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::vtable::{
  23     count_own_vtable_entries, prepare_vtable_segments, vtable_trait_first_method_offset,
  24     VtblSegment,
  25 };
  26 use crate::traits::{
  27     BuiltinDerivedObligation, ImplDerivedObligation, ImplDerivedObligationCause, ImplSource,
  28     ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
  29     ImplSourceConstDestructData, ImplSourceFnPointerData, ImplSourceFutureData,
  30     ImplSourceGeneratorData, ImplSourceObjectData, ImplSourceTraitAliasData,
  31     ImplSourceTraitUpcastingData, ImplSourceUserDefinedData, Normalized, ObjectCastObligation,
  32     Obligation, ObligationCause, OutputTypeParameterMismatch, PredicateObligation, Selection,
  33     SelectionError, TraitNotObjectSafe, TraitObligation, Unimplemented,
  34 };
  35
  36 use super::BuiltinImplConditions;
  37 use super::SelectionCandidate::{self, *};
  38 use super::SelectionContext;
  39
  40 use std::iter;
  41 use std::ops::ControlFlow;
  42
  43 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
  44     #[instrument(level = "debug", skip(self))]
  45     pub(super) fn confirm_candidate(
  46         &mut self,
  47         obligation: &TraitObligation<'tcx>,
  48         candidate: SelectionCandidate<'tcx>,
  49     ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
  50         let mut impl_src = match candidate {
  51             BuiltinCandidate { has_nested } => {
  52                 let data = self.confirm_builtin_candidate(obligation, has_nested);
  53                 ImplSource::Builtin(data)
  54             }
  55
  56             TransmutabilityCandidate => {
  57                 let data = self.confirm_transmutability_candidate(obligation)?;
  58                 ImplSource::Builtin(data)
  59             }
  60
  61             ParamCandidate(param) => {
  62                 let obligations =
  63                     self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
  64                 ImplSource::Param(obligations, param.skip_binder().constness)
  65             }
  66
  67             ImplCandidate(impl_def_id) => {
  68                 ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id))
  69             }
  70
  71             AutoImplCandidate => {
  72                 let data = self.confirm_auto_impl_candidate(obligation);
  73                 ImplSource::AutoImpl(data)
  74             }
  75
  76             ProjectionCandidate(idx, constness) => {
  77                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
  78                 ImplSource::Param(obligations, constness)
  79             }
  80
  81             ObjectCandidate(idx) => {
  82                 let data = self.confirm_object_candidate(obligation, idx)?;
  83                 ImplSource::Object(data)
  84             }
  85
  86             ClosureCandidate => {
  87                 let vtable_closure = self.confirm_closure_candidate(obligation)?;
  88                 ImplSource::Closure(vtable_closure)
  89             }
  90
  91             GeneratorCandidate => {
  92                 let vtable_generator = self.confirm_generator_candidate(obligation)?;
  93                 ImplSource::Generator(vtable_generator)
  94             }
  95
  96             FutureCandidate => {
  97                 let vtable_future = self.confirm_future_candidate(obligation)?;
  98                 ImplSource::Future(vtable_future)
  99             }
 100
 101             FnPointerCandidate { .. } => {
 102                 let data = self.confirm_fn_pointer_candidate(obligation)?;
 103                 ImplSource::FnPointer(data)
 104             }
 105
 106             TraitAliasCandidate => {
 107                 let data = self.confirm_trait_alias_candidate(obligation);
 108                 ImplSource::TraitAlias(data)
 109             }
 110
 111             BuiltinObjectCandidate => {
 112                 // This indicates something like `Trait + Send: Send`. In this case, we know that
 113                 // this holds because that's what the object type is telling us, and there's really
 114                 // no additional obligations to prove and no types in particular to unify, etc.
 115                 ImplSource::Param(Vec::new(), ty::BoundConstness::NotConst)
 116             }
 117
 118             BuiltinUnsizeCandidate => {
 119                 let data = self.confirm_builtin_unsize_candidate(obligation)?;
 120                 ImplSource::Builtin(data)
 121             }
 122
 123             TraitUpcastingUnsizeCandidate(idx) => {
 124                 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
 125                 ImplSource::TraitUpcasting(data)
 126             }
 127
 128             ConstDestructCandidate(def_id) => {
 129                 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
 130                 ImplSource::ConstDestruct(data)
 131             }
 132         };
 133
 134         if !obligation.predicate.is_const_if_const() {
 135             // normalize nested predicates according to parent predicate's constness.
 136             impl_src = impl_src.map(|mut o| {
 137                 o.predicate = o.predicate.without_const(self.tcx());
 138                 o
 139             });
 140         }
 141
 142         Ok(impl_src)
 143     }
 144
 145     fn confirm_projection_candidate(
 146         &mut self,
 147         obligation: &TraitObligation<'tcx>,
 148         idx: usize,
 149     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 150         let tcx = self.tcx();
 151
 152         let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
 153         let placeholder_trait_predicate =
 154             self.infcx.replace_bound_vars_with_placeholders(trait_predicate).trait_ref;
 155         let placeholder_self_ty = placeholder_trait_predicate.self_ty();
 156         let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
 157         let (def_id, substs) = match *placeholder_self_ty.kind() {
 158             ty::Alias(_, ty::AliasTy { 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::Alias(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                     self.tcx(),
 200                     obligation.cause.clone(),
 201                     obligation.recursion_depth + 1,
 202                     obligation.param_env,
 203                     normalized,
 204                 ));
 205             }
 206         }
 207
 208         Ok(obligations)
 209     }
 210
 211     fn confirm_param_candidate(
 212         &mut self,
 213         obligation: &TraitObligation<'tcx>,
 214         param: ty::PolyTraitRef<'tcx>,
 215     ) -> Vec<PredicateObligation<'tcx>> {
 216         debug!(?obligation, ?param, "confirm_param_candidate");
 217
 218         // During evaluation, we already checked that this
 219         // where-clause trait-ref could be unified with the obligation
 220         // trait-ref. Repeat that unification now without any
 221         // transactional boundary; it should not fail.
 222         match self.match_where_clause_trait_ref(obligation, param) {
 223             Ok(obligations) => obligations,
 224             Err(()) => {
 225                 bug!(
 226                     "Where clause `{:?}` was applicable to `{:?}` but now is not",
 227                     param,
 228                     obligation
 229                 );
 230             }
 231         }
 232     }
 233
 234     fn confirm_builtin_candidate(
 235         &mut self,
 236         obligation: &TraitObligation<'tcx>,
 237         has_nested: bool,
 238     ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
 239         debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
 240
 241         let lang_items = self.tcx().lang_items();
 242         let obligations = if has_nested {
 243             let trait_def = obligation.predicate.def_id();
 244             let conditions = if Some(trait_def) == lang_items.sized_trait() {
 245                 self.sized_conditions(obligation)
 246             } else if Some(trait_def) == lang_items.copy_trait() {
 247                 self.copy_clone_conditions(obligation)
 248             } else if Some(trait_def) == lang_items.clone_trait() {
 249                 self.copy_clone_conditions(obligation)
 250             } else {
 251                 bug!("unexpected builtin trait {:?}", trait_def)
 252             };
 253             let BuiltinImplConditions::Where(nested) = conditions else {
 254                 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
 255             };
 256
 257             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 258             ensure_sufficient_stack(|| {
 259                 self.collect_predicates_for_types(
 260                     obligation.param_env,
 261                     cause,
 262                     obligation.recursion_depth + 1,
 263                     trait_def,
 264                     nested,
 265                 )
 266             })
 267         } else {
 268             vec![]
 269         };
 270
 271         debug!(?obligations);
 272
 273         ImplSourceBuiltinData { nested: obligations }
 274     }
 275
 276     fn confirm_transmutability_candidate(
 277         &mut self,
 278         obligation: &TraitObligation<'tcx>,
 279     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 280         debug!(?obligation, "confirm_transmutability_candidate");
 281
 282         let predicate = obligation.predicate;
 283
 284         let type_at = |i| predicate.map_bound(|p| p.trait_ref.substs.type_at(i));
 285         let const_at = |i| predicate.skip_binder().trait_ref.substs.const_at(i);
 286
 287         let src_and_dst = predicate.map_bound(|p| rustc_transmute::Types {
 288             dst: p.trait_ref.substs.type_at(0),
 289             src: p.trait_ref.substs.type_at(1),
 290         });
 291
 292         let scope = type_at(2).skip_binder();
 293
 294         let Some(assume) =
 295             rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3)) else {
 296                 return Err(Unimplemented);
 297             };
 298
 299         let cause = obligation.cause.clone();
 300
 301         let mut transmute_env = rustc_transmute::TransmuteTypeEnv::new(self.infcx);
 302
 303         let maybe_transmutable = transmute_env.is_transmutable(cause, src_and_dst, scope, assume);
 304
 305         use rustc_transmute::Answer;
 306
 307         match maybe_transmutable {
 308             Answer::Yes => Ok(ImplSourceBuiltinData { nested: vec![] }),
 309             _ => Err(Unimplemented),
 310         }
 311     }
 312
 313     /// This handles the case where an `auto trait Foo` impl is being used.
 314     /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
 315     ///
 316     /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
 317     /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
 318     fn confirm_auto_impl_candidate(
 319         &mut self,
 320         obligation: &TraitObligation<'tcx>,
 321     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 322         debug!(?obligation, "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, obligation.predicate.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.with(tcx, normalized_super_trait));
 489         }
 490
 491         let assoc_types: Vec<_> = tcx
 492             .associated_items(trait_predicate.def_id())
 493             .in_definition_order()
 494             .filter_map(
 495                 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
 496             )
 497             .collect();
 498
 499         for assoc_type in assoc_types {
 500             let defs: &ty::Generics = tcx.generics_of(assoc_type);
 501
 502             if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
 503                 tcx.sess.delay_span_bug(
 504                     obligation.cause.span,
 505                     "GATs in trait object shouldn't have been considered",
 506                 );
 507                 return Err(SelectionError::Unimplemented);
 508             }
 509
 510             // This maybe belongs in wf, but that can't (doesn't) handle
 511             // higher-ranked things.
 512             // Prevent, e.g., `dyn Iterator<Item = str>`.
 513             for bound in self.tcx().bound_item_bounds(assoc_type).transpose_iter() {
 514                 let subst_bound =
 515                     if defs.count() == 0 {
 516                         bound.subst(tcx, trait_predicate.trait_ref.substs)
 517                     } else {
 518                         let mut substs = smallvec::SmallVec::with_capacity(defs.count());
 519                         substs.extend(trait_predicate.trait_ref.substs.iter());
 520                         let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
 521                             smallvec::SmallVec::with_capacity(
 522                                 bound.0.kind().bound_vars().len() + defs.count(),
 523                             );
 524                         bound_vars.extend(bound.0.kind().bound_vars().into_iter());
 525                         InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
 526                             .kind
 527                         {
 528                             GenericParamDefKind::Type { .. } => {
 529                                 let kind = ty::BoundTyKind::Param(param.name);
 530                                 let bound_var = ty::BoundVariableKind::Ty(kind);
 531                                 bound_vars.push(bound_var);
 532                                 tcx.mk_ty(ty::Bound(
 533                                     ty::INNERMOST,
 534                                     ty::BoundTy {
 535                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 536                                         kind,
 537                                     },
 538                                 ))
 539                                 .into()
 540                             }
 541                             GenericParamDefKind::Lifetime => {
 542                                 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
 543                                 let bound_var = ty::BoundVariableKind::Region(kind);
 544                                 bound_vars.push(bound_var);
 545                                 tcx.mk_region(ty::ReLateBound(
 546                                     ty::INNERMOST,
 547                                     ty::BoundRegion {
 548                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 549                                         kind,
 550                                     },
 551                                 ))
 552                                 .into()
 553                             }
 554                             GenericParamDefKind::Const { .. } => {
 555                                 let bound_var = ty::BoundVariableKind::Const;
 556                                 bound_vars.push(bound_var);
 557                                 tcx.mk_const(
 558                                     ty::ConstKind::Bound(
 559                                         ty::INNERMOST,
 560                                         ty::BoundVar::from_usize(bound_vars.len() - 1),
 561                                     ),
 562                                     tcx.type_of(param.def_id),
 563                                 )
 564                                 .into()
 565                             }
 566                         });
 567                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 568                         let assoc_ty_substs = tcx.intern_substs(&substs);
 569
 570                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 571                         let bound =
 572                             bound.map_bound(|b| b.kind().skip_binder()).subst(tcx, assoc_ty_substs);
 573                         tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
 574                     };
 575                 let normalized_bound = normalize_with_depth_to(
 576                     self,
 577                     obligation.param_env,
 578                     obligation.cause.clone(),
 579                     obligation.recursion_depth + 1,
 580                     subst_bound,
 581                     &mut nested,
 582                 );
 583                 nested.push(obligation.with(tcx, normalized_bound));
 584             }
 585         }
 586
 587         debug!(?nested, "object nested obligations");
 588
 589         let vtable_base = vtable_trait_first_method_offset(
 590             tcx,
 591             (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
 592         );
 593
 594         Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
 595     }
 596
 597     fn confirm_fn_pointer_candidate(
 598         &mut self,
 599         obligation: &TraitObligation<'tcx>,
 600     ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 601     {
 602         debug!(?obligation, "confirm_fn_pointer_candidate");
 603
 604         let self_ty = self
 605             .infcx
 606             .shallow_resolve(obligation.self_ty().no_bound_vars())
 607             .expect("fn pointer should not capture bound vars from predicate");
 608         let sig = self_ty.fn_sig(self.tcx());
 609         let trait_ref = closure_trait_ref_and_return_type(
 610             self.tcx(),
 611             obligation.predicate.def_id(),
 612             self_ty,
 613             sig,
 614             util::TupleArgumentsFlag::Yes,
 615         )
 616         .map_bound(|(trait_ref, _)| trait_ref);
 617
 618         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 619
 620         // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
 621         let cause = obligation.derived_cause(BuiltinDerivedObligation);
 622         let output_ty = self.infcx.replace_bound_vars_with_placeholders(sig.output());
 623         let output_ty = normalize_with_depth_to(
 624             self,
 625             obligation.param_env,
 626             cause.clone(),
 627             obligation.recursion_depth,
 628             output_ty,
 629             &mut nested,
 630         );
 631         let tr =
 632             ty::Binder::dummy(self.tcx().at(cause.span).mk_trait_ref(LangItem::Sized, [output_ty]));
 633         nested.push(Obligation::new(self.infcx.tcx, cause, obligation.param_env, tr));
 634
 635         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
 636     }
 637
 638     fn confirm_trait_alias_candidate(
 639         &mut self,
 640         obligation: &TraitObligation<'tcx>,
 641     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
 642         debug!(?obligation, "confirm_trait_alias_candidate");
 643
 644         let alias_def_id = obligation.predicate.def_id();
 645         let predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
 646         let trait_ref = predicate.trait_ref;
 647         let trait_def_id = trait_ref.def_id;
 648         let substs = trait_ref.substs;
 649
 650         let trait_obligations = self.impl_or_trait_obligations(
 651             &obligation.cause,
 652             obligation.recursion_depth,
 653             obligation.param_env,
 654             trait_def_id,
 655             &substs,
 656             obligation.predicate,
 657         );
 658
 659         debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
 660
 661         ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
 662     }
 663
 664     fn confirm_generator_candidate(
 665         &mut self,
 666         obligation: &TraitObligation<'tcx>,
 667     ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 668     {
 669         // Okay to skip binder because the substs on generator types never
 670         // touch bound regions, they just capture the in-scope
 671         // type/region parameters.
 672         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 673         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 674             bug!("closure candidate for non-closure {:?}", obligation);
 675         };
 676
 677         debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
 678
 679         let gen_sig = substs.as_generator().poly_sig();
 680
 681         // NOTE: The self-type is a generator type and hence is
 682         // in fact unparameterized (or at least does not reference any
 683         // regions bound in the obligation).
 684         let self_ty = obligation
 685             .predicate
 686             .self_ty()
 687             .no_bound_vars()
 688             .expect("unboxed closure type should not capture bound vars from the predicate");
 689
 690         let trait_ref = super::util::generator_trait_ref_and_outputs(
 691             self.tcx(),
 692             obligation.predicate.def_id(),
 693             self_ty,
 694             gen_sig,
 695         )
 696         .map_bound(|(trait_ref, ..)| trait_ref);
 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     fn confirm_future_candidate(
 705         &mut self,
 706         obligation: &TraitObligation<'tcx>,
 707     ) -> Result<ImplSourceFutureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 708         // Okay to skip binder because the substs on generator types never
 709         // touch bound regions, they just capture the in-scope
 710         // type/region parameters.
 711         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 712         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 713             bug!("closure candidate for non-closure {:?}", obligation);
 714         };
 715
 716         debug!(?obligation, ?generator_def_id, ?substs, "confirm_future_candidate");
 717
 718         let gen_sig = substs.as_generator().poly_sig();
 719
 720         let trait_ref = super::util::future_trait_ref_and_outputs(
 721             self.tcx(),
 722             obligation.predicate.def_id(),
 723             obligation.predicate.no_bound_vars().expect("future has no bound vars").self_ty(),
 724             gen_sig,
 725         )
 726         .map_bound(|(trait_ref, ..)| trait_ref);
 727
 728         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 729         debug!(?trait_ref, ?nested, "future candidate obligations");
 730
 731         Ok(ImplSourceFutureData { generator_def_id, substs, nested })
 732     }
 733
 734     #[instrument(skip(self), level = "debug")]
 735     fn confirm_closure_candidate(
 736         &mut self,
 737         obligation: &TraitObligation<'tcx>,
 738     ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 739         let kind = self
 740             .tcx()
 741             .fn_trait_kind_from_def_id(obligation.predicate.def_id())
 742             .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
 743
 744         // Okay to skip binder because the substs on closure types never
 745         // touch bound regions, they just capture the in-scope
 746         // type/region parameters.
 747         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 748         let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
 749             bug!("closure candidate for non-closure {:?}", obligation);
 750         };
 751
 752         let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
 753         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 754
 755         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 756
 757         // FIXME: Chalk
 758
 759         if !self.tcx().sess.opts.unstable_opts.chalk {
 760             nested.push(obligation.with(
 761                 self.tcx(),
 762                 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind)),
 763             ));
 764         }
 765
 766         Ok(ImplSourceClosureData { closure_def_id, substs, nested })
 767     }
 768
 769     /// In the case of closure types and fn pointers,
 770     /// we currently treat the input type parameters on the trait as
 771     /// outputs. This means that when we have a match we have only
 772     /// considered the self type, so we have to go back and make sure
 773     /// to relate the argument types too. This is kind of wrong, but
 774     /// since we control the full set of impls, also not that wrong,
 775     /// and it DOES yield better error messages (since we don't report
 776     /// errors as if there is no applicable impl, but rather report
 777     /// errors are about mismatched argument types.
 778     ///
 779     /// Here is an example. Imagine we have a closure expression
 780     /// and we desugared it so that the type of the expression is
 781     /// `Closure`, and `Closure` expects `i32` as argument. Then it
 782     /// is "as if" the compiler generated this impl:
 783     /// ```ignore (illustrative)
 784     /// impl Fn(i32) for Closure { ... }
 785     /// ```
 786     /// Now imagine our obligation is `Closure: Fn(usize)`. So far
 787     /// we have matched the self type `Closure`. At this point we'll
 788     /// compare the `i32` to `usize` and generate an error.
 789     ///
 790     /// Note that this checking occurs *after* the impl has selected,
 791     /// because these output type parameters should not affect the
 792     /// selection of the impl. Therefore, if there is a mismatch, we
 793     /// report an error to the user.
 794     #[instrument(skip(self), level = "trace")]
 795     fn confirm_poly_trait_refs(
 796         &mut self,
 797         obligation: &TraitObligation<'tcx>,
 798         expected_trait_ref: ty::PolyTraitRef<'tcx>,
 799     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 800         let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
 801         // Normalize the obligation and expected trait refs together, because why not
 802         let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
 803             ensure_sufficient_stack(|| {
 804                 normalize_with_depth(
 805                     self,
 806                     obligation.param_env,
 807                     obligation.cause.clone(),
 808                     obligation.recursion_depth + 1,
 809                     (obligation_trait_ref, expected_trait_ref),
 810                 )
 811             });
 812
 813         self.infcx
 814             .at(&obligation.cause, obligation.param_env)
 815             .sup(obligation_trait_ref, expected_trait_ref)
 816             .map(|InferOk { mut obligations, .. }| {
 817                 obligations.extend(nested);
 818                 obligations
 819             })
 820             .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
 821     }
 822
 823     fn confirm_trait_upcasting_unsize_candidate(
 824         &mut self,
 825         obligation: &TraitObligation<'tcx>,
 826         idx: usize,
 827     ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 828     {
 829         let tcx = self.tcx();
 830
 831         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 832         // regions here. See the comment there for more details.
 833         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 834         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 835         let target = self.infcx.shallow_resolve(target);
 836
 837         debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
 838
 839         let mut nested = vec![];
 840         let source_trait_ref;
 841         let upcast_trait_ref;
 842         match (source.kind(), target.kind()) {
 843             // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
 844             (
 845                 &ty::Dynamic(ref data_a, r_a, repr_a @ ty::Dyn),
 846                 &ty::Dynamic(ref data_b, r_b, ty::Dyn),
 847             ) => {
 848                 // See `assemble_candidates_for_unsizing` for more info.
 849                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 850                 let principal_a = data_a.principal().unwrap();
 851                 source_trait_ref = principal_a.with_self_ty(tcx, source);
 852                 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
 853                 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
 854                 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
 855                     ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
 856                         tcx, trait_ref,
 857                     ))
 858                 });
 859                 let iter = Some(existential_predicate)
 860                     .into_iter()
 861                     .chain(
 862                         data_a
 863                             .projection_bounds()
 864                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 865                     )
 866                     .chain(
 867                         data_b
 868                             .auto_traits()
 869                             .map(ty::ExistentialPredicate::AutoTrait)
 870                             .map(ty::Binder::dummy),
 871                     );
 872                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 873                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_a);
 874
 875                 // Require that the traits involved in this upcast are **equal**;
 876                 // only the **lifetime bound** is changed.
 877                 let InferOk { obligations, .. } = self
 878                     .infcx
 879                     .at(&obligation.cause, obligation.param_env)
 880                     .sup(target, source_trait)
 881                     .map_err(|_| Unimplemented)?;
 882                 nested.extend(obligations);
 883
 884                 // Register one obligation for 'a: 'b.
 885                 let cause = ObligationCause::new(
 886                     obligation.cause.span,
 887                     obligation.cause.body_id,
 888                     ObjectCastObligation(source, target),
 889                 );
 890                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 891                 nested.push(Obligation::with_depth(
 892                     tcx,
 893                     cause,
 894                     obligation.recursion_depth + 1,
 895                     obligation.param_env,
 896                     obligation.predicate.rebind(outlives),
 897                 ));
 898             }
 899             _ => bug!(),
 900         };
 901
 902         let vtable_segment_callback = {
 903             let mut vptr_offset = 0;
 904             move |segment| {
 905                 match segment {
 906                     VtblSegment::MetadataDSA => {
 907                         vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
 908                     }
 909                     VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 910                         vptr_offset += count_own_vtable_entries(tcx, trait_ref);
 911                         if trait_ref == upcast_trait_ref {
 912                             if emit_vptr {
 913                                 return ControlFlow::Break(Some(vptr_offset));
 914                             } else {
 915                                 return ControlFlow::Break(None);
 916                             }
 917                         }
 918
 919                         if emit_vptr {
 920                             vptr_offset += 1;
 921                         }
 922                     }
 923                 }
 924                 ControlFlow::Continue(())
 925             }
 926         };
 927
 928         let vtable_vptr_slot =
 929             prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback).unwrap();
 930
 931         Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
 932     }
 933
 934     fn confirm_builtin_unsize_candidate(
 935         &mut self,
 936         obligation: &TraitObligation<'tcx>,
 937     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 938         let tcx = self.tcx();
 939
 940         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 941         // regions here. See the comment there for more details.
 942         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 943         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 944         let target = self.infcx.shallow_resolve(target);
 945
 946         debug!(?source, ?target, "confirm_builtin_unsize_candidate");
 947
 948         let mut nested = vec![];
 949         match (source.kind(), target.kind()) {
 950             // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
 951             (&ty::Dynamic(ref data_a, r_a, dyn_a), &ty::Dynamic(ref data_b, r_b, dyn_b))
 952                 if dyn_a == dyn_b =>
 953             {
 954                 // See `assemble_candidates_for_unsizing` for more info.
 955                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 956                 let iter = data_a
 957                     .principal()
 958                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
 959                     .into_iter()
 960                     .chain(
 961                         data_a
 962                             .projection_bounds()
 963                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 964                     )
 965                     .chain(
 966                         data_b
 967                             .auto_traits()
 968                             .map(ty::ExistentialPredicate::AutoTrait)
 969                             .map(ty::Binder::dummy),
 970                     );
 971                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 972                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, dyn_a);
 973
 974                 // Require that the traits involved in this upcast are **equal**;
 975                 // only the **lifetime bound** is changed.
 976                 let InferOk { obligations, .. } = self
 977                     .infcx
 978                     .at(&obligation.cause, obligation.param_env)
 979                     .sup(target, source_trait)
 980                     .map_err(|_| Unimplemented)?;
 981                 nested.extend(obligations);
 982
 983                 // Register one obligation for 'a: 'b.
 984                 let cause = ObligationCause::new(
 985                     obligation.cause.span,
 986                     obligation.cause.body_id,
 987                     ObjectCastObligation(source, target),
 988                 );
 989                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 990                 nested.push(Obligation::with_depth(
 991                     tcx,
 992                     cause,
 993                     obligation.recursion_depth + 1,
 994                     obligation.param_env,
 995                     obligation.predicate.rebind(outlives),
 996                 ));
 997             }
 998
 999             // `T` -> `Trait`
1000             (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
1001                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
1002                 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
1003                     return Err(TraitNotObjectSafe(did));
1004                 }
1005
1006                 let cause = ObligationCause::new(
1007                     obligation.cause.span,
1008                     obligation.cause.body_id,
1009                     ObjectCastObligation(source, target),
1010                 );
1011
1012                 let predicate_to_obligation = |predicate| {
1013                     Obligation::with_depth(
1014                         tcx,
1015                         cause.clone(),
1016                         obligation.recursion_depth + 1,
1017                         obligation.param_env,
1018                         predicate,
1019                     )
1020                 };
1021
1022                 // Create obligations:
1023                 //  - Casting `T` to `Trait`
1024                 //  - For all the various builtin bounds attached to the object cast. (In other
1025                 //  words, if the object type is `Foo + Send`, this would create an obligation for
1026                 //  the `Send` check.)
1027                 //  - Projection predicates
1028                 nested.extend(
1029                     data.iter().map(|predicate| {
1030                         predicate_to_obligation(predicate.with_self_ty(tcx, source))
1031                     }),
1032                 );
1033
1034                 // We can only make objects from sized types.
1035                 let tr =
1036                     ty::Binder::dummy(tcx.at(cause.span).mk_trait_ref(LangItem::Sized, [source]));
1037                 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
1038
1039                 // If the type is `Foo + 'a`, ensure that the type
1040                 // being cast to `Foo + 'a` outlives `'a`:
1041                 let outlives = ty::OutlivesPredicate(source, r);
1042                 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
1043             }
1044
1045             // `[T; n]` -> `[T]`
1046             (&ty::Array(a, _), &ty::Slice(b)) => {
1047                 let InferOk { obligations, .. } = self
1048                     .infcx
1049                     .at(&obligation.cause, obligation.param_env)
1050                     .eq(b, a)
1051                     .map_err(|_| Unimplemented)?;
1052                 nested.extend(obligations);
1053             }
1054
1055             // `Struct<T>` -> `Struct<U>`
1056             (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
1057                 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
1058                     GenericArgKind::Type(ty) => match ty.kind() {
1059                         ty::Param(p) => Some(p.index),
1060                         _ => None,
1061                     },
1062
1063                     // Lifetimes aren't allowed to change during unsizing.
1064                     GenericArgKind::Lifetime(_) => None,
1065
1066                     GenericArgKind::Const(ct) => match ct.kind() {
1067                         ty::ConstKind::Param(p) => Some(p.index),
1068                         _ => None,
1069                     },
1070                 };
1071
1072                 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1073                 // by putting it in a query; it would only need the `DefId` as it
1074                 // looks at declared field types, not anything substituted.
1075
1076                 // The last field of the structure has to exist and contain type/const parameters.
1077                 let (tail_field, prefix_fields) =
1078                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1079                 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1080
1081                 let mut unsizing_params = GrowableBitSet::new_empty();
1082                 for arg in tail_field_ty.0.walk() {
1083                     if let Some(i) = maybe_unsizing_param_idx(arg) {
1084                         unsizing_params.insert(i);
1085                     }
1086                 }
1087
1088                 // Ensure none of the other fields mention the parameters used
1089                 // in unsizing.
1090                 for field in prefix_fields {
1091                     for arg in tcx.type_of(field.did).walk() {
1092                         if let Some(i) = maybe_unsizing_param_idx(arg) {
1093                             unsizing_params.remove(i);
1094                         }
1095                     }
1096                 }
1097
1098                 if unsizing_params.is_empty() {
1099                     return Err(Unimplemented);
1100                 }
1101
1102                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
1103                 // normalizing in the process, since `type_of` returns something directly from
1104                 // astconv (which means it's un-normalized).
1105                 let source_tail = normalize_with_depth_to(
1106                     self,
1107                     obligation.param_env,
1108                     obligation.cause.clone(),
1109                     obligation.recursion_depth + 1,
1110                     tail_field_ty.subst(tcx, substs_a),
1111                     &mut nested,
1112                 );
1113                 let target_tail = normalize_with_depth_to(
1114                     self,
1115                     obligation.param_env,
1116                     obligation.cause.clone(),
1117                     obligation.recursion_depth + 1,
1118                     tail_field_ty.subst(tcx, substs_b),
1119                     &mut nested,
1120                 );
1121
1122                 // Check that the source struct with the target's
1123                 // unsizing parameters is equal to the target.
1124                 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1125                     if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1126                 }));
1127                 let new_struct = tcx.mk_adt(def, substs);
1128                 let InferOk { obligations, .. } = self
1129                     .infcx
1130                     .at(&obligation.cause, obligation.param_env)
1131                     .eq(target, new_struct)
1132                     .map_err(|_| Unimplemented)?;
1133                 nested.extend(obligations);
1134
1135                 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1136                 nested.push(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                     [source_tail, target_tail],
1143                 ));
1144             }
1145
1146             // `(.., T)` -> `(.., U)`
1147             (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1148                 assert_eq!(tys_a.len(), tys_b.len());
1149
1150                 // The last field of the tuple has to exist.
1151                 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1152                 let &b_last = tys_b.last().unwrap();
1153
1154                 // Check that the source tuple with the target's
1155                 // last element is equal to the target.
1156                 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1157                 let InferOk { obligations, .. } = self
1158                     .infcx
1159                     .at(&obligation.cause, obligation.param_env)
1160                     .eq(target, new_tuple)
1161                     .map_err(|_| Unimplemented)?;
1162                 nested.extend(obligations);
1163
1164                 // Construct the nested `T: Unsize<U>` predicate.
1165                 nested.push(ensure_sufficient_stack(|| {
1166                     predicate_for_trait_def(
1167                         tcx,
1168                         obligation.param_env,
1169                         obligation.cause.clone(),
1170                         obligation.predicate.def_id(),
1171                         obligation.recursion_depth + 1,
1172                         [a_last, b_last],
1173                     )
1174                 }));
1175             }
1176
1177             _ => bug!("source: {source}, target: {target}"),
1178         };
1179
1180         Ok(ImplSourceBuiltinData { nested })
1181     }
1182
1183     fn confirm_const_destruct_candidate(
1184         &mut self,
1185         obligation: &TraitObligation<'tcx>,
1186         impl_def_id: Option<DefId>,
1187     ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1188         // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1189         if !obligation.is_const() {
1190             return Ok(ImplSourceConstDestructData { nested: vec![] });
1191         }
1192
1193         let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1194
1195         let tcx = self.tcx();
1196         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1197
1198         let mut nested = vec![];
1199         let cause = obligation.derived_cause(BuiltinDerivedObligation);
1200
1201         // If we have a custom `impl const Drop`, then
1202         // first check it like a regular impl candidate.
1203         // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1204         if let Some(impl_def_id) = impl_def_id {
1205             let mut new_obligation = obligation.clone();
1206             new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1207                 trait_pred.trait_ref.def_id = drop_trait;
1208                 trait_pred
1209             });
1210             let substs = self.rematch_impl(impl_def_id, &new_obligation);
1211             debug!(?substs, "impl substs");
1212
1213             let cause = obligation.derived_cause(|derived| {
1214                 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1215                     derived,
1216                     impl_def_id,
1217                     span: obligation.cause.span,
1218                 }))
1219             });
1220             let obligations = ensure_sufficient_stack(|| {
1221                 self.vtable_impl(
1222                     impl_def_id,
1223                     substs,
1224                     &cause,
1225                     new_obligation.recursion_depth + 1,
1226                     new_obligation.param_env,
1227                     obligation.predicate,
1228                 )
1229             });
1230             nested.extend(obligations.nested);
1231         }
1232
1233         // We want to confirm the ADT's fields if we have an ADT
1234         let mut stack = match *self_ty.skip_binder().kind() {
1235             ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1236             _ => vec![self_ty.skip_binder()],
1237         };
1238
1239         while let Some(nested_ty) = stack.pop() {
1240             match *nested_ty.kind() {
1241                 // We know these types are trivially drop
1242                 ty::Bool
1243                 | ty::Char
1244                 | ty::Int(_)
1245                 | ty::Uint(_)
1246                 | ty::Float(_)
1247                 | ty::Infer(ty::IntVar(_))
1248                 | ty::Infer(ty::FloatVar(_))
1249                 | ty::Str
1250                 | ty::RawPtr(_)
1251                 | ty::Ref(..)
1252                 | ty::FnDef(..)
1253                 | ty::FnPtr(_)
1254                 | ty::Never
1255                 | ty::Foreign(_) => {}
1256
1257                 // `ManuallyDrop` is trivially drop
1258                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
1259
1260                 // These types are built-in, so we can fast-track by registering
1261                 // nested predicates for their constituent type(s)
1262                 ty::Array(ty, _) | ty::Slice(ty) => {
1263                     stack.push(ty);
1264                 }
1265                 ty::Tuple(tys) => {
1266                     stack.extend(tys.iter());
1267                 }
1268                 ty::Closure(_, substs) => {
1269                     stack.push(substs.as_closure().tupled_upvars_ty());
1270                 }
1271                 ty::Generator(_, substs, _) => {
1272                     let generator = substs.as_generator();
1273                     stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1274                 }
1275                 ty::GeneratorWitness(tys) => {
1276                     stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1277                 }
1278
1279                 // If we have a projection type, make sure to normalize it so we replace it
1280                 // with a fresh infer variable
1281                 ty::Alias(ty::Projection, ..) => {
1282                     let predicate = normalize_with_depth_to(
1283                         self,
1284                         obligation.param_env,
1285                         cause.clone(),
1286                         obligation.recursion_depth + 1,
1287                         self_ty.rebind(ty::TraitPredicate {
1288                             trait_ref: self
1289                                 .tcx()
1290                                 .at(cause.span)
1291                                 .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1292                             constness: ty::BoundConstness::ConstIfConst,
1293                             polarity: ty::ImplPolarity::Positive,
1294                         }),
1295                         &mut nested,
1296                     );
1297
1298                     nested.push(Obligation::with_depth(
1299                         tcx,
1300                         cause.clone(),
1301                         obligation.recursion_depth + 1,
1302                         obligation.param_env,
1303                         predicate,
1304                     ));
1305                 }
1306
1307                 // If we have any other type (e.g. an ADT), just register a nested obligation
1308                 // since it's either not `const Drop` (and we raise an error during selection),
1309                 // or it's an ADT (and we need to check for a custom impl during selection)
1310                 _ => {
1311                     let predicate = self_ty.rebind(ty::TraitPredicate {
1312                         trait_ref: self
1313                             .tcx()
1314                             .at(cause.span)
1315                             .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1316                         constness: ty::BoundConstness::ConstIfConst,
1317                         polarity: ty::ImplPolarity::Positive,
1318                     });
1319
1320                     nested.push(Obligation::with_depth(
1321                         tcx,
1322                         cause.clone(),
1323                         obligation.recursion_depth + 1,
1324                         obligation.param_env,
1325                         predicate,
1326                     ));
1327                 }
1328             }
1329         }
1330
1331         Ok(ImplSourceConstDestructData { nested })
1332     }
1333 }