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, InternalSubsts, Subst, SubstsRef};
  15 use rustc_middle::ty::{self, EarlyBinder, GenericParamDefKind, Ty, TyCtxt};
  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::util::{self, closure_trait_ref_and_return_type, predicate_for_trait_def};
  21 use crate::traits::{
  22     BuiltinDerivedObligation, ImplDerivedObligation, ImplDerivedObligationCause, ImplSource,
  23     ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
  24     ImplSourceConstDestructData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
  25     ImplSourceGeneratorData, ImplSourceObjectData, ImplSourcePointeeData, ImplSourceTraitAliasData,
  26     ImplSourceTraitUpcastingData, ImplSourceUserDefinedData, Normalized, ObjectCastObligation,
  27     Obligation, ObligationCause, OutputTypeParameterMismatch, PredicateObligation, Selection,
  28     SelectionError, TraitNotObjectSafe, TraitObligation, Unimplemented, VtblSegment,
  29 };
  30
  31 use super::BuiltinImplConditions;
  32 use super::SelectionCandidate::{self, *};
  33 use super::SelectionContext;
  34
  35 use std::iter;
  36 use std::ops::ControlFlow;
  37
  38 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
  39     #[instrument(level = "debug", skip(self))]
  40     pub(super) fn confirm_candidate(
  41         &mut self,
  42         obligation: &TraitObligation<'tcx>,
  43         candidate: SelectionCandidate<'tcx>,
  44     ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
  45         let mut obligation = obligation;
  46         let new_obligation;
  47
  48         // HACK(const_trait_impl): the surrounding environment is remapped to a non-const context
  49         // because nested obligations might be actually `~const` then (incorrectly) requiring
  50         // const impls. for example:
  51         // ```
  52         // pub trait Super {}
  53         // pub trait Sub: Super {}
  54         //
  55         // impl<A> const Super for &A where A: ~const Super {}
  56         // impl<A> const Sub for &A where A: ~const Sub {}
  57         // ```
  58         //
  59         // The procedure to check the code above without the remapping code is as follows:
  60         // ```
  61         // CheckWf(impl const Sub for &A where A: ~const Sub) // <- const env
  62         // CheckPredicate(&A: Super)
  63         // CheckPredicate(A: ~const Super) // <- still const env, failure
  64         // ```
  65         if obligation.param_env.is_const() && !obligation.predicate.is_const_if_const() {
  66             new_obligation = TraitObligation {
  67                 cause: obligation.cause.clone(),
  68                 param_env: obligation.param_env.without_const(),
  69                 ..*obligation
  70             };
  71             obligation = &new_obligation;
  72         }
  73
  74         match candidate {
  75             BuiltinCandidate { has_nested } => {
  76                 let data = self.confirm_builtin_candidate(obligation, has_nested);
  77                 Ok(ImplSource::Builtin(data))
  78             }
  79
  80             ParamCandidate(param) => {
  81                 let obligations =
  82                     self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
  83                 Ok(ImplSource::Param(obligations, param.skip_binder().constness))
  84             }
  85
  86             ImplCandidate(impl_def_id) => {
  87                 Ok(ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id)))
  88             }
  89
  90             AutoImplCandidate(trait_def_id) => {
  91                 let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
  92                 Ok(ImplSource::AutoImpl(data))
  93             }
  94
  95             ProjectionCandidate(idx) => {
  96                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
  97                 // FIXME(jschievink): constness
  98                 Ok(ImplSource::Param(obligations, ty::BoundConstness::NotConst))
  99             }
 100
 101             ObjectCandidate(idx) => {
 102                 let data = self.confirm_object_candidate(obligation, idx)?;
 103                 Ok(ImplSource::Object(data))
 104             }
 105
 106             ClosureCandidate => {
 107                 let vtable_closure = self.confirm_closure_candidate(obligation)?;
 108                 Ok(ImplSource::Closure(vtable_closure))
 109             }
 110
 111             GeneratorCandidate => {
 112                 let vtable_generator = self.confirm_generator_candidate(obligation)?;
 113                 Ok(ImplSource::Generator(vtable_generator))
 114             }
 115
 116             FnPointerCandidate { .. } => {
 117                 let data = self.confirm_fn_pointer_candidate(obligation)?;
 118                 Ok(ImplSource::FnPointer(data))
 119             }
 120
 121             DiscriminantKindCandidate => {
 122                 Ok(ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData))
 123             }
 124
 125             PointeeCandidate => Ok(ImplSource::Pointee(ImplSourcePointeeData)),
 126
 127             TraitAliasCandidate(alias_def_id) => {
 128                 let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
 129                 Ok(ImplSource::TraitAlias(data))
 130             }
 131
 132             BuiltinObjectCandidate => {
 133                 // This indicates something like `Trait + Send: Send`. In this case, we know that
 134                 // this holds because that's what the object type is telling us, and there's really
 135                 // no additional obligations to prove and no types in particular to unify, etc.
 136                 Ok(ImplSource::Param(Vec::new(), ty::BoundConstness::NotConst))
 137             }
 138
 139             BuiltinUnsizeCandidate => {
 140                 let data = self.confirm_builtin_unsize_candidate(obligation)?;
 141                 Ok(ImplSource::Builtin(data))
 142             }
 143
 144             TraitUpcastingUnsizeCandidate(idx) => {
 145                 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
 146                 Ok(ImplSource::TraitUpcasting(data))
 147             }
 148
 149             ConstDestructCandidate(def_id) => {
 150                 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
 151                 Ok(ImplSource::ConstDestruct(data))
 152             }
 153         }
 154     }
 155
 156     fn confirm_projection_candidate(
 157         &mut self,
 158         obligation: &TraitObligation<'tcx>,
 159         idx: usize,
 160     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 161         self.infcx.commit_unconditionally(|_| {
 162             let tcx = self.tcx();
 163
 164             let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
 165             let placeholder_trait_predicate =
 166                 self.infcx().replace_bound_vars_with_placeholders(trait_predicate).trait_ref;
 167             let placeholder_self_ty = placeholder_trait_predicate.self_ty();
 168             let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
 169             let (def_id, substs) = match *placeholder_self_ty.kind() {
 170                 ty::Projection(proj) => (proj.item_def_id, proj.substs),
 171                 ty::Opaque(def_id, substs) => (def_id, substs),
 172                 _ => bug!("projection candidate for unexpected type: {:?}", placeholder_self_ty),
 173             };
 174
 175             let candidate_predicate =
 176                 tcx.bound_item_bounds(def_id).map_bound(|i| i[idx]).subst(tcx, substs);
 177             let candidate = candidate_predicate
 178                 .to_opt_poly_trait_pred()
 179                 .expect("projection candidate is not a trait predicate")
 180                 .map_bound(|t| t.trait_ref);
 181             let mut obligations = Vec::new();
 182             let candidate = normalize_with_depth_to(
 183                 self,
 184                 obligation.param_env,
 185                 obligation.cause.clone(),
 186                 obligation.recursion_depth + 1,
 187                 candidate,
 188                 &mut obligations,
 189             );
 190
 191             obligations.extend(self.infcx.commit_if_ok(|_| {
 192                 self.infcx
 193                     .at(&obligation.cause, obligation.param_env)
 194                     .sup(placeholder_trait_predicate, candidate)
 195                     .map(|InferOk { obligations, .. }| obligations)
 196                     .map_err(|_| Unimplemented)
 197             })?);
 198
 199             if let ty::Projection(..) = placeholder_self_ty.kind() {
 200                 let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates;
 201                 debug!(?predicates, "projection predicates");
 202                 for predicate in predicates {
 203                     let normalized = normalize_with_depth_to(
 204                         self,
 205                         obligation.param_env,
 206                         obligation.cause.clone(),
 207                         obligation.recursion_depth + 1,
 208                         predicate,
 209                         &mut obligations,
 210                     );
 211                     obligations.push(Obligation::with_depth(
 212                         obligation.cause.clone(),
 213                         obligation.recursion_depth + 1,
 214                         obligation.param_env,
 215                         normalized,
 216                     ));
 217                 }
 218             }
 219
 220             Ok(obligations)
 221         })
 222     }
 223
 224     fn confirm_param_candidate(
 225         &mut self,
 226         obligation: &TraitObligation<'tcx>,
 227         param: ty::PolyTraitRef<'tcx>,
 228     ) -> Vec<PredicateObligation<'tcx>> {
 229         debug!(?obligation, ?param, "confirm_param_candidate");
 230
 231         // During evaluation, we already checked that this
 232         // where-clause trait-ref could be unified with the obligation
 233         // trait-ref. Repeat that unification now without any
 234         // transactional boundary; it should not fail.
 235         match self.match_where_clause_trait_ref(obligation, param) {
 236             Ok(obligations) => obligations,
 237             Err(()) => {
 238                 bug!(
 239                     "Where clause `{:?}` was applicable to `{:?}` but now is not",
 240                     param,
 241                     obligation
 242                 );
 243             }
 244         }
 245     }
 246
 247     fn confirm_builtin_candidate(
 248         &mut self,
 249         obligation: &TraitObligation<'tcx>,
 250         has_nested: bool,
 251     ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
 252         debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
 253
 254         let lang_items = self.tcx().lang_items();
 255         let obligations = if has_nested {
 256             let trait_def = obligation.predicate.def_id();
 257             let conditions = if Some(trait_def) == lang_items.sized_trait() {
 258                 self.sized_conditions(obligation)
 259             } else if Some(trait_def) == lang_items.copy_trait() {
 260                 self.copy_clone_conditions(obligation)
 261             } else if Some(trait_def) == lang_items.clone_trait() {
 262                 self.copy_clone_conditions(obligation)
 263             } else {
 264                 bug!("unexpected builtin trait {:?}", trait_def)
 265             };
 266             let BuiltinImplConditions::Where(nested) = conditions else {
 267                 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
 268             };
 269
 270             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 271             ensure_sufficient_stack(|| {
 272                 self.collect_predicates_for_types(
 273                     obligation.param_env,
 274                     cause,
 275                     obligation.recursion_depth + 1,
 276                     trait_def,
 277                     nested,
 278                 )
 279             })
 280         } else {
 281             vec![]
 282         };
 283
 284         debug!(?obligations);
 285
 286         ImplSourceBuiltinData { nested: obligations }
 287     }
 288
 289     /// This handles the case where an `auto trait Foo` impl is being used.
 290     /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
 291     ///
 292     /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
 293     /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
 294     fn confirm_auto_impl_candidate(
 295         &mut self,
 296         obligation: &TraitObligation<'tcx>,
 297         trait_def_id: DefId,
 298     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 299         debug!(?obligation, ?trait_def_id, "confirm_auto_impl_candidate");
 300
 301         let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
 302         let types = self.constituent_types_for_ty(self_ty);
 303         self.vtable_auto_impl(obligation, trait_def_id, types)
 304     }
 305
 306     /// See `confirm_auto_impl_candidate`.
 307     fn vtable_auto_impl(
 308         &mut self,
 309         obligation: &TraitObligation<'tcx>,
 310         trait_def_id: DefId,
 311         nested: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
 312     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 313         debug!(?nested, "vtable_auto_impl");
 314         ensure_sufficient_stack(|| {
 315             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 316
 317             let trait_obligations: Vec<PredicateObligation<'_>> =
 318                 self.infcx.commit_unconditionally(|_| {
 319                     let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
 320                     let trait_ref = self.infcx.replace_bound_vars_with_placeholders(poly_trait_ref);
 321                     self.impl_or_trait_obligations(
 322                         &cause,
 323                         obligation.recursion_depth + 1,
 324                         obligation.param_env,
 325                         trait_def_id,
 326                         &trait_ref.substs,
 327                         obligation.predicate,
 328                     )
 329                 });
 330
 331             let mut obligations = self.collect_predicates_for_types(
 332                 obligation.param_env,
 333                 cause,
 334                 obligation.recursion_depth + 1,
 335                 trait_def_id,
 336                 nested,
 337             );
 338
 339             // Adds the predicates from the trait.  Note that this contains a `Self: Trait`
 340             // predicate as usual.  It won't have any effect since auto traits are coinductive.
 341             obligations.extend(trait_obligations);
 342
 343             debug!(?obligations, "vtable_auto_impl");
 344
 345             ImplSourceAutoImplData { trait_def_id, nested: obligations }
 346         })
 347     }
 348
 349     fn confirm_impl_candidate(
 350         &mut self,
 351         obligation: &TraitObligation<'tcx>,
 352         impl_def_id: DefId,
 353     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 354         debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");
 355
 356         // First, create the substitutions by matching the impl again,
 357         // this time not in a probe.
 358         self.infcx.commit_unconditionally(|_| {
 359             let substs = self.rematch_impl(impl_def_id, obligation);
 360             debug!(?substs, "impl substs");
 361             ensure_sufficient_stack(|| {
 362                 self.vtable_impl(
 363                     impl_def_id,
 364                     substs,
 365                     &obligation.cause,
 366                     obligation.recursion_depth + 1,
 367                     obligation.param_env,
 368                     obligation.predicate,
 369                 )
 370             })
 371         })
 372     }
 373
 374     fn vtable_impl(
 375         &mut self,
 376         impl_def_id: DefId,
 377         substs: Normalized<'tcx, SubstsRef<'tcx>>,
 378         cause: &ObligationCause<'tcx>,
 379         recursion_depth: usize,
 380         param_env: ty::ParamEnv<'tcx>,
 381         parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
 382     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 383         debug!(?impl_def_id, ?substs, ?recursion_depth, "vtable_impl");
 384
 385         let mut impl_obligations = self.impl_or_trait_obligations(
 386             cause,
 387             recursion_depth,
 388             param_env,
 389             impl_def_id,
 390             &substs.value,
 391             parent_trait_pred,
 392         );
 393
 394         debug!(?impl_obligations, "vtable_impl");
 395
 396         // Because of RFC447, the impl-trait-ref and obligations
 397         // are sufficient to determine the impl substs, without
 398         // relying on projections in the impl-trait-ref.
 399         //
 400         // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
 401         impl_obligations.extend(substs.obligations);
 402
 403         ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations }
 404     }
 405
 406     fn confirm_object_candidate(
 407         &mut self,
 408         obligation: &TraitObligation<'tcx>,
 409         index: usize,
 410     ) -> Result<ImplSourceObjectData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 411         let tcx = self.tcx();
 412         debug!(?obligation, ?index, "confirm_object_candidate");
 413
 414         let trait_predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
 415         let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
 416         let obligation_trait_ref = ty::Binder::dummy(trait_predicate.trait_ref);
 417         let ty::Dynamic(data, ..) = *self_ty.kind() else {
 418             span_bug!(obligation.cause.span, "object candidate with non-object");
 419         };
 420
 421         let object_trait_ref = data.principal().unwrap_or_else(|| {
 422             span_bug!(obligation.cause.span, "object candidate with no principal")
 423         });
 424         let object_trait_ref = self.infcx.replace_bound_vars_with_fresh_vars(
 425             obligation.cause.span,
 426             HigherRankedType,
 427             object_trait_ref,
 428         );
 429         let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);
 430
 431         let mut nested = vec![];
 432
 433         let mut supertraits = util::supertraits(tcx, ty::Binder::dummy(object_trait_ref));
 434         let unnormalized_upcast_trait_ref =
 435             supertraits.nth(index).expect("supertraits iterator no longer has as many elements");
 436
 437         let upcast_trait_ref = normalize_with_depth_to(
 438             self,
 439             obligation.param_env,
 440             obligation.cause.clone(),
 441             obligation.recursion_depth + 1,
 442             unnormalized_upcast_trait_ref,
 443             &mut nested,
 444         );
 445
 446         nested.extend(self.infcx.commit_if_ok(|_| {
 447             self.infcx
 448                 .at(&obligation.cause, obligation.param_env)
 449                 .sup(obligation_trait_ref, upcast_trait_ref)
 450                 .map(|InferOk { obligations, .. }| obligations)
 451                 .map_err(|_| Unimplemented)
 452         })?);
 453
 454         // Check supertraits hold. This is so that their associated type bounds
 455         // will be checked in the code below.
 456         for super_trait in tcx
 457             .super_predicates_of(trait_predicate.def_id())
 458             .instantiate(tcx, trait_predicate.trait_ref.substs)
 459             .predicates
 460             .into_iter()
 461         {
 462             let normalized_super_trait = normalize_with_depth_to(
 463                 self,
 464                 obligation.param_env,
 465                 obligation.cause.clone(),
 466                 obligation.recursion_depth + 1,
 467                 super_trait,
 468                 &mut nested,
 469             );
 470             nested.push(Obligation::new(
 471                 obligation.cause.clone(),
 472                 obligation.param_env,
 473                 normalized_super_trait,
 474             ));
 475         }
 476
 477         let assoc_types: Vec<_> = tcx
 478             .associated_items(trait_predicate.def_id())
 479             .in_definition_order()
 480             .filter_map(
 481                 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
 482             )
 483             .collect();
 484
 485         for assoc_type in assoc_types {
 486             let defs: &ty::Generics = tcx.generics_of(assoc_type);
 487
 488             if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
 489                 tcx.sess.delay_span_bug(
 490                     obligation.cause.span,
 491                     "GATs in trait object shouldn't have been considered",
 492                 );
 493                 return Err(SelectionError::Unimplemented);
 494             }
 495
 496             // This maybe belongs in wf, but that can't (doesn't) handle
 497             // higher-ranked things.
 498             // Prevent, e.g., `dyn Iterator<Item = str>`.
 499             for bound in self.tcx().bound_item_bounds(assoc_type).transpose_iter() {
 500                 let subst_bound =
 501                     if defs.count() == 0 {
 502                         bound.subst(tcx, trait_predicate.trait_ref.substs)
 503                     } else {
 504                         let mut substs = smallvec::SmallVec::with_capacity(defs.count());
 505                         substs.extend(trait_predicate.trait_ref.substs.iter());
 506                         let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
 507                             smallvec::SmallVec::with_capacity(
 508                                 bound.0.kind().bound_vars().len() + defs.count(),
 509                             );
 510                         bound_vars.extend(bound.0.kind().bound_vars().into_iter());
 511                         InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
 512                             .kind
 513                         {
 514                             GenericParamDefKind::Type { .. } => {
 515                                 let kind = ty::BoundTyKind::Param(param.name);
 516                                 let bound_var = ty::BoundVariableKind::Ty(kind);
 517                                 bound_vars.push(bound_var);
 518                                 tcx.mk_ty(ty::Bound(
 519                                     ty::INNERMOST,
 520                                     ty::BoundTy {
 521                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 522                                         kind,
 523                                     },
 524                                 ))
 525                                 .into()
 526                             }
 527                             GenericParamDefKind::Lifetime => {
 528                                 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
 529                                 let bound_var = ty::BoundVariableKind::Region(kind);
 530                                 bound_vars.push(bound_var);
 531                                 tcx.mk_region(ty::ReLateBound(
 532                                     ty::INNERMOST,
 533                                     ty::BoundRegion {
 534                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 535                                         kind,
 536                                     },
 537                                 ))
 538                                 .into()
 539                             }
 540                             GenericParamDefKind::Const { .. } => {
 541                                 let bound_var = ty::BoundVariableKind::Const;
 542                                 bound_vars.push(bound_var);
 543                                 tcx.mk_const(ty::ConstS {
 544                                     ty: tcx.type_of(param.def_id),
 545                                     kind: ty::ConstKind::Bound(
 546                                         ty::INNERMOST,
 547                                         ty::BoundVar::from_usize(bound_vars.len() - 1),
 548                                     ),
 549                                 })
 550                                 .into()
 551                             }
 552                         });
 553                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 554                         let assoc_ty_substs = tcx.intern_substs(&substs);
 555
 556                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 557                         let bound =
 558                             EarlyBinder(bound.0.kind().skip_binder()).subst(tcx, assoc_ty_substs);
 559                         tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
 560                     };
 561                 let normalized_bound = normalize_with_depth_to(
 562                     self,
 563                     obligation.param_env,
 564                     obligation.cause.clone(),
 565                     obligation.recursion_depth + 1,
 566                     subst_bound,
 567                     &mut nested,
 568                 );
 569                 nested.push(Obligation::new(
 570                     obligation.cause.clone(),
 571                     obligation.param_env,
 572                     normalized_bound,
 573                 ));
 574             }
 575         }
 576
 577         debug!(?nested, "object nested obligations");
 578
 579         let vtable_base = super::super::vtable_trait_first_method_offset(
 580             tcx,
 581             (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
 582         );
 583
 584         Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
 585     }
 586
 587     fn confirm_fn_pointer_candidate(
 588         &mut self,
 589         obligation: &TraitObligation<'tcx>,
 590     ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 591     {
 592         debug!(?obligation, "confirm_fn_pointer_candidate");
 593
 594         // Okay to skip binder; it is reintroduced below.
 595         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 596         let sig = self_ty.fn_sig(self.tcx());
 597         let trait_ref = closure_trait_ref_and_return_type(
 598             self.tcx(),
 599             obligation.predicate.def_id(),
 600             self_ty,
 601             sig,
 602             util::TupleArgumentsFlag::Yes,
 603         )
 604         .map_bound(|(trait_ref, _)| trait_ref);
 605
 606         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 607         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
 608     }
 609
 610     fn confirm_trait_alias_candidate(
 611         &mut self,
 612         obligation: &TraitObligation<'tcx>,
 613         alias_def_id: DefId,
 614     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
 615         debug!(?obligation, ?alias_def_id, "confirm_trait_alias_candidate");
 616
 617         self.infcx.commit_unconditionally(|_| {
 618             let predicate = self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
 619             let trait_ref = predicate.trait_ref;
 620             let trait_def_id = trait_ref.def_id;
 621             let substs = trait_ref.substs;
 622
 623             let trait_obligations = self.impl_or_trait_obligations(
 624                 &obligation.cause,
 625                 obligation.recursion_depth,
 626                 obligation.param_env,
 627                 trait_def_id,
 628                 &substs,
 629                 obligation.predicate,
 630             );
 631
 632             debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
 633
 634             ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
 635         })
 636     }
 637
 638     fn confirm_generator_candidate(
 639         &mut self,
 640         obligation: &TraitObligation<'tcx>,
 641     ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 642     {
 643         // Okay to skip binder because the substs on generator types never
 644         // touch bound regions, they just capture the in-scope
 645         // type/region parameters.
 646         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 647         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 648             bug!("closure candidate for non-closure {:?}", obligation);
 649         };
 650
 651         debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
 652
 653         let trait_ref = self.generator_trait_ref_unnormalized(obligation, substs);
 654
 655         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 656         debug!(?trait_ref, ?nested, "generator candidate obligations");
 657
 658         Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
 659     }
 660
 661     #[instrument(skip(self), level = "debug")]
 662     fn confirm_closure_candidate(
 663         &mut self,
 664         obligation: &TraitObligation<'tcx>,
 665     ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 666         let kind = self
 667             .tcx()
 668             .fn_trait_kind_from_lang_item(obligation.predicate.def_id())
 669             .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
 670
 671         // Okay to skip binder because the substs on closure types never
 672         // touch bound regions, they just capture the in-scope
 673         // type/region parameters.
 674         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 675         let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
 676             bug!("closure candidate for non-closure {:?}", obligation);
 677         };
 678
 679         let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
 680         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 681
 682         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 683
 684         // FIXME: Chalk
 685
 686         if !self.tcx().sess.opts.debugging_opts.chalk {
 687             nested.push(Obligation::new(
 688                 obligation.cause.clone(),
 689                 obligation.param_env,
 690                 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind))
 691                     .to_predicate(self.tcx()),
 692             ));
 693         }
 694
 695         Ok(ImplSourceClosureData { closure_def_id, substs, nested })
 696     }
 697
 698     /// In the case of closure types and fn pointers,
 699     /// we currently treat the input type parameters on the trait as
 700     /// outputs. This means that when we have a match we have only
 701     /// considered the self type, so we have to go back and make sure
 702     /// to relate the argument types too. This is kind of wrong, but
 703     /// since we control the full set of impls, also not that wrong,
 704     /// and it DOES yield better error messages (since we don't report
 705     /// errors as if there is no applicable impl, but rather report
 706     /// errors are about mismatched argument types.
 707     ///
 708     /// Here is an example. Imagine we have a closure expression
 709     /// and we desugared it so that the type of the expression is
 710     /// `Closure`, and `Closure` expects `i32` as argument. Then it
 711     /// is "as if" the compiler generated this impl:
 712     /// ```ignore (illustrative)
 713     /// impl Fn(i32) for Closure { ... }
 714     /// ```
 715     /// Now imagine our obligation is `Closure: Fn(usize)`. So far
 716     /// we have matched the self type `Closure`. At this point we'll
 717     /// compare the `i32` to `usize` and generate an error.
 718     ///
 719     /// Note that this checking occurs *after* the impl has selected,
 720     /// because these output type parameters should not affect the
 721     /// selection of the impl. Therefore, if there is a mismatch, we
 722     /// report an error to the user.
 723     #[instrument(skip(self), level = "trace")]
 724     fn confirm_poly_trait_refs(
 725         &mut self,
 726         obligation: &TraitObligation<'tcx>,
 727         expected_trait_ref: ty::PolyTraitRef<'tcx>,
 728     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 729         let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
 730         // Normalize the obligation and expected trait refs together, because why not
 731         let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
 732             ensure_sufficient_stack(|| {
 733                 self.infcx.commit_unconditionally(|_| {
 734                     normalize_with_depth(
 735                         self,
 736                         obligation.param_env,
 737                         obligation.cause.clone(),
 738                         obligation.recursion_depth + 1,
 739                         (obligation_trait_ref, expected_trait_ref),
 740                     )
 741                 })
 742             });
 743
 744         self.infcx
 745             .at(&obligation.cause, obligation.param_env)
 746             .sup(obligation_trait_ref, expected_trait_ref)
 747             .map(|InferOk { mut obligations, .. }| {
 748                 obligations.extend(nested);
 749                 obligations
 750             })
 751             .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
 752     }
 753
 754     fn confirm_trait_upcasting_unsize_candidate(
 755         &mut self,
 756         obligation: &TraitObligation<'tcx>,
 757         idx: usize,
 758     ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 759     {
 760         let tcx = self.tcx();
 761
 762         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 763         // regions here. See the comment there for more details.
 764         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 765         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 766         let target = self.infcx.shallow_resolve(target);
 767
 768         debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
 769
 770         let mut nested = vec![];
 771         let source_trait_ref;
 772         let upcast_trait_ref;
 773         match (source.kind(), target.kind()) {
 774             // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
 775             (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
 776                 // See `assemble_candidates_for_unsizing` for more info.
 777                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 778                 let principal_a = data_a.principal().unwrap();
 779                 source_trait_ref = principal_a.with_self_ty(tcx, source);
 780                 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
 781                 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
 782                 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
 783                     ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
 784                         tcx, trait_ref,
 785                     ))
 786                 });
 787                 let iter = Some(existential_predicate)
 788                     .into_iter()
 789                     .chain(
 790                         data_a
 791                             .projection_bounds()
 792                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 793                     )
 794                     .chain(
 795                         data_b
 796                             .auto_traits()
 797                             .map(ty::ExistentialPredicate::AutoTrait)
 798                             .map(ty::Binder::dummy),
 799                     );
 800                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 801                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
 802
 803                 // Require that the traits involved in this upcast are **equal**;
 804                 // only the **lifetime bound** is changed.
 805                 let InferOk { obligations, .. } = self
 806                     .infcx
 807                     .at(&obligation.cause, obligation.param_env)
 808                     .sup(target, source_trait)
 809                     .map_err(|_| Unimplemented)?;
 810                 nested.extend(obligations);
 811
 812                 // Register one obligation for 'a: 'b.
 813                 let cause = ObligationCause::new(
 814                     obligation.cause.span,
 815                     obligation.cause.body_id,
 816                     ObjectCastObligation(source, target),
 817                 );
 818                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 819                 nested.push(Obligation::with_depth(
 820                     cause,
 821                     obligation.recursion_depth + 1,
 822                     obligation.param_env,
 823                     obligation.predicate.rebind(outlives).to_predicate(tcx),
 824                 ));
 825             }
 826             _ => bug!(),
 827         };
 828
 829         let vtable_segment_callback = {
 830             let mut vptr_offset = 0;
 831             move |segment| {
 832                 match segment {
 833                     VtblSegment::MetadataDSA => {
 834                         vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
 835                     }
 836                     VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 837                         vptr_offset += util::count_own_vtable_entries(tcx, trait_ref);
 838                         if trait_ref == upcast_trait_ref {
 839                             if emit_vptr {
 840                                 return ControlFlow::Break(Some(vptr_offset));
 841                             } else {
 842                                 return ControlFlow::Break(None);
 843                             }
 844                         }
 845
 846                         if emit_vptr {
 847                             vptr_offset += 1;
 848                         }
 849                     }
 850                 }
 851                 ControlFlow::Continue(())
 852             }
 853         };
 854
 855         let vtable_vptr_slot =
 856             super::super::prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback)
 857                 .unwrap();
 858
 859         Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
 860     }
 861
 862     fn confirm_builtin_unsize_candidate(
 863         &mut self,
 864         obligation: &TraitObligation<'tcx>,
 865     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 866         let tcx = self.tcx();
 867
 868         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 869         // regions here. See the comment there for more details.
 870         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 871         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 872         let target = self.infcx.shallow_resolve(target);
 873
 874         debug!(?source, ?target, "confirm_builtin_unsize_candidate");
 875
 876         let mut nested = vec![];
 877         match (source.kind(), target.kind()) {
 878             // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
 879             (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
 880                 // See `assemble_candidates_for_unsizing` for more info.
 881                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 882                 let iter = data_a
 883                     .principal()
 884                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
 885                     .into_iter()
 886                     .chain(
 887                         data_a
 888                             .projection_bounds()
 889                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 890                     )
 891                     .chain(
 892                         data_b
 893                             .auto_traits()
 894                             .map(ty::ExistentialPredicate::AutoTrait)
 895                             .map(ty::Binder::dummy),
 896                     );
 897                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 898                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
 899
 900                 // Require that the traits involved in this upcast are **equal**;
 901                 // only the **lifetime bound** is changed.
 902                 let InferOk { obligations, .. } = self
 903                     .infcx
 904                     .at(&obligation.cause, obligation.param_env)
 905                     .sup(target, source_trait)
 906                     .map_err(|_| Unimplemented)?;
 907                 nested.extend(obligations);
 908
 909                 // Register one obligation for 'a: 'b.
 910                 let cause = ObligationCause::new(
 911                     obligation.cause.span,
 912                     obligation.cause.body_id,
 913                     ObjectCastObligation(source, target),
 914                 );
 915                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 916                 nested.push(Obligation::with_depth(
 917                     cause,
 918                     obligation.recursion_depth + 1,
 919                     obligation.param_env,
 920                     obligation.predicate.rebind(outlives).to_predicate(tcx),
 921                 ));
 922             }
 923
 924             // `T` -> `Trait`
 925             (_, &ty::Dynamic(ref data, r)) => {
 926                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
 927                 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
 928                     return Err(TraitNotObjectSafe(did));
 929                 }
 930
 931                 let cause = ObligationCause::new(
 932                     obligation.cause.span,
 933                     obligation.cause.body_id,
 934                     ObjectCastObligation(source, target),
 935                 );
 936
 937                 let predicate_to_obligation = |predicate| {
 938                     Obligation::with_depth(
 939                         cause.clone(),
 940                         obligation.recursion_depth + 1,
 941                         obligation.param_env,
 942                         predicate,
 943                     )
 944                 };
 945
 946                 // Create obligations:
 947                 //  - Casting `T` to `Trait`
 948                 //  - For all the various builtin bounds attached to the object cast. (In other
 949                 //  words, if the object type is `Foo + Send`, this would create an obligation for
 950                 //  the `Send` check.)
 951                 //  - Projection predicates
 952                 nested.extend(
 953                     data.iter().map(|predicate| {
 954                         predicate_to_obligation(predicate.with_self_ty(tcx, source))
 955                     }),
 956                 );
 957
 958                 // We can only make objects from sized types.
 959                 let tr = ty::Binder::dummy(ty::TraitRef::new(
 960                     tcx.require_lang_item(LangItem::Sized, None),
 961                     tcx.mk_substs_trait(source, &[]),
 962                 ));
 963                 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
 964
 965                 // If the type is `Foo + 'a`, ensure that the type
 966                 // being cast to `Foo + 'a` outlives `'a`:
 967                 let outlives = ty::OutlivesPredicate(source, r);
 968                 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
 969             }
 970
 971             // `[T; n]` -> `[T]`
 972             (&ty::Array(a, _), &ty::Slice(b)) => {
 973                 let InferOk { obligations, .. } = self
 974                     .infcx
 975                     .at(&obligation.cause, obligation.param_env)
 976                     .eq(b, a)
 977                     .map_err(|_| Unimplemented)?;
 978                 nested.extend(obligations);
 979             }
 980
 981             // `Struct<T>` -> `Struct<U>`
 982             (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
 983                 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
 984                     GenericArgKind::Type(ty) => match ty.kind() {
 985                         ty::Param(p) => Some(p.index),
 986                         _ => None,
 987                     },
 988
 989                     // Lifetimes aren't allowed to change during unsizing.
 990                     GenericArgKind::Lifetime(_) => None,
 991
 992                     GenericArgKind::Const(ct) => match ct.kind() {
 993                         ty::ConstKind::Param(p) => Some(p.index),
 994                         _ => None,
 995                     },
 996                 };
 997
 998                 // FIXME(eddyb) cache this (including computing `unsizing_params`)
 999                 // by putting it in a query; it would only need the `DefId` as it
1000                 // looks at declared field types, not anything substituted.
1001
1002                 // The last field of the structure has to exist and contain type/const parameters.
1003                 let (tail_field, prefix_fields) =
1004                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1005                 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1006
1007                 let mut unsizing_params = GrowableBitSet::new_empty();
1008                 for arg in tail_field_ty.0.walk() {
1009                     if let Some(i) = maybe_unsizing_param_idx(arg) {
1010                         unsizing_params.insert(i);
1011                     }
1012                 }
1013
1014                 // Ensure none of the other fields mention the parameters used
1015                 // in unsizing.
1016                 for field in prefix_fields {
1017                     for arg in tcx.type_of(field.did).walk() {
1018                         if let Some(i) = maybe_unsizing_param_idx(arg) {
1019                             unsizing_params.remove(i);
1020                         }
1021                     }
1022                 }
1023
1024                 if unsizing_params.is_empty() {
1025                     return Err(Unimplemented);
1026                 }
1027
1028                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`.
1029                 let source_tail = tail_field_ty.subst(tcx, substs_a);
1030                 let target_tail = tail_field_ty.subst(tcx, substs_b);
1031
1032                 // Check that the source struct with the target's
1033                 // unsizing parameters is equal to the target.
1034                 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1035                     if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1036                 }));
1037                 let new_struct = tcx.mk_adt(def, substs);
1038                 let InferOk { obligations, .. } = self
1039                     .infcx
1040                     .at(&obligation.cause, obligation.param_env)
1041                     .eq(target, new_struct)
1042                     .map_err(|_| Unimplemented)?;
1043                 nested.extend(obligations);
1044
1045                 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1046                 nested.push(predicate_for_trait_def(
1047                     tcx,
1048                     obligation.param_env,
1049                     obligation.cause.clone(),
1050                     obligation.predicate.def_id(),
1051                     obligation.recursion_depth + 1,
1052                     source_tail,
1053                     &[target_tail.into()],
1054                 ));
1055             }
1056
1057             // `(.., T)` -> `(.., U)`
1058             (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1059                 assert_eq!(tys_a.len(), tys_b.len());
1060
1061                 // The last field of the tuple has to exist.
1062                 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1063                 let &b_last = tys_b.last().unwrap();
1064
1065                 // Check that the source tuple with the target's
1066                 // last element is equal to the target.
1067                 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1068                 let InferOk { obligations, .. } = self
1069                     .infcx
1070                     .at(&obligation.cause, obligation.param_env)
1071                     .eq(target, new_tuple)
1072                     .map_err(|_| Unimplemented)?;
1073                 nested.extend(obligations);
1074
1075                 // Construct the nested `T: Unsize<U>` predicate.
1076                 nested.push(ensure_sufficient_stack(|| {
1077                     predicate_for_trait_def(
1078                         tcx,
1079                         obligation.param_env,
1080                         obligation.cause.clone(),
1081                         obligation.predicate.def_id(),
1082                         obligation.recursion_depth + 1,
1083                         a_last,
1084                         &[b_last.into()],
1085                     )
1086                 }));
1087             }
1088
1089             _ => bug!(),
1090         };
1091
1092         Ok(ImplSourceBuiltinData { nested })
1093     }
1094
1095     fn confirm_const_destruct_candidate(
1096         &mut self,
1097         obligation: &TraitObligation<'tcx>,
1098         impl_def_id: Option<DefId>,
1099     ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1100         // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1101         if !obligation.is_const() {
1102             return Ok(ImplSourceConstDestructData { nested: vec![] });
1103         }
1104
1105         let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1106
1107         let tcx = self.tcx();
1108         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1109
1110         let mut nested = vec![];
1111         let cause = obligation.derived_cause(BuiltinDerivedObligation);
1112
1113         // If we have a custom `impl const Drop`, then
1114         // first check it like a regular impl candidate.
1115         // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1116         if let Some(impl_def_id) = impl_def_id {
1117             let obligations = self.infcx.commit_unconditionally(|_| {
1118                 let mut new_obligation = obligation.clone();
1119                 new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1120                     trait_pred.trait_ref.def_id = drop_trait;
1121                     trait_pred
1122                 });
1123                 let substs = self.rematch_impl(impl_def_id, &new_obligation);
1124                 debug!(?substs, "impl substs");
1125
1126                 let cause = obligation.derived_cause(|derived| {
1127                     ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1128                         derived,
1129                         impl_def_id,
1130                         span: obligation.cause.span,
1131                     }))
1132                 });
1133                 ensure_sufficient_stack(|| {
1134                     self.vtable_impl(
1135                         impl_def_id,
1136                         substs,
1137                         &cause,
1138                         new_obligation.recursion_depth + 1,
1139                         new_obligation.param_env,
1140                         obligation.predicate,
1141                     )
1142                 })
1143             });
1144             nested.extend(obligations.nested);
1145         }
1146
1147         // We want to confirm the ADT's fields if we have an ADT
1148         let mut stack = match *self_ty.skip_binder().kind() {
1149             ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1150             _ => vec![self_ty.skip_binder()],
1151         };
1152
1153         while let Some(nested_ty) = stack.pop() {
1154             match *nested_ty.kind() {
1155                 // We know these types are trivially drop
1156                 ty::Bool
1157                 | ty::Char
1158                 | ty::Int(_)
1159                 | ty::Uint(_)
1160                 | ty::Float(_)
1161                 | ty::Infer(ty::IntVar(_))
1162                 | ty::Infer(ty::FloatVar(_))
1163                 | ty::Str
1164                 | ty::RawPtr(_)
1165                 | ty::Ref(..)
1166                 | ty::FnDef(..)
1167                 | ty::FnPtr(_)
1168                 | ty::Never
1169                 | ty::Foreign(_) => {}
1170
1171                 // These types are built-in, so we can fast-track by registering
1172                 // nested predicates for their constituent type(s)
1173                 ty::Array(ty, _) | ty::Slice(ty) => {
1174                     stack.push(ty);
1175                 }
1176                 ty::Tuple(tys) => {
1177                     stack.extend(tys.iter());
1178                 }
1179                 ty::Closure(_, substs) => {
1180                     stack.push(substs.as_closure().tupled_upvars_ty());
1181                 }
1182                 ty::Generator(_, substs, _) => {
1183                     let generator = substs.as_generator();
1184                     stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1185                 }
1186                 ty::GeneratorWitness(tys) => {
1187                     stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1188                 }
1189
1190                 // If we have a projection type, make sure to normalize it so we replace it
1191                 // with a fresh infer variable
1192                 ty::Projection(..) => {
1193                     self.infcx.commit_unconditionally(|_| {
1194                         let predicate = normalize_with_depth_to(
1195                             self,
1196                             obligation.param_env,
1197                             cause.clone(),
1198                             obligation.recursion_depth + 1,
1199                             self_ty
1200                                 .rebind(ty::TraitPredicate {
1201                                     trait_ref: ty::TraitRef {
1202                                         def_id: self
1203                                             .tcx()
1204                                             .require_lang_item(LangItem::Destruct, None),
1205                                         substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1206                                     },
1207                                     constness: ty::BoundConstness::ConstIfConst,
1208                                     polarity: ty::ImplPolarity::Positive,
1209                                 })
1210                                 .to_predicate(tcx),
1211                             &mut nested,
1212                         );
1213
1214                         nested.push(Obligation::with_depth(
1215                             cause.clone(),
1216                             obligation.recursion_depth + 1,
1217                             obligation.param_env,
1218                             predicate,
1219                         ));
1220                     });
1221                 }
1222
1223                 // If we have any other type (e.g. an ADT), just register a nested obligation
1224                 // since it's either not `const Drop` (and we raise an error during selection),
1225                 // or it's an ADT (and we need to check for a custom impl during selection)
1226                 _ => {
1227                     let predicate = self_ty
1228                         .rebind(ty::TraitPredicate {
1229                             trait_ref: ty::TraitRef {
1230                                 def_id: self.tcx().require_lang_item(LangItem::Destruct, None),
1231                                 substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1232                             },
1233                             constness: ty::BoundConstness::ConstIfConst,
1234                             polarity: ty::ImplPolarity::Positive,
1235                         })
1236                         .to_predicate(tcx);
1237
1238                     nested.push(Obligation::with_depth(
1239                         cause.clone(),
1240                         obligation.recursion_depth + 1,
1241                         obligation.param_env,
1242                         predicate,
1243                     ));
1244                 }
1245             }
1246         }
1247
1248         Ok(ImplSourceConstDestructData { nested })
1249     }
1250 }