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, Binder, GenericArg, GenericArgKind, GenericParamDefKind, InternalSubsts, SubstsRef,
  16     ToPolyTraitRef, ToPredicate, TraitRef, 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 { is_const } => {
 102                 let data = self.confirm_fn_pointer_candidate(obligation, is_const)?;
 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         is_const: bool,
 601     ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 602     {
 603         debug!(?obligation, "confirm_fn_pointer_candidate");
 604
 605         let tcx = self.tcx();
 606         let self_ty = self
 607             .infcx
 608             .shallow_resolve(obligation.self_ty().no_bound_vars())
 609             .expect("fn pointer should not capture bound vars from predicate");
 610         let sig = self_ty.fn_sig(tcx);
 611         let trait_ref = closure_trait_ref_and_return_type(
 612             tcx,
 613             obligation.predicate.def_id(),
 614             self_ty,
 615             sig,
 616             util::TupleArgumentsFlag::Yes,
 617         )
 618         .map_bound(|(trait_ref, _)| trait_ref);
 619
 620         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 621         let cause = obligation.derived_cause(BuiltinDerivedObligation);
 622
 623         if obligation.is_const() && !is_const {
 624             // function is a trait method
 625             if let ty::FnDef(def_id, substs) = self_ty.kind() && let Some(trait_id) = tcx.trait_of_item(*def_id) {
 626                 let trait_ref = TraitRef::from_method(tcx, trait_id, *substs);
 627                 let poly_trait_pred = Binder::dummy(trait_ref).with_constness(ty::BoundConstness::ConstIfConst);
 628                 let obligation = Obligation::new(tcx, cause.clone(), obligation.param_env, poly_trait_pred);
 629                 nested.push(obligation);
 630             }
 631         }
 632
 633         // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
 634         let output_ty = self.infcx.replace_bound_vars_with_placeholders(sig.output());
 635         let output_ty = normalize_with_depth_to(
 636             self,
 637             obligation.param_env,
 638             cause.clone(),
 639             obligation.recursion_depth,
 640             output_ty,
 641             &mut nested,
 642         );
 643         let tr =
 644             ty::Binder::dummy(self.tcx().at(cause.span).mk_trait_ref(LangItem::Sized, [output_ty]));
 645         nested.push(Obligation::new(self.infcx.tcx, cause, obligation.param_env, tr));
 646
 647         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
 648     }
 649
 650     fn confirm_trait_alias_candidate(
 651         &mut self,
 652         obligation: &TraitObligation<'tcx>,
 653     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
 654         debug!(?obligation, "confirm_trait_alias_candidate");
 655
 656         let alias_def_id = obligation.predicate.def_id();
 657         let predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
 658         let trait_ref = predicate.trait_ref;
 659         let trait_def_id = trait_ref.def_id;
 660         let substs = trait_ref.substs;
 661
 662         let trait_obligations = self.impl_or_trait_obligations(
 663             &obligation.cause,
 664             obligation.recursion_depth,
 665             obligation.param_env,
 666             trait_def_id,
 667             &substs,
 668             obligation.predicate,
 669         );
 670
 671         debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
 672
 673         ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
 674     }
 675
 676     fn confirm_generator_candidate(
 677         &mut self,
 678         obligation: &TraitObligation<'tcx>,
 679     ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 680     {
 681         // Okay to skip binder because the substs on generator types never
 682         // touch bound regions, they just capture the in-scope
 683         // type/region parameters.
 684         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 685         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 686             bug!("closure candidate for non-closure {:?}", obligation);
 687         };
 688
 689         debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
 690
 691         let gen_sig = substs.as_generator().poly_sig();
 692
 693         // NOTE: The self-type is a generator type and hence is
 694         // in fact unparameterized (or at least does not reference any
 695         // regions bound in the obligation).
 696         let self_ty = obligation
 697             .predicate
 698             .self_ty()
 699             .no_bound_vars()
 700             .expect("unboxed closure type should not capture bound vars from the predicate");
 701
 702         let trait_ref = super::util::generator_trait_ref_and_outputs(
 703             self.tcx(),
 704             obligation.predicate.def_id(),
 705             self_ty,
 706             gen_sig,
 707         )
 708         .map_bound(|(trait_ref, ..)| trait_ref);
 709
 710         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 711         debug!(?trait_ref, ?nested, "generator candidate obligations");
 712
 713         Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
 714     }
 715
 716     fn confirm_future_candidate(
 717         &mut self,
 718         obligation: &TraitObligation<'tcx>,
 719     ) -> Result<ImplSourceFutureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 720         // Okay to skip binder because the substs on generator types never
 721         // touch bound regions, they just capture the in-scope
 722         // type/region parameters.
 723         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 724         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 725             bug!("closure candidate for non-closure {:?}", obligation);
 726         };
 727
 728         debug!(?obligation, ?generator_def_id, ?substs, "confirm_future_candidate");
 729
 730         let gen_sig = substs.as_generator().poly_sig();
 731
 732         let trait_ref = super::util::future_trait_ref_and_outputs(
 733             self.tcx(),
 734             obligation.predicate.def_id(),
 735             obligation.predicate.no_bound_vars().expect("future has no bound vars").self_ty(),
 736             gen_sig,
 737         )
 738         .map_bound(|(trait_ref, ..)| trait_ref);
 739
 740         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 741         debug!(?trait_ref, ?nested, "future candidate obligations");
 742
 743         Ok(ImplSourceFutureData { generator_def_id, substs, nested })
 744     }
 745
 746     #[instrument(skip(self), level = "debug")]
 747     fn confirm_closure_candidate(
 748         &mut self,
 749         obligation: &TraitObligation<'tcx>,
 750     ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 751         let kind = self
 752             .tcx()
 753             .fn_trait_kind_from_def_id(obligation.predicate.def_id())
 754             .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
 755
 756         // Okay to skip binder because the substs on closure types never
 757         // touch bound regions, they just capture the in-scope
 758         // type/region parameters.
 759         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 760         let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
 761             bug!("closure candidate for non-closure {:?}", obligation);
 762         };
 763
 764         let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
 765         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 766
 767         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 768
 769         // FIXME: Chalk
 770
 771         if !self.tcx().sess.opts.unstable_opts.chalk {
 772             nested.push(obligation.with(
 773                 self.tcx(),
 774                 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind)),
 775             ));
 776         }
 777
 778         Ok(ImplSourceClosureData { closure_def_id, substs, nested })
 779     }
 780
 781     /// In the case of closure types and fn pointers,
 782     /// we currently treat the input type parameters on the trait as
 783     /// outputs. This means that when we have a match we have only
 784     /// considered the self type, so we have to go back and make sure
 785     /// to relate the argument types too. This is kind of wrong, but
 786     /// since we control the full set of impls, also not that wrong,
 787     /// and it DOES yield better error messages (since we don't report
 788     /// errors as if there is no applicable impl, but rather report
 789     /// errors are about mismatched argument types.
 790     ///
 791     /// Here is an example. Imagine we have a closure expression
 792     /// and we desugared it so that the type of the expression is
 793     /// `Closure`, and `Closure` expects `i32` as argument. Then it
 794     /// is "as if" the compiler generated this impl:
 795     /// ```ignore (illustrative)
 796     /// impl Fn(i32) for Closure { ... }
 797     /// ```
 798     /// Now imagine our obligation is `Closure: Fn(usize)`. So far
 799     /// we have matched the self type `Closure`. At this point we'll
 800     /// compare the `i32` to `usize` and generate an error.
 801     ///
 802     /// Note that this checking occurs *after* the impl has selected,
 803     /// because these output type parameters should not affect the
 804     /// selection of the impl. Therefore, if there is a mismatch, we
 805     /// report an error to the user.
 806     #[instrument(skip(self), level = "trace")]
 807     fn confirm_poly_trait_refs(
 808         &mut self,
 809         obligation: &TraitObligation<'tcx>,
 810         expected_trait_ref: ty::PolyTraitRef<'tcx>,
 811     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 812         let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
 813         // Normalize the obligation and expected trait refs together, because why not
 814         let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
 815             ensure_sufficient_stack(|| {
 816                 normalize_with_depth(
 817                     self,
 818                     obligation.param_env,
 819                     obligation.cause.clone(),
 820                     obligation.recursion_depth + 1,
 821                     (obligation_trait_ref, expected_trait_ref),
 822                 )
 823             });
 824
 825         self.infcx
 826             .at(&obligation.cause, obligation.param_env)
 827             .sup(obligation_trait_ref, expected_trait_ref)
 828             .map(|InferOk { mut obligations, .. }| {
 829                 obligations.extend(nested);
 830                 obligations
 831             })
 832             .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
 833     }
 834
 835     fn confirm_trait_upcasting_unsize_candidate(
 836         &mut self,
 837         obligation: &TraitObligation<'tcx>,
 838         idx: usize,
 839     ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 840     {
 841         let tcx = self.tcx();
 842
 843         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 844         // regions here. See the comment there for more details.
 845         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 846         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 847         let target = self.infcx.shallow_resolve(target);
 848
 849         debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
 850
 851         let mut nested = vec![];
 852         let source_trait_ref;
 853         let upcast_trait_ref;
 854         match (source.kind(), target.kind()) {
 855             // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
 856             (
 857                 &ty::Dynamic(ref data_a, r_a, repr_a @ ty::Dyn),
 858                 &ty::Dynamic(ref data_b, r_b, ty::Dyn),
 859             ) => {
 860                 // See `assemble_candidates_for_unsizing` for more info.
 861                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 862                 let principal_a = data_a.principal().unwrap();
 863                 source_trait_ref = principal_a.with_self_ty(tcx, source);
 864                 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
 865                 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
 866                 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
 867                     ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
 868                         tcx, trait_ref,
 869                     ))
 870                 });
 871                 let iter = Some(existential_predicate)
 872                     .into_iter()
 873                     .chain(
 874                         data_a
 875                             .projection_bounds()
 876                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 877                     )
 878                     .chain(
 879                         data_b
 880                             .auto_traits()
 881                             .map(ty::ExistentialPredicate::AutoTrait)
 882                             .map(ty::Binder::dummy),
 883                     );
 884                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 885                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_a);
 886
 887                 // Require that the traits involved in this upcast are **equal**;
 888                 // only the **lifetime bound** is changed.
 889                 let InferOk { obligations, .. } = self
 890                     .infcx
 891                     .at(&obligation.cause, obligation.param_env)
 892                     .sup(target, source_trait)
 893                     .map_err(|_| Unimplemented)?;
 894                 nested.extend(obligations);
 895
 896                 // Register one obligation for 'a: 'b.
 897                 let cause = ObligationCause::new(
 898                     obligation.cause.span,
 899                     obligation.cause.body_id,
 900                     ObjectCastObligation(source, target),
 901                 );
 902                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 903                 nested.push(Obligation::with_depth(
 904                     tcx,
 905                     cause,
 906                     obligation.recursion_depth + 1,
 907                     obligation.param_env,
 908                     obligation.predicate.rebind(outlives),
 909                 ));
 910             }
 911             _ => bug!(),
 912         };
 913
 914         let vtable_segment_callback = {
 915             let mut vptr_offset = 0;
 916             move |segment| {
 917                 match segment {
 918                     VtblSegment::MetadataDSA => {
 919                         vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
 920                     }
 921                     VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 922                         vptr_offset += count_own_vtable_entries(tcx, trait_ref);
 923                         if trait_ref == upcast_trait_ref {
 924                             if emit_vptr {
 925                                 return ControlFlow::Break(Some(vptr_offset));
 926                             } else {
 927                                 return ControlFlow::Break(None);
 928                             }
 929                         }
 930
 931                         if emit_vptr {
 932                             vptr_offset += 1;
 933                         }
 934                     }
 935                 }
 936                 ControlFlow::Continue(())
 937             }
 938         };
 939
 940         let vtable_vptr_slot =
 941             prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback).unwrap();
 942
 943         Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
 944     }
 945
 946     fn confirm_builtin_unsize_candidate(
 947         &mut self,
 948         obligation: &TraitObligation<'tcx>,
 949     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 950         let tcx = self.tcx();
 951
 952         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 953         // regions here. See the comment there for more details.
 954         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 955         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 956         let target = self.infcx.shallow_resolve(target);
 957
 958         debug!(?source, ?target, "confirm_builtin_unsize_candidate");
 959
 960         let mut nested = vec![];
 961         match (source.kind(), target.kind()) {
 962             // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
 963             (&ty::Dynamic(ref data_a, r_a, dyn_a), &ty::Dynamic(ref data_b, r_b, dyn_b))
 964                 if dyn_a == dyn_b =>
 965             {
 966                 // See `assemble_candidates_for_unsizing` for more info.
 967                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 968                 let iter = data_a
 969                     .principal()
 970                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
 971                     .into_iter()
 972                     .chain(
 973                         data_a
 974                             .projection_bounds()
 975                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 976                     )
 977                     .chain(
 978                         data_b
 979                             .auto_traits()
 980                             .map(ty::ExistentialPredicate::AutoTrait)
 981                             .map(ty::Binder::dummy),
 982                     );
 983                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 984                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, dyn_a);
 985
 986                 // Require that the traits involved in this upcast are **equal**;
 987                 // only the **lifetime bound** is changed.
 988                 let InferOk { obligations, .. } = self
 989                     .infcx
 990                     .at(&obligation.cause, obligation.param_env)
 991                     .sup(target, source_trait)
 992                     .map_err(|_| Unimplemented)?;
 993                 nested.extend(obligations);
 994
 995                 // Register one obligation for 'a: 'b.
 996                 let cause = ObligationCause::new(
 997                     obligation.cause.span,
 998                     obligation.cause.body_id,
 999                     ObjectCastObligation(source, target),
1000                 );
1001                 let outlives = ty::OutlivesPredicate(r_a, r_b);
1002                 nested.push(Obligation::with_depth(
1003                     tcx,
1004                     cause,
1005                     obligation.recursion_depth + 1,
1006                     obligation.param_env,
1007                     obligation.predicate.rebind(outlives),
1008                 ));
1009             }
1010
1011             // `T` -> `Trait`
1012             (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
1013                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
1014                 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
1015                     return Err(TraitNotObjectSafe(did));
1016                 }
1017
1018                 let cause = ObligationCause::new(
1019                     obligation.cause.span,
1020                     obligation.cause.body_id,
1021                     ObjectCastObligation(source, target),
1022                 );
1023
1024                 let predicate_to_obligation = |predicate| {
1025                     Obligation::with_depth(
1026                         tcx,
1027                         cause.clone(),
1028                         obligation.recursion_depth + 1,
1029                         obligation.param_env,
1030                         predicate,
1031                     )
1032                 };
1033
1034                 // Create obligations:
1035                 //  - Casting `T` to `Trait`
1036                 //  - For all the various builtin bounds attached to the object cast. (In other
1037                 //  words, if the object type is `Foo + Send`, this would create an obligation for
1038                 //  the `Send` check.)
1039                 //  - Projection predicates
1040                 nested.extend(
1041                     data.iter().map(|predicate| {
1042                         predicate_to_obligation(predicate.with_self_ty(tcx, source))
1043                     }),
1044                 );
1045
1046                 // We can only make objects from sized types.
1047                 let tr =
1048                     ty::Binder::dummy(tcx.at(cause.span).mk_trait_ref(LangItem::Sized, [source]));
1049                 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
1050
1051                 // If the type is `Foo + 'a`, ensure that the type
1052                 // being cast to `Foo + 'a` outlives `'a`:
1053                 let outlives = ty::OutlivesPredicate(source, r);
1054                 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
1055             }
1056
1057             // `[T; n]` -> `[T]`
1058             (&ty::Array(a, _), &ty::Slice(b)) => {
1059                 let InferOk { obligations, .. } = self
1060                     .infcx
1061                     .at(&obligation.cause, obligation.param_env)
1062                     .eq(b, a)
1063                     .map_err(|_| Unimplemented)?;
1064                 nested.extend(obligations);
1065             }
1066
1067             // `Struct<T>` -> `Struct<U>`
1068             (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
1069                 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
1070                     GenericArgKind::Type(ty) => match ty.kind() {
1071                         ty::Param(p) => Some(p.index),
1072                         _ => None,
1073                     },
1074
1075                     // Lifetimes aren't allowed to change during unsizing.
1076                     GenericArgKind::Lifetime(_) => None,
1077
1078                     GenericArgKind::Const(ct) => match ct.kind() {
1079                         ty::ConstKind::Param(p) => Some(p.index),
1080                         _ => None,
1081                     },
1082                 };
1083
1084                 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1085                 // by putting it in a query; it would only need the `DefId` as it
1086                 // looks at declared field types, not anything substituted.
1087
1088                 // The last field of the structure has to exist and contain type/const parameters.
1089                 let (tail_field, prefix_fields) =
1090                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1091                 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1092
1093                 let mut unsizing_params = GrowableBitSet::new_empty();
1094                 for arg in tail_field_ty.0.walk() {
1095                     if let Some(i) = maybe_unsizing_param_idx(arg) {
1096                         unsizing_params.insert(i);
1097                     }
1098                 }
1099
1100                 // Ensure none of the other fields mention the parameters used
1101                 // in unsizing.
1102                 for field in prefix_fields {
1103                     for arg in tcx.type_of(field.did).walk() {
1104                         if let Some(i) = maybe_unsizing_param_idx(arg) {
1105                             unsizing_params.remove(i);
1106                         }
1107                     }
1108                 }
1109
1110                 if unsizing_params.is_empty() {
1111                     return Err(Unimplemented);
1112                 }
1113
1114                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
1115                 // normalizing in the process, since `type_of` returns something directly from
1116                 // astconv (which means it's un-normalized).
1117                 let source_tail = normalize_with_depth_to(
1118                     self,
1119                     obligation.param_env,
1120                     obligation.cause.clone(),
1121                     obligation.recursion_depth + 1,
1122                     tail_field_ty.subst(tcx, substs_a),
1123                     &mut nested,
1124                 );
1125                 let target_tail = normalize_with_depth_to(
1126                     self,
1127                     obligation.param_env,
1128                     obligation.cause.clone(),
1129                     obligation.recursion_depth + 1,
1130                     tail_field_ty.subst(tcx, substs_b),
1131                     &mut nested,
1132                 );
1133
1134                 // Check that the source struct with the target's
1135                 // unsizing parameters is equal to the target.
1136                 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1137                     if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1138                 }));
1139                 let new_struct = tcx.mk_adt(def, substs);
1140                 let InferOk { obligations, .. } = self
1141                     .infcx
1142                     .at(&obligation.cause, obligation.param_env)
1143                     .eq(target, new_struct)
1144                     .map_err(|_| Unimplemented)?;
1145                 nested.extend(obligations);
1146
1147                 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1148                 nested.push(predicate_for_trait_def(
1149                     tcx,
1150                     obligation.param_env,
1151                     obligation.cause.clone(),
1152                     obligation.predicate.def_id(),
1153                     obligation.recursion_depth + 1,
1154                     [source_tail, target_tail],
1155                 ));
1156             }
1157
1158             // `(.., T)` -> `(.., U)`
1159             (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1160                 assert_eq!(tys_a.len(), tys_b.len());
1161
1162                 // The last field of the tuple has to exist.
1163                 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1164                 let &b_last = tys_b.last().unwrap();
1165
1166                 // Check that the source tuple with the target's
1167                 // last element is equal to the target.
1168                 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1169                 let InferOk { obligations, .. } = self
1170                     .infcx
1171                     .at(&obligation.cause, obligation.param_env)
1172                     .eq(target, new_tuple)
1173                     .map_err(|_| Unimplemented)?;
1174                 nested.extend(obligations);
1175
1176                 // Construct the nested `T: Unsize<U>` predicate.
1177                 nested.push(ensure_sufficient_stack(|| {
1178                     predicate_for_trait_def(
1179                         tcx,
1180                         obligation.param_env,
1181                         obligation.cause.clone(),
1182                         obligation.predicate.def_id(),
1183                         obligation.recursion_depth + 1,
1184                         [a_last, b_last],
1185                     )
1186                 }));
1187             }
1188
1189             _ => bug!("source: {source}, target: {target}"),
1190         };
1191
1192         Ok(ImplSourceBuiltinData { nested })
1193     }
1194
1195     fn confirm_const_destruct_candidate(
1196         &mut self,
1197         obligation: &TraitObligation<'tcx>,
1198         impl_def_id: Option<DefId>,
1199     ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1200         // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1201         if !obligation.is_const() {
1202             return Ok(ImplSourceConstDestructData { nested: vec![] });
1203         }
1204
1205         let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1206
1207         let tcx = self.tcx();
1208         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1209
1210         let mut nested = vec![];
1211         let cause = obligation.derived_cause(BuiltinDerivedObligation);
1212
1213         // If we have a custom `impl const Drop`, then
1214         // first check it like a regular impl candidate.
1215         // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1216         if let Some(impl_def_id) = impl_def_id {
1217             let mut new_obligation = obligation.clone();
1218             new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1219                 trait_pred.trait_ref.def_id = drop_trait;
1220                 trait_pred
1221             });
1222             let substs = self.rematch_impl(impl_def_id, &new_obligation);
1223             debug!(?substs, "impl substs");
1224
1225             let cause = obligation.derived_cause(|derived| {
1226                 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1227                     derived,
1228                     impl_def_id,
1229                     span: obligation.cause.span,
1230                 }))
1231             });
1232             let obligations = ensure_sufficient_stack(|| {
1233                 self.vtable_impl(
1234                     impl_def_id,
1235                     substs,
1236                     &cause,
1237                     new_obligation.recursion_depth + 1,
1238                     new_obligation.param_env,
1239                     obligation.predicate,
1240                 )
1241             });
1242             nested.extend(obligations.nested);
1243         }
1244
1245         // We want to confirm the ADT's fields if we have an ADT
1246         let mut stack = match *self_ty.skip_binder().kind() {
1247             ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1248             _ => vec![self_ty.skip_binder()],
1249         };
1250
1251         while let Some(nested_ty) = stack.pop() {
1252             match *nested_ty.kind() {
1253                 // We know these types are trivially drop
1254                 ty::Bool
1255                 | ty::Char
1256                 | ty::Int(_)
1257                 | ty::Uint(_)
1258                 | ty::Float(_)
1259                 | ty::Infer(ty::IntVar(_))
1260                 | ty::Infer(ty::FloatVar(_))
1261                 | ty::Str
1262                 | ty::RawPtr(_)
1263                 | ty::Ref(..)
1264                 | ty::FnDef(..)
1265                 | ty::FnPtr(_)
1266                 | ty::Never
1267                 | ty::Foreign(_) => {}
1268
1269                 // `ManuallyDrop` is trivially drop
1270                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
1271
1272                 // These types are built-in, so we can fast-track by registering
1273                 // nested predicates for their constituent type(s)
1274                 ty::Array(ty, _) | ty::Slice(ty) => {
1275                     stack.push(ty);
1276                 }
1277                 ty::Tuple(tys) => {
1278                     stack.extend(tys.iter());
1279                 }
1280                 ty::Closure(_, substs) => {
1281                     stack.push(substs.as_closure().tupled_upvars_ty());
1282                 }
1283                 ty::Generator(_, substs, _) => {
1284                     let generator = substs.as_generator();
1285                     stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1286                 }
1287                 ty::GeneratorWitness(tys) => {
1288                     stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1289                 }
1290
1291                 // If we have a projection type, make sure to normalize it so we replace it
1292                 // with a fresh infer variable
1293                 ty::Alias(ty::Projection, ..) => {
1294                     let predicate = normalize_with_depth_to(
1295                         self,
1296                         obligation.param_env,
1297                         cause.clone(),
1298                         obligation.recursion_depth + 1,
1299                         self_ty.rebind(ty::TraitPredicate {
1300                             trait_ref: self
1301                                 .tcx()
1302                                 .at(cause.span)
1303                                 .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1304                             constness: ty::BoundConstness::ConstIfConst,
1305                             polarity: ty::ImplPolarity::Positive,
1306                         }),
1307                         &mut nested,
1308                     );
1309
1310                     nested.push(Obligation::with_depth(
1311                         tcx,
1312                         cause.clone(),
1313                         obligation.recursion_depth + 1,
1314                         obligation.param_env,
1315                         predicate,
1316                     ));
1317                 }
1318
1319                 // If we have any other type (e.g. an ADT), just register a nested obligation
1320                 // since it's either not `const Drop` (and we raise an error during selection),
1321                 // or it's an ADT (and we need to check for a custom impl during selection)
1322                 _ => {
1323                     let predicate = self_ty.rebind(ty::TraitPredicate {
1324                         trait_ref: self
1325                             .tcx()
1326                             .at(cause.span)
1327                             .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1328                         constness: ty::BoundConstness::ConstIfConst,
1329                         polarity: ty::ImplPolarity::Positive,
1330                     });
1331
1332                     nested.push(Obligation::with_depth(
1333                         tcx,
1334                         cause.clone(),
1335                         obligation.recursion_depth + 1,
1336                         obligation.param_env,
1337                         predicate,
1338                     ));
1339                 }
1340             }
1341         }
1342
1343         Ok(ImplSourceConstDestructData { nested })
1344     }
1345 }