compiler/rustc_trait_selection/src/traits/select/confirmation.rs

   1 //! Confirmation.
   2 //!
   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.
   6 //!
   7 //! [rustc dev guide]:
   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, Subst, SubstsRef};
  15 use rustc_middle::ty::{self, Ty};
  16 use rustc_middle::ty::{ToPolyTraitRef, ToPredicate};
  17 use rustc_span::def_id::DefId;
  18
  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};
  22 use crate::traits::{
  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,
  31 };
  32
  33 use super::BuiltinImplConditions;
  34 use super::SelectionCandidate::{self, *};
  35 use super::SelectionContext;
  36
  37 use std::iter;
  38 use std::ops::ControlFlow;
  39
  40 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
  41     #[instrument(level = "debug", skip(self))]
  42     pub(super) fn confirm_candidate(
  43         &mut self,
  44         obligation: &TraitObligation<'tcx>,
  45         candidate: SelectionCandidate<'tcx>,
  46     ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
  47         let mut obligation = obligation;
  48         let new_obligation;
  49
  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:
  53         // ```
  54         // pub trait Super {}
  55         // pub trait Sub: Super {}
  56         //
  57         // impl<A> const Super for &A where A: ~const Super {}
  58         // impl<A> const Sub for &A where A: ~const Sub {}
  59         // ```
  60         //
  61         // The procedure to check the code above without the remapping code is as follows:
  62         // ```
  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
  66         // ```
  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(),
  71                 ..*obligation
  72             };
  73             obligation = &new_obligation;
  74         }
  75
  76         match candidate {
  77             BuiltinCandidate { has_nested } => {
  78                 let data = self.confirm_builtin_candidate(obligation, has_nested);
  79                 Ok(ImplSource::Builtin(data))
  80             }
  81
  82             ParamCandidate(param) => {
  83                 let obligations =
  84                     self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
  85                 Ok(ImplSource::Param(obligations, param.skip_binder().constness))
  86             }
  87
  88             ImplCandidate(impl_def_id) => {
  89                 Ok(ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id)))
  90             }
  91
  92             AutoImplCandidate(trait_def_id) => {
  93                 let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
  94                 Ok(ImplSource::AutoImpl(data))
  95             }
  96
  97             ProjectionCandidate(idx) => {
  98                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
  99                 // FIXME(jschievink): constness
 100                 Ok(ImplSource::Param(obligations, ty::BoundConstness::NotConst))
 101             }
 102
 103             ObjectCandidate(idx) => {
 104                 let data = self.confirm_object_candidate(obligation, idx)?;
 105                 Ok(ImplSource::Object(data))
 106             }
 107
 108             ClosureCandidate => {
 109                 let vtable_closure = self.confirm_closure_candidate(obligation)?;
 110                 Ok(ImplSource::Closure(vtable_closure))
 111             }
 112
 113             GeneratorCandidate => {
 114                 let vtable_generator = self.confirm_generator_candidate(obligation)?;
 115                 Ok(ImplSource::Generator(vtable_generator))
 116             }
 117
 118             FnPointerCandidate { .. } => {
 119                 let data = self.confirm_fn_pointer_candidate(obligation)?;
 120                 Ok(ImplSource::FnPointer(data))
 121             }
 122
 123             DiscriminantKindCandidate => {
 124                 Ok(ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData))
 125             }
 126
 127             PointeeCandidate => Ok(ImplSource::Pointee(ImplSourcePointeeData)),
 128
 129             TraitAliasCandidate(alias_def_id) => {
 130                 let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
 131                 Ok(ImplSource::TraitAlias(data))
 132             }
 133
 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))
 139             }
 140
 141             BuiltinUnsizeCandidate => {
 142                 let data = self.confirm_builtin_unsize_candidate(obligation)?;
 143                 Ok(ImplSource::Builtin(data))
 144             }
 145
 146             TraitUpcastingUnsizeCandidate(idx) => {
 147                 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
 148                 Ok(ImplSource::TraitUpcasting(data))
 149             }
 150
 151             ConstDestructCandidate(def_id) => {
 152                 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
 153                 Ok(ImplSource::ConstDestruct(data))
 154             }
 155         }
 156     }
 157
 158     fn confirm_projection_candidate(
 159         &mut self,
 160         obligation: &TraitObligation<'tcx>,
 161         idx: usize,
 162     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 163         self.infcx.commit_unconditionally(|_| {
 164             let tcx = self.tcx();
 165
 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),
 175             };
 176
 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(
 184                 self,
 185                 obligation.param_env,
 186                 obligation.cause.clone(),
 187                 obligation.recursion_depth + 1,
 188                 candidate,
 189                 &mut obligations,
 190             );
 191
 192             obligations.extend(self.infcx.commit_if_ok(|_| {
 193                 self.infcx
 194                     .at(&obligation.cause, obligation.param_env)
 195                     .sup(placeholder_trait_predicate, candidate)
 196                     .map(|InferOk { obligations, .. }| obligations)
 197                     .map_err(|_| Unimplemented)
 198             })?);
 199
 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(
 205                         self,
 206                         obligation.param_env,
 207                         obligation.cause.clone(),
 208                         obligation.recursion_depth + 1,
 209                         predicate,
 210                         &mut obligations,
 211                     );
 212                     obligations.push(Obligation::with_depth(
 213                         obligation.cause.clone(),
 214                         obligation.recursion_depth + 1,
 215                         obligation.param_env,
 216                         normalized,
 217                     ));
 218                 }
 219             }
 220
 221             Ok(obligations)
 222         })
 223     }
 224
 225     fn confirm_param_candidate(
 226         &mut self,
 227         obligation: &TraitObligation<'tcx>,
 228         param: ty::PolyTraitRef<'tcx>,
 229     ) -> Vec<PredicateObligation<'tcx>> {
 230         debug!(?obligation, ?param, "confirm_param_candidate");
 231
 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,
 238             Err(()) => {
 239                 bug!(
 240                     "Where clause `{:?}` was applicable to `{:?}` but now is not",
 241                     param,
 242                     obligation
 243                 );
 244             }
 245         }
 246     }
 247
 248     fn confirm_builtin_candidate(
 249         &mut self,
 250         obligation: &TraitObligation<'tcx>,
 251         has_nested: bool,
 252     ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
 253         debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
 254
 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)
 264             } else {
 265                 bug!("unexpected builtin trait {:?}", trait_def)
 266             };
 267             let BuiltinImplConditions::Where(nested) = conditions else {
 268                 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
 269             };
 270
 271             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 272             ensure_sufficient_stack(|| {
 273                 self.collect_predicates_for_types(
 274                     obligation.param_env,
 275                     cause,
 276                     obligation.recursion_depth + 1,
 277                     trait_def,
 278                     nested,
 279                 )
 280             })
 281         } else {
 282             vec![]
 283         };
 284
 285         debug!(?obligations);
 286
 287         ImplSourceBuiltinData { nested: obligations }
 288     }
 289
 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:
 292     ///
 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(
 296         &mut self,
 297         obligation: &TraitObligation<'tcx>,
 298         trait_def_id: DefId,
 299     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 300         debug!(?obligation, ?trait_def_id, "confirm_auto_impl_candidate");
 301
 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)
 305     }
 306
 307     /// See `confirm_auto_impl_candidate`.
 308     fn vtable_auto_impl(
 309         &mut self,
 310         obligation: &TraitObligation<'tcx>,
 311         trait_def_id: DefId,
 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);
 317
 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(
 323                         &cause,
 324                         obligation.recursion_depth + 1,
 325                         obligation.param_env,
 326                         trait_def_id,
 327                         &trait_ref.substs,
 328                         obligation.predicate,
 329                     )
 330                 });
 331
 332             let mut obligations = self.collect_predicates_for_types(
 333                 obligation.param_env,
 334                 cause,
 335                 obligation.recursion_depth + 1,
 336                 trait_def_id,
 337                 nested,
 338             );
 339
 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);
 343
 344             debug!(?obligations, "vtable_auto_impl");
 345
 346             ImplSourceAutoImplData { trait_def_id, nested: obligations }
 347         })
 348     }
 349
 350     fn confirm_impl_candidate(
 351         &mut self,
 352         obligation: &TraitObligation<'tcx>,
 353         impl_def_id: DefId,
 354     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 355         debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");
 356
 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(|| {
 363                 self.vtable_impl(
 364                     impl_def_id,
 365                     substs,
 366                     &obligation.cause,
 367                     obligation.recursion_depth + 1,
 368                     obligation.param_env,
 369                     obligation.predicate,
 370                 )
 371             })
 372         })
 373     }
 374
 375     fn vtable_impl(
 376         &mut self,
 377         impl_def_id: DefId,
 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");
 385
 386         let mut impl_obligations = self.impl_or_trait_obligations(
 387             cause,
 388             recursion_depth,
 389             param_env,
 390             impl_def_id,
 391             &substs.value,
 392             parent_trait_pred,
 393         );
 394
 395         debug!(?impl_obligations, "vtable_impl");
 396
 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.
 400         //
 401         // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
 402         impl_obligations.extend(substs.obligations);
 403
 404         ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations }
 405     }
 406
 407     fn confirm_object_candidate(
 408         &mut self,
 409         obligation: &TraitObligation<'tcx>,
 410         index: usize,
 411     ) -> Result<ImplSourceObjectData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 412         let tcx = self.tcx();
 413         debug!(?obligation, ?index, "confirm_object_candidate");
 414
 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");
 420         };
 421
 422         let object_trait_ref = data.principal().unwrap_or_else(|| {
 423             span_bug!(obligation.cause.span, "object candidate with no principal")
 424         });
 425         let object_trait_ref = self
 426             .infcx
 427             .replace_bound_vars_with_fresh_vars(
 428                 obligation.cause.span,
 429                 HigherRankedType,
 430                 object_trait_ref,
 431             )
 432             .0;
 433         let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);
 434
 435         let mut nested = vec![];
 436
 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");
 440
 441         let upcast_trait_ref = normalize_with_depth_to(
 442             self,
 443             obligation.param_env,
 444             obligation.cause.clone(),
 445             obligation.recursion_depth + 1,
 446             unnormalized_upcast_trait_ref,
 447             &mut nested,
 448         );
 449
 450         nested.extend(self.infcx.commit_if_ok(|_| {
 451             self.infcx
 452                 .at(&obligation.cause, obligation.param_env)
 453                 .sup(obligation_trait_ref, upcast_trait_ref)
 454                 .map(|InferOk { obligations, .. }| obligations)
 455                 .map_err(|_| Unimplemented)
 456         })?);
 457
 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)
 463             .predicates
 464             .into_iter()
 465         {
 466             let normalized_super_trait = normalize_with_depth_to(
 467                 self,
 468                 obligation.param_env,
 469                 obligation.cause.clone(),
 470                 obligation.recursion_depth + 1,
 471                 super_trait,
 472                 &mut nested,
 473             );
 474             nested.push(Obligation::new(
 475                 obligation.cause.clone(),
 476                 obligation.param_env,
 477                 normalized_super_trait,
 478             ));
 479         }
 480
 481         let assoc_types: Vec<_> = tcx
 482             .associated_items(trait_predicate.def_id())
 483             .in_definition_order()
 484             .filter_map(
 485                 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
 486             )
 487             .collect();
 488
 489         for assoc_type in assoc_types {
 490             if !tcx.generics_of(assoc_type).params.is_empty() {
 491                 tcx.sess.delay_span_bug(
 492                     obligation.cause.span,
 493                     "GATs in trait object shouldn't have been considered",
 494                 );
 495                 return Err(SelectionError::Unimplemented);
 496             }
 497             // This maybe belongs in wf, but that can't (doesn't) handle
 498             // higher-ranked things.
 499             // Prevent, e.g., `dyn Iterator<Item = str>`.
 500             for bound in self.tcx().item_bounds(assoc_type) {
 501                 let subst_bound = bound.subst(tcx, trait_predicate.trait_ref.substs);
 502                 let normalized_bound = normalize_with_depth_to(
 503                     self,
 504                     obligation.param_env,
 505                     obligation.cause.clone(),
 506                     obligation.recursion_depth + 1,
 507                     subst_bound,
 508                     &mut nested,
 509                 );
 510                 nested.push(Obligation::new(
 511                     obligation.cause.clone(),
 512                     obligation.param_env,
 513                     normalized_bound,
 514                 ));
 515             }
 516         }
 517
 518         debug!(?nested, "object nested obligations");
 519
 520         let vtable_base = super::super::vtable_trait_first_method_offset(
 521             tcx,
 522             (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
 523         );
 524
 525         Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
 526     }
 527
 528     fn confirm_fn_pointer_candidate(
 529         &mut self,
 530         obligation: &TraitObligation<'tcx>,
 531     ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 532     {
 533         debug!(?obligation, "confirm_fn_pointer_candidate");
 534
 535         // Okay to skip binder; it is reintroduced below.
 536         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 537         let sig = self_ty.fn_sig(self.tcx());
 538         let trait_ref = closure_trait_ref_and_return_type(
 539             self.tcx(),
 540             obligation.predicate.def_id(),
 541             self_ty,
 542             sig,
 543             util::TupleArgumentsFlag::Yes,
 544         )
 545         .map_bound(|(trait_ref, _)| trait_ref);
 546
 547         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 548         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
 549     }
 550
 551     fn confirm_trait_alias_candidate(
 552         &mut self,
 553         obligation: &TraitObligation<'tcx>,
 554         alias_def_id: DefId,
 555     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
 556         debug!(?obligation, ?alias_def_id, "confirm_trait_alias_candidate");
 557
 558         self.infcx.commit_unconditionally(|_| {
 559             let predicate = self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
 560             let trait_ref = predicate.trait_ref;
 561             let trait_def_id = trait_ref.def_id;
 562             let substs = trait_ref.substs;
 563
 564             let trait_obligations = self.impl_or_trait_obligations(
 565                 &obligation.cause,
 566                 obligation.recursion_depth,
 567                 obligation.param_env,
 568                 trait_def_id,
 569                 &substs,
 570                 obligation.predicate,
 571             );
 572
 573             debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
 574
 575             ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
 576         })
 577     }
 578
 579     fn confirm_generator_candidate(
 580         &mut self,
 581         obligation: &TraitObligation<'tcx>,
 582     ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 583     {
 584         // Okay to skip binder because the substs on generator types never
 585         // touch bound regions, they just capture the in-scope
 586         // type/region parameters.
 587         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 588         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 589             bug!("closure candidate for non-closure {:?}", obligation);
 590         };
 591
 592         debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
 593
 594         let trait_ref = self.generator_trait_ref_unnormalized(obligation, substs);
 595
 596         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 597         debug!(?trait_ref, ?nested, "generator candidate obligations");
 598
 599         Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
 600     }
 601
 602     #[instrument(skip(self), level = "debug")]
 603     fn confirm_closure_candidate(
 604         &mut self,
 605         obligation: &TraitObligation<'tcx>,
 606     ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 607         let kind = self
 608             .tcx()
 609             .fn_trait_kind_from_lang_item(obligation.predicate.def_id())
 610             .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
 611
 612         // Okay to skip binder because the substs on closure types never
 613         // touch bound regions, they just capture the in-scope
 614         // type/region parameters.
 615         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 616         let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
 617             bug!("closure candidate for non-closure {:?}", obligation);
 618         };
 619
 620         let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
 621         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 622
 623         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 624
 625         // FIXME: Chalk
 626
 627         if !self.tcx().sess.opts.debugging_opts.chalk {
 628             nested.push(Obligation::new(
 629                 obligation.cause.clone(),
 630                 obligation.param_env,
 631                 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind))
 632                     .to_predicate(self.tcx()),
 633             ));
 634         }
 635
 636         Ok(ImplSourceClosureData { closure_def_id, substs, nested })
 637     }
 638
 639     /// In the case of closure types and fn pointers,
 640     /// we currently treat the input type parameters on the trait as
 641     /// outputs. This means that when we have a match we have only
 642     /// considered the self type, so we have to go back and make sure
 643     /// to relate the argument types too. This is kind of wrong, but
 644     /// since we control the full set of impls, also not that wrong,
 645     /// and it DOES yield better error messages (since we don't report
 646     /// errors as if there is no applicable impl, but rather report
 647     /// errors are about mismatched argument types.
 648     ///
 649     /// Here is an example. Imagine we have a closure expression
 650     /// and we desugared it so that the type of the expression is
 651     /// `Closure`, and `Closure` expects `i32` as argument. Then it
 652     /// is "as if" the compiler generated this impl:
 653     ///
 654     ///     impl Fn(i32) for Closure { ... }
 655     ///
 656     /// Now imagine our obligation is `Closure: Fn(usize)`. So far
 657     /// we have matched the self type `Closure`. At this point we'll
 658     /// compare the `i32` to `usize` and generate an error.
 659     ///
 660     /// Note that this checking occurs *after* the impl has selected,
 661     /// because these output type parameters should not affect the
 662     /// selection of the impl. Therefore, if there is a mismatch, we
 663     /// report an error to the user.
 664     #[instrument(skip(self), level = "trace")]
 665     fn confirm_poly_trait_refs(
 666         &mut self,
 667         obligation: &TraitObligation<'tcx>,
 668         expected_trait_ref: ty::PolyTraitRef<'tcx>,
 669     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 670         let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
 671         // Normalize the obligation and expected trait refs together, because why not
 672         let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
 673             ensure_sufficient_stack(|| {
 674                 self.infcx.commit_unconditionally(|_| {
 675                     normalize_with_depth(
 676                         self,
 677                         obligation.param_env,
 678                         obligation.cause.clone(),
 679                         obligation.recursion_depth + 1,
 680                         (obligation_trait_ref, expected_trait_ref),
 681                     )
 682                 })
 683             });
 684
 685         self.infcx
 686             .at(&obligation.cause, obligation.param_env)
 687             .sup(obligation_trait_ref, expected_trait_ref)
 688             .map(|InferOk { mut obligations, .. }| {
 689                 obligations.extend(nested);
 690                 obligations
 691             })
 692             .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
 693     }
 694
 695     fn confirm_trait_upcasting_unsize_candidate(
 696         &mut self,
 697         obligation: &TraitObligation<'tcx>,
 698         idx: usize,
 699     ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 700     {
 701         let tcx = self.tcx();
 702
 703         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 704         // regions here. See the comment there for more details.
 705         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 706         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 707         let target = self.infcx.shallow_resolve(target);
 708
 709         debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
 710
 711         let mut nested = vec![];
 712         let source_trait_ref;
 713         let upcast_trait_ref;
 714         match (source.kind(), target.kind()) {
 715             // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
 716             (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
 717                 // See `assemble_candidates_for_unsizing` for more info.
 718                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 719                 let principal_a = data_a.principal().unwrap();
 720                 source_trait_ref = principal_a.with_self_ty(tcx, source);
 721                 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
 722                 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
 723                 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
 724                     ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
 725                         tcx, trait_ref,
 726                     ))
 727                 });
 728                 let iter = Some(existential_predicate)
 729                     .into_iter()
 730                     .chain(
 731                         data_a
 732                             .projection_bounds()
 733                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 734                     )
 735                     .chain(
 736                         data_b
 737                             .auto_traits()
 738                             .map(ty::ExistentialPredicate::AutoTrait)
 739                             .map(ty::Binder::dummy),
 740                     );
 741                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 742                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
 743
 744                 // Require that the traits involved in this upcast are **equal**;
 745                 // only the **lifetime bound** is changed.
 746                 let InferOk { obligations, .. } = self
 747                     .infcx
 748                     .at(&obligation.cause, obligation.param_env)
 749                     .sup(target, source_trait)
 750                     .map_err(|_| Unimplemented)?;
 751                 nested.extend(obligations);
 752
 753                 // Register one obligation for 'a: 'b.
 754                 let cause = ObligationCause::new(
 755                     obligation.cause.span,
 756                     obligation.cause.body_id,
 757                     ObjectCastObligation(target),
 758                 );
 759                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 760                 nested.push(Obligation::with_depth(
 761                     cause,
 762                     obligation.recursion_depth + 1,
 763                     obligation.param_env,
 764                     obligation.predicate.rebind(outlives).to_predicate(tcx),
 765                 ));
 766             }
 767             _ => bug!(),
 768         };
 769
 770         let vtable_segment_callback = {
 771             let mut vptr_offset = 0;
 772             move |segment| {
 773                 match segment {
 774                     VtblSegment::MetadataDSA => {
 775                         vptr_offset += ty::COMMON_VTABLE_ENTRIES.len();
 776                     }
 777                     VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 778                         vptr_offset += util::count_own_vtable_entries(tcx, trait_ref);
 779                         if trait_ref == upcast_trait_ref {
 780                             if emit_vptr {
 781                                 return ControlFlow::Break(Some(vptr_offset));
 782                             } else {
 783                                 return ControlFlow::Break(None);
 784                             }
 785                         }
 786
 787                         if emit_vptr {
 788                             vptr_offset += 1;
 789                         }
 790                     }
 791                 }
 792                 ControlFlow::Continue(())
 793             }
 794         };
 795
 796         let vtable_vptr_slot =
 797             super::super::prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback)
 798                 .unwrap();
 799
 800         Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
 801     }
 802
 803     fn confirm_builtin_unsize_candidate(
 804         &mut self,
 805         obligation: &TraitObligation<'tcx>,
 806     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 807         let tcx = self.tcx();
 808
 809         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 810         // regions here. See the comment there for more details.
 811         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 812         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 813         let target = self.infcx.shallow_resolve(target);
 814
 815         debug!(?source, ?target, "confirm_builtin_unsize_candidate");
 816
 817         let mut nested = vec![];
 818         match (source.kind(), target.kind()) {
 819             // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
 820             (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_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 iter = data_a
 824                     .principal()
 825                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
 826                     .into_iter()
 827                     .chain(
 828                         data_a
 829                             .projection_bounds()
 830                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 831                     )
 832                     .chain(
 833                         data_b
 834                             .auto_traits()
 835                             .map(ty::ExistentialPredicate::AutoTrait)
 836                             .map(ty::Binder::dummy),
 837                     );
 838                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 839                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
 840
 841                 // Require that the traits involved in this upcast are **equal**;
 842                 // only the **lifetime bound** is changed.
 843                 let InferOk { obligations, .. } = self
 844                     .infcx
 845                     .at(&obligation.cause, obligation.param_env)
 846                     .sup(target, source_trait)
 847                     .map_err(|_| Unimplemented)?;
 848                 nested.extend(obligations);
 849
 850                 // Register one obligation for 'a: 'b.
 851                 let cause = ObligationCause::new(
 852                     obligation.cause.span,
 853                     obligation.cause.body_id,
 854                     ObjectCastObligation(target),
 855                 );
 856                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 857                 nested.push(Obligation::with_depth(
 858                     cause,
 859                     obligation.recursion_depth + 1,
 860                     obligation.param_env,
 861                     obligation.predicate.rebind(outlives).to_predicate(tcx),
 862                 ));
 863             }
 864
 865             // `T` -> `Trait`
 866             (_, &ty::Dynamic(ref data, r)) => {
 867                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
 868                 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
 869                     return Err(TraitNotObjectSafe(did));
 870                 }
 871
 872                 let cause = ObligationCause::new(
 873                     obligation.cause.span,
 874                     obligation.cause.body_id,
 875                     ObjectCastObligation(target),
 876                 );
 877
 878                 let predicate_to_obligation = |predicate| {
 879                     Obligation::with_depth(
 880                         cause.clone(),
 881                         obligation.recursion_depth + 1,
 882                         obligation.param_env,
 883                         predicate,
 884                     )
 885                 };
 886
 887                 // Create obligations:
 888                 //  - Casting `T` to `Trait`
 889                 //  - For all the various builtin bounds attached to the object cast. (In other
 890                 //  words, if the object type is `Foo + Send`, this would create an obligation for
 891                 //  the `Send` check.)
 892                 //  - Projection predicates
 893                 nested.extend(
 894                     data.iter().map(|predicate| {
 895                         predicate_to_obligation(predicate.with_self_ty(tcx, source))
 896                     }),
 897                 );
 898
 899                 // We can only make objects from sized types.
 900                 let tr = ty::Binder::dummy(ty::TraitRef::new(
 901                     tcx.require_lang_item(LangItem::Sized, None),
 902                     tcx.mk_substs_trait(source, &[]),
 903                 ));
 904                 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
 905
 906                 // If the type is `Foo + 'a`, ensure that the type
 907                 // being cast to `Foo + 'a` outlives `'a`:
 908                 let outlives = ty::OutlivesPredicate(source, r);
 909                 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
 910             }
 911
 912             // `[T; n]` -> `[T]`
 913             (&ty::Array(a, _), &ty::Slice(b)) => {
 914                 let InferOk { obligations, .. } = self
 915                     .infcx
 916                     .at(&obligation.cause, obligation.param_env)
 917                     .eq(b, a)
 918                     .map_err(|_| Unimplemented)?;
 919                 nested.extend(obligations);
 920             }
 921
 922             // `Struct<T>` -> `Struct<U>`
 923             (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
 924                 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
 925                     GenericArgKind::Type(ty) => match ty.kind() {
 926                         ty::Param(p) => Some(p.index),
 927                         _ => None,
 928                     },
 929
 930                     // Lifetimes aren't allowed to change during unsizing.
 931                     GenericArgKind::Lifetime(_) => None,
 932
 933                     GenericArgKind::Const(ct) => match ct.val() {
 934                         ty::ConstKind::Param(p) => Some(p.index),
 935                         _ => None,
 936                     },
 937                 };
 938
 939                 // FIXME(eddyb) cache this (including computing `unsizing_params`)
 940                 // by putting it in a query; it would only need the `DefId` as it
 941                 // looks at declared field types, not anything substituted.
 942
 943                 // The last field of the structure has to exist and contain type/const parameters.
 944                 let (tail_field, prefix_fields) =
 945                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
 946                 let tail_field_ty = tcx.type_of(tail_field.did);
 947
 948                 let mut unsizing_params = GrowableBitSet::new_empty();
 949                 for arg in tail_field_ty.walk() {
 950                     if let Some(i) = maybe_unsizing_param_idx(arg) {
 951                         unsizing_params.insert(i);
 952                     }
 953                 }
 954
 955                 // Ensure none of the other fields mention the parameters used
 956                 // in unsizing.
 957                 for field in prefix_fields {
 958                     for arg in tcx.type_of(field.did).walk() {
 959                         if let Some(i) = maybe_unsizing_param_idx(arg) {
 960                             unsizing_params.remove(i);
 961                         }
 962                     }
 963                 }
 964
 965                 if unsizing_params.is_empty() {
 966                     return Err(Unimplemented);
 967                 }
 968
 969                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`.
 970                 let source_tail = tail_field_ty.subst(tcx, substs_a);
 971                 let target_tail = tail_field_ty.subst(tcx, substs_b);
 972
 973                 // Check that the source struct with the target's
 974                 // unsizing parameters is equal to the target.
 975                 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
 976                     if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
 977                 }));
 978                 let new_struct = tcx.mk_adt(def, substs);
 979                 let InferOk { obligations, .. } = self
 980                     .infcx
 981                     .at(&obligation.cause, obligation.param_env)
 982                     .eq(target, new_struct)
 983                     .map_err(|_| Unimplemented)?;
 984                 nested.extend(obligations);
 985
 986                 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
 987                 nested.push(predicate_for_trait_def(
 988                     tcx,
 989                     obligation.param_env,
 990                     obligation.cause.clone(),
 991                     obligation.predicate.def_id(),
 992                     obligation.recursion_depth + 1,
 993                     source_tail,
 994                     &[target_tail.into()],
 995                 ));
 996             }
 997
 998             // `(.., T)` -> `(.., U)`
 999             (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1000                 assert_eq!(tys_a.len(), tys_b.len());
1001
1002                 // The last field of the tuple has to exist.
1003                 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1004                 let &b_last = tys_b.last().unwrap();
1005
1006                 // Check that the source tuple with the target's
1007                 // last element is equal to the target.
1008                 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1009                 let InferOk { obligations, .. } = self
1010                     .infcx
1011                     .at(&obligation.cause, obligation.param_env)
1012                     .eq(target, new_tuple)
1013                     .map_err(|_| Unimplemented)?;
1014                 nested.extend(obligations);
1015
1016                 // Construct the nested `T: Unsize<U>` predicate.
1017                 nested.push(ensure_sufficient_stack(|| {
1018                     predicate_for_trait_def(
1019                         tcx,
1020                         obligation.param_env,
1021                         obligation.cause.clone(),
1022                         obligation.predicate.def_id(),
1023                         obligation.recursion_depth + 1,
1024                         a_last,
1025                         &[b_last.into()],
1026                     )
1027                 }));
1028             }
1029
1030             _ => bug!(),
1031         };
1032
1033         Ok(ImplSourceBuiltinData { nested })
1034     }
1035
1036     fn confirm_const_destruct_candidate(
1037         &mut self,
1038         obligation: &TraitObligation<'tcx>,
1039         impl_def_id: Option<DefId>,
1040     ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1041         // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1042         if !obligation.is_const() {
1043             return Ok(ImplSourceConstDestructData { nested: vec![] });
1044         }
1045
1046         let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1047         // FIXME: remove if statement below when beta is bumped
1048         #[cfg(bootstrap)]
1049         {}
1050
1051         if obligation.predicate.skip_binder().def_id() == drop_trait {
1052             return Ok(ImplSourceConstDestructData { nested: vec![] });
1053         }
1054
1055         let tcx = self.tcx();
1056         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1057
1058         let mut nested = vec![];
1059         let cause = obligation.derived_cause(BuiltinDerivedObligation);
1060
1061         // If we have a custom `impl const Drop`, then
1062         // first check it like a regular impl candidate.
1063         // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1064         if let Some(impl_def_id) = impl_def_id {
1065             let obligations = self.infcx.commit_unconditionally(|_| {
1066                 let mut new_obligation = obligation.clone();
1067                 new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1068                     trait_pred.trait_ref.def_id = drop_trait;
1069                     trait_pred
1070                 });
1071                 let substs = self.rematch_impl(impl_def_id, &new_obligation);
1072                 debug!(?substs, "impl substs");
1073
1074                 let derived = DerivedObligationCause {
1075                     parent_trait_pred: obligation.predicate,
1076                     parent_code: obligation.cause.clone_code(),
1077                 };
1078                 let derived_code = ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1079                     derived,
1080                     impl_def_id,
1081                     span: obligation.cause.span,
1082                 }));
1083
1084                 let cause = ObligationCause::new(
1085                     obligation.cause.span,
1086                     obligation.cause.body_id,
1087                     derived_code,
1088                 );
1089                 ensure_sufficient_stack(|| {
1090                     self.vtable_impl(
1091                         impl_def_id,
1092                         substs,
1093                         &cause,
1094                         new_obligation.recursion_depth + 1,
1095                         new_obligation.param_env,
1096                         obligation.predicate,
1097                     )
1098                 })
1099             });
1100             nested.extend(obligations.nested);
1101         }
1102
1103         // We want to confirm the ADT's fields if we have an ADT
1104         let mut stack = match *self_ty.skip_binder().kind() {
1105             ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1106             _ => vec![self_ty.skip_binder()],
1107         };
1108
1109         while let Some(nested_ty) = stack.pop() {
1110             match *nested_ty.kind() {
1111                 // We know these types are trivially drop
1112                 ty::Bool
1113                 | ty::Char
1114                 | ty::Int(_)
1115                 | ty::Uint(_)
1116                 | ty::Float(_)
1117                 | ty::Infer(ty::IntVar(_))
1118                 | ty::Infer(ty::FloatVar(_))
1119                 | ty::Str
1120                 | ty::RawPtr(_)
1121                 | ty::Ref(..)
1122                 | ty::FnDef(..)
1123                 | ty::FnPtr(_)
1124                 | ty::Never
1125                 | ty::Foreign(_) => {}
1126
1127                 // These types are built-in, so we can fast-track by registering
1128                 // nested predicates for their constituent type(s)
1129                 ty::Array(ty, _) | ty::Slice(ty) => {
1130                     stack.push(ty);
1131                 }
1132                 ty::Tuple(tys) => {
1133                     stack.extend(tys.iter());
1134                 }
1135                 ty::Closure(_, substs) => {
1136                     stack.push(substs.as_closure().tupled_upvars_ty());
1137                 }
1138                 ty::Generator(_, substs, _) => {
1139                     let generator = substs.as_generator();
1140                     stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1141                 }
1142                 ty::GeneratorWitness(tys) => {
1143                     stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1144                 }
1145
1146                 // If we have a projection type, make sure to normalize it so we replace it
1147                 // with a fresh infer variable
1148                 ty::Projection(..) => {
1149                     self.infcx.commit_unconditionally(|_| {
1150                         let predicate = normalize_with_depth_to(
1151                             self,
1152                             obligation.param_env,
1153                             cause.clone(),
1154                             obligation.recursion_depth + 1,
1155                             self_ty
1156                                 .rebind(ty::TraitPredicate {
1157                                     trait_ref: ty::TraitRef {
1158                                         def_id: self
1159                                             .tcx()
1160                                             .require_lang_item(LangItem::Destruct, None),
1161                                         substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1162                                     },
1163                                     constness: ty::BoundConstness::ConstIfConst,
1164                                     polarity: ty::ImplPolarity::Positive,
1165                                 })
1166                                 .to_predicate(tcx),
1167                             &mut nested,
1168                         );
1169
1170                         nested.push(Obligation::with_depth(
1171                             cause.clone(),
1172                             obligation.recursion_depth + 1,
1173                             obligation.param_env,
1174                             predicate,
1175                         ));
1176                     });
1177                 }
1178
1179                 // If we have any other type (e.g. an ADT), just register a nested obligation
1180                 // since it's either not `const Drop` (and we raise an error during selection),
1181                 // or it's an ADT (and we need to check for a custom impl during selection)
1182                 _ => {
1183                     let predicate = self_ty
1184                         .rebind(ty::TraitPredicate {
1185                             trait_ref: ty::TraitRef {
1186                                 def_id: self.tcx().require_lang_item(LangItem::Destruct, None),
1187                                 substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1188                             },
1189                             constness: ty::BoundConstness::ConstIfConst,
1190                             polarity: ty::ImplPolarity::Positive,
1191                         })
1192                         .to_predicate(tcx);
1193
1194                     nested.push(Obligation::with_depth(
1195                         cause.clone(),
1196                         obligation.recursion_depth + 1,
1197                         obligation.param_env,
1198                         predicate,
1199                     ));
1200                 }
1201             }
1202         }
1203
1204         Ok(ImplSourceConstDestructData { nested })
1205     }
1206 }