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, GenericArg, GenericArgKind, GenericParamDefKind, InternalSubsts, SubstsRef,
16 ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
18 use rustc_span::def_id::DefId;
20 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
21 use crate::traits::util::{self, closure_trait_ref_and_return_type, predicate_for_trait_def};
23 BuiltinDerivedObligation, ImplDerivedObligation, ImplDerivedObligationCause, ImplSource,
24 ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
25 ImplSourceConstDestructData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
26 ImplSourceGeneratorData, ImplSourceObjectData, ImplSourcePointeeData, ImplSourceTraitAliasData,
27 ImplSourceTraitUpcastingData, ImplSourceUserDefinedData, Normalized, ObjectCastObligation,
28 Obligation, ObligationCause, OutputTypeParameterMismatch, PredicateObligation, Selection,
29 SelectionError, TraitNotObjectSafe, TraitObligation, Unimplemented, VtblSegment,
32 use super::BuiltinImplConditions;
33 use super::SelectionCandidate::{self, *};
34 use super::SelectionContext;
37 use std::ops::ControlFlow;
39 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
40 #[instrument(level = "debug", skip(self))]
41 pub(super) fn confirm_candidate(
43 obligation: &TraitObligation<'tcx>,
44 candidate: SelectionCandidate<'tcx>,
45 ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
46 let mut impl_src = match candidate {
47 BuiltinCandidate { has_nested } => {
48 let data = self.confirm_builtin_candidate(obligation, has_nested);
49 ImplSource::Builtin(data)
52 TransmutabilityCandidate => {
53 let data = self.confirm_transmutability_candidate(obligation)?;
54 ImplSource::Builtin(data)
57 ParamCandidate(param) => {
59 self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
60 ImplSource::Param(obligations, param.skip_binder().constness)
63 ImplCandidate(impl_def_id) => {
64 ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id))
67 AutoImplCandidate => {
68 let data = self.confirm_auto_impl_candidate(obligation);
69 ImplSource::AutoImpl(data)
72 ProjectionCandidate(idx, constness) => {
73 let obligations = self.confirm_projection_candidate(obligation, idx)?;
74 ImplSource::Param(obligations, constness)
77 ObjectCandidate(idx) => {
78 let data = self.confirm_object_candidate(obligation, idx)?;
79 ImplSource::Object(data)
83 let vtable_closure = self.confirm_closure_candidate(obligation)?;
84 ImplSource::Closure(vtable_closure)
87 GeneratorCandidate => {
88 let vtable_generator = self.confirm_generator_candidate(obligation)?;
89 ImplSource::Generator(vtable_generator)
92 FnPointerCandidate { .. } => {
93 let data = self.confirm_fn_pointer_candidate(obligation)?;
94 ImplSource::FnPointer(data)
97 DiscriminantKindCandidate => {
98 ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
101 PointeeCandidate => ImplSource::Pointee(ImplSourcePointeeData),
103 TraitAliasCandidate => {
104 let data = self.confirm_trait_alias_candidate(obligation);
105 ImplSource::TraitAlias(data)
108 BuiltinObjectCandidate => {
109 // This indicates something like `Trait + Send: Send`. In this case, we know that
110 // this holds because that's what the object type is telling us, and there's really
111 // no additional obligations to prove and no types in particular to unify, etc.
112 ImplSource::Param(Vec::new(), ty::BoundConstness::NotConst)
115 BuiltinUnsizeCandidate => {
116 let data = self.confirm_builtin_unsize_candidate(obligation)?;
117 ImplSource::Builtin(data)
120 TraitUpcastingUnsizeCandidate(idx) => {
121 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
122 ImplSource::TraitUpcasting(data)
125 ConstDestructCandidate(def_id) => {
126 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
127 ImplSource::ConstDestruct(data)
130 TupleCandidate => ImplSource::Tuple,
133 if !obligation.predicate.is_const_if_const() {
134 // normalize nested predicates according to parent predicate's constness.
135 impl_src = impl_src.map(|mut o| {
136 o.predicate = o.predicate.without_const(self.tcx());
144 fn confirm_projection_candidate(
146 obligation: &TraitObligation<'tcx>,
148 ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
149 let tcx = self.tcx();
151 let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
152 let placeholder_trait_predicate =
153 self.infcx().replace_bound_vars_with_placeholders(trait_predicate).trait_ref;
154 let placeholder_self_ty = placeholder_trait_predicate.self_ty();
155 let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
156 let (def_id, substs) = match *placeholder_self_ty.kind() {
157 ty::Projection(proj) => (proj.item_def_id, proj.substs),
158 ty::Opaque(def_id, substs) => (def_id, substs),
159 _ => bug!("projection candidate for unexpected type: {:?}", placeholder_self_ty),
162 let candidate_predicate =
163 tcx.bound_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::Projection(..) = placeholder_self_ty.kind() {
187 let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates;
188 debug!(?predicates, "projection predicates");
189 for predicate in predicates {
190 let normalized = normalize_with_depth_to(
192 obligation.param_env,
193 obligation.cause.clone(),
194 obligation.recursion_depth + 1,
198 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::new(
488 obligation.cause.clone(),
489 obligation.param_env,
490 normalized_super_trait,
494 let assoc_types: Vec<_> = tcx
495 .associated_items(trait_predicate.def_id())
496 .in_definition_order()
498 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
502 for assoc_type in assoc_types {
503 let defs: &ty::Generics = tcx.generics_of(assoc_type);
505 if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
506 tcx.sess.delay_span_bug(
507 obligation.cause.span,
508 "GATs in trait object shouldn't have been considered",
510 return Err(SelectionError::Unimplemented);
513 // This maybe belongs in wf, but that can't (doesn't) handle
514 // higher-ranked things.
515 // Prevent, e.g., `dyn Iterator<Item = str>`.
516 for bound in self.tcx().bound_item_bounds(assoc_type).transpose_iter() {
518 if defs.count() == 0 {
519 bound.subst(tcx, trait_predicate.trait_ref.substs)
521 let mut substs = smallvec::SmallVec::with_capacity(defs.count());
522 substs.extend(trait_predicate.trait_ref.substs.iter());
523 let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
524 smallvec::SmallVec::with_capacity(
525 bound.0.kind().bound_vars().len() + defs.count(),
527 bound_vars.extend(bound.0.kind().bound_vars().into_iter());
528 InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
531 GenericParamDefKind::Type { .. } => {
532 let kind = ty::BoundTyKind::Param(param.name);
533 let bound_var = ty::BoundVariableKind::Ty(kind);
534 bound_vars.push(bound_var);
538 var: ty::BoundVar::from_usize(bound_vars.len() - 1),
544 GenericParamDefKind::Lifetime => {
545 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
546 let bound_var = ty::BoundVariableKind::Region(kind);
547 bound_vars.push(bound_var);
548 tcx.mk_region(ty::ReLateBound(
551 var: ty::BoundVar::from_usize(bound_vars.len() - 1),
557 GenericParamDefKind::Const { .. } => {
558 let bound_var = ty::BoundVariableKind::Const;
559 bound_vars.push(bound_var);
560 tcx.mk_const(ty::ConstS {
561 ty: tcx.type_of(param.def_id),
562 kind: ty::ConstKind::Bound(
564 ty::BoundVar::from_usize(bound_vars.len() - 1),
570 let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
571 let assoc_ty_substs = tcx.intern_substs(&substs);
573 let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
575 bound.map_bound(|b| b.kind().skip_binder()).subst(tcx, assoc_ty_substs);
576 tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
578 let normalized_bound = normalize_with_depth_to(
580 obligation.param_env,
581 obligation.cause.clone(),
582 obligation.recursion_depth + 1,
586 nested.push(Obligation::new(
587 obligation.cause.clone(),
588 obligation.param_env,
594 debug!(?nested, "object nested obligations");
596 let vtable_base = super::super::vtable_trait_first_method_offset(
598 (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
601 Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
604 fn confirm_fn_pointer_candidate(
606 obligation: &TraitObligation<'tcx>,
607 ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
609 debug!(?obligation, "confirm_fn_pointer_candidate");
611 // Okay to skip binder; it is reintroduced below.
612 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
613 let sig = self_ty.fn_sig(self.tcx());
614 let trait_ref = closure_trait_ref_and_return_type(
616 obligation.predicate.def_id(),
619 util::TupleArgumentsFlag::Yes,
621 .map_bound(|(trait_ref, _)| trait_ref);
623 let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
625 // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
626 let cause = obligation.derived_cause(BuiltinDerivedObligation);
627 // The binder on the Fn obligation is "less" important than the one on
628 // the signature, as evidenced by how we treat it during projection.
629 // The safe thing to do here is to liberate it, though, which should
630 // have no worse effect than skipping the binder here.
631 let liberated_fn_ty =
632 self.infcx.replace_bound_vars_with_placeholders(obligation.predicate.rebind(self_ty));
635 .replace_bound_vars_with_placeholders(liberated_fn_ty.fn_sig(self.tcx()).output());
636 let output_ty = normalize_with_depth_to(
638 obligation.param_env,
640 obligation.recursion_depth,
644 let tr = ty::Binder::dummy(ty::TraitRef::new(
645 self.tcx().require_lang_item(LangItem::Sized, None),
646 self.tcx().mk_substs_trait(output_ty, &[]),
648 nested.push(Obligation::new(
650 obligation.param_env,
651 tr.to_poly_trait_predicate().to_predicate(self.tcx()),
654 Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
657 fn confirm_trait_alias_candidate(
659 obligation: &TraitObligation<'tcx>,
660 ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
661 debug!(?obligation, "confirm_trait_alias_candidate");
663 let alias_def_id = obligation.predicate.def_id();
664 let predicate = self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
665 let trait_ref = predicate.trait_ref;
666 let trait_def_id = trait_ref.def_id;
667 let substs = trait_ref.substs;
669 let trait_obligations = self.impl_or_trait_obligations(
671 obligation.recursion_depth,
672 obligation.param_env,
675 obligation.predicate,
678 debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
680 ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
683 fn confirm_generator_candidate(
685 obligation: &TraitObligation<'tcx>,
686 ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
688 // Okay to skip binder because the substs on generator types never
689 // touch bound regions, they just capture the in-scope
690 // type/region parameters.
691 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
692 let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
693 bug!("closure candidate for non-closure {:?}", obligation);
696 debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
698 let trait_ref = self.generator_trait_ref_unnormalized(obligation, substs);
700 let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
701 debug!(?trait_ref, ?nested, "generator candidate obligations");
703 Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
706 #[instrument(skip(self), level = "debug")]
707 fn confirm_closure_candidate(
709 obligation: &TraitObligation<'tcx>,
710 ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
713 .fn_trait_kind_from_lang_item(obligation.predicate.def_id())
714 .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
716 // Okay to skip binder because the substs on closure types never
717 // touch bound regions, they just capture the in-scope
718 // type/region parameters.
719 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
720 let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
721 bug!("closure candidate for non-closure {:?}", obligation);
724 let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
725 let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
727 debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
731 if !self.tcx().sess.opts.unstable_opts.chalk {
732 nested.push(Obligation::new(
733 obligation.cause.clone(),
734 obligation.param_env,
735 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind))
736 .to_predicate(self.tcx()),
740 Ok(ImplSourceClosureData { closure_def_id, substs, nested })
743 /// In the case of closure types and fn pointers,
744 /// we currently treat the input type parameters on the trait as
745 /// outputs. This means that when we have a match we have only
746 /// considered the self type, so we have to go back and make sure
747 /// to relate the argument types too. This is kind of wrong, but
748 /// since we control the full set of impls, also not that wrong,
749 /// and it DOES yield better error messages (since we don't report
750 /// errors as if there is no applicable impl, but rather report
751 /// errors are about mismatched argument types.
753 /// Here is an example. Imagine we have a closure expression
754 /// and we desugared it so that the type of the expression is
755 /// `Closure`, and `Closure` expects `i32` as argument. Then it
756 /// is "as if" the compiler generated this impl:
757 /// ```ignore (illustrative)
758 /// impl Fn(i32) for Closure { ... }
760 /// Now imagine our obligation is `Closure: Fn(usize)`. So far
761 /// we have matched the self type `Closure`. At this point we'll
762 /// compare the `i32` to `usize` and generate an error.
764 /// Note that this checking occurs *after* the impl has selected,
765 /// because these output type parameters should not affect the
766 /// selection of the impl. Therefore, if there is a mismatch, we
767 /// report an error to the user.
768 #[instrument(skip(self), level = "trace")]
769 fn confirm_poly_trait_refs(
771 obligation: &TraitObligation<'tcx>,
772 expected_trait_ref: ty::PolyTraitRef<'tcx>,
773 ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
774 let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
775 // Normalize the obligation and expected trait refs together, because why not
776 let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
777 ensure_sufficient_stack(|| {
778 normalize_with_depth(
780 obligation.param_env,
781 obligation.cause.clone(),
782 obligation.recursion_depth + 1,
783 (obligation_trait_ref, expected_trait_ref),
788 .at(&obligation.cause, obligation.param_env)
789 .sup(obligation_trait_ref, expected_trait_ref)
790 .map(|InferOk { mut obligations, .. }| {
791 obligations.extend(nested);
794 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
797 fn confirm_trait_upcasting_unsize_candidate(
799 obligation: &TraitObligation<'tcx>,
801 ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
803 let tcx = self.tcx();
805 // `assemble_candidates_for_unsizing` should ensure there are no late-bound
806 // regions here. See the comment there for more details.
807 let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
808 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
809 let target = self.infcx.shallow_resolve(target);
811 debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
813 let mut nested = vec![];
814 let source_trait_ref;
815 let upcast_trait_ref;
816 match (source.kind(), target.kind()) {
817 // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
818 (&ty::Dynamic(ref data_a, r_a, repr_a), &ty::Dynamic(ref data_b, r_b, repr_b))
819 if repr_a == repr_b =>
821 // See `assemble_candidates_for_unsizing` for more info.
822 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
823 let principal_a = data_a.principal().unwrap();
824 source_trait_ref = principal_a.with_self_ty(tcx, source);
825 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
826 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
827 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
828 ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
832 let iter = Some(existential_predicate)
837 .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
842 .map(ty::ExistentialPredicate::AutoTrait)
843 .map(ty::Binder::dummy),
845 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
846 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_b);
848 // Require that the traits involved in this upcast are **equal**;
849 // only the **lifetime bound** is changed.
850 let InferOk { obligations, .. } = self
852 .at(&obligation.cause, obligation.param_env)
853 .sup(target, source_trait)
854 .map_err(|_| Unimplemented)?;
855 nested.extend(obligations);
857 // Register one obligation for 'a: 'b.
858 let cause = ObligationCause::new(
859 obligation.cause.span,
860 obligation.cause.body_id,
861 ObjectCastObligation(source, target),
863 let outlives = ty::OutlivesPredicate(r_a, r_b);
864 nested.push(Obligation::with_depth(
866 obligation.recursion_depth + 1,
867 obligation.param_env,
868 obligation.predicate.rebind(outlives).to_predicate(tcx),
874 let vtable_segment_callback = {
875 let mut vptr_offset = 0;
878 VtblSegment::MetadataDSA => {
879 vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
881 VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
882 vptr_offset += util::count_own_vtable_entries(tcx, trait_ref);
883 if trait_ref == upcast_trait_ref {
885 return ControlFlow::Break(Some(vptr_offset));
887 return ControlFlow::Break(None);
896 ControlFlow::Continue(())
900 let vtable_vptr_slot =
901 super::super::prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback)
904 Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
907 fn confirm_builtin_unsize_candidate(
909 obligation: &TraitObligation<'tcx>,
910 ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
911 let tcx = self.tcx();
913 // `assemble_candidates_for_unsizing` should ensure there are no late-bound
914 // regions here. See the comment there for more details.
915 let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
916 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
917 let target = self.infcx.shallow_resolve(target);
919 debug!(?source, ?target, "confirm_builtin_unsize_candidate");
921 let mut nested = vec![];
922 match (source.kind(), target.kind()) {
923 // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
924 (&ty::Dynamic(ref data_a, r_a, ty::Dyn), &ty::Dynamic(ref data_b, r_b, ty::Dyn)) => {
925 // See `assemble_candidates_for_unsizing` for more info.
926 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
929 .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
934 .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
939 .map(ty::ExistentialPredicate::AutoTrait)
940 .map(ty::Binder::dummy),
942 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
943 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, ty::Dyn);
945 // Require that the traits involved in this upcast are **equal**;
946 // only the **lifetime bound** is changed.
947 let InferOk { obligations, .. } = self
949 .at(&obligation.cause, obligation.param_env)
950 .sup(target, source_trait)
951 .map_err(|_| Unimplemented)?;
952 nested.extend(obligations);
954 // Register one obligation for 'a: 'b.
955 let cause = ObligationCause::new(
956 obligation.cause.span,
957 obligation.cause.body_id,
958 ObjectCastObligation(source, target),
960 let outlives = ty::OutlivesPredicate(r_a, r_b);
961 nested.push(Obligation::with_depth(
963 obligation.recursion_depth + 1,
964 obligation.param_env,
965 obligation.predicate.rebind(outlives).to_predicate(tcx),
970 (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
971 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
972 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
973 return Err(TraitNotObjectSafe(did));
976 let cause = ObligationCause::new(
977 obligation.cause.span,
978 obligation.cause.body_id,
979 ObjectCastObligation(source, target),
982 let predicate_to_obligation = |predicate| {
983 Obligation::with_depth(
985 obligation.recursion_depth + 1,
986 obligation.param_env,
991 // Create obligations:
992 // - Casting `T` to `Trait`
993 // - For all the various builtin bounds attached to the object cast. (In other
994 // words, if the object type is `Foo + Send`, this would create an obligation for
995 // the `Send` check.)
996 // - Projection predicates
998 data.iter().map(|predicate| {
999 predicate_to_obligation(predicate.with_self_ty(tcx, source))
1003 // We can only make objects from sized types.
1004 let tr = ty::Binder::dummy(ty::TraitRef::new(
1005 tcx.require_lang_item(LangItem::Sized, None),
1006 tcx.mk_substs_trait(source, &[]),
1008 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
1010 // If the type is `Foo + 'a`, ensure that the type
1011 // being cast to `Foo + 'a` outlives `'a`:
1012 let outlives = ty::OutlivesPredicate(source, r);
1013 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
1016 // `[T; n]` -> `[T]`
1017 (&ty::Array(a, _), &ty::Slice(b)) => {
1018 let InferOk { obligations, .. } = self
1020 .at(&obligation.cause, obligation.param_env)
1022 .map_err(|_| Unimplemented)?;
1023 nested.extend(obligations);
1026 // `Struct<T>` -> `Struct<U>`
1027 (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
1028 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
1029 GenericArgKind::Type(ty) => match ty.kind() {
1030 ty::Param(p) => Some(p.index),
1034 // Lifetimes aren't allowed to change during unsizing.
1035 GenericArgKind::Lifetime(_) => None,
1037 GenericArgKind::Const(ct) => match ct.kind() {
1038 ty::ConstKind::Param(p) => Some(p.index),
1043 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1044 // by putting it in a query; it would only need the `DefId` as it
1045 // looks at declared field types, not anything substituted.
1047 // The last field of the structure has to exist and contain type/const parameters.
1048 let (tail_field, prefix_fields) =
1049 def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1050 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1052 let mut unsizing_params = GrowableBitSet::new_empty();
1053 for arg in tail_field_ty.0.walk() {
1054 if let Some(i) = maybe_unsizing_param_idx(arg) {
1055 unsizing_params.insert(i);
1059 // Ensure none of the other fields mention the parameters used
1061 for field in prefix_fields {
1062 for arg in tcx.type_of(field.did).walk() {
1063 if let Some(i) = maybe_unsizing_param_idx(arg) {
1064 unsizing_params.remove(i);
1069 if unsizing_params.is_empty() {
1070 return Err(Unimplemented);
1073 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
1074 // normalizing in the process, since `type_of` returns something directly from
1075 // astconv (which means it's un-normalized).
1076 let source_tail = normalize_with_depth_to(
1078 obligation.param_env,
1079 obligation.cause.clone(),
1080 obligation.recursion_depth + 1,
1081 tail_field_ty.subst(tcx, substs_a),
1084 let target_tail = normalize_with_depth_to(
1086 obligation.param_env,
1087 obligation.cause.clone(),
1088 obligation.recursion_depth + 1,
1089 tail_field_ty.subst(tcx, substs_b),
1093 // Check that the source struct with the target's
1094 // unsizing parameters is equal to the target.
1095 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1096 if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1098 let new_struct = tcx.mk_adt(def, substs);
1099 let InferOk { obligations, .. } = self
1101 .at(&obligation.cause, obligation.param_env)
1102 .eq(target, new_struct)
1103 .map_err(|_| Unimplemented)?;
1104 nested.extend(obligations);
1106 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1107 nested.push(predicate_for_trait_def(
1109 obligation.param_env,
1110 obligation.cause.clone(),
1111 obligation.predicate.def_id(),
1112 obligation.recursion_depth + 1,
1114 &[target_tail.into()],
1118 // `(.., T)` -> `(.., U)`
1119 (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1120 assert_eq!(tys_a.len(), tys_b.len());
1122 // The last field of the tuple has to exist.
1123 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1124 let &b_last = tys_b.last().unwrap();
1126 // Check that the source tuple with the target's
1127 // last element is equal to the target.
1128 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1129 let InferOk { obligations, .. } = self
1131 .at(&obligation.cause, obligation.param_env)
1132 .eq(target, new_tuple)
1133 .map_err(|_| Unimplemented)?;
1134 nested.extend(obligations);
1136 // Construct the nested `T: Unsize<U>` predicate.
1137 nested.push(ensure_sufficient_stack(|| {
1138 predicate_for_trait_def(
1140 obligation.param_env,
1141 obligation.cause.clone(),
1142 obligation.predicate.def_id(),
1143 obligation.recursion_depth + 1,
1153 Ok(ImplSourceBuiltinData { nested })
1156 fn confirm_const_destruct_candidate(
1158 obligation: &TraitObligation<'tcx>,
1159 impl_def_id: Option<DefId>,
1160 ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1161 // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1162 if !obligation.is_const() {
1163 return Ok(ImplSourceConstDestructData { nested: vec![] });
1166 let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1168 let tcx = self.tcx();
1169 let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1171 let mut nested = vec![];
1172 let cause = obligation.derived_cause(BuiltinDerivedObligation);
1174 // If we have a custom `impl const Drop`, then
1175 // first check it like a regular impl candidate.
1176 // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1177 if let Some(impl_def_id) = impl_def_id {
1178 let mut new_obligation = obligation.clone();
1179 new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1180 trait_pred.trait_ref.def_id = drop_trait;
1183 let substs = self.rematch_impl(impl_def_id, &new_obligation);
1184 debug!(?substs, "impl substs");
1186 let cause = obligation.derived_cause(|derived| {
1187 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1190 span: obligation.cause.span,
1193 let obligations = ensure_sufficient_stack(|| {
1198 new_obligation.recursion_depth + 1,
1199 new_obligation.param_env,
1200 obligation.predicate,
1203 nested.extend(obligations.nested);
1206 // We want to confirm the ADT's fields if we have an ADT
1207 let mut stack = match *self_ty.skip_binder().kind() {
1208 ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1209 _ => vec![self_ty.skip_binder()],
1212 while let Some(nested_ty) = stack.pop() {
1213 match *nested_ty.kind() {
1214 // We know these types are trivially drop
1220 | ty::Infer(ty::IntVar(_))
1221 | ty::Infer(ty::FloatVar(_))
1228 | ty::Foreign(_) => {}
1230 // `ManuallyDrop` is trivially drop
1231 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
1233 // These types are built-in, so we can fast-track by registering
1234 // nested predicates for their constituent type(s)
1235 ty::Array(ty, _) | ty::Slice(ty) => {
1239 stack.extend(tys.iter());
1241 ty::Closure(_, substs) => {
1242 stack.push(substs.as_closure().tupled_upvars_ty());
1244 ty::Generator(_, substs, _) => {
1245 let generator = substs.as_generator();
1246 stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1248 ty::GeneratorWitness(tys) => {
1249 stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1252 // If we have a projection type, make sure to normalize it so we replace it
1253 // with a fresh infer variable
1254 ty::Projection(..) => {
1255 let predicate = normalize_with_depth_to(
1257 obligation.param_env,
1259 obligation.recursion_depth + 1,
1261 .rebind(ty::TraitPredicate {
1262 trait_ref: ty::TraitRef {
1263 def_id: self.tcx().require_lang_item(LangItem::Destruct, None),
1264 substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1266 constness: ty::BoundConstness::ConstIfConst,
1267 polarity: ty::ImplPolarity::Positive,
1273 nested.push(Obligation::with_depth(
1275 obligation.recursion_depth + 1,
1276 obligation.param_env,
1281 // If we have any other type (e.g. an ADT), just register a nested obligation
1282 // since it's either not `const Drop` (and we raise an error during selection),
1283 // or it's an ADT (and we need to check for a custom impl during selection)
1285 let predicate = self_ty
1286 .rebind(ty::TraitPredicate {
1287 trait_ref: ty::TraitRef {
1288 def_id: self.tcx().require_lang_item(LangItem::Destruct, None),
1289 substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1291 constness: ty::BoundConstness::ConstIfConst,
1292 polarity: ty::ImplPolarity::Positive,
1296 nested.push(Obligation::with_depth(
1298 obligation.recursion_depth + 1,
1299 obligation.param_env,
1306 Ok(ImplSourceConstDestructData { nested })