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::{
  15     self, GenericArg, GenericArgKind, GenericParamDefKind, InternalSubsts, SubstsRef,
  16     ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
  17 };
  18 use rustc_span::def_id::DefId;
  19
  20 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
  21 use crate::traits::util::{self, closure_trait_ref_and_return_type, predicate_for_trait_def};
  22 use crate::traits::{
  23     BuiltinDerivedObligation, ImplDerivedObligation, ImplDerivedObligationCause, ImplSource,
  24     ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
  25     ImplSourceConstDestructData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
  26     ImplSourceGeneratorData, ImplSourceObjectData, ImplSourcePointeeData, ImplSourceTraitAliasData,
  27     ImplSourceTraitUpcastingData, ImplSourceUserDefinedData, Normalized, ObjectCastObligation,
  28     Obligation, ObligationCause, OutputTypeParameterMismatch, PredicateObligation, Selection,
  29     SelectionError, TraitNotObjectSafe, TraitObligation, Unimplemented, VtblSegment,
  30 };
  31
  32 use super::BuiltinImplConditions;
  33 use super::SelectionCandidate::{self, *};
  34 use super::SelectionContext;
  35
  36 use std::iter;
  37 use std::ops::ControlFlow;
  38
  39 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
  40     #[instrument(level = "debug", skip(self))]
  41     pub(super) fn confirm_candidate(
  42         &mut self,
  43         obligation: &TraitObligation<'tcx>,
  44         candidate: SelectionCandidate<'tcx>,
  45     ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
  46         let mut impl_src = match candidate {
  47             BuiltinCandidate { has_nested } => {
  48                 let data = self.confirm_builtin_candidate(obligation, has_nested);
  49                 ImplSource::Builtin(data)
  50             }
  51
  52             TransmutabilityCandidate => {
  53                 let data = self.confirm_transmutability_candidate(obligation)?;
  54                 ImplSource::Builtin(data)
  55             }
  56
  57             ParamCandidate(param) => {
  58                 let obligations =
  59                     self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
  60                 ImplSource::Param(obligations, param.skip_binder().constness)
  61             }
  62
  63             ImplCandidate(impl_def_id) => {
  64                 ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id))
  65             }
  66
  67             AutoImplCandidate(trait_def_id) => {
  68                 let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
  69                 ImplSource::AutoImpl(data)
  70             }
  71
  72             ProjectionCandidate(idx, constness) => {
  73                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
  74                 ImplSource::Param(obligations, constness)
  75             }
  76
  77             ObjectCandidate(idx) => {
  78                 let data = self.confirm_object_candidate(obligation, idx)?;
  79                 ImplSource::Object(data)
  80             }
  81
  82             ClosureCandidate => {
  83                 let vtable_closure = self.confirm_closure_candidate(obligation)?;
  84                 ImplSource::Closure(vtable_closure)
  85             }
  86
  87             GeneratorCandidate => {
  88                 let vtable_generator = self.confirm_generator_candidate(obligation)?;
  89                 ImplSource::Generator(vtable_generator)
  90             }
  91
  92             FnPointerCandidate { .. } => {
  93                 let data = self.confirm_fn_pointer_candidate(obligation)?;
  94                 ImplSource::FnPointer(data)
  95             }
  96
  97             DiscriminantKindCandidate => {
  98                 ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
  99             }
 100
 101             PointeeCandidate => ImplSource::Pointee(ImplSourcePointeeData),
 102
 103             TraitAliasCandidate(alias_def_id) => {
 104                 let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
 105                 ImplSource::TraitAlias(data)
 106             }
 107
 108             BuiltinObjectCandidate => {
 109                 // This indicates something like `Trait + Send: Send`. In this case, we know that
 110                 // this holds because that's what the object type is telling us, and there's really
 111                 // no additional obligations to prove and no types in particular to unify, etc.
 112                 ImplSource::Param(Vec::new(), ty::BoundConstness::NotConst)
 113             }
 114
 115             BuiltinUnsizeCandidate => {
 116                 let data = self.confirm_builtin_unsize_candidate(obligation)?;
 117                 ImplSource::Builtin(data)
 118             }
 119
 120             TraitUpcastingUnsizeCandidate(idx) => {
 121                 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
 122                 ImplSource::TraitUpcasting(data)
 123             }
 124
 125             ConstDestructCandidate(def_id) => {
 126                 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
 127                 ImplSource::ConstDestruct(data)
 128             }
 129
 130             TupleCandidate => ImplSource::Tuple,
 131         };
 132
 133         if !obligation.predicate.is_const_if_const() {
 134             // normalize nested predicates according to parent predicate's constness.
 135             impl_src = impl_src.map(|mut o| {
 136                 o.predicate = o.predicate.without_const(self.tcx());
 137                 o
 138             });
 139         }
 140
 141         Ok(impl_src)
 142     }
 143
 144     fn confirm_projection_candidate(
 145         &mut self,
 146         obligation: &TraitObligation<'tcx>,
 147         idx: usize,
 148     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 149         let tcx = self.tcx();
 150
 151         let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
 152         let placeholder_trait_predicate =
 153             self.infcx().replace_bound_vars_with_placeholders(trait_predicate).trait_ref;
 154         let placeholder_self_ty = placeholder_trait_predicate.self_ty();
 155         let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
 156         let (def_id, substs) = match *placeholder_self_ty.kind() {
 157             ty::Projection(proj) => (proj.item_def_id, proj.substs),
 158             ty::Opaque(def_id, substs) => (def_id, substs),
 159             _ => bug!("projection candidate for unexpected type: {:?}", placeholder_self_ty),
 160         };
 161
 162         let candidate_predicate =
 163             tcx.bound_item_bounds(def_id).map_bound(|i| i[idx]).subst(tcx, substs);
 164         let candidate = candidate_predicate
 165             .to_opt_poly_trait_pred()
 166             .expect("projection candidate is not a trait predicate")
 167             .map_bound(|t| t.trait_ref);
 168         let mut obligations = Vec::new();
 169         let candidate = normalize_with_depth_to(
 170             self,
 171             obligation.param_env,
 172             obligation.cause.clone(),
 173             obligation.recursion_depth + 1,
 174             candidate,
 175             &mut obligations,
 176         );
 177
 178         obligations.extend(self.infcx.commit_if_ok(|_| {
 179             self.infcx
 180                 .at(&obligation.cause, obligation.param_env)
 181                 .sup(placeholder_trait_predicate, candidate)
 182                 .map(|InferOk { obligations, .. }| obligations)
 183                 .map_err(|_| Unimplemented)
 184         })?);
 185
 186         if let ty::Projection(..) = placeholder_self_ty.kind() {
 187             let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates;
 188             debug!(?predicates, "projection predicates");
 189             for predicate in predicates {
 190                 let normalized = normalize_with_depth_to(
 191                     self,
 192                     obligation.param_env,
 193                     obligation.cause.clone(),
 194                     obligation.recursion_depth + 1,
 195                     predicate,
 196                     &mut obligations,
 197                 );
 198                 obligations.push(Obligation::with_depth(
 199                     obligation.cause.clone(),
 200                     obligation.recursion_depth + 1,
 201                     obligation.param_env,
 202                     normalized,
 203                 ));
 204             }
 205         }
 206
 207         Ok(obligations)
 208     }
 209
 210     fn confirm_param_candidate(
 211         &mut self,
 212         obligation: &TraitObligation<'tcx>,
 213         param: ty::PolyTraitRef<'tcx>,
 214     ) -> Vec<PredicateObligation<'tcx>> {
 215         debug!(?obligation, ?param, "confirm_param_candidate");
 216
 217         // During evaluation, we already checked that this
 218         // where-clause trait-ref could be unified with the obligation
 219         // trait-ref. Repeat that unification now without any
 220         // transactional boundary; it should not fail.
 221         match self.match_where_clause_trait_ref(obligation, param) {
 222             Ok(obligations) => obligations,
 223             Err(()) => {
 224                 bug!(
 225                     "Where clause `{:?}` was applicable to `{:?}` but now is not",
 226                     param,
 227                     obligation
 228                 );
 229             }
 230         }
 231     }
 232
 233     fn confirm_builtin_candidate(
 234         &mut self,
 235         obligation: &TraitObligation<'tcx>,
 236         has_nested: bool,
 237     ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
 238         debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
 239
 240         let lang_items = self.tcx().lang_items();
 241         let obligations = if has_nested {
 242             let trait_def = obligation.predicate.def_id();
 243             let conditions = if Some(trait_def) == lang_items.sized_trait() {
 244                 self.sized_conditions(obligation)
 245             } else if Some(trait_def) == lang_items.copy_trait() {
 246                 self.copy_clone_conditions(obligation)
 247             } else if Some(trait_def) == lang_items.clone_trait() {
 248                 self.copy_clone_conditions(obligation)
 249             } else {
 250                 bug!("unexpected builtin trait {:?}", trait_def)
 251             };
 252             let BuiltinImplConditions::Where(nested) = conditions else {
 253                 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
 254             };
 255
 256             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 257             ensure_sufficient_stack(|| {
 258                 self.collect_predicates_for_types(
 259                     obligation.param_env,
 260                     cause,
 261                     obligation.recursion_depth + 1,
 262                     trait_def,
 263                     nested,
 264                 )
 265             })
 266         } else {
 267             vec![]
 268         };
 269
 270         debug!(?obligations);
 271
 272         ImplSourceBuiltinData { nested: obligations }
 273     }
 274
 275     fn confirm_transmutability_candidate(
 276         &mut self,
 277         obligation: &TraitObligation<'tcx>,
 278     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 279         debug!(?obligation, "confirm_transmutability_candidate");
 280
 281         let predicate = obligation.predicate;
 282
 283         let type_at = |i| predicate.map_bound(|p| p.trait_ref.substs.type_at(i));
 284         let const_at = |i| predicate.skip_binder().trait_ref.substs.const_at(i);
 285
 286         let src_and_dst = predicate.map_bound(|p| rustc_transmute::Types {
 287             dst: p.trait_ref.substs.type_at(0),
 288             src: p.trait_ref.substs.type_at(1),
 289         });
 290
 291         let scope = type_at(2).skip_binder();
 292
 293         let Some(assume) =
 294             rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3)) else {
 295                 return Err(Unimplemented);
 296             };
 297
 298         let cause = obligation.cause.clone();
 299
 300         let mut transmute_env = rustc_transmute::TransmuteTypeEnv::new(self.infcx);
 301
 302         let maybe_transmutable = transmute_env.is_transmutable(cause, src_and_dst, scope, assume);
 303
 304         use rustc_transmute::Answer;
 305
 306         match maybe_transmutable {
 307             Answer::Yes => Ok(ImplSourceBuiltinData { nested: vec![] }),
 308             _ => Err(Unimplemented),
 309         }
 310     }
 311
 312     /// This handles the case where an `auto trait Foo` impl is being used.
 313     /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
 314     ///
 315     /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
 316     /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
 317     fn confirm_auto_impl_candidate(
 318         &mut self,
 319         obligation: &TraitObligation<'tcx>,
 320         trait_def_id: DefId,
 321     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 322         debug!(?obligation, ?trait_def_id, "confirm_auto_impl_candidate");
 323
 324         let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
 325         let types = self.constituent_types_for_ty(self_ty);
 326         self.vtable_auto_impl(obligation, trait_def_id, types)
 327     }
 328
 329     /// See `confirm_auto_impl_candidate`.
 330     fn vtable_auto_impl(
 331         &mut self,
 332         obligation: &TraitObligation<'tcx>,
 333         trait_def_id: DefId,
 334         nested: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
 335     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 336         debug!(?nested, "vtable_auto_impl");
 337         ensure_sufficient_stack(|| {
 338             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 339
 340             let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
 341             let trait_ref = self.infcx.replace_bound_vars_with_placeholders(poly_trait_ref);
 342             let trait_obligations: Vec<PredicateObligation<'_>> = self.impl_or_trait_obligations(
 343                 &cause,
 344                 obligation.recursion_depth + 1,
 345                 obligation.param_env,
 346                 trait_def_id,
 347                 &trait_ref.substs,
 348                 obligation.predicate,
 349             );
 350
 351             let mut obligations = self.collect_predicates_for_types(
 352                 obligation.param_env,
 353                 cause,
 354                 obligation.recursion_depth + 1,
 355                 trait_def_id,
 356                 nested,
 357             );
 358
 359             // Adds the predicates from the trait.  Note that this contains a `Self: Trait`
 360             // predicate as usual.  It won't have any effect since auto traits are coinductive.
 361             obligations.extend(trait_obligations);
 362
 363             debug!(?obligations, "vtable_auto_impl");
 364
 365             ImplSourceAutoImplData { trait_def_id, nested: obligations }
 366         })
 367     }
 368
 369     fn confirm_impl_candidate(
 370         &mut self,
 371         obligation: &TraitObligation<'tcx>,
 372         impl_def_id: DefId,
 373     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 374         debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");
 375
 376         // First, create the substitutions by matching the impl again,
 377         // this time not in a probe.
 378         let substs = self.rematch_impl(impl_def_id, obligation);
 379         debug!(?substs, "impl substs");
 380         ensure_sufficient_stack(|| {
 381             self.vtable_impl(
 382                 impl_def_id,
 383                 substs,
 384                 &obligation.cause,
 385                 obligation.recursion_depth + 1,
 386                 obligation.param_env,
 387                 obligation.predicate,
 388             )
 389         })
 390     }
 391
 392     fn vtable_impl(
 393         &mut self,
 394         impl_def_id: DefId,
 395         substs: Normalized<'tcx, SubstsRef<'tcx>>,
 396         cause: &ObligationCause<'tcx>,
 397         recursion_depth: usize,
 398         param_env: ty::ParamEnv<'tcx>,
 399         parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
 400     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 401         debug!(?impl_def_id, ?substs, ?recursion_depth, "vtable_impl");
 402
 403         let mut impl_obligations = self.impl_or_trait_obligations(
 404             cause,
 405             recursion_depth,
 406             param_env,
 407             impl_def_id,
 408             &substs.value,
 409             parent_trait_pred,
 410         );
 411
 412         debug!(?impl_obligations, "vtable_impl");
 413
 414         // Because of RFC447, the impl-trait-ref and obligations
 415         // are sufficient to determine the impl substs, without
 416         // relying on projections in the impl-trait-ref.
 417         //
 418         // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
 419         impl_obligations.extend(substs.obligations);
 420
 421         ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations }
 422     }
 423
 424     fn confirm_object_candidate(
 425         &mut self,
 426         obligation: &TraitObligation<'tcx>,
 427         index: usize,
 428     ) -> Result<ImplSourceObjectData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 429         let tcx = self.tcx();
 430         debug!(?obligation, ?index, "confirm_object_candidate");
 431
 432         let trait_predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
 433         let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
 434         let obligation_trait_ref = ty::Binder::dummy(trait_predicate.trait_ref);
 435         let ty::Dynamic(data, ..) = *self_ty.kind() else {
 436             span_bug!(obligation.cause.span, "object candidate with non-object");
 437         };
 438
 439         let object_trait_ref = data.principal().unwrap_or_else(|| {
 440             span_bug!(obligation.cause.span, "object candidate with no principal")
 441         });
 442         let object_trait_ref = self.infcx.replace_bound_vars_with_fresh_vars(
 443             obligation.cause.span,
 444             HigherRankedType,
 445             object_trait_ref,
 446         );
 447         let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);
 448
 449         let mut nested = vec![];
 450
 451         let mut supertraits = util::supertraits(tcx, ty::Binder::dummy(object_trait_ref));
 452         let unnormalized_upcast_trait_ref =
 453             supertraits.nth(index).expect("supertraits iterator no longer has as many elements");
 454
 455         let upcast_trait_ref = normalize_with_depth_to(
 456             self,
 457             obligation.param_env,
 458             obligation.cause.clone(),
 459             obligation.recursion_depth + 1,
 460             unnormalized_upcast_trait_ref,
 461             &mut nested,
 462         );
 463
 464         nested.extend(self.infcx.commit_if_ok(|_| {
 465             self.infcx
 466                 .at(&obligation.cause, obligation.param_env)
 467                 .sup(obligation_trait_ref, upcast_trait_ref)
 468                 .map(|InferOk { obligations, .. }| obligations)
 469                 .map_err(|_| Unimplemented)
 470         })?);
 471
 472         // Check supertraits hold. This is so that their associated type bounds
 473         // will be checked in the code below.
 474         for super_trait in tcx
 475             .super_predicates_of(trait_predicate.def_id())
 476             .instantiate(tcx, trait_predicate.trait_ref.substs)
 477             .predicates
 478             .into_iter()
 479         {
 480             let normalized_super_trait = normalize_with_depth_to(
 481                 self,
 482                 obligation.param_env,
 483                 obligation.cause.clone(),
 484                 obligation.recursion_depth + 1,
 485                 super_trait,
 486                 &mut nested,
 487             );
 488             nested.push(Obligation::new(
 489                 obligation.cause.clone(),
 490                 obligation.param_env,
 491                 normalized_super_trait,
 492             ));
 493         }
 494
 495         let assoc_types: Vec<_> = tcx
 496             .associated_items(trait_predicate.def_id())
 497             .in_definition_order()
 498             .filter_map(
 499                 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
 500             )
 501             .collect();
 502
 503         for assoc_type in assoc_types {
 504             let defs: &ty::Generics = tcx.generics_of(assoc_type);
 505
 506             if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
 507                 tcx.sess.delay_span_bug(
 508                     obligation.cause.span,
 509                     "GATs in trait object shouldn't have been considered",
 510                 );
 511                 return Err(SelectionError::Unimplemented);
 512             }
 513
 514             // This maybe belongs in wf, but that can't (doesn't) handle
 515             // higher-ranked things.
 516             // Prevent, e.g., `dyn Iterator<Item = str>`.
 517             for bound in self.tcx().bound_item_bounds(assoc_type).transpose_iter() {
 518                 let subst_bound =
 519                     if defs.count() == 0 {
 520                         bound.subst(tcx, trait_predicate.trait_ref.substs)
 521                     } else {
 522                         let mut substs = smallvec::SmallVec::with_capacity(defs.count());
 523                         substs.extend(trait_predicate.trait_ref.substs.iter());
 524                         let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
 525                             smallvec::SmallVec::with_capacity(
 526                                 bound.0.kind().bound_vars().len() + defs.count(),
 527                             );
 528                         bound_vars.extend(bound.0.kind().bound_vars().into_iter());
 529                         InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
 530                             .kind
 531                         {
 532                             GenericParamDefKind::Type { .. } => {
 533                                 let kind = ty::BoundTyKind::Param(param.name);
 534                                 let bound_var = ty::BoundVariableKind::Ty(kind);
 535                                 bound_vars.push(bound_var);
 536                                 tcx.mk_ty(ty::Bound(
 537                                     ty::INNERMOST,
 538                                     ty::BoundTy {
 539                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 540                                         kind,
 541                                     },
 542                                 ))
 543                                 .into()
 544                             }
 545                             GenericParamDefKind::Lifetime => {
 546                                 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
 547                                 let bound_var = ty::BoundVariableKind::Region(kind);
 548                                 bound_vars.push(bound_var);
 549                                 tcx.mk_region(ty::ReLateBound(
 550                                     ty::INNERMOST,
 551                                     ty::BoundRegion {
 552                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 553                                         kind,
 554                                     },
 555                                 ))
 556                                 .into()
 557                             }
 558                             GenericParamDefKind::Const { .. } => {
 559                                 let bound_var = ty::BoundVariableKind::Const;
 560                                 bound_vars.push(bound_var);
 561                                 tcx.mk_const(ty::ConstS {
 562                                     ty: tcx.type_of(param.def_id),
 563                                     kind: ty::ConstKind::Bound(
 564                                         ty::INNERMOST,
 565                                         ty::BoundVar::from_usize(bound_vars.len() - 1),
 566                                     ),
 567                                 })
 568                                 .into()
 569                             }
 570                         });
 571                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 572                         let assoc_ty_substs = tcx.intern_substs(&substs);
 573
 574                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 575                         let bound =
 576                             bound.map_bound(|b| b.kind().skip_binder()).subst(tcx, assoc_ty_substs);
 577                         tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
 578                     };
 579                 let normalized_bound = normalize_with_depth_to(
 580                     self,
 581                     obligation.param_env,
 582                     obligation.cause.clone(),
 583                     obligation.recursion_depth + 1,
 584                     subst_bound,
 585                     &mut nested,
 586                 );
 587                 nested.push(Obligation::new(
 588                     obligation.cause.clone(),
 589                     obligation.param_env,
 590                     normalized_bound,
 591                 ));
 592             }
 593         }
 594
 595         debug!(?nested, "object nested obligations");
 596
 597         let vtable_base = super::super::vtable_trait_first_method_offset(
 598             tcx,
 599             (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
 600         );
 601
 602         Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
 603     }
 604
 605     fn confirm_fn_pointer_candidate(
 606         &mut self,
 607         obligation: &TraitObligation<'tcx>,
 608     ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 609     {
 610         debug!(?obligation, "confirm_fn_pointer_candidate");
 611
 612         // Okay to skip binder; it is reintroduced below.
 613         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 614         let sig = self_ty.fn_sig(self.tcx());
 615         let trait_ref = closure_trait_ref_and_return_type(
 616             self.tcx(),
 617             obligation.predicate.def_id(),
 618             self_ty,
 619             sig,
 620             util::TupleArgumentsFlag::Yes,
 621         )
 622         .map_bound(|(trait_ref, _)| trait_ref);
 623
 624         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 625
 626         // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
 627         let cause = obligation.derived_cause(BuiltinDerivedObligation);
 628         // The binder on the Fn obligation is "less" important than the one on
 629         // the signature, as evidenced by how we treat it during projection.
 630         // The safe thing to do here is to liberate it, though, which should
 631         // have no worse effect than skipping the binder here.
 632         let liberated_fn_ty =
 633             self.infcx.replace_bound_vars_with_placeholders(obligation.predicate.rebind(self_ty));
 634         let output_ty = self
 635             .infcx
 636             .replace_bound_vars_with_placeholders(liberated_fn_ty.fn_sig(self.tcx()).output());
 637         let output_ty = normalize_with_depth_to(
 638             self,
 639             obligation.param_env,
 640             cause.clone(),
 641             obligation.recursion_depth,
 642             output_ty,
 643             &mut nested,
 644         );
 645         let tr = ty::Binder::dummy(ty::TraitRef::new(
 646             self.tcx().require_lang_item(LangItem::Sized, None),
 647             self.tcx().mk_substs_trait(output_ty, &[]),
 648         ));
 649         nested.push(Obligation::new(
 650             cause,
 651             obligation.param_env,
 652             tr.to_poly_trait_predicate().to_predicate(self.tcx()),
 653         ));
 654
 655         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
 656     }
 657
 658     fn confirm_trait_alias_candidate(
 659         &mut self,
 660         obligation: &TraitObligation<'tcx>,
 661         alias_def_id: DefId,
 662     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
 663         debug!(?obligation, ?alias_def_id, "confirm_trait_alias_candidate");
 664
 665         let predicate = self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
 666         let trait_ref = predicate.trait_ref;
 667         let trait_def_id = trait_ref.def_id;
 668         let substs = trait_ref.substs;
 669
 670         let trait_obligations = self.impl_or_trait_obligations(
 671             &obligation.cause,
 672             obligation.recursion_depth,
 673             obligation.param_env,
 674             trait_def_id,
 675             &substs,
 676             obligation.predicate,
 677         );
 678
 679         debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
 680
 681         ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
 682     }
 683
 684     fn confirm_generator_candidate(
 685         &mut self,
 686         obligation: &TraitObligation<'tcx>,
 687     ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 688     {
 689         // Okay to skip binder because the substs on generator types never
 690         // touch bound regions, they just capture the in-scope
 691         // type/region parameters.
 692         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 693         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 694             bug!("closure candidate for non-closure {:?}", obligation);
 695         };
 696
 697         debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
 698
 699         let trait_ref = self.generator_trait_ref_unnormalized(obligation, substs);
 700
 701         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 702         debug!(?trait_ref, ?nested, "generator candidate obligations");
 703
 704         Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
 705     }
 706
 707     #[instrument(skip(self), level = "debug")]
 708     fn confirm_closure_candidate(
 709         &mut self,
 710         obligation: &TraitObligation<'tcx>,
 711     ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 712         let kind = self
 713             .tcx()
 714             .fn_trait_kind_from_lang_item(obligation.predicate.def_id())
 715             .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
 716
 717         // Okay to skip binder because the substs on closure types never
 718         // touch bound regions, they just capture the in-scope
 719         // type/region parameters.
 720         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 721         let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
 722             bug!("closure candidate for non-closure {:?}", obligation);
 723         };
 724
 725         let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
 726         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 727
 728         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 729
 730         // FIXME: Chalk
 731
 732         if !self.tcx().sess.opts.unstable_opts.chalk {
 733             nested.push(Obligation::new(
 734                 obligation.cause.clone(),
 735                 obligation.param_env,
 736                 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind))
 737                     .to_predicate(self.tcx()),
 738             ));
 739         }
 740
 741         Ok(ImplSourceClosureData { closure_def_id, substs, nested })
 742     }
 743
 744     /// In the case of closure types and fn pointers,
 745     /// we currently treat the input type parameters on the trait as
 746     /// outputs. This means that when we have a match we have only
 747     /// considered the self type, so we have to go back and make sure
 748     /// to relate the argument types too. This is kind of wrong, but
 749     /// since we control the full set of impls, also not that wrong,
 750     /// and it DOES yield better error messages (since we don't report
 751     /// errors as if there is no applicable impl, but rather report
 752     /// errors are about mismatched argument types.
 753     ///
 754     /// Here is an example. Imagine we have a closure expression
 755     /// and we desugared it so that the type of the expression is
 756     /// `Closure`, and `Closure` expects `i32` as argument. Then it
 757     /// is "as if" the compiler generated this impl:
 758     /// ```ignore (illustrative)
 759     /// impl Fn(i32) for Closure { ... }
 760     /// ```
 761     /// Now imagine our obligation is `Closure: Fn(usize)`. So far
 762     /// we have matched the self type `Closure`. At this point we'll
 763     /// compare the `i32` to `usize` and generate an error.
 764     ///
 765     /// Note that this checking occurs *after* the impl has selected,
 766     /// because these output type parameters should not affect the
 767     /// selection of the impl. Therefore, if there is a mismatch, we
 768     /// report an error to the user.
 769     #[instrument(skip(self), level = "trace")]
 770     fn confirm_poly_trait_refs(
 771         &mut self,
 772         obligation: &TraitObligation<'tcx>,
 773         expected_trait_ref: ty::PolyTraitRef<'tcx>,
 774     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 775         let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
 776         // Normalize the obligation and expected trait refs together, because why not
 777         let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
 778             ensure_sufficient_stack(|| {
 779                 normalize_with_depth(
 780                     self,
 781                     obligation.param_env,
 782                     obligation.cause.clone(),
 783                     obligation.recursion_depth + 1,
 784                     (obligation_trait_ref, expected_trait_ref),
 785                 )
 786             });
 787
 788         self.infcx
 789             .at(&obligation.cause, obligation.param_env)
 790             .sup(obligation_trait_ref, expected_trait_ref)
 791             .map(|InferOk { mut obligations, .. }| {
 792                 obligations.extend(nested);
 793                 obligations
 794             })
 795             .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
 796     }
 797
 798     fn confirm_trait_upcasting_unsize_candidate(
 799         &mut self,
 800         obligation: &TraitObligation<'tcx>,
 801         idx: usize,
 802     ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 803     {
 804         let tcx = self.tcx();
 805
 806         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 807         // regions here. See the comment there for more details.
 808         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 809         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 810         let target = self.infcx.shallow_resolve(target);
 811
 812         debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
 813
 814         let mut nested = vec![];
 815         let source_trait_ref;
 816         let upcast_trait_ref;
 817         match (source.kind(), target.kind()) {
 818             // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
 819             (&ty::Dynamic(ref data_a, r_a, repr_a), &ty::Dynamic(ref data_b, r_b, repr_b))
 820                 if repr_a == repr_b =>
 821             {
 822                 // See `assemble_candidates_for_unsizing` for more info.
 823                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 824                 let principal_a = data_a.principal().unwrap();
 825                 source_trait_ref = principal_a.with_self_ty(tcx, source);
 826                 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
 827                 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
 828                 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
 829                     ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
 830                         tcx, trait_ref,
 831                     ))
 832                 });
 833                 let iter = Some(existential_predicate)
 834                     .into_iter()
 835                     .chain(
 836                         data_a
 837                             .projection_bounds()
 838                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 839                     )
 840                     .chain(
 841                         data_b
 842                             .auto_traits()
 843                             .map(ty::ExistentialPredicate::AutoTrait)
 844                             .map(ty::Binder::dummy),
 845                     );
 846                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 847                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_b);
 848
 849                 // Require that the traits involved in this upcast are **equal**;
 850                 // only the **lifetime bound** is changed.
 851                 let InferOk { obligations, .. } = self
 852                     .infcx
 853                     .at(&obligation.cause, obligation.param_env)
 854                     .sup(target, source_trait)
 855                     .map_err(|_| Unimplemented)?;
 856                 nested.extend(obligations);
 857
 858                 // Register one obligation for 'a: 'b.
 859                 let cause = ObligationCause::new(
 860                     obligation.cause.span,
 861                     obligation.cause.body_id,
 862                     ObjectCastObligation(source, target),
 863                 );
 864                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 865                 nested.push(Obligation::with_depth(
 866                     cause,
 867                     obligation.recursion_depth + 1,
 868                     obligation.param_env,
 869                     obligation.predicate.rebind(outlives).to_predicate(tcx),
 870                 ));
 871             }
 872             _ => bug!(),
 873         };
 874
 875         let vtable_segment_callback = {
 876             let mut vptr_offset = 0;
 877             move |segment| {
 878                 match segment {
 879                     VtblSegment::MetadataDSA => {
 880                         vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
 881                     }
 882                     VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 883                         vptr_offset += util::count_own_vtable_entries(tcx, trait_ref);
 884                         if trait_ref == upcast_trait_ref {
 885                             if emit_vptr {
 886                                 return ControlFlow::Break(Some(vptr_offset));
 887                             } else {
 888                                 return ControlFlow::Break(None);
 889                             }
 890                         }
 891
 892                         if emit_vptr {
 893                             vptr_offset += 1;
 894                         }
 895                     }
 896                 }
 897                 ControlFlow::Continue(())
 898             }
 899         };
 900
 901         let vtable_vptr_slot =
 902             super::super::prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback)
 903                 .unwrap();
 904
 905         Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
 906     }
 907
 908     fn confirm_builtin_unsize_candidate(
 909         &mut self,
 910         obligation: &TraitObligation<'tcx>,
 911     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 912         let tcx = self.tcx();
 913
 914         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 915         // regions here. See the comment there for more details.
 916         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 917         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 918         let target = self.infcx.shallow_resolve(target);
 919
 920         debug!(?source, ?target, "confirm_builtin_unsize_candidate");
 921
 922         let mut nested = vec![];
 923         match (source.kind(), target.kind()) {
 924             // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
 925             (&ty::Dynamic(ref data_a, r_a, ty::Dyn), &ty::Dynamic(ref data_b, r_b, ty::Dyn)) => {
 926                 // See `assemble_candidates_for_unsizing` for more info.
 927                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 928                 let iter = data_a
 929                     .principal()
 930                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
 931                     .into_iter()
 932                     .chain(
 933                         data_a
 934                             .projection_bounds()
 935                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 936                     )
 937                     .chain(
 938                         data_b
 939                             .auto_traits()
 940                             .map(ty::ExistentialPredicate::AutoTrait)
 941                             .map(ty::Binder::dummy),
 942                     );
 943                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 944                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, ty::Dyn);
 945
 946                 // Require that the traits involved in this upcast are **equal**;
 947                 // only the **lifetime bound** is changed.
 948                 let InferOk { obligations, .. } = self
 949                     .infcx
 950                     .at(&obligation.cause, obligation.param_env)
 951                     .sup(target, source_trait)
 952                     .map_err(|_| Unimplemented)?;
 953                 nested.extend(obligations);
 954
 955                 // Register one obligation for 'a: 'b.
 956                 let cause = ObligationCause::new(
 957                     obligation.cause.span,
 958                     obligation.cause.body_id,
 959                     ObjectCastObligation(source, target),
 960                 );
 961                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 962                 nested.push(Obligation::with_depth(
 963                     cause,
 964                     obligation.recursion_depth + 1,
 965                     obligation.param_env,
 966                     obligation.predicate.rebind(outlives).to_predicate(tcx),
 967                 ));
 968             }
 969
 970             // `T` -> `Trait`
 971             (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
 972                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
 973                 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
 974                     return Err(TraitNotObjectSafe(did));
 975                 }
 976
 977                 let cause = ObligationCause::new(
 978                     obligation.cause.span,
 979                     obligation.cause.body_id,
 980                     ObjectCastObligation(source, target),
 981                 );
 982
 983                 let predicate_to_obligation = |predicate| {
 984                     Obligation::with_depth(
 985                         cause.clone(),
 986                         obligation.recursion_depth + 1,
 987                         obligation.param_env,
 988                         predicate,
 989                     )
 990                 };
 991
 992                 // Create obligations:
 993                 //  - Casting `T` to `Trait`
 994                 //  - For all the various builtin bounds attached to the object cast. (In other
 995                 //  words, if the object type is `Foo + Send`, this would create an obligation for
 996                 //  the `Send` check.)
 997                 //  - Projection predicates
 998                 nested.extend(
 999                     data.iter().map(|predicate| {
1000                         predicate_to_obligation(predicate.with_self_ty(tcx, source))
1001                     }),
1002                 );
1003
1004                 // We can only make objects from sized types.
1005                 let tr = ty::Binder::dummy(ty::TraitRef::new(
1006                     tcx.require_lang_item(LangItem::Sized, None),
1007                     tcx.mk_substs_trait(source, &[]),
1008                 ));
1009                 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
1010
1011                 // If the type is `Foo + 'a`, ensure that the type
1012                 // being cast to `Foo + 'a` outlives `'a`:
1013                 let outlives = ty::OutlivesPredicate(source, r);
1014                 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
1015             }
1016
1017             // `[T; n]` -> `[T]`
1018             (&ty::Array(a, _), &ty::Slice(b)) => {
1019                 let InferOk { obligations, .. } = self
1020                     .infcx
1021                     .at(&obligation.cause, obligation.param_env)
1022                     .eq(b, a)
1023                     .map_err(|_| Unimplemented)?;
1024                 nested.extend(obligations);
1025             }
1026
1027             // `Struct<T>` -> `Struct<U>`
1028             (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
1029                 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
1030                     GenericArgKind::Type(ty) => match ty.kind() {
1031                         ty::Param(p) => Some(p.index),
1032                         _ => None,
1033                     },
1034
1035                     // Lifetimes aren't allowed to change during unsizing.
1036                     GenericArgKind::Lifetime(_) => None,
1037
1038                     GenericArgKind::Const(ct) => match ct.kind() {
1039                         ty::ConstKind::Param(p) => Some(p.index),
1040                         _ => None,
1041                     },
1042                 };
1043
1044                 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1045                 // by putting it in a query; it would only need the `DefId` as it
1046                 // looks at declared field types, not anything substituted.
1047
1048                 // The last field of the structure has to exist and contain type/const parameters.
1049                 let (tail_field, prefix_fields) =
1050                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1051                 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1052
1053                 let mut unsizing_params = GrowableBitSet::new_empty();
1054                 for arg in tail_field_ty.0.walk() {
1055                     if let Some(i) = maybe_unsizing_param_idx(arg) {
1056                         unsizing_params.insert(i);
1057                     }
1058                 }
1059
1060                 // Ensure none of the other fields mention the parameters used
1061                 // in unsizing.
1062                 for field in prefix_fields {
1063                     for arg in tcx.type_of(field.did).walk() {
1064                         if let Some(i) = maybe_unsizing_param_idx(arg) {
1065                             unsizing_params.remove(i);
1066                         }
1067                     }
1068                 }
1069
1070                 if unsizing_params.is_empty() {
1071                     return Err(Unimplemented);
1072                 }
1073
1074                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
1075                 // normalizing in the process, since `type_of` returns something directly from
1076                 // astconv (which means it's un-normalized).
1077                 let source_tail = normalize_with_depth_to(
1078                     self,
1079                     obligation.param_env,
1080                     obligation.cause.clone(),
1081                     obligation.recursion_depth + 1,
1082                     tail_field_ty.subst(tcx, substs_a),
1083                     &mut nested,
1084                 );
1085                 let target_tail = normalize_with_depth_to(
1086                     self,
1087                     obligation.param_env,
1088                     obligation.cause.clone(),
1089                     obligation.recursion_depth + 1,
1090                     tail_field_ty.subst(tcx, substs_b),
1091                     &mut nested,
1092                 );
1093
1094                 // Check that the source struct with the target's
1095                 // unsizing parameters is equal to the target.
1096                 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1097                     if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1098                 }));
1099                 let new_struct = tcx.mk_adt(def, substs);
1100                 let InferOk { obligations, .. } = self
1101                     .infcx
1102                     .at(&obligation.cause, obligation.param_env)
1103                     .eq(target, new_struct)
1104                     .map_err(|_| Unimplemented)?;
1105                 nested.extend(obligations);
1106
1107                 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1108                 nested.push(predicate_for_trait_def(
1109                     tcx,
1110                     obligation.param_env,
1111                     obligation.cause.clone(),
1112                     obligation.predicate.def_id(),
1113                     obligation.recursion_depth + 1,
1114                     source_tail,
1115                     &[target_tail.into()],
1116                 ));
1117             }
1118
1119             // `(.., T)` -> `(.., U)`
1120             (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1121                 assert_eq!(tys_a.len(), tys_b.len());
1122
1123                 // The last field of the tuple has to exist.
1124                 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1125                 let &b_last = tys_b.last().unwrap();
1126
1127                 // Check that the source tuple with the target's
1128                 // last element is equal to the target.
1129                 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1130                 let InferOk { obligations, .. } = self
1131                     .infcx
1132                     .at(&obligation.cause, obligation.param_env)
1133                     .eq(target, new_tuple)
1134                     .map_err(|_| Unimplemented)?;
1135                 nested.extend(obligations);
1136
1137                 // Construct the nested `T: Unsize<U>` predicate.
1138                 nested.push(ensure_sufficient_stack(|| {
1139                     predicate_for_trait_def(
1140                         tcx,
1141                         obligation.param_env,
1142                         obligation.cause.clone(),
1143                         obligation.predicate.def_id(),
1144                         obligation.recursion_depth + 1,
1145                         a_last,
1146                         &[b_last.into()],
1147                     )
1148                 }));
1149             }
1150
1151             _ => bug!(),
1152         };
1153
1154         Ok(ImplSourceBuiltinData { nested })
1155     }
1156
1157     fn confirm_const_destruct_candidate(
1158         &mut self,
1159         obligation: &TraitObligation<'tcx>,
1160         impl_def_id: Option<DefId>,
1161     ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1162         // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1163         if !obligation.is_const() {
1164             return Ok(ImplSourceConstDestructData { nested: vec![] });
1165         }
1166
1167         let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1168
1169         let tcx = self.tcx();
1170         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1171
1172         let mut nested = vec![];
1173         let cause = obligation.derived_cause(BuiltinDerivedObligation);
1174
1175         // If we have a custom `impl const Drop`, then
1176         // first check it like a regular impl candidate.
1177         // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1178         if let Some(impl_def_id) = impl_def_id {
1179             let mut new_obligation = obligation.clone();
1180             new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1181                 trait_pred.trait_ref.def_id = drop_trait;
1182                 trait_pred
1183             });
1184             let substs = self.rematch_impl(impl_def_id, &new_obligation);
1185             debug!(?substs, "impl substs");
1186
1187             let cause = obligation.derived_cause(|derived| {
1188                 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1189                     derived,
1190                     impl_def_id,
1191                     span: obligation.cause.span,
1192                 }))
1193             });
1194             let obligations = ensure_sufficient_stack(|| {
1195                 self.vtable_impl(
1196                     impl_def_id,
1197                     substs,
1198                     &cause,
1199                     new_obligation.recursion_depth + 1,
1200                     new_obligation.param_env,
1201                     obligation.predicate,
1202                 )
1203             });
1204             nested.extend(obligations.nested);
1205         }
1206
1207         // We want to confirm the ADT's fields if we have an ADT
1208         let mut stack = match *self_ty.skip_binder().kind() {
1209             ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1210             _ => vec![self_ty.skip_binder()],
1211         };
1212
1213         while let Some(nested_ty) = stack.pop() {
1214             match *nested_ty.kind() {
1215                 // We know these types are trivially drop
1216                 ty::Bool
1217                 | ty::Char
1218                 | ty::Int(_)
1219                 | ty::Uint(_)
1220                 | ty::Float(_)
1221                 | ty::Infer(ty::IntVar(_))
1222                 | ty::Infer(ty::FloatVar(_))
1223                 | ty::Str
1224                 | ty::RawPtr(_)
1225                 | ty::Ref(..)
1226                 | ty::FnDef(..)
1227                 | ty::FnPtr(_)
1228                 | ty::Never
1229                 | ty::Foreign(_) => {}
1230
1231                 // `ManuallyDrop` is trivially drop
1232                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
1233
1234                 // These types are built-in, so we can fast-track by registering
1235                 // nested predicates for their constituent type(s)
1236                 ty::Array(ty, _) | ty::Slice(ty) => {
1237                     stack.push(ty);
1238                 }
1239                 ty::Tuple(tys) => {
1240                     stack.extend(tys.iter());
1241                 }
1242                 ty::Closure(_, substs) => {
1243                     stack.push(substs.as_closure().tupled_upvars_ty());
1244                 }
1245                 ty::Generator(_, substs, _) => {
1246                     let generator = substs.as_generator();
1247                     stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1248                 }
1249                 ty::GeneratorWitness(tys) => {
1250                     stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1251                 }
1252
1253                 // If we have a projection type, make sure to normalize it so we replace it
1254                 // with a fresh infer variable
1255                 ty::Projection(..) => {
1256                     let predicate = normalize_with_depth_to(
1257                         self,
1258                         obligation.param_env,
1259                         cause.clone(),
1260                         obligation.recursion_depth + 1,
1261                         self_ty
1262                             .rebind(ty::TraitPredicate {
1263                                 trait_ref: ty::TraitRef {
1264                                     def_id: self.tcx().require_lang_item(LangItem::Destruct, None),
1265                                     substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1266                                 },
1267                                 constness: ty::BoundConstness::ConstIfConst,
1268                                 polarity: ty::ImplPolarity::Positive,
1269                             })
1270                             .to_predicate(tcx),
1271                         &mut nested,
1272                     );
1273
1274                     nested.push(Obligation::with_depth(
1275                         cause.clone(),
1276                         obligation.recursion_depth + 1,
1277                         obligation.param_env,
1278                         predicate,
1279                     ));
1280                 }
1281
1282                 // If we have any other type (e.g. an ADT), just register a nested obligation
1283                 // since it's either not `const Drop` (and we raise an error during selection),
1284                 // or it's an ADT (and we need to check for a custom impl during selection)
1285                 _ => {
1286                     let predicate = self_ty
1287                         .rebind(ty::TraitPredicate {
1288                             trait_ref: ty::TraitRef {
1289                                 def_id: self.tcx().require_lang_item(LangItem::Destruct, None),
1290                                 substs: self.tcx().mk_substs_trait(nested_ty, &[]),
1291                             },
1292                             constness: ty::BoundConstness::ConstIfConst,
1293                             polarity: ty::ImplPolarity::Positive,
1294                         })
1295                         .to_predicate(tcx);
1296
1297                     nested.push(Obligation::with_depth(
1298                         cause.clone(),
1299                         obligation.recursion_depth + 1,
1300                         obligation.param_env,
1301                         predicate,
1302                     ));
1303                 }
1304             }
1305         }
1306
1307         Ok(ImplSourceConstDestructData { nested })
1308     }
1309 }