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::subst::{GenericArg, GenericArgKind, InternalSubsts, Subst, SubstsRef};
15 use rustc_middle::ty::{self, GenericParamDefKind, Ty};
16 use rustc_middle::ty::{ToPolyTraitRef, ToPredicate};
17 use rustc_span::def_id::DefId;
19 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
20 use crate::traits::select::TraitObligationExt;
21 use crate::traits::util::{self, closure_trait_ref_and_return_type, predicate_for_trait_def};
23 BuiltinDerivedObligation, DerivedObligationCause, ImplDerivedObligation,
24 ImplDerivedObligationCause, ImplSource, ImplSourceAutoImplData, ImplSourceBuiltinData,
25 ImplSourceClosureData, ImplSourceConstDestructData, ImplSourceDiscriminantKindData,
26 ImplSourceFnPointerData, ImplSourceGeneratorData, ImplSourceObjectData, ImplSourcePointeeData,
27 ImplSourceTraitAliasData, ImplSourceTraitUpcastingData, ImplSourceUserDefinedData, Normalized,
28 ObjectCastObligation, Obligation, ObligationCause, OutputTypeParameterMismatch,
29 PredicateObligation, Selection, SelectionError, TraitNotObjectSafe, TraitObligation,
30 Unimplemented, VtblSegment,
33 use super::BuiltinImplConditions;
34 use super::SelectionCandidate::{self, *};
35 use super::SelectionContext;
38 use std::ops::ControlFlow;
40 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
41 #[instrument(level = "debug", skip(self))]
42 pub(super) fn confirm_candidate(
44 obligation: &TraitObligation<'tcx>,
45 candidate: SelectionCandidate<'tcx>,
46 ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
47 let mut obligation = obligation;
50 // HACK(const_trait_impl): the surrounding environment is remapped to a non-const context
51 // because nested obligations might be actually `~const` then (incorrectly) requiring
52 // const impls. for example:
55 // pub trait Sub: Super {}
57 // impl<A> const Super for &A where A: ~const Super {}
58 // impl<A> const Sub for &A where A: ~const Sub {}
61 // The procedure to check the code above without the remapping code is as follows:
63 // CheckWf(impl const Sub for &A where A: ~const Sub) // <- const env
64 // CheckPredicate(&A: Super)
65 // CheckPredicate(A: ~const Super) // <- still const env, failure
67 if obligation.param_env.is_const() && !obligation.predicate.is_const_if_const() {
68 new_obligation = TraitObligation {
69 cause: obligation.cause.clone(),
70 param_env: obligation.param_env.without_const(),
73 obligation = &new_obligation;
77 BuiltinCandidate { has_nested } => {
78 let data = self.confirm_builtin_candidate(obligation, has_nested);
79 Ok(ImplSource::Builtin(data))
82 ParamCandidate(param) => {
84 self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
85 Ok(ImplSource::Param(obligations, param.skip_binder().constness))
88 ImplCandidate(impl_def_id) => {
89 Ok(ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id)))
92 AutoImplCandidate(trait_def_id) => {
93 let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
94 Ok(ImplSource::AutoImpl(data))
97 ProjectionCandidate(idx) => {
98 let obligations = self.confirm_projection_candidate(obligation, idx)?;
99 // FIXME(jschievink): constness
100 Ok(ImplSource::Param(obligations, ty::BoundConstness::NotConst))
103 ObjectCandidate(idx) => {
104 let data = self.confirm_object_candidate(obligation, idx)?;
105 Ok(ImplSource::Object(data))
108 ClosureCandidate => {
109 let vtable_closure = self.confirm_closure_candidate(obligation)?;
110 Ok(ImplSource::Closure(vtable_closure))
113 GeneratorCandidate => {
114 let vtable_generator = self.confirm_generator_candidate(obligation)?;
115 Ok(ImplSource::Generator(vtable_generator))
118 FnPointerCandidate { .. } => {
119 let data = self.confirm_fn_pointer_candidate(obligation)?;
120 Ok(ImplSource::FnPointer(data))
123 DiscriminantKindCandidate => {
124 Ok(ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData))
127 PointeeCandidate => Ok(ImplSource::Pointee(ImplSourcePointeeData)),
129 TraitAliasCandidate(alias_def_id) => {
130 let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
131 Ok(ImplSource::TraitAlias(data))
134 BuiltinObjectCandidate => {
135 // This indicates something like `Trait + Send: Send`. In this case, we know that
136 // this holds because that's what the object type is telling us, and there's really
137 // no additional obligations to prove and no types in particular to unify, etc.
138 Ok(ImplSource::Param(Vec::new(), ty::BoundConstness::NotConst))
141 BuiltinUnsizeCandidate => {
142 let data = self.confirm_builtin_unsize_candidate(obligation)?;
143 Ok(ImplSource::Builtin(data))
146 TraitUpcastingUnsizeCandidate(idx) => {
147 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
148 Ok(ImplSource::TraitUpcasting(data))
151 ConstDestructCandidate(def_id) => {
152 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
153 Ok(ImplSource::ConstDestruct(data))
158 fn confirm_projection_candidate(
160 obligation: &TraitObligation<'tcx>,
162 ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
163 self.infcx.commit_unconditionally(|_| {
164 let tcx = self.tcx();
166 let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
167 let placeholder_trait_predicate =
168 self.infcx().replace_bound_vars_with_placeholders(trait_predicate).trait_ref;
169 let placeholder_self_ty = placeholder_trait_predicate.self_ty();
170 let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
171 let (def_id, substs) = match *placeholder_self_ty.kind() {
172 ty::Projection(proj) => (proj.item_def_id, proj.substs),
173 ty::Opaque(def_id, substs) => (def_id, substs),
174 _ => bug!("projection candidate for unexpected type: {:?}", placeholder_self_ty),
177 let candidate_predicate = tcx.item_bounds(def_id)[idx].subst(tcx, substs);
178 let candidate = candidate_predicate
179 .to_opt_poly_trait_pred()
180 .expect("projection candidate is not a trait predicate")
181 .map_bound(|t| t.trait_ref);
182 let mut obligations = Vec::new();
183 let candidate = normalize_with_depth_to(
185 obligation.param_env,
186 obligation.cause.clone(),
187 obligation.recursion_depth + 1,
192 obligations.extend(self.infcx.commit_if_ok(|_| {
194 .at(&obligation.cause, obligation.param_env)
195 .sup(placeholder_trait_predicate, candidate)
196 .map(|InferOk { obligations, .. }| obligations)
197 .map_err(|_| Unimplemented)
200 if let ty::Projection(..) = placeholder_self_ty.kind() {
201 let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates;
202 debug!(?predicates, "projection predicates");
203 for predicate in predicates {
204 let normalized = normalize_with_depth_to(
206 obligation.param_env,
207 obligation.cause.clone(),
208 obligation.recursion_depth + 1,
212 obligations.push(Obligation::with_depth(
213 obligation.cause.clone(),
214 obligation.recursion_depth + 1,
215 obligation.param_env,
225 fn confirm_param_candidate(
227 obligation: &TraitObligation<'tcx>,
228 param: ty::PolyTraitRef<'tcx>,
229 ) -> Vec<PredicateObligation<'tcx>> {
230 debug!(?obligation, ?param, "confirm_param_candidate");
232 // During evaluation, we already checked that this
233 // where-clause trait-ref could be unified with the obligation
234 // trait-ref. Repeat that unification now without any
235 // transactional boundary; it should not fail.
236 match self.match_where_clause_trait_ref(obligation, param) {
237 Ok(obligations) => obligations,
240 "Where clause `{:?}` was applicable to `{:?}` but now is not",
248 fn confirm_builtin_candidate(
250 obligation: &TraitObligation<'tcx>,
252 ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
253 debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
255 let lang_items = self.tcx().lang_items();
256 let obligations = if has_nested {
257 let trait_def = obligation.predicate.def_id();
258 let conditions = if Some(trait_def) == lang_items.sized_trait() {
259 self.sized_conditions(obligation)
260 } else if Some(trait_def) == lang_items.copy_trait() {
261 self.copy_clone_conditions(obligation)
262 } else if Some(trait_def) == lang_items.clone_trait() {
263 self.copy_clone_conditions(obligation)
265 bug!("unexpected builtin trait {:?}", trait_def)
267 let BuiltinImplConditions::Where(nested) = conditions else {
268 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
271 let cause = obligation.derived_cause(BuiltinDerivedObligation);
272 ensure_sufficient_stack(|| {
273 self.collect_predicates_for_types(
274 obligation.param_env,
276 obligation.recursion_depth + 1,
285 debug!(?obligations);
287 ImplSourceBuiltinData { nested: obligations }
290 /// This handles the case where an `auto trait Foo` impl is being used.
291 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
293 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
294 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
295 fn confirm_auto_impl_candidate(
297 obligation: &TraitObligation<'tcx>,
299 ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
300 debug!(?obligation, ?trait_def_id, "confirm_auto_impl_candidate");
302 let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
303 let types = self.constituent_types_for_ty(self_ty);
304 self.vtable_auto_impl(obligation, trait_def_id, types)
307 /// See `confirm_auto_impl_candidate`.
310 obligation: &TraitObligation<'tcx>,
312 nested: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
313 ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
314 debug!(?nested, "vtable_auto_impl");
315 ensure_sufficient_stack(|| {
316 let cause = obligation.derived_cause(BuiltinDerivedObligation);
318 let trait_obligations: Vec<PredicateObligation<'_>> =
319 self.infcx.commit_unconditionally(|_| {
320 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
321 let trait_ref = self.infcx.replace_bound_vars_with_placeholders(poly_trait_ref);
322 self.impl_or_trait_obligations(
324 obligation.recursion_depth + 1,
325 obligation.param_env,
328 obligation.predicate,
332 let mut obligations = self.collect_predicates_for_types(
333 obligation.param_env,
335 obligation.recursion_depth + 1,
340 // Adds the predicates from the trait. Note that this contains a `Self: Trait`
341 // predicate as usual. It won't have any effect since auto traits are coinductive.
342 obligations.extend(trait_obligations);
344 debug!(?obligations, "vtable_auto_impl");
346 ImplSourceAutoImplData { trait_def_id, nested: obligations }
350 fn confirm_impl_candidate(
352 obligation: &TraitObligation<'tcx>,
354 ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
355 debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");
357 // First, create the substitutions by matching the impl again,
358 // this time not in a probe.
359 self.infcx.commit_unconditionally(|_| {
360 let substs = self.rematch_impl(impl_def_id, obligation);
361 debug!(?substs, "impl substs");
362 ensure_sufficient_stack(|| {
367 obligation.recursion_depth + 1,
368 obligation.param_env,
369 obligation.predicate,
378 substs: Normalized<'tcx, SubstsRef<'tcx>>,
379 cause: &ObligationCause<'tcx>,
380 recursion_depth: usize,
381 param_env: ty::ParamEnv<'tcx>,
382 parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
383 ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
384 debug!(?impl_def_id, ?substs, ?recursion_depth, "vtable_impl");
386 let mut impl_obligations = self.impl_or_trait_obligations(
395 debug!(?impl_obligations, "vtable_impl");
397 // Because of RFC447, the impl-trait-ref and obligations
398 // are sufficient to determine the impl substs, without
399 // relying on projections in the impl-trait-ref.
401 // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
402 impl_obligations.extend(substs.obligations);
404 ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations }
407 fn confirm_object_candidate(
409 obligation: &TraitObligation<'tcx>,
411 ) -> Result<ImplSourceObjectData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
412 let tcx = self.tcx();
413 debug!(?obligation, ?index, "confirm_object_candidate");
415 let trait_predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
416 let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
417 let obligation_trait_ref = ty::Binder::dummy(trait_predicate.trait_ref);
418 let ty::Dynamic(data, ..) = *self_ty.kind() else {
419 span_bug!(obligation.cause.span, "object candidate with non-object");
422 let object_trait_ref = data.principal().unwrap_or_else(|| {
423 span_bug!(obligation.cause.span, "object candidate with no principal")
425 let object_trait_ref = self
427 .replace_bound_vars_with_fresh_vars(
428 obligation.cause.span,
433 let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);
435 let mut nested = vec![];
437 let mut supertraits = util::supertraits(tcx, ty::Binder::dummy(object_trait_ref));
438 let unnormalized_upcast_trait_ref =
439 supertraits.nth(index).expect("supertraits iterator no longer has as many elements");
441 let upcast_trait_ref = normalize_with_depth_to(
443 obligation.param_env,
444 obligation.cause.clone(),
445 obligation.recursion_depth + 1,
446 unnormalized_upcast_trait_ref,
450 nested.extend(self.infcx.commit_if_ok(|_| {
452 .at(&obligation.cause, obligation.param_env)
453 .sup(obligation_trait_ref, upcast_trait_ref)
454 .map(|InferOk { obligations, .. }| obligations)
455 .map_err(|_| Unimplemented)
458 // Check supertraits hold. This is so that their associated type bounds
459 // will be checked in the code below.
460 for super_trait in tcx
461 .super_predicates_of(trait_predicate.def_id())
462 .instantiate(tcx, trait_predicate.trait_ref.substs)
466 let normalized_super_trait = normalize_with_depth_to(
468 obligation.param_env,
469 obligation.cause.clone(),
470 obligation.recursion_depth + 1,
474 nested.push(Obligation::new(
475 obligation.cause.clone(),
476 obligation.param_env,
477 normalized_super_trait,
481 let assoc_types: Vec<_> = tcx
482 .associated_items(trait_predicate.def_id())
483 .in_definition_order()
485 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
489 for assoc_type in assoc_types {
490 let defs: &ty::Generics = tcx.generics_of(assoc_type);
492 if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
493 tcx.sess.delay_span_bug(
494 obligation.cause.span,
495 "GATs in trait object shouldn't have been considered",
497 return Err(SelectionError::Unimplemented);
500 // This maybe belongs in wf, but that can't (doesn't) handle
501 // higher-ranked things.
502 // Prevent, e.g., `dyn Iterator<Item = str>`.
503 for bound in self.tcx().item_bounds(assoc_type) {
505 if defs.count() == 0 {
506 bound.subst(tcx, trait_predicate.trait_ref.substs)
508 let mut substs = smallvec::SmallVec::with_capacity(defs.count());
509 substs.extend(trait_predicate.trait_ref.substs.iter());
510 let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
511 smallvec::SmallVec::with_capacity(
512 bound.kind().bound_vars().len() + defs.count(),
514 bound_vars.extend(bound.kind().bound_vars().into_iter());
515 InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
518 GenericParamDefKind::Type { .. } => {
519 let kind = ty::BoundTyKind::Param(param.name);
520 let bound_var = ty::BoundVariableKind::Ty(kind);
521 bound_vars.push(bound_var);
525 var: ty::BoundVar::from_usize(bound_vars.len() - 1),
531 GenericParamDefKind::Lifetime => {
532 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
533 let bound_var = ty::BoundVariableKind::Region(kind);
534 bound_vars.push(bound_var);
535 tcx.mk_region(ty::ReLateBound(
538 var: ty::BoundVar::from_usize(bound_vars.len() - 1),
544 GenericParamDefKind::Const { .. } => {
545 let bound_var = ty::BoundVariableKind::Const;
546 bound_vars.push(bound_var);
547 tcx.mk_const(ty::ConstS {
548 ty: tcx.type_of(param.def_id),
549 val: ty::ConstKind::Bound(
551 ty::BoundVar::from_usize(bound_vars.len() - 1),
557 let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
558 let assoc_ty_substs = tcx.intern_substs(&substs);
560 let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
561 let bound = bound.kind().skip_binder().subst(tcx, assoc_ty_substs);
562 tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
564 let normalized_bound = normalize_with_depth_to(
566 obligation.param_env,
567 obligation.cause.clone(),
568 obligation.recursion_depth + 1,
572 nested.push(Obligation::new(
573 obligation.cause.clone(),
574 obligation.param_env,
580 debug!(?nested, "object nested obligations");
582 let vtable_base = super::super::vtable_trait_first_method_offset(
584 (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
587 Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
590 fn confirm_fn_pointer_candidate(
592 obligation: &TraitObligation<'tcx>,
593 ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
595 debug!(?obligation, "confirm_fn_pointer_candidate");
597 // Okay to skip binder; it is reintroduced below.
598 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
599 let sig = self_ty.fn_sig(self.tcx());
600 let trait_ref = closure_trait_ref_and_return_type(
602 obligation.predicate.def_id(),
605 util::TupleArgumentsFlag::Yes,
607 .map_bound(|(trait_ref, _)| trait_ref);
609 let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
610 Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
613 fn confirm_trait_alias_candidate(
615 obligation: &TraitObligation<'tcx>,
617 ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
618 debug!(?obligation, ?alias_def_id, "confirm_trait_alias_candidate");
620 self.infcx.commit_unconditionally(|_| {
621 let predicate = self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
622 let trait_ref = predicate.trait_ref;
623 let trait_def_id = trait_ref.def_id;
624 let substs = trait_ref.substs;
626 let trait_obligations = self.impl_or_trait_obligations(
628 obligation.recursion_depth,
629 obligation.param_env,
632 obligation.predicate,
635 debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
637 ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
641 fn confirm_generator_candidate(
643 obligation: &TraitObligation<'tcx>,
644 ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
646 // Okay to skip binder because the substs on generator types never
647 // touch bound regions, they just capture the in-scope
648 // type/region parameters.
649 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
650 let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
651 bug!("closure candidate for non-closure {:?}", obligation);
654 debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
656 let trait_ref = self.generator_trait_ref_unnormalized(obligation, substs);
658 let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
659 debug!(?trait_ref, ?nested, "generator candidate obligations");
661 Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
664 #[instrument(skip(self), level = "debug")]
665 fn confirm_closure_candidate(
667 obligation: &TraitObligation<'tcx>,
668 ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
671 .fn_trait_kind_from_lang_item(obligation.predicate.def_id())
672 .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
674 // Okay to skip binder because the substs on closure types never
675 // touch bound regions, they just capture the in-scope
676 // type/region parameters.
677 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
678 let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
679 bug!("closure candidate for non-closure {:?}", obligation);
682 let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
683 let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
685 debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
689 if !self.tcx().sess.opts.debugging_opts.chalk {
690 nested.push(Obligation::new(
691 obligation.cause.clone(),
692 obligation.param_env,
693 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind))
694 .to_predicate(self.tcx()),
698 Ok(ImplSourceClosureData { closure_def_id, substs, nested })
701 /// In the case of closure types and fn pointers,
702 /// we currently treat the input type parameters on the trait as
703 /// outputs. This means that when we have a match we have only
704 /// considered the self type, so we have to go back and make sure
705 /// to relate the argument types too. This is kind of wrong, but
706 /// since we control the full set of impls, also not that wrong,
707 /// and it DOES yield better error messages (since we don't report
708 /// errors as if there is no applicable impl, but rather report
709 /// errors are about mismatched argument types.
711 /// Here is an example. Imagine we have a closure expression
712 /// and we desugared it so that the type of the expression is
713 /// `Closure`, and `Closure` expects `i32` as argument. Then it
714 /// is "as if" the compiler generated this impl:
716 /// impl Fn(i32) for Closure { ... }
718 /// Now imagine our obligation is `Closure: Fn(usize)`. So far
719 /// we have matched the self type `Closure`. At this point we'll
720 /// compare the `i32` to `usize` and generate an error.
722 /// Note that this checking occurs *after* the impl has selected,
723 /// because these output type parameters should not affect the
724 /// selection of the impl. Therefore, if there is a mismatch, we
725 /// report an error to the user.
726 #[instrument(skip(self), level = "trace")]
727 fn confirm_poly_trait_refs(
729 obligation: &TraitObligation<'tcx>,
730 expected_trait_ref: ty::PolyTraitRef<'tcx>,
731 ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
732 let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
733 // Normalize the obligation and expected trait refs together, because why not
734 let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
735 ensure_sufficient_stack(|| {
736 self.infcx.commit_unconditionally(|_| {
737 normalize_with_depth(
739 obligation.param_env,
740 obligation.cause.clone(),
741 obligation.recursion_depth + 1,
742 (obligation_trait_ref, expected_trait_ref),
748 .at(&obligation.cause, obligation.param_env)
749 .sup(obligation_trait_ref, expected_trait_ref)
750 .map(|InferOk { mut obligations, .. }| {
751 obligations.extend(nested);
754 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
757 fn confirm_trait_upcasting_unsize_candidate(
759 obligation: &TraitObligation<'tcx>,
761 ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
763 let tcx = self.tcx();
765 // `assemble_candidates_for_unsizing` should ensure there are no late-bound
766 // regions here. See the comment there for more details.
767 let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
768 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
769 let target = self.infcx.shallow_resolve(target);
771 debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
773 let mut nested = vec![];
774 let source_trait_ref;
775 let upcast_trait_ref;
776 match (source.kind(), target.kind()) {
777 // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
778 (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
779 // See `assemble_candidates_for_unsizing` for more info.
780 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
781 let principal_a = data_a.principal().unwrap();
782 source_trait_ref = principal_a.with_self_ty(tcx, source);
783 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
784 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
785 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
786 ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
790 let iter = Some(existential_predicate)
795 .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
800 .map(ty::ExistentialPredicate::AutoTrait)
801 .map(ty::Binder::dummy),
803 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
804 let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
806 // Require that the traits involved in this upcast are **equal**;
807 // only the **lifetime bound** is changed.
808 let InferOk { obligations, .. } = self
810 .at(&obligation.cause, obligation.param_env)
811 .sup(target, source_trait)
812 .map_err(|_| Unimplemented)?;
813 nested.extend(obligations);
815 // Register one obligation for 'a: 'b.
816 let cause = ObligationCause::new(
817 obligation.cause.span,
818 obligation.cause.body_id,
819 ObjectCastObligation(target),
821 let outlives = ty::OutlivesPredicate(r_a, r_b);
822 nested.push(Obligation::with_depth(
824 obligation.recursion_depth + 1,
825 obligation.param_env,
826 obligation.predicate.rebind(outlives).to_predicate(tcx),
832 let vtable_segment_callback = {
833 let mut vptr_offset = 0;
836 VtblSegment::MetadataDSA => {
837 vptr_offset += ty::COMMON_VTABLE_ENTRIES.len();
839 VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
840 vptr_offset += util::count_own_vtable_entries(tcx, trait_ref);
841 if trait_ref == upcast_trait_ref {
843 return ControlFlow::Break(Some(vptr_offset));
845 return ControlFlow::Break(None);
854 ControlFlow::Continue(())
858 let vtable_vptr_slot =
859 super::super::prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback)
862 Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
865 fn confirm_builtin_unsize_candidate(
867 obligation: &TraitObligation<'tcx>,
868 ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
869 let tcx = self.tcx();
871 // `assemble_candidates_for_unsizing` should ensure there are no late-bound
872 // regions here. See the comment there for more details.
873 let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
874 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
875 let target = self.infcx.shallow_resolve(target);
877 debug!(?source, ?target, "confirm_builtin_unsize_candidate");
879 let mut nested = vec![];
880 match (source.kind(), target.kind()) {
881 // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
882 (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
883 // See `assemble_candidates_for_unsizing` for more info.
884 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
887 .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
892 .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
897 .map(ty::ExistentialPredicate::AutoTrait)
898 .map(ty::Binder::dummy),
900 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
901 let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
903 // Require that the traits involved in this upcast are **equal**;
904 // only the **lifetime bound** is changed.
905 let InferOk { obligations, .. } = self
907 .at(&obligation.cause, obligation.param_env)
908 .sup(target, source_trait)
909 .map_err(|_| Unimplemented)?;
910 nested.extend(obligations);
912 // Register one obligation for 'a: 'b.
913 let cause = ObligationCause::new(
914 obligation.cause.span,
915 obligation.cause.body_id,
916 ObjectCastObligation(target),
918 let outlives = ty::OutlivesPredicate(r_a, r_b);
919 nested.push(Obligation::with_depth(
921 obligation.recursion_depth + 1,
922 obligation.param_env,
923 obligation.predicate.rebind(outlives).to_predicate(tcx),
928 (_, &ty::Dynamic(ref data, r)) => {
929 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
930 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
931 return Err(TraitNotObjectSafe(did));
934 let cause = ObligationCause::new(
935 obligation.cause.span,
936 obligation.cause.body_id,
937 ObjectCastObligation(target),
940 let predicate_to_obligation = |predicate| {
941 Obligation::with_depth(
943 obligation.recursion_depth + 1,
944 obligation.param_env,
949 // Create obligations:
950 // - Casting `T` to `Trait`
951 // - For all the various builtin bounds attached to the object cast. (In other
952 // words, if the object type is `Foo + Send`, this would create an obligation for
953 // the `Send` check.)
954 // - Projection predicates
956 data.iter().map(|predicate| {
957 predicate_to_obligation(predicate.with_self_ty(tcx, source))
961 // We can only make objects from sized types.
962 let tr = ty::Binder::dummy(ty::TraitRef::new(
963 tcx.require_lang_item(LangItem::Sized, None),
964 tcx.mk_substs_trait(source, &[]),
966 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
968 // If the type is `Foo + 'a`, ensure that the type
969 // being cast to `Foo + 'a` outlives `'a`:
970 let outlives = ty::OutlivesPredicate(source, r);
971 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
975 (&ty::Array(a, _), &ty::Slice(b)) => {
976 let InferOk { obligations, .. } = self
978 .at(&obligation.cause, obligation.param_env)
980 .map_err(|_| Unimplemented)?;
981 nested.extend(obligations);
984 // `Struct<T>` -> `Struct<U>`
985 (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
986 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
987 GenericArgKind::Type(ty) => match ty.kind() {
988 ty::Param(p) => Some(p.index),
992 // Lifetimes aren't allowed to change during unsizing.
993 GenericArgKind::Lifetime(_) => None,
995 GenericArgKind::Const(ct) => match ct.val() {
996 ty::ConstKind::Param(p) => Some(p.index),
1001 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1002 // by putting it in a query; it would only need the `DefId` as it
1003 // looks at declared field types, not anything substituted.
1005 // The last field of the structure has to exist and contain type/const parameters.
1006 let (tail_field, prefix_fields) =
1007 def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1008 let tail_field_ty = tcx.type_of(tail_field.did);
1010 let mut unsizing_params = GrowableBitSet::new_empty();
1011 for arg in tail_field_ty.walk() {
1012 if let Some(i) = maybe_unsizing_param_idx(arg) {
1013 unsizing_params.insert(i);
1017 // Ensure none of the other fields mention the parameters used
1019 for field in prefix_fields {
1020 for arg in tcx.type_of(field.did).walk() {
1021 if let Some(i) = maybe_unsizing_param_idx(arg) {
1022 unsizing_params.remove(i);
1027 if unsizing_params.is_empty() {
1028 return Err(Unimplemented);
1031 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`.
1032 let source_tail = tail_field_ty.subst(tcx, substs_a);
1033 let target_tail = tail_field_ty.subst(tcx, substs_b);
1035 // Check that the source struct with the target's
1036 // unsizing parameters is equal to the target.
1037 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1038 if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1040 let new_struct = tcx.mk_adt(def, substs);
1041 let InferOk { obligations, .. } = self
1043 .at(&obligation.cause, obligation.param_env)
1044 .eq(target, new_struct)
1045 .map_err(|_| Unimplemented)?;
1046 nested.extend(obligations);
1048 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1049 nested.push(predicate_for_trait_def(
1051 obligation.param_env,
1052 obligation.cause.clone(),
1053 obligation.predicate.def_id(),
1054 obligation.recursion_depth + 1,
1056 &[target_tail.into()],
1060 // `(.., T)` -> `(.., U)`
1061 (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1062 assert_eq!(tys_a.len(), tys_b.len());
1064 // The last field of the tuple has to exist.
1065 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1066 let &b_last = tys_b.last().unwrap();
1068 // Check that the source tuple with the target's
1069 // last element is equal to the target.
1070 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1071 let InferOk { obligations, .. } = self
1073 .at(&obligation.cause, obligation.param_env)
1074 .eq(target, new_tuple)
1075 .map_err(|_| Unimplemented)?;
1076 nested.extend(obligations);
1078 // Construct the nested `T: Unsize<U>` predicate.
1079 nested.push(ensure_sufficient_stack(|| {
1080 predicate_for_trait_def(
1082 obligation.param_env,
1083 obligation.cause.clone(),
1084 obligation.predicate.def_id(),
1085 obligation.recursion_depth + 1,
1095 Ok(ImplSourceBuiltinData { nested })
1098 fn confirm_const_destruct_candidate(
1100 obligation: &TraitObligation<'tcx>,
1101 impl_def_id: Option<DefId>,
1102 ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1103 // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1104 if !obligation.is_const() {
1105 return Ok(ImplSourceConstDestructData { nested: vec![] });
1108 let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1109 // FIXME: remove if statement below when beta is bumped
1113 if obligation.predicate.skip_binder().def_id() == drop_trait {
1114 return Ok(ImplSourceConstDestructData { nested: vec![] });
1117 let tcx = self.tcx();
1118 let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1120 let mut nested = vec![];
1121 let cause = obligation.derived_cause(BuiltinDerivedObligation);
1123 // If we have a custom `impl const Drop`, then
1124 // first check it like a regular impl candidate.
1125 // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1126 if let Some(impl_def_id) = impl_def_id {
1127 let obligations = self.infcx.commit_unconditionally(|_| {
1128 let mut new_obligation = obligation.clone();
1129 new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1130 trait_pred.trait_ref.def_id = drop_trait;
1133 let substs = self.rematch_impl(impl_def_id, &new_obligation);
1134 debug!(?substs, "impl substs");
1136 let derived = DerivedObligationCause {
1137 parent_trait_pred: obligation.predicate,
1138 parent_code: obligation.cause.clone_code(),
1140 let derived_code = ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1143 span: obligation.cause.span,
1146 let cause = ObligationCause::new(
1147 obligation.cause.span,
1148 obligation.cause.body_id,
1151 ensure_sufficient_stack(|| {
1156 new_obligation.recursion_depth + 1,
1157 new_obligation.param_env,
1158 obligation.predicate,
1162 nested.extend(obligations.nested);
1165 // We want to confirm the ADT's fields if we have an ADT
1166 let mut stack = match *self_ty.skip_binder().kind() {
1167 ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1168 _ => vec![self_ty.skip_binder()],
1171 while let Some(nested_ty) = stack.pop() {
1172 match *nested_ty.kind() {
1173 // We know these types are trivially drop
1179 | ty::Infer(ty::IntVar(_))
1180 | ty::Infer(ty::FloatVar(_))
1187 | ty::Foreign(_) => {}
1189 // These types are built-in, so we can fast-track by registering
1190 // nested predicates for their constituent type(s)
1191 ty::Array(ty, _) | ty::Slice(ty) => {
1195 stack.extend(tys.iter());
1197 ty::Closure(_, substs) => {
1198 stack.push(substs.as_closure().tupled_upvars_ty());
1200 ty::Generator(_, substs, _) => {
1201 let generator = substs.as_generator();
1202 stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1204 ty::GeneratorWitness(tys) => {
1205 stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1208 // If we have a projection type, make sure to normalize it so we replace it
1209 // with a fresh infer variable
1210 ty::Projection(..) => {
1211 self.infcx.commit_unconditionally(|_| {
1212 let predicate = normalize_with_depth_to(
1214 obligation.param_env,
1216 obligation.recursion_depth + 1,
1218 .rebind(ty::TraitPredicate {
1219 trait_ref: ty::TraitRef {
1222 .require_lang_item(LangItem::Destruct, None),
1223 substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1225 constness: ty::BoundConstness::ConstIfConst,
1226 polarity: ty::ImplPolarity::Positive,
1232 nested.push(Obligation::with_depth(
1234 obligation.recursion_depth + 1,
1235 obligation.param_env,
1241 // If we have any other type (e.g. an ADT), just register a nested obligation
1242 // since it's either not `const Drop` (and we raise an error during selection),
1243 // or it's an ADT (and we need to check for a custom impl during selection)
1245 let predicate = self_ty
1246 .rebind(ty::TraitPredicate {
1247 trait_ref: ty::TraitRef {
1248 def_id: self.tcx().require_lang_item(LangItem::Destruct, None),
1249 substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1251 constness: ty::BoundConstness::ConstIfConst,
1252 polarity: ty::ImplPolarity::Positive,
1256 nested.push(Obligation::with_depth(
1258 obligation.recursion_depth + 1,
1259 obligation.param_env,
1266 Ok(ImplSourceConstDestructData { nested })