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::{self, GenericParamDefKind, Ty, TyCtxt};
  15 use rustc_middle::ty::{GenericArg, GenericArgKind, InternalSubsts, SubstsRef};
  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 impl_src = match candidate {
  46             BuiltinCandidate { has_nested } => {
  47                 let data = self.confirm_builtin_candidate(obligation, has_nested);
  48                 ImplSource::Builtin(data)
  49             }
  50
  51             TransmutabilityCandidate => {
  52                 let data = self.confirm_transmutability_candidate(obligation)?;
  53                 ImplSource::Builtin(data)
  54             }
  55
  56             ParamCandidate(param) => {
  57                 let obligations =
  58                     self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
  59                 ImplSource::Param(obligations, param.skip_binder().constness)
  60             }
  61
  62             ImplCandidate(impl_def_id) => {
  63                 ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id))
  64             }
  65
  66             AutoImplCandidate(trait_def_id) => {
  67                 let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
  68                 ImplSource::AutoImpl(data)
  69             }
  70
  71             ProjectionCandidate(idx, constness) => {
  72                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
  73                 ImplSource::Param(obligations, constness)
  74             }
  75
  76             ObjectCandidate(idx) => {
  77                 let data = self.confirm_object_candidate(obligation, idx)?;
  78                 ImplSource::Object(data)
  79             }
  80
  81             ClosureCandidate => {
  82                 let vtable_closure = self.confirm_closure_candidate(obligation)?;
  83                 ImplSource::Closure(vtable_closure)
  84             }
  85
  86             GeneratorCandidate => {
  87                 let vtable_generator = self.confirm_generator_candidate(obligation)?;
  88                 ImplSource::Generator(vtable_generator)
  89             }
  90
  91             FnPointerCandidate { .. } => {
  92                 let data = self.confirm_fn_pointer_candidate(obligation)?;
  93                 ImplSource::FnPointer(data)
  94             }
  95
  96             DiscriminantKindCandidate => {
  97                 ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
  98             }
  99
 100             PointeeCandidate => ImplSource::Pointee(ImplSourcePointeeData),
 101
 102             TraitAliasCandidate(alias_def_id) => {
 103                 let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
 104                 ImplSource::TraitAlias(data)
 105             }
 106
 107             BuiltinObjectCandidate => {
 108                 // This indicates something like `Trait + Send: Send`. In this case, we know that
 109                 // this holds because that's what the object type is telling us, and there's really
 110                 // no additional obligations to prove and no types in particular to unify, etc.
 111                 ImplSource::Param(Vec::new(), ty::BoundConstness::NotConst)
 112             }
 113
 114             BuiltinUnsizeCandidate => {
 115                 let data = self.confirm_builtin_unsize_candidate(obligation)?;
 116                 ImplSource::Builtin(data)
 117             }
 118
 119             TraitUpcastingUnsizeCandidate(idx) => {
 120                 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
 121                 ImplSource::TraitUpcasting(data)
 122             }
 123
 124             ConstDestructCandidate(def_id) => {
 125                 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
 126                 ImplSource::ConstDestruct(data)
 127             }
 128
 129             TupleCandidate => ImplSource::Tuple,
 130         };
 131
 132         if !obligation.predicate.is_const_if_const() {
 133             // normalize nested predicates according to parent predicate's constness.
 134             impl_src = impl_src.map(|mut o| {
 135                 o.predicate = o.predicate.without_const(self.tcx());
 136                 o
 137             });
 138         }
 139
 140         Ok(impl_src)
 141     }
 142
 143     fn confirm_projection_candidate(
 144         &mut self,
 145         obligation: &TraitObligation<'tcx>,
 146         idx: usize,
 147     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 148         let tcx = self.tcx();
 149
 150         let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
 151         let placeholder_trait_predicate =
 152             self.infcx().replace_bound_vars_with_placeholders(trait_predicate).trait_ref;
 153         let placeholder_self_ty = placeholder_trait_predicate.self_ty();
 154         let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
 155         let (def_id, substs) = match *placeholder_self_ty.kind() {
 156             ty::Projection(proj) => (proj.item_def_id, proj.substs),
 157             ty::Opaque(def_id, substs) => (def_id, substs),
 158             _ => bug!("projection candidate for unexpected type: {:?}", placeholder_self_ty),
 159         };
 160
 161         let candidate_predicate =
 162             tcx.bound_item_bounds(def_id).map_bound(|i| i[idx]).subst(tcx, substs);
 163         let candidate = candidate_predicate
 164             .to_opt_poly_trait_pred()
 165             .expect("projection candidate is not a trait predicate")
 166             .map_bound(|t| t.trait_ref);
 167         let mut obligations = Vec::new();
 168         let candidate = normalize_with_depth_to(
 169             self,
 170             obligation.param_env,
 171             obligation.cause.clone(),
 172             obligation.recursion_depth + 1,
 173             candidate,
 174             &mut obligations,
 175         );
 176
 177         obligations.extend(self.infcx.commit_if_ok(|_| {
 178             self.infcx
 179                 .at(&obligation.cause, obligation.param_env)
 180                 .sup(placeholder_trait_predicate, candidate)
 181                 .map(|InferOk { obligations, .. }| obligations)
 182                 .map_err(|_| Unimplemented)
 183         })?);
 184
 185         if let ty::Projection(..) = placeholder_self_ty.kind() {
 186             let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates;
 187             debug!(?predicates, "projection predicates");
 188             for predicate in predicates {
 189                 let normalized = normalize_with_depth_to(
 190                     self,
 191                     obligation.param_env,
 192                     obligation.cause.clone(),
 193                     obligation.recursion_depth + 1,
 194                     predicate,
 195                     &mut obligations,
 196                 );
 197                 obligations.push(Obligation::with_depth(
 198                     obligation.cause.clone(),
 199                     obligation.recursion_depth + 1,
 200                     obligation.param_env,
 201                     normalized,
 202                 ));
 203             }
 204         }
 205
 206         Ok(obligations)
 207     }
 208
 209     fn confirm_param_candidate(
 210         &mut self,
 211         obligation: &TraitObligation<'tcx>,
 212         param: ty::PolyTraitRef<'tcx>,
 213     ) -> Vec<PredicateObligation<'tcx>> {
 214         debug!(?obligation, ?param, "confirm_param_candidate");
 215
 216         // During evaluation, we already checked that this
 217         // where-clause trait-ref could be unified with the obligation
 218         // trait-ref. Repeat that unification now without any
 219         // transactional boundary; it should not fail.
 220         match self.match_where_clause_trait_ref(obligation, param) {
 221             Ok(obligations) => obligations,
 222             Err(()) => {
 223                 bug!(
 224                     "Where clause `{:?}` was applicable to `{:?}` but now is not",
 225                     param,
 226                     obligation
 227                 );
 228             }
 229         }
 230     }
 231
 232     fn confirm_builtin_candidate(
 233         &mut self,
 234         obligation: &TraitObligation<'tcx>,
 235         has_nested: bool,
 236     ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
 237         debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
 238
 239         let lang_items = self.tcx().lang_items();
 240         let obligations = if has_nested {
 241             let trait_def = obligation.predicate.def_id();
 242             let conditions = if Some(trait_def) == lang_items.sized_trait() {
 243                 self.sized_conditions(obligation)
 244             } else if Some(trait_def) == lang_items.copy_trait() {
 245                 self.copy_clone_conditions(obligation)
 246             } else if Some(trait_def) == lang_items.clone_trait() {
 247                 self.copy_clone_conditions(obligation)
 248             } else {
 249                 bug!("unexpected builtin trait {:?}", trait_def)
 250             };
 251             let BuiltinImplConditions::Where(nested) = conditions else {
 252                 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
 253             };
 254
 255             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 256             ensure_sufficient_stack(|| {
 257                 self.collect_predicates_for_types(
 258                     obligation.param_env,
 259                     cause,
 260                     obligation.recursion_depth + 1,
 261                     trait_def,
 262                     nested,
 263                 )
 264             })
 265         } else {
 266             vec![]
 267         };
 268
 269         debug!(?obligations);
 270
 271         ImplSourceBuiltinData { nested: obligations }
 272     }
 273
 274     fn confirm_transmutability_candidate(
 275         &mut self,
 276         obligation: &TraitObligation<'tcx>,
 277     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 278         debug!(?obligation, "confirm_transmutability_candidate");
 279
 280         let predicate = obligation.predicate;
 281
 282         let type_at = |i| predicate.map_bound(|p| p.trait_ref.substs.type_at(i));
 283         let const_at = |i| predicate.skip_binder().trait_ref.substs.const_at(i);
 284
 285         let src_and_dst = predicate.map_bound(|p| rustc_transmute::Types {
 286             dst: p.trait_ref.substs.type_at(0),
 287             src: p.trait_ref.substs.type_at(1),
 288         });
 289
 290         let scope = type_at(2).skip_binder();
 291
 292         let assume =
 293             rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3));
 294
 295         let cause = obligation.cause.clone();
 296
 297         let mut transmute_env = rustc_transmute::TransmuteTypeEnv::new(self.infcx);
 298
 299         let maybe_transmutable = transmute_env.is_transmutable(cause, src_and_dst, scope, assume);
 300
 301         use rustc_transmute::Answer;
 302
 303         match maybe_transmutable {
 304             Answer::Yes => Ok(ImplSourceBuiltinData { nested: vec![] }),
 305             _ => Err(Unimplemented),
 306         }
 307     }
 308
 309     /// This handles the case where an `auto trait Foo` impl is being used.
 310     /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
 311     ///
 312     /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
 313     /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
 314     fn confirm_auto_impl_candidate(
 315         &mut self,
 316         obligation: &TraitObligation<'tcx>,
 317         trait_def_id: DefId,
 318     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 319         debug!(?obligation, ?trait_def_id, "confirm_auto_impl_candidate");
 320
 321         let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
 322         let types = self.constituent_types_for_ty(self_ty);
 323         self.vtable_auto_impl(obligation, trait_def_id, types)
 324     }
 325
 326     /// See `confirm_auto_impl_candidate`.
 327     fn vtable_auto_impl(
 328         &mut self,
 329         obligation: &TraitObligation<'tcx>,
 330         trait_def_id: DefId,
 331         nested: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
 332     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 333         debug!(?nested, "vtable_auto_impl");
 334         ensure_sufficient_stack(|| {
 335             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 336
 337             let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
 338             let trait_ref = self.infcx.replace_bound_vars_with_placeholders(poly_trait_ref);
 339             let trait_obligations: Vec<PredicateObligation<'_>> = self.impl_or_trait_obligations(
 340                 &cause,
 341                 obligation.recursion_depth + 1,
 342                 obligation.param_env,
 343                 trait_def_id,
 344                 &trait_ref.substs,
 345                 obligation.predicate,
 346             );
 347
 348             let mut obligations = self.collect_predicates_for_types(
 349                 obligation.param_env,
 350                 cause,
 351                 obligation.recursion_depth + 1,
 352                 trait_def_id,
 353                 nested,
 354             );
 355
 356             // Adds the predicates from the trait.  Note that this contains a `Self: Trait`
 357             // predicate as usual.  It won't have any effect since auto traits are coinductive.
 358             obligations.extend(trait_obligations);
 359
 360             debug!(?obligations, "vtable_auto_impl");
 361
 362             ImplSourceAutoImplData { trait_def_id, nested: obligations }
 363         })
 364     }
 365
 366     fn confirm_impl_candidate(
 367         &mut self,
 368         obligation: &TraitObligation<'tcx>,
 369         impl_def_id: DefId,
 370     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 371         debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");
 372
 373         // First, create the substitutions by matching the impl again,
 374         // this time not in a probe.
 375         let substs = self.rematch_impl(impl_def_id, obligation);
 376         debug!(?substs, "impl substs");
 377         ensure_sufficient_stack(|| {
 378             self.vtable_impl(
 379                 impl_def_id,
 380                 substs,
 381                 &obligation.cause,
 382                 obligation.recursion_depth + 1,
 383                 obligation.param_env,
 384                 obligation.predicate,
 385             )
 386         })
 387     }
 388
 389     fn vtable_impl(
 390         &mut self,
 391         impl_def_id: DefId,
 392         substs: Normalized<'tcx, SubstsRef<'tcx>>,
 393         cause: &ObligationCause<'tcx>,
 394         recursion_depth: usize,
 395         param_env: ty::ParamEnv<'tcx>,
 396         parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
 397     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 398         debug!(?impl_def_id, ?substs, ?recursion_depth, "vtable_impl");
 399
 400         let mut impl_obligations = self.impl_or_trait_obligations(
 401             cause,
 402             recursion_depth,
 403             param_env,
 404             impl_def_id,
 405             &substs.value,
 406             parent_trait_pred,
 407         );
 408
 409         debug!(?impl_obligations, "vtable_impl");
 410
 411         // Because of RFC447, the impl-trait-ref and obligations
 412         // are sufficient to determine the impl substs, without
 413         // relying on projections in the impl-trait-ref.
 414         //
 415         // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
 416         impl_obligations.extend(substs.obligations);
 417
 418         ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations }
 419     }
 420
 421     fn confirm_object_candidate(
 422         &mut self,
 423         obligation: &TraitObligation<'tcx>,
 424         index: usize,
 425     ) -> Result<ImplSourceObjectData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 426         let tcx = self.tcx();
 427         debug!(?obligation, ?index, "confirm_object_candidate");
 428
 429         let trait_predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
 430         let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
 431         let obligation_trait_ref = ty::Binder::dummy(trait_predicate.trait_ref);
 432         let ty::Dynamic(data, ..) = *self_ty.kind() else {
 433             span_bug!(obligation.cause.span, "object candidate with non-object");
 434         };
 435
 436         let object_trait_ref = data.principal().unwrap_or_else(|| {
 437             span_bug!(obligation.cause.span, "object candidate with no principal")
 438         });
 439         let object_trait_ref = self.infcx.replace_bound_vars_with_fresh_vars(
 440             obligation.cause.span,
 441             HigherRankedType,
 442             object_trait_ref,
 443         );
 444         let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);
 445
 446         let mut nested = vec![];
 447
 448         let mut supertraits = util::supertraits(tcx, ty::Binder::dummy(object_trait_ref));
 449         let unnormalized_upcast_trait_ref =
 450             supertraits.nth(index).expect("supertraits iterator no longer has as many elements");
 451
 452         let upcast_trait_ref = normalize_with_depth_to(
 453             self,
 454             obligation.param_env,
 455             obligation.cause.clone(),
 456             obligation.recursion_depth + 1,
 457             unnormalized_upcast_trait_ref,
 458             &mut nested,
 459         );
 460
 461         nested.extend(self.infcx.commit_if_ok(|_| {
 462             self.infcx
 463                 .at(&obligation.cause, obligation.param_env)
 464                 .sup(obligation_trait_ref, upcast_trait_ref)
 465                 .map(|InferOk { obligations, .. }| obligations)
 466                 .map_err(|_| Unimplemented)
 467         })?);
 468
 469         // Check supertraits hold. This is so that their associated type bounds
 470         // will be checked in the code below.
 471         for super_trait in tcx
 472             .super_predicates_of(trait_predicate.def_id())
 473             .instantiate(tcx, trait_predicate.trait_ref.substs)
 474             .predicates
 475             .into_iter()
 476         {
 477             let normalized_super_trait = normalize_with_depth_to(
 478                 self,
 479                 obligation.param_env,
 480                 obligation.cause.clone(),
 481                 obligation.recursion_depth + 1,
 482                 super_trait,
 483                 &mut nested,
 484             );
 485             nested.push(Obligation::new(
 486                 obligation.cause.clone(),
 487                 obligation.param_env,
 488                 normalized_super_trait,
 489             ));
 490         }
 491
 492         let assoc_types: Vec<_> = tcx
 493             .associated_items(trait_predicate.def_id())
 494             .in_definition_order()
 495             .filter_map(
 496                 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
 497             )
 498             .collect();
 499
 500         for assoc_type in assoc_types {
 501             let defs: &ty::Generics = tcx.generics_of(assoc_type);
 502
 503             if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
 504                 tcx.sess.delay_span_bug(
 505                     obligation.cause.span,
 506                     "GATs in trait object shouldn't have been considered",
 507                 );
 508                 return Err(SelectionError::Unimplemented);
 509             }
 510
 511             // This maybe belongs in wf, but that can't (doesn't) handle
 512             // higher-ranked things.
 513             // Prevent, e.g., `dyn Iterator<Item = str>`.
 514             for bound in self.tcx().bound_item_bounds(assoc_type).transpose_iter() {
 515                 let subst_bound =
 516                     if defs.count() == 0 {
 517                         bound.subst(tcx, trait_predicate.trait_ref.substs)
 518                     } else {
 519                         let mut substs = smallvec::SmallVec::with_capacity(defs.count());
 520                         substs.extend(trait_predicate.trait_ref.substs.iter());
 521                         let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
 522                             smallvec::SmallVec::with_capacity(
 523                                 bound.0.kind().bound_vars().len() + defs.count(),
 524                             );
 525                         bound_vars.extend(bound.0.kind().bound_vars().into_iter());
 526                         InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
 527                             .kind
 528                         {
 529                             GenericParamDefKind::Type { .. } => {
 530                                 let kind = ty::BoundTyKind::Param(param.name);
 531                                 let bound_var = ty::BoundVariableKind::Ty(kind);
 532                                 bound_vars.push(bound_var);
 533                                 tcx.mk_ty(ty::Bound(
 534                                     ty::INNERMOST,
 535                                     ty::BoundTy {
 536                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 537                                         kind,
 538                                     },
 539                                 ))
 540                                 .into()
 541                             }
 542                             GenericParamDefKind::Lifetime => {
 543                                 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
 544                                 let bound_var = ty::BoundVariableKind::Region(kind);
 545                                 bound_vars.push(bound_var);
 546                                 tcx.mk_region(ty::ReLateBound(
 547                                     ty::INNERMOST,
 548                                     ty::BoundRegion {
 549                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 550                                         kind,
 551                                     },
 552                                 ))
 553                                 .into()
 554                             }
 555                             GenericParamDefKind::Const { .. } => {
 556                                 let bound_var = ty::BoundVariableKind::Const;
 557                                 bound_vars.push(bound_var);
 558                                 tcx.mk_const(ty::ConstS {
 559                                     ty: tcx.type_of(param.def_id),
 560                                     kind: ty::ConstKind::Bound(
 561                                         ty::INNERMOST,
 562                                         ty::BoundVar::from_usize(bound_vars.len() - 1),
 563                                     ),
 564                                 })
 565                                 .into()
 566                             }
 567                         });
 568                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 569                         let assoc_ty_substs = tcx.intern_substs(&substs);
 570
 571                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 572                         let bound =
 573                             bound.map_bound(|b| b.kind().skip_binder()).subst(tcx, assoc_ty_substs);
 574                         tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
 575                     };
 576                 let normalized_bound = normalize_with_depth_to(
 577                     self,
 578                     obligation.param_env,
 579                     obligation.cause.clone(),
 580                     obligation.recursion_depth + 1,
 581                     subst_bound,
 582                     &mut nested,
 583                 );
 584                 nested.push(Obligation::new(
 585                     obligation.cause.clone(),
 586                     obligation.param_env,
 587                     normalized_bound,
 588                 ));
 589             }
 590         }
 591
 592         debug!(?nested, "object nested obligations");
 593
 594         let vtable_base = super::super::vtable_trait_first_method_offset(
 595             tcx,
 596             (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
 597         );
 598
 599         Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
 600     }
 601
 602     fn confirm_fn_pointer_candidate(
 603         &mut self,
 604         obligation: &TraitObligation<'tcx>,
 605     ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 606     {
 607         debug!(?obligation, "confirm_fn_pointer_candidate");
 608
 609         // Okay to skip binder; it is reintroduced below.
 610         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 611         let sig = self_ty.fn_sig(self.tcx());
 612         let trait_ref = closure_trait_ref_and_return_type(
 613             self.tcx(),
 614             obligation.predicate.def_id(),
 615             self_ty,
 616             sig,
 617             util::TupleArgumentsFlag::Yes,
 618         )
 619         .map_bound(|(trait_ref, _)| trait_ref);
 620
 621         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 622
 623         // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
 624         let cause = obligation.derived_cause(BuiltinDerivedObligation);
 625         // The binder on the Fn obligation is "less" important than the one on
 626         // the signature, as evidenced by how we treat it during projection.
 627         // The safe thing to do here is to liberate it, though, which should
 628         // have no worse effect than skipping the binder here.
 629         let liberated_fn_ty =
 630             self.infcx.replace_bound_vars_with_placeholders(obligation.predicate.rebind(self_ty));
 631         let output_ty = self
 632             .infcx
 633             .replace_bound_vars_with_placeholders(liberated_fn_ty.fn_sig(self.tcx()).output());
 634         let output_ty = normalize_with_depth_to(
 635             self,
 636             obligation.param_env,
 637             cause.clone(),
 638             obligation.recursion_depth,
 639             output_ty,
 640             &mut nested,
 641         );
 642         let tr = ty::Binder::dummy(ty::TraitRef::new(
 643             self.tcx().require_lang_item(LangItem::Sized, None),
 644             self.tcx().mk_substs_trait(output_ty, &[]),
 645         ));
 646         nested.push(Obligation::new(
 647             cause,
 648             obligation.param_env,
 649             tr.to_poly_trait_predicate().to_predicate(self.tcx()),
 650         ));
 651
 652         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
 653     }
 654
 655     fn confirm_trait_alias_candidate(
 656         &mut self,
 657         obligation: &TraitObligation<'tcx>,
 658         alias_def_id: DefId,
 659     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
 660         debug!(?obligation, ?alias_def_id, "confirm_trait_alias_candidate");
 661
 662         let predicate = self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
 663         let trait_ref = predicate.trait_ref;
 664         let trait_def_id = trait_ref.def_id;
 665         let substs = trait_ref.substs;
 666
 667         let trait_obligations = self.impl_or_trait_obligations(
 668             &obligation.cause,
 669             obligation.recursion_depth,
 670             obligation.param_env,
 671             trait_def_id,
 672             &substs,
 673             obligation.predicate,
 674         );
 675
 676         debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
 677
 678         ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
 679     }
 680
 681     fn confirm_generator_candidate(
 682         &mut self,
 683         obligation: &TraitObligation<'tcx>,
 684     ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 685     {
 686         // Okay to skip binder because the substs on generator types never
 687         // touch bound regions, they just capture the in-scope
 688         // type/region parameters.
 689         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 690         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 691             bug!("closure candidate for non-closure {:?}", obligation);
 692         };
 693
 694         debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
 695
 696         let trait_ref = self.generator_trait_ref_unnormalized(obligation, substs);
 697
 698         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 699         debug!(?trait_ref, ?nested, "generator candidate obligations");
 700
 701         Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
 702     }
 703
 704     #[instrument(skip(self), level = "debug")]
 705     fn confirm_closure_candidate(
 706         &mut self,
 707         obligation: &TraitObligation<'tcx>,
 708     ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 709         let kind = self
 710             .tcx()
 711             .fn_trait_kind_from_lang_item(obligation.predicate.def_id())
 712             .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
 713
 714         // Okay to skip binder because the substs on closure types never
 715         // touch bound regions, they just capture the in-scope
 716         // type/region parameters.
 717         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 718         let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
 719             bug!("closure candidate for non-closure {:?}", obligation);
 720         };
 721
 722         let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
 723         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 724
 725         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 726
 727         // FIXME: Chalk
 728
 729         if !self.tcx().sess.opts.unstable_opts.chalk {
 730             nested.push(Obligation::new(
 731                 obligation.cause.clone(),
 732                 obligation.param_env,
 733                 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind))
 734                     .to_predicate(self.tcx()),
 735             ));
 736         }
 737
 738         Ok(ImplSourceClosureData { closure_def_id, substs, nested })
 739     }
 740
 741     /// In the case of closure types and fn pointers,
 742     /// we currently treat the input type parameters on the trait as
 743     /// outputs. This means that when we have a match we have only
 744     /// considered the self type, so we have to go back and make sure
 745     /// to relate the argument types too. This is kind of wrong, but
 746     /// since we control the full set of impls, also not that wrong,
 747     /// and it DOES yield better error messages (since we don't report
 748     /// errors as if there is no applicable impl, but rather report
 749     /// errors are about mismatched argument types.
 750     ///
 751     /// Here is an example. Imagine we have a closure expression
 752     /// and we desugared it so that the type of the expression is
 753     /// `Closure`, and `Closure` expects `i32` as argument. Then it
 754     /// is "as if" the compiler generated this impl:
 755     /// ```ignore (illustrative)
 756     /// impl Fn(i32) for Closure { ... }
 757     /// ```
 758     /// Now imagine our obligation is `Closure: Fn(usize)`. So far
 759     /// we have matched the self type `Closure`. At this point we'll
 760     /// compare the `i32` to `usize` and generate an error.
 761     ///
 762     /// Note that this checking occurs *after* the impl has selected,
 763     /// because these output type parameters should not affect the
 764     /// selection of the impl. Therefore, if there is a mismatch, we
 765     /// report an error to the user.
 766     #[instrument(skip(self), level = "trace")]
 767     fn confirm_poly_trait_refs(
 768         &mut self,
 769         obligation: &TraitObligation<'tcx>,
 770         expected_trait_ref: ty::PolyTraitRef<'tcx>,
 771     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 772         let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
 773         // Normalize the obligation and expected trait refs together, because why not
 774         let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
 775             ensure_sufficient_stack(|| {
 776                 normalize_with_depth(
 777                     self,
 778                     obligation.param_env,
 779                     obligation.cause.clone(),
 780                     obligation.recursion_depth + 1,
 781                     (obligation_trait_ref, expected_trait_ref),
 782                 )
 783             });
 784
 785         self.infcx
 786             .at(&obligation.cause, obligation.param_env)
 787             .sup(obligation_trait_ref, expected_trait_ref)
 788             .map(|InferOk { mut obligations, .. }| {
 789                 obligations.extend(nested);
 790                 obligations
 791             })
 792             .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
 793     }
 794
 795     fn confirm_trait_upcasting_unsize_candidate(
 796         &mut self,
 797         obligation: &TraitObligation<'tcx>,
 798         idx: usize,
 799     ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 800     {
 801         let tcx = self.tcx();
 802
 803         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 804         // regions here. See the comment there for more details.
 805         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 806         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 807         let target = self.infcx.shallow_resolve(target);
 808
 809         debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
 810
 811         let mut nested = vec![];
 812         let source_trait_ref;
 813         let upcast_trait_ref;
 814         match (source.kind(), target.kind()) {
 815             // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
 816             (&ty::Dynamic(ref data_a, r_a, repr_a), &ty::Dynamic(ref data_b, r_b, repr_b))
 817                 if repr_a == repr_b =>
 818             {
 819                 // See `assemble_candidates_for_unsizing` for more info.
 820                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 821                 let principal_a = data_a.principal().unwrap();
 822                 source_trait_ref = principal_a.with_self_ty(tcx, source);
 823                 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
 824                 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
 825                 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
 826                     ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
 827                         tcx, trait_ref,
 828                     ))
 829                 });
 830                 let iter = Some(existential_predicate)
 831                     .into_iter()
 832                     .chain(
 833                         data_a
 834                             .projection_bounds()
 835                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 836                     )
 837                     .chain(
 838                         data_b
 839                             .auto_traits()
 840                             .map(ty::ExistentialPredicate::AutoTrait)
 841                             .map(ty::Binder::dummy),
 842                     );
 843                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 844                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_b);
 845
 846                 // Require that the traits involved in this upcast are **equal**;
 847                 // only the **lifetime bound** is changed.
 848                 let InferOk { obligations, .. } = self
 849                     .infcx
 850                     .at(&obligation.cause, obligation.param_env)
 851                     .sup(target, source_trait)
 852                     .map_err(|_| Unimplemented)?;
 853                 nested.extend(obligations);
 854
 855                 // Register one obligation for 'a: 'b.
 856                 let cause = ObligationCause::new(
 857                     obligation.cause.span,
 858                     obligation.cause.body_id,
 859                     ObjectCastObligation(source, target),
 860                 );
 861                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 862                 nested.push(Obligation::with_depth(
 863                     cause,
 864                     obligation.recursion_depth + 1,
 865                     obligation.param_env,
 866                     obligation.predicate.rebind(outlives).to_predicate(tcx),
 867                 ));
 868             }
 869             _ => bug!(),
 870         };
 871
 872         let vtable_segment_callback = {
 873             let mut vptr_offset = 0;
 874             move |segment| {
 875                 match segment {
 876                     VtblSegment::MetadataDSA => {
 877                         vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
 878                     }
 879                     VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 880                         vptr_offset += util::count_own_vtable_entries(tcx, trait_ref);
 881                         if trait_ref == upcast_trait_ref {
 882                             if emit_vptr {
 883                                 return ControlFlow::Break(Some(vptr_offset));
 884                             } else {
 885                                 return ControlFlow::Break(None);
 886                             }
 887                         }
 888
 889                         if emit_vptr {
 890                             vptr_offset += 1;
 891                         }
 892                     }
 893                 }
 894                 ControlFlow::Continue(())
 895             }
 896         };
 897
 898         let vtable_vptr_slot =
 899             super::super::prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback)
 900                 .unwrap();
 901
 902         Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
 903     }
 904
 905     fn confirm_builtin_unsize_candidate(
 906         &mut self,
 907         obligation: &TraitObligation<'tcx>,
 908     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 909         let tcx = self.tcx();
 910
 911         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 912         // regions here. See the comment there for more details.
 913         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 914         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 915         let target = self.infcx.shallow_resolve(target);
 916
 917         debug!(?source, ?target, "confirm_builtin_unsize_candidate");
 918
 919         let mut nested = vec![];
 920         match (source.kind(), target.kind()) {
 921             // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
 922             (&ty::Dynamic(ref data_a, r_a, ty::Dyn), &ty::Dynamic(ref data_b, r_b, ty::Dyn)) => {
 923                 // See `assemble_candidates_for_unsizing` for more info.
 924                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 925                 let iter = data_a
 926                     .principal()
 927                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
 928                     .into_iter()
 929                     .chain(
 930                         data_a
 931                             .projection_bounds()
 932                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 933                     )
 934                     .chain(
 935                         data_b
 936                             .auto_traits()
 937                             .map(ty::ExistentialPredicate::AutoTrait)
 938                             .map(ty::Binder::dummy),
 939                     );
 940                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 941                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, ty::Dyn);
 942
 943                 // Require that the traits involved in this upcast are **equal**;
 944                 // only the **lifetime bound** is changed.
 945                 let InferOk { obligations, .. } = self
 946                     .infcx
 947                     .at(&obligation.cause, obligation.param_env)
 948                     .sup(target, source_trait)
 949                     .map_err(|_| Unimplemented)?;
 950                 nested.extend(obligations);
 951
 952                 // Register one obligation for 'a: 'b.
 953                 let cause = ObligationCause::new(
 954                     obligation.cause.span,
 955                     obligation.cause.body_id,
 956                     ObjectCastObligation(source, target),
 957                 );
 958                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 959                 nested.push(Obligation::with_depth(
 960                     cause,
 961                     obligation.recursion_depth + 1,
 962                     obligation.param_env,
 963                     obligation.predicate.rebind(outlives).to_predicate(tcx),
 964                 ));
 965             }
 966
 967             // `T` -> `Trait`
 968             (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
 969                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
 970                 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
 971                     return Err(TraitNotObjectSafe(did));
 972                 }
 973
 974                 let cause = ObligationCause::new(
 975                     obligation.cause.span,
 976                     obligation.cause.body_id,
 977                     ObjectCastObligation(source, target),
 978                 );
 979
 980                 let predicate_to_obligation = |predicate| {
 981                     Obligation::with_depth(
 982                         cause.clone(),
 983                         obligation.recursion_depth + 1,
 984                         obligation.param_env,
 985                         predicate,
 986                     )
 987                 };
 988
 989                 // Create obligations:
 990                 //  - Casting `T` to `Trait`
 991                 //  - For all the various builtin bounds attached to the object cast. (In other
 992                 //  words, if the object type is `Foo + Send`, this would create an obligation for
 993                 //  the `Send` check.)
 994                 //  - Projection predicates
 995                 nested.extend(
 996                     data.iter().map(|predicate| {
 997                         predicate_to_obligation(predicate.with_self_ty(tcx, source))
 998                     }),
 999                 );
1000
1001                 // We can only make objects from sized types.
1002                 let tr = ty::Binder::dummy(ty::TraitRef::new(
1003                     tcx.require_lang_item(LangItem::Sized, None),
1004                     tcx.mk_substs_trait(source, &[]),
1005                 ));
1006                 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
1007
1008                 // If the type is `Foo + 'a`, ensure that the type
1009                 // being cast to `Foo + 'a` outlives `'a`:
1010                 let outlives = ty::OutlivesPredicate(source, r);
1011                 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
1012             }
1013
1014             // `[T; n]` -> `[T]`
1015             (&ty::Array(a, _), &ty::Slice(b)) => {
1016                 let InferOk { obligations, .. } = self
1017                     .infcx
1018                     .at(&obligation.cause, obligation.param_env)
1019                     .eq(b, a)
1020                     .map_err(|_| Unimplemented)?;
1021                 nested.extend(obligations);
1022             }
1023
1024             // `Struct<T>` -> `Struct<U>`
1025             (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
1026                 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
1027                     GenericArgKind::Type(ty) => match ty.kind() {
1028                         ty::Param(p) => Some(p.index),
1029                         _ => None,
1030                     },
1031
1032                     // Lifetimes aren't allowed to change during unsizing.
1033                     GenericArgKind::Lifetime(_) => None,
1034
1035                     GenericArgKind::Const(ct) => match ct.kind() {
1036                         ty::ConstKind::Param(p) => Some(p.index),
1037                         _ => None,
1038                     },
1039                 };
1040
1041                 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1042                 // by putting it in a query; it would only need the `DefId` as it
1043                 // looks at declared field types, not anything substituted.
1044
1045                 // The last field of the structure has to exist and contain type/const parameters.
1046                 let (tail_field, prefix_fields) =
1047                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1048                 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1049
1050                 let mut unsizing_params = GrowableBitSet::new_empty();
1051                 for arg in tail_field_ty.0.walk() {
1052                     if let Some(i) = maybe_unsizing_param_idx(arg) {
1053                         unsizing_params.insert(i);
1054                     }
1055                 }
1056
1057                 // Ensure none of the other fields mention the parameters used
1058                 // in unsizing.
1059                 for field in prefix_fields {
1060                     for arg in tcx.type_of(field.did).walk() {
1061                         if let Some(i) = maybe_unsizing_param_idx(arg) {
1062                             unsizing_params.remove(i);
1063                         }
1064                     }
1065                 }
1066
1067                 if unsizing_params.is_empty() {
1068                     return Err(Unimplemented);
1069                 }
1070
1071                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
1072                 // normalizing in the process, since `type_of` returns something directly from
1073                 // astconv (which means it's un-normalized).
1074                 let source_tail = normalize_with_depth_to(
1075                     self,
1076                     obligation.param_env,
1077                     obligation.cause.clone(),
1078                     obligation.recursion_depth + 1,
1079                     tail_field_ty.subst(tcx, substs_a),
1080                     &mut nested,
1081                 );
1082                 let target_tail = normalize_with_depth_to(
1083                     self,
1084                     obligation.param_env,
1085                     obligation.cause.clone(),
1086                     obligation.recursion_depth + 1,
1087                     tail_field_ty.subst(tcx, substs_b),
1088                     &mut nested,
1089                 );
1090
1091                 // Check that the source struct with the target's
1092                 // unsizing parameters is equal to the target.
1093                 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1094                     if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1095                 }));
1096                 let new_struct = tcx.mk_adt(def, substs);
1097                 let InferOk { obligations, .. } = self
1098                     .infcx
1099                     .at(&obligation.cause, obligation.param_env)
1100                     .eq(target, new_struct)
1101                     .map_err(|_| Unimplemented)?;
1102                 nested.extend(obligations);
1103
1104                 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1105                 nested.push(predicate_for_trait_def(
1106                     tcx,
1107                     obligation.param_env,
1108                     obligation.cause.clone(),
1109                     obligation.predicate.def_id(),
1110                     obligation.recursion_depth + 1,
1111                     source_tail,
1112                     &[target_tail.into()],
1113                 ));
1114             }
1115
1116             // `(.., T)` -> `(.., U)`
1117             (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1118                 assert_eq!(tys_a.len(), tys_b.len());
1119
1120                 // The last field of the tuple has to exist.
1121                 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1122                 let &b_last = tys_b.last().unwrap();
1123
1124                 // Check that the source tuple with the target's
1125                 // last element is equal to the target.
1126                 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1127                 let InferOk { obligations, .. } = self
1128                     .infcx
1129                     .at(&obligation.cause, obligation.param_env)
1130                     .eq(target, new_tuple)
1131                     .map_err(|_| Unimplemented)?;
1132                 nested.extend(obligations);
1133
1134                 // Construct the nested `T: Unsize<U>` predicate.
1135                 nested.push(ensure_sufficient_stack(|| {
1136                     predicate_for_trait_def(
1137                         tcx,
1138                         obligation.param_env,
1139                         obligation.cause.clone(),
1140                         obligation.predicate.def_id(),
1141                         obligation.recursion_depth + 1,
1142                         a_last,
1143                         &[b_last.into()],
1144                     )
1145                 }));
1146             }
1147
1148             _ => bug!(),
1149         };
1150
1151         Ok(ImplSourceBuiltinData { nested })
1152     }
1153
1154     fn confirm_const_destruct_candidate(
1155         &mut self,
1156         obligation: &TraitObligation<'tcx>,
1157         impl_def_id: Option<DefId>,
1158     ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1159         // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1160         if !obligation.is_const() {
1161             return Ok(ImplSourceConstDestructData { nested: vec![] });
1162         }
1163
1164         let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1165
1166         let tcx = self.tcx();
1167         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1168
1169         let mut nested = vec![];
1170         let cause = obligation.derived_cause(BuiltinDerivedObligation);
1171
1172         // If we have a custom `impl const Drop`, then
1173         // first check it like a regular impl candidate.
1174         // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1175         if let Some(impl_def_id) = impl_def_id {
1176             let mut new_obligation = obligation.clone();
1177             new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1178                 trait_pred.trait_ref.def_id = drop_trait;
1179                 trait_pred
1180             });
1181             let substs = self.rematch_impl(impl_def_id, &new_obligation);
1182             debug!(?substs, "impl substs");
1183
1184             let cause = obligation.derived_cause(|derived| {
1185                 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1186                     derived,
1187                     impl_def_id,
1188                     span: obligation.cause.span,
1189                 }))
1190             });
1191             let obligations = ensure_sufficient_stack(|| {
1192                 self.vtable_impl(
1193                     impl_def_id,
1194                     substs,
1195                     &cause,
1196                     new_obligation.recursion_depth + 1,
1197                     new_obligation.param_env,
1198                     obligation.predicate,
1199                 )
1200             });
1201             nested.extend(obligations.nested);
1202         }
1203
1204         // We want to confirm the ADT's fields if we have an ADT
1205         let mut stack = match *self_ty.skip_binder().kind() {
1206             ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1207             _ => vec![self_ty.skip_binder()],
1208         };
1209
1210         while let Some(nested_ty) = stack.pop() {
1211             match *nested_ty.kind() {
1212                 // We know these types are trivially drop
1213                 ty::Bool
1214                 | ty::Char
1215                 | ty::Int(_)
1216                 | ty::Uint(_)
1217                 | ty::Float(_)
1218                 | ty::Infer(ty::IntVar(_))
1219                 | ty::Infer(ty::FloatVar(_))
1220                 | ty::Str
1221                 | ty::RawPtr(_)
1222                 | ty::Ref(..)
1223                 | ty::FnDef(..)
1224                 | ty::FnPtr(_)
1225                 | ty::Never
1226                 | ty::Foreign(_) => {}
1227
1228                 // `ManuallyDrop` is trivially drop
1229                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
1230
1231                 // These types are built-in, so we can fast-track by registering
1232                 // nested predicates for their constituent type(s)
1233                 ty::Array(ty, _) | ty::Slice(ty) => {
1234                     stack.push(ty);
1235                 }
1236                 ty::Tuple(tys) => {
1237                     stack.extend(tys.iter());
1238                 }
1239                 ty::Closure(_, substs) => {
1240                     stack.push(substs.as_closure().tupled_upvars_ty());
1241                 }
1242                 ty::Generator(_, substs, _) => {
1243                     let generator = substs.as_generator();
1244                     stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1245                 }
1246                 ty::GeneratorWitness(tys) => {
1247                     stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1248                 }
1249
1250                 // If we have a projection type, make sure to normalize it so we replace it
1251                 // with a fresh infer variable
1252                 ty::Projection(..) => {
1253                     let predicate = normalize_with_depth_to(
1254                         self,
1255                         obligation.param_env,
1256                         cause.clone(),
1257                         obligation.recursion_depth + 1,
1258                         self_ty
1259                             .rebind(ty::TraitPredicate {
1260                                 trait_ref: ty::TraitRef {
1261                                     def_id: self.tcx().require_lang_item(LangItem::Destruct, None),
1262                                     substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1263                                 },
1264                                 constness: ty::BoundConstness::ConstIfConst,
1265                                 polarity: ty::ImplPolarity::Positive,
1266                             })
1267                             .to_predicate(tcx),
1268                         &mut nested,
1269                     );
1270
1271                     nested.push(Obligation::with_depth(
1272                         cause.clone(),
1273                         obligation.recursion_depth + 1,
1274                         obligation.param_env,
1275                         predicate,
1276                     ));
1277                 }
1278
1279                 // If we have any other type (e.g. an ADT), just register a nested obligation
1280                 // since it's either not `const Drop` (and we raise an error during selection),
1281                 // or it's an ADT (and we need to check for a custom impl during selection)
1282                 _ => {
1283                     let predicate = self_ty
1284                         .rebind(ty::TraitPredicate {
1285                             trait_ref: ty::TraitRef {
1286                                 def_id: self.tcx().require_lang_item(LangItem::Destruct, None),
1287                                 substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1288                             },
1289                             constness: ty::BoundConstness::ConstIfConst,
1290                             polarity: ty::ImplPolarity::Positive,
1291                         })
1292                         .to_predicate(tcx);
1293
1294                     nested.push(Obligation::with_depth(
1295                         cause.clone(),
1296                         obligation.recursion_depth + 1,
1297                         obligation.param_env,
1298                         predicate,
1299                     ));
1300                 }
1301             }
1302         }
1303
1304         Ok(ImplSourceConstDestructData { nested })
1305     }
1306 }