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, EarlyBinder, 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 =
178 tcx.bound_item_bounds(def_id).map_bound(|i| i[idx]).subst(tcx, substs);
179 let candidate = candidate_predicate
180 .to_opt_poly_trait_pred()
181 .expect("projection candidate is not a trait predicate")
182 .map_bound(|t| t.trait_ref);
183 let mut obligations = Vec::new();
184 let candidate = normalize_with_depth_to(
186 obligation.param_env,
187 obligation.cause.clone(),
188 obligation.recursion_depth + 1,
193 obligations.extend(self.infcx.commit_if_ok(|_| {
195 .at(&obligation.cause, obligation.param_env)
196 .sup(placeholder_trait_predicate, candidate)
197 .map(|InferOk { obligations, .. }| obligations)
198 .map_err(|_| Unimplemented)
201 if let ty::Projection(..) = placeholder_self_ty.kind() {
202 let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates;
203 debug!(?predicates, "projection predicates");
204 for predicate in predicates {
205 let normalized = normalize_with_depth_to(
207 obligation.param_env,
208 obligation.cause.clone(),
209 obligation.recursion_depth + 1,
213 obligations.push(Obligation::with_depth(
214 obligation.cause.clone(),
215 obligation.recursion_depth + 1,
216 obligation.param_env,
226 fn confirm_param_candidate(
228 obligation: &TraitObligation<'tcx>,
229 param: ty::PolyTraitRef<'tcx>,
230 ) -> Vec<PredicateObligation<'tcx>> {
231 debug!(?obligation, ?param, "confirm_param_candidate");
233 // During evaluation, we already checked that this
234 // where-clause trait-ref could be unified with the obligation
235 // trait-ref. Repeat that unification now without any
236 // transactional boundary; it should not fail.
237 match self.match_where_clause_trait_ref(obligation, param) {
238 Ok(obligations) => obligations,
241 "Where clause `{:?}` was applicable to `{:?}` but now is not",
249 fn confirm_builtin_candidate(
251 obligation: &TraitObligation<'tcx>,
253 ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
254 debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
256 let lang_items = self.tcx().lang_items();
257 let obligations = if has_nested {
258 let trait_def = obligation.predicate.def_id();
259 let conditions = if Some(trait_def) == lang_items.sized_trait() {
260 self.sized_conditions(obligation)
261 } else if Some(trait_def) == lang_items.copy_trait() {
262 self.copy_clone_conditions(obligation)
263 } else if Some(trait_def) == lang_items.clone_trait() {
264 self.copy_clone_conditions(obligation)
266 bug!("unexpected builtin trait {:?}", trait_def)
268 let BuiltinImplConditions::Where(nested) = conditions else {
269 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
272 let cause = obligation.derived_cause(BuiltinDerivedObligation);
273 ensure_sufficient_stack(|| {
274 self.collect_predicates_for_types(
275 obligation.param_env,
277 obligation.recursion_depth + 1,
286 debug!(?obligations);
288 ImplSourceBuiltinData { nested: obligations }
291 /// This handles the case where an `auto trait Foo` impl is being used.
292 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
294 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
295 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
296 fn confirm_auto_impl_candidate(
298 obligation: &TraitObligation<'tcx>,
300 ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
301 debug!(?obligation, ?trait_def_id, "confirm_auto_impl_candidate");
303 let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
304 let types = self.constituent_types_for_ty(self_ty);
305 self.vtable_auto_impl(obligation, trait_def_id, types)
308 /// See `confirm_auto_impl_candidate`.
311 obligation: &TraitObligation<'tcx>,
313 nested: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
314 ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
315 debug!(?nested, "vtable_auto_impl");
316 ensure_sufficient_stack(|| {
317 let cause = obligation.derived_cause(BuiltinDerivedObligation);
319 let trait_obligations: Vec<PredicateObligation<'_>> =
320 self.infcx.commit_unconditionally(|_| {
321 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
322 let trait_ref = self.infcx.replace_bound_vars_with_placeholders(poly_trait_ref);
323 self.impl_or_trait_obligations(
325 obligation.recursion_depth + 1,
326 obligation.param_env,
329 obligation.predicate,
333 let mut obligations = self.collect_predicates_for_types(
334 obligation.param_env,
336 obligation.recursion_depth + 1,
341 // Adds the predicates from the trait. Note that this contains a `Self: Trait`
342 // predicate as usual. It won't have any effect since auto traits are coinductive.
343 obligations.extend(trait_obligations);
345 debug!(?obligations, "vtable_auto_impl");
347 ImplSourceAutoImplData { trait_def_id, nested: obligations }
351 fn confirm_impl_candidate(
353 obligation: &TraitObligation<'tcx>,
355 ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
356 debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");
358 // First, create the substitutions by matching the impl again,
359 // this time not in a probe.
360 self.infcx.commit_unconditionally(|_| {
361 let substs = self.rematch_impl(impl_def_id, obligation);
362 debug!(?substs, "impl substs");
363 ensure_sufficient_stack(|| {
368 obligation.recursion_depth + 1,
369 obligation.param_env,
370 obligation.predicate,
379 substs: Normalized<'tcx, SubstsRef<'tcx>>,
380 cause: &ObligationCause<'tcx>,
381 recursion_depth: usize,
382 param_env: ty::ParamEnv<'tcx>,
383 parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
384 ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
385 debug!(?impl_def_id, ?substs, ?recursion_depth, "vtable_impl");
387 let mut impl_obligations = self.impl_or_trait_obligations(
396 debug!(?impl_obligations, "vtable_impl");
398 // Because of RFC447, the impl-trait-ref and obligations
399 // are sufficient to determine the impl substs, without
400 // relying on projections in the impl-trait-ref.
402 // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
403 impl_obligations.extend(substs.obligations);
405 ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations }
408 fn confirm_object_candidate(
410 obligation: &TraitObligation<'tcx>,
412 ) -> Result<ImplSourceObjectData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
413 let tcx = self.tcx();
414 debug!(?obligation, ?index, "confirm_object_candidate");
416 let trait_predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
417 let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
418 let obligation_trait_ref = ty::Binder::dummy(trait_predicate.trait_ref);
419 let ty::Dynamic(data, ..) = *self_ty.kind() else {
420 span_bug!(obligation.cause.span, "object candidate with non-object");
423 let object_trait_ref = data.principal().unwrap_or_else(|| {
424 span_bug!(obligation.cause.span, "object candidate with no principal")
426 let object_trait_ref = self
428 .replace_bound_vars_with_fresh_vars(
429 obligation.cause.span,
434 let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);
436 let mut nested = vec![];
438 let mut supertraits = util::supertraits(tcx, ty::Binder::dummy(object_trait_ref));
439 let unnormalized_upcast_trait_ref =
440 supertraits.nth(index).expect("supertraits iterator no longer has as many elements");
442 let upcast_trait_ref = normalize_with_depth_to(
444 obligation.param_env,
445 obligation.cause.clone(),
446 obligation.recursion_depth + 1,
447 unnormalized_upcast_trait_ref,
451 nested.extend(self.infcx.commit_if_ok(|_| {
453 .at(&obligation.cause, obligation.param_env)
454 .sup(obligation_trait_ref, upcast_trait_ref)
455 .map(|InferOk { obligations, .. }| obligations)
456 .map_err(|_| Unimplemented)
459 // Check supertraits hold. This is so that their associated type bounds
460 // will be checked in the code below.
461 for super_trait in tcx
462 .super_predicates_of(trait_predicate.def_id())
463 .instantiate(tcx, trait_predicate.trait_ref.substs)
467 let normalized_super_trait = normalize_with_depth_to(
469 obligation.param_env,
470 obligation.cause.clone(),
471 obligation.recursion_depth + 1,
475 nested.push(Obligation::new(
476 obligation.cause.clone(),
477 obligation.param_env,
478 normalized_super_trait,
482 let assoc_types: Vec<_> = tcx
483 .associated_items(trait_predicate.def_id())
484 .in_definition_order()
486 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
490 for assoc_type in assoc_types {
491 let defs: &ty::Generics = tcx.generics_of(assoc_type);
493 if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
494 tcx.sess.delay_span_bug(
495 obligation.cause.span,
496 "GATs in trait object shouldn't have been considered",
498 return Err(SelectionError::Unimplemented);
501 // This maybe belongs in wf, but that can't (doesn't) handle
502 // higher-ranked things.
503 // Prevent, e.g., `dyn Iterator<Item = str>`.
504 for bound in self.tcx().bound_item_bounds(assoc_type).transpose_iter() {
506 if defs.count() == 0 {
507 bound.subst(tcx, trait_predicate.trait_ref.substs)
509 let mut substs = smallvec::SmallVec::with_capacity(defs.count());
510 substs.extend(trait_predicate.trait_ref.substs.iter());
511 let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
512 smallvec::SmallVec::with_capacity(
513 bound.0.kind().bound_vars().len() + defs.count(),
515 bound_vars.extend(bound.0.kind().bound_vars().into_iter());
516 InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
519 GenericParamDefKind::Type { .. } => {
520 let kind = ty::BoundTyKind::Param(param.name);
521 let bound_var = ty::BoundVariableKind::Ty(kind);
522 bound_vars.push(bound_var);
526 var: ty::BoundVar::from_usize(bound_vars.len() - 1),
532 GenericParamDefKind::Lifetime => {
533 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
534 let bound_var = ty::BoundVariableKind::Region(kind);
535 bound_vars.push(bound_var);
536 tcx.mk_region(ty::ReLateBound(
539 var: ty::BoundVar::from_usize(bound_vars.len() - 1),
545 GenericParamDefKind::Const { .. } => {
546 let bound_var = ty::BoundVariableKind::Const;
547 bound_vars.push(bound_var);
548 tcx.mk_const(ty::ConstS {
549 ty: tcx.type_of(param.def_id),
550 val: ty::ConstKind::Bound(
552 ty::BoundVar::from_usize(bound_vars.len() - 1),
558 let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
559 let assoc_ty_substs = tcx.intern_substs(&substs);
561 let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
563 EarlyBinder(bound.0.kind().skip_binder()).subst(tcx, assoc_ty_substs);
564 tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
566 let normalized_bound = normalize_with_depth_to(
568 obligation.param_env,
569 obligation.cause.clone(),
570 obligation.recursion_depth + 1,
574 nested.push(Obligation::new(
575 obligation.cause.clone(),
576 obligation.param_env,
582 debug!(?nested, "object nested obligations");
584 let vtable_base = super::super::vtable_trait_first_method_offset(
586 (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
589 Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
592 fn confirm_fn_pointer_candidate(
594 obligation: &TraitObligation<'tcx>,
595 ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
597 debug!(?obligation, "confirm_fn_pointer_candidate");
599 // Okay to skip binder; it is reintroduced below.
600 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
601 let sig = self_ty.fn_sig(self.tcx());
602 let trait_ref = closure_trait_ref_and_return_type(
604 obligation.predicate.def_id(),
607 util::TupleArgumentsFlag::Yes,
609 .map_bound(|(trait_ref, _)| trait_ref);
611 let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
612 Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
615 fn confirm_trait_alias_candidate(
617 obligation: &TraitObligation<'tcx>,
619 ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
620 debug!(?obligation, ?alias_def_id, "confirm_trait_alias_candidate");
622 self.infcx.commit_unconditionally(|_| {
623 let predicate = self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
624 let trait_ref = predicate.trait_ref;
625 let trait_def_id = trait_ref.def_id;
626 let substs = trait_ref.substs;
628 let trait_obligations = self.impl_or_trait_obligations(
630 obligation.recursion_depth,
631 obligation.param_env,
634 obligation.predicate,
637 debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
639 ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
643 fn confirm_generator_candidate(
645 obligation: &TraitObligation<'tcx>,
646 ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
648 // Okay to skip binder because the substs on generator types never
649 // touch bound regions, they just capture the in-scope
650 // type/region parameters.
651 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
652 let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
653 bug!("closure candidate for non-closure {:?}", obligation);
656 debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
658 let trait_ref = self.generator_trait_ref_unnormalized(obligation, substs);
660 let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
661 debug!(?trait_ref, ?nested, "generator candidate obligations");
663 Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
666 #[instrument(skip(self), level = "debug")]
667 fn confirm_closure_candidate(
669 obligation: &TraitObligation<'tcx>,
670 ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
673 .fn_trait_kind_from_lang_item(obligation.predicate.def_id())
674 .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
676 // Okay to skip binder because the substs on closure types never
677 // touch bound regions, they just capture the in-scope
678 // type/region parameters.
679 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
680 let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
681 bug!("closure candidate for non-closure {:?}", obligation);
684 let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
685 let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
687 debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
691 if !self.tcx().sess.opts.debugging_opts.chalk {
692 nested.push(Obligation::new(
693 obligation.cause.clone(),
694 obligation.param_env,
695 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind))
696 .to_predicate(self.tcx()),
700 Ok(ImplSourceClosureData { closure_def_id, substs, nested })
703 /// In the case of closure types and fn pointers,
704 /// we currently treat the input type parameters on the trait as
705 /// outputs. This means that when we have a match we have only
706 /// considered the self type, so we have to go back and make sure
707 /// to relate the argument types too. This is kind of wrong, but
708 /// since we control the full set of impls, also not that wrong,
709 /// and it DOES yield better error messages (since we don't report
710 /// errors as if there is no applicable impl, but rather report
711 /// errors are about mismatched argument types.
713 /// Here is an example. Imagine we have a closure expression
714 /// and we desugared it so that the type of the expression is
715 /// `Closure`, and `Closure` expects `i32` as argument. Then it
716 /// is "as if" the compiler generated this impl:
717 /// ```ignore (illustrative)
718 /// impl Fn(i32) for Closure { ... }
720 /// Now imagine our obligation is `Closure: Fn(usize)`. So far
721 /// we have matched the self type `Closure`. At this point we'll
722 /// compare the `i32` to `usize` and generate an error.
724 /// Note that this checking occurs *after* the impl has selected,
725 /// because these output type parameters should not affect the
726 /// selection of the impl. Therefore, if there is a mismatch, we
727 /// report an error to the user.
728 #[instrument(skip(self), level = "trace")]
729 fn confirm_poly_trait_refs(
731 obligation: &TraitObligation<'tcx>,
732 expected_trait_ref: ty::PolyTraitRef<'tcx>,
733 ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
734 let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
735 // Normalize the obligation and expected trait refs together, because why not
736 let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
737 ensure_sufficient_stack(|| {
738 self.infcx.commit_unconditionally(|_| {
739 normalize_with_depth(
741 obligation.param_env,
742 obligation.cause.clone(),
743 obligation.recursion_depth + 1,
744 (obligation_trait_ref, expected_trait_ref),
750 .at(&obligation.cause, obligation.param_env)
751 .sup(obligation_trait_ref, expected_trait_ref)
752 .map(|InferOk { mut obligations, .. }| {
753 obligations.extend(nested);
756 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
759 fn confirm_trait_upcasting_unsize_candidate(
761 obligation: &TraitObligation<'tcx>,
763 ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
765 let tcx = self.tcx();
767 // `assemble_candidates_for_unsizing` should ensure there are no late-bound
768 // regions here. See the comment there for more details.
769 let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
770 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
771 let target = self.infcx.shallow_resolve(target);
773 debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
775 let mut nested = vec![];
776 let source_trait_ref;
777 let upcast_trait_ref;
778 match (source.kind(), target.kind()) {
779 // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
780 (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
781 // See `assemble_candidates_for_unsizing` for more info.
782 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
783 let principal_a = data_a.principal().unwrap();
784 source_trait_ref = principal_a.with_self_ty(tcx, source);
785 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
786 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
787 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
788 ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
792 let iter = Some(existential_predicate)
797 .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
802 .map(ty::ExistentialPredicate::AutoTrait)
803 .map(ty::Binder::dummy),
805 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
806 let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
808 // Require that the traits involved in this upcast are **equal**;
809 // only the **lifetime bound** is changed.
810 let InferOk { obligations, .. } = self
812 .at(&obligation.cause, obligation.param_env)
813 .sup(target, source_trait)
814 .map_err(|_| Unimplemented)?;
815 nested.extend(obligations);
817 // Register one obligation for 'a: 'b.
818 let cause = ObligationCause::new(
819 obligation.cause.span,
820 obligation.cause.body_id,
821 ObjectCastObligation(target),
823 let outlives = ty::OutlivesPredicate(r_a, r_b);
824 nested.push(Obligation::with_depth(
826 obligation.recursion_depth + 1,
827 obligation.param_env,
828 obligation.predicate.rebind(outlives).to_predicate(tcx),
834 let vtable_segment_callback = {
835 let mut vptr_offset = 0;
838 VtblSegment::MetadataDSA => {
839 vptr_offset += ty::COMMON_VTABLE_ENTRIES.len();
841 VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
842 vptr_offset += util::count_own_vtable_entries(tcx, trait_ref);
843 if trait_ref == upcast_trait_ref {
845 return ControlFlow::Break(Some(vptr_offset));
847 return ControlFlow::Break(None);
856 ControlFlow::Continue(())
860 let vtable_vptr_slot =
861 super::super::prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback)
864 Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
867 fn confirm_builtin_unsize_candidate(
869 obligation: &TraitObligation<'tcx>,
870 ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
871 let tcx = self.tcx();
873 // `assemble_candidates_for_unsizing` should ensure there are no late-bound
874 // regions here. See the comment there for more details.
875 let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
876 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
877 let target = self.infcx.shallow_resolve(target);
879 debug!(?source, ?target, "confirm_builtin_unsize_candidate");
881 let mut nested = vec![];
882 match (source.kind(), target.kind()) {
883 // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
884 (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
885 // See `assemble_candidates_for_unsizing` for more info.
886 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
889 .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
894 .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
899 .map(ty::ExistentialPredicate::AutoTrait)
900 .map(ty::Binder::dummy),
902 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
903 let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
905 // Require that the traits involved in this upcast are **equal**;
906 // only the **lifetime bound** is changed.
907 let InferOk { obligations, .. } = self
909 .at(&obligation.cause, obligation.param_env)
910 .sup(target, source_trait)
911 .map_err(|_| Unimplemented)?;
912 nested.extend(obligations);
914 // Register one obligation for 'a: 'b.
915 let cause = ObligationCause::new(
916 obligation.cause.span,
917 obligation.cause.body_id,
918 ObjectCastObligation(target),
920 let outlives = ty::OutlivesPredicate(r_a, r_b);
921 nested.push(Obligation::with_depth(
923 obligation.recursion_depth + 1,
924 obligation.param_env,
925 obligation.predicate.rebind(outlives).to_predicate(tcx),
930 (_, &ty::Dynamic(ref data, r)) => {
931 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
932 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
933 return Err(TraitNotObjectSafe(did));
936 let cause = ObligationCause::new(
937 obligation.cause.span,
938 obligation.cause.body_id,
939 ObjectCastObligation(target),
942 let predicate_to_obligation = |predicate| {
943 Obligation::with_depth(
945 obligation.recursion_depth + 1,
946 obligation.param_env,
951 // Create obligations:
952 // - Casting `T` to `Trait`
953 // - For all the various builtin bounds attached to the object cast. (In other
954 // words, if the object type is `Foo + Send`, this would create an obligation for
955 // the `Send` check.)
956 // - Projection predicates
958 data.iter().map(|predicate| {
959 predicate_to_obligation(predicate.with_self_ty(tcx, source))
963 // We can only make objects from sized types.
964 let tr = ty::Binder::dummy(ty::TraitRef::new(
965 tcx.require_lang_item(LangItem::Sized, None),
966 tcx.mk_substs_trait(source, &[]),
968 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
970 // If the type is `Foo + 'a`, ensure that the type
971 // being cast to `Foo + 'a` outlives `'a`:
972 let outlives = ty::OutlivesPredicate(source, r);
973 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
977 (&ty::Array(a, _), &ty::Slice(b)) => {
978 let InferOk { obligations, .. } = self
980 .at(&obligation.cause, obligation.param_env)
982 .map_err(|_| Unimplemented)?;
983 nested.extend(obligations);
986 // `Struct<T>` -> `Struct<U>`
987 (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
988 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
989 GenericArgKind::Type(ty) => match ty.kind() {
990 ty::Param(p) => Some(p.index),
994 // Lifetimes aren't allowed to change during unsizing.
995 GenericArgKind::Lifetime(_) => None,
997 GenericArgKind::Const(ct) => match ct.val() {
998 ty::ConstKind::Param(p) => Some(p.index),
1003 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1004 // by putting it in a query; it would only need the `DefId` as it
1005 // looks at declared field types, not anything substituted.
1007 // The last field of the structure has to exist and contain type/const parameters.
1008 let (tail_field, prefix_fields) =
1009 def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1010 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1012 let mut unsizing_params = GrowableBitSet::new_empty();
1013 for arg in tail_field_ty.0.walk() {
1014 if let Some(i) = maybe_unsizing_param_idx(arg) {
1015 unsizing_params.insert(i);
1019 // Ensure none of the other fields mention the parameters used
1021 for field in prefix_fields {
1022 for arg in tcx.type_of(field.did).walk() {
1023 if let Some(i) = maybe_unsizing_param_idx(arg) {
1024 unsizing_params.remove(i);
1029 if unsizing_params.is_empty() {
1030 return Err(Unimplemented);
1033 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`.
1034 let source_tail = tail_field_ty.subst(tcx, substs_a);
1035 let target_tail = tail_field_ty.subst(tcx, substs_b);
1037 // Check that the source struct with the target's
1038 // unsizing parameters is equal to the target.
1039 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1040 if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1042 let new_struct = tcx.mk_adt(def, substs);
1043 let InferOk { obligations, .. } = self
1045 .at(&obligation.cause, obligation.param_env)
1046 .eq(target, new_struct)
1047 .map_err(|_| Unimplemented)?;
1048 nested.extend(obligations);
1050 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1051 nested.push(predicate_for_trait_def(
1053 obligation.param_env,
1054 obligation.cause.clone(),
1055 obligation.predicate.def_id(),
1056 obligation.recursion_depth + 1,
1058 &[target_tail.into()],
1062 // `(.., T)` -> `(.., U)`
1063 (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1064 assert_eq!(tys_a.len(), tys_b.len());
1066 // The last field of the tuple has to exist.
1067 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1068 let &b_last = tys_b.last().unwrap();
1070 // Check that the source tuple with the target's
1071 // last element is equal to the target.
1072 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1073 let InferOk { obligations, .. } = self
1075 .at(&obligation.cause, obligation.param_env)
1076 .eq(target, new_tuple)
1077 .map_err(|_| Unimplemented)?;
1078 nested.extend(obligations);
1080 // Construct the nested `T: Unsize<U>` predicate.
1081 nested.push(ensure_sufficient_stack(|| {
1082 predicate_for_trait_def(
1084 obligation.param_env,
1085 obligation.cause.clone(),
1086 obligation.predicate.def_id(),
1087 obligation.recursion_depth + 1,
1097 Ok(ImplSourceBuiltinData { nested })
1100 fn confirm_const_destruct_candidate(
1102 obligation: &TraitObligation<'tcx>,
1103 impl_def_id: Option<DefId>,
1104 ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1105 // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1106 if !obligation.is_const() {
1107 return Ok(ImplSourceConstDestructData { nested: vec![] });
1110 let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1112 let tcx = self.tcx();
1113 let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1115 let mut nested = vec![];
1116 let cause = obligation.derived_cause(BuiltinDerivedObligation);
1118 // If we have a custom `impl const Drop`, then
1119 // first check it like a regular impl candidate.
1120 // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1121 if let Some(impl_def_id) = impl_def_id {
1122 let obligations = self.infcx.commit_unconditionally(|_| {
1123 let mut new_obligation = obligation.clone();
1124 new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1125 trait_pred.trait_ref.def_id = drop_trait;
1128 let substs = self.rematch_impl(impl_def_id, &new_obligation);
1129 debug!(?substs, "impl substs");
1131 let derived = DerivedObligationCause {
1132 parent_trait_pred: obligation.predicate,
1133 parent_code: obligation.cause.clone_code(),
1135 let derived_code = ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1138 span: obligation.cause.span,
1141 let cause = ObligationCause::new(
1142 obligation.cause.span,
1143 obligation.cause.body_id,
1146 ensure_sufficient_stack(|| {
1151 new_obligation.recursion_depth + 1,
1152 new_obligation.param_env,
1153 obligation.predicate,
1157 nested.extend(obligations.nested);
1160 // We want to confirm the ADT's fields if we have an ADT
1161 let mut stack = match *self_ty.skip_binder().kind() {
1162 ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1163 _ => vec![self_ty.skip_binder()],
1166 while let Some(nested_ty) = stack.pop() {
1167 match *nested_ty.kind() {
1168 // We know these types are trivially drop
1174 | ty::Infer(ty::IntVar(_))
1175 | ty::Infer(ty::FloatVar(_))
1182 | ty::Foreign(_) => {}
1184 // These types are built-in, so we can fast-track by registering
1185 // nested predicates for their constituent type(s)
1186 ty::Array(ty, _) | ty::Slice(ty) => {
1190 stack.extend(tys.iter());
1192 ty::Closure(_, substs) => {
1193 stack.push(substs.as_closure().tupled_upvars_ty());
1195 ty::Generator(_, substs, _) => {
1196 let generator = substs.as_generator();
1197 stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1199 ty::GeneratorWitness(tys) => {
1200 stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1203 // If we have a projection type, make sure to normalize it so we replace it
1204 // with a fresh infer variable
1205 ty::Projection(..) => {
1206 self.infcx.commit_unconditionally(|_| {
1207 let predicate = normalize_with_depth_to(
1209 obligation.param_env,
1211 obligation.recursion_depth + 1,
1213 .rebind(ty::TraitPredicate {
1214 trait_ref: ty::TraitRef {
1217 .require_lang_item(LangItem::Destruct, None),
1218 substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1220 constness: ty::BoundConstness::ConstIfConst,
1221 polarity: ty::ImplPolarity::Positive,
1227 nested.push(Obligation::with_depth(
1229 obligation.recursion_depth + 1,
1230 obligation.param_env,
1236 // If we have any other type (e.g. an ADT), just register a nested obligation
1237 // since it's either not `const Drop` (and we raise an error during selection),
1238 // or it's an ADT (and we need to check for a custom impl during selection)
1240 let predicate = self_ty
1241 .rebind(ty::TraitPredicate {
1242 trait_ref: ty::TraitRef {
1243 def_id: self.tcx().require_lang_item(LangItem::Destruct, None),
1244 substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1246 constness: ty::BoundConstness::ConstIfConst,
1247 polarity: ty::ImplPolarity::Positive,
1251 nested.push(Obligation::with_depth(
1253 obligation.recursion_depth + 1,
1254 obligation.param_env,
1261 Ok(ImplSourceConstDestructData { nested })