligation for 'a: 'b.
994 let cause = ObligationCause::new(
995 obligation.cause.span,
996 obligation.cause.body_id,
997 ObjectCastObligation(source, target),
999 let outlives = ty::OutlivesPredicate(r_a, r_b);
1000 nested.push(Obligation::with_depth(
1003 obligation.recursion_depth + 1,
1004 obligation.param_env,
1005 obligation.predicate.rebind(outlives),
1010 (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
1011 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
1012 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
1013 return Err(TraitNotObjectSafe(did));
1016 let cause = ObligationCause::new(
1017 obligation.cause.span,
1018 obligation.cause.body_id,
1019 ObjectCastObligation(source, target),
1022 let predicate_to_obligation = |predicate| {
1023 Obligation::with_depth(
1026 obligation.recursion_depth + 1,
1027 obligation.param_env,
1032 // Create obligations:
1033 // - Casting `T` to `Trait`
1034 // - For all the various builtin bounds attached to the object cast. (In other
1035 // words, if the object type is `Foo + Send`, this would create an obligation for
1036 // the `Send` check.)
1037 // - Projection predicates
1039 data.iter().map(|predicate| {
1040 predicate_to_obligation(predicate.with_self_ty(tcx, source))
1044 // We can only make objects from sized types.
1046 ty::Binder::dummy(tcx.at(cause.span).mk_trait_ref(LangItem::Sized, [source]));
1047 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
1049 // If the type is `Foo + 'a`, ensure that the type
1050 // being cast to `Foo + 'a` outlives `'a`:
1051 let outlives = ty::OutlivesPredicate(source, r);
1052 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
1055 // `[T; n]` -> `[T]`
1056 (&ty::Array(a, _), &ty::Slice(b)) => {
1057 let InferOk { obligations, .. } = self
1059 .at(&obligation.cause, obligation.param_env)
1061 .map_err(|_| Unimplemented)?;
1062 nested.extend(obligations);
1065 // `Struct<T>` -> `Struct<U>`
1066 (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
1067 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
1068 GenericArgKind::Type(ty) => match ty.kind() {
1069 ty::Param(p) => Some(p.index),
1073 // Lifetimes aren't allowed to change during unsizing.
1074 GenericArgKind::Lifetime(_) => None,
1076 GenericArgKind::Const(ct) => match ct.kind() {
1077 ty::ConstKind::Param(p) => Some(p.index),
1082 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1083 // by putting it in a query; it would only need the `DefId` as it
1084 // looks at declared field types, not anything substituted.
1086 // The last field of the structure has to exist and contain type/const parameters.
1087 let (tail_field, prefix_fields) =
1088 def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1089 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1091 let mut unsizing_params = GrowableBitSet::new_empty();
1092 for arg in tail_field_ty.0.walk() {
1093 if let Some(i) = maybe_unsizing_param_idx(arg) {
1094 unsizing_params.insert(i);
1098 // Ensure none of the other fields mention the parameters used
1100 for field in prefix_fields {
1101 for arg in tcx.type_of(field.did).walk() {
1102 if let Some(i) = maybe_unsizing_param_idx(arg) {
1103 unsizing_params.remove(i);
1108 if unsizing_params.is_empty() {
1109 return Err(Unimplemented);
1112 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
1113 // normalizing in the process, since `type_of` returns something directly from
1114 // astconv (which means it's un-normalized).
1115 let source_tail = normalize_with_depth_to(
1117 obligation.param_env,
1118 obligation.cause.clone(),
1119 obligation.recursion_depth + 1,
1120 tail_field_ty.subst(tcx, substs_a),
1123 let target_tail = normalize_with_depth_to(
1125 obligation.param_env,
1126 obligation.cause.clone(),
1127 obligation.recursion_depth + 1,
1128 tail_field_ty.subst(tcx, substs_b),
1132 // Check that the source struct with the target's
1133 // unsizing parameters is equal to the target.
1134 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1135 if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1137 let new_struct = tcx.mk_adt(def, substs);
1138 let InferOk { obligations, .. } = self
1140 .at(&obligation.cause, obligation.param_env)
1141 .eq(target, new_struct)
1142 .map_err(|_| Unimplemented)?;
1143 nested.extend(obligations);
1145 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1146 nested.push(predicate_for_trait_def(
1148 obligation.param_env,
1149 obligation.cause.clone(),
1150 obligation.predicate.def_id(),
1151 obligation.recursion_depth + 1,
1152 [source_tail, target_tail],
1156 // `(.., T)` -> `(.., U)`
1157 (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1158 assert_eq!(tys_a.len(), tys_b.len());
1160 // The last field of the tuple has to exist.
1161 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1162 let &b_last = tys_b.last().unwrap();
1164 // Check that the source tuple with the target's
1165 // last element is equal to the target.
1166 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1167 let InferOk { obligations, .. } = self
1169 .at(&obligation.cause, obligation.param_env)
1170 .eq(target, new_tuple)
1171 .map_err(|_| Unimplemented)?;
1172 nested.extend(obligations);
1174 // Construct the nested `T: Unsize<U>` predicate.
1175 nested.push(ensure_sufficient_stack(|| {
1176 predicate_for_trait_def(
1178 obligation.param_env,
1179 obligation.cause.clone(),
1180 obligation.predicate.def_id(),
1181 obligation.recursion_depth + 1,
1187 _ => bug!("source: {source}, target: {target}"),
1190 Ok(ImplSourceBuiltinData { nested })
1193 fn confirm_const_destruct_candidate(
1195 obligation: &TraitObligation<'tcx>,
1196 impl_def_id: Option<DefId>,
1197 ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1198 // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1199 if !obligation.is_const() {
1200 return Ok(ImplSourceConstDestructData { nested: vec![] });
1203 let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1205 let tcx = self.tcx();
1206 let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1208 let mut nested = vec![];
1209 let cause = obligation.derived_cause(BuiltinDerivedObligation);
1211 // If we have a custom `impl const Drop`, then
1212 // first check it like a regular impl candidate.
1213 // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1214 if let Some(impl_def_id) = impl_def_id {
1215 let mut new_obligation = obligation.clone();
1216 new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1217 trait_pred.trait_ref.def_id = drop_trait;
1220 let substs = self.rematch_impl(impl_def_id, &new_obligation);
1221 debug!(?substs, "impl substs");
1223 let cause = obligation.derived_cause(|derived| {
1224 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1227 span: obligation.cause.span,
1230 let obligations = ensure_sufficient_stack(|| {
1235 new_obligation.recursion_depth + 1,
1236 new_obligation.param_env,
1237 obligation.predicate,
1240 nested.extend(obligations.nested);
1243 // We want to confirm the ADT's fields if we have an ADT
1244 let mut stack = match *self_ty.skip_binder().kind() {
1245 ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1246 _ => vec![self_ty.skip_binder()],
1249 while let Some(nested_ty) = stack.pop() {
1250 match *nested_ty.kind() {
1251 // We know these types are trivially drop
1257 | ty::Infer(ty::IntVar(_))
1258 | ty::Infer(ty::FloatVar(_))
1265 | ty::Foreign(_) => {}
1267 // `ManuallyDrop` is trivially drop
1268 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
1270 // These types are built-in, so we can fast-track by registering
1271 // nested predicates for their constituent type(s)
1272 ty::Array(ty, _) | ty::Slice(ty) => {
1276 stack.extend(tys.iter());
1278 ty::Closure(_, substs) => {
1279 stack.push(substs.as_closure().tupled_upvars_ty());
1281 ty::Generator(_, substs, _) => {
1282 let generator = substs.as_generator();
1283 stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1285 ty::GeneratorWitness(tys) => {
1286 stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1289 // If we have a projection type, make sure to normalize it so we replace it
1290 // with a fresh infer variable
1291 ty::Alias(ty::Projection, ..) => {
1292 let predicate = normalize_with_depth_to(
1294 obligation.param_env,
1296 obligation.recursion_depth + 1,
1297 self_ty.rebind(ty::TraitPredicate {
1301 .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1302 constness: ty::BoundConstness::ConstIfConst,
1303 polarity: ty::ImplPolarity::Positive,
1308 nested.push(Obligation::with_depth(
1311 obligation.recursion_depth + 1,
1312 obligation.param_env,
1317 // If we have any other type (e.g. an ADT), just register a nested obligation
1318 // since it's either not `const Drop` (and we raise an error during selection),
1319 // or it's an ADT (and we need to check for a custom impl during selection)
1321 let predicate = self_ty.rebind(ty::TraitPredicate {
1325 .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1326 constness: ty::BoundConstness::ConstIfConst,
1327 polarity: ty::ImplPolarity::Positive,
1330 nested.push(Obligation::with_depth(
1333 obligation.recursion_depth + 1,
1334 obligation.param_env,
1341 Ok(ImplSourceConstDestructData { nested })