compiler/rustc_trait_selection/src/traits/select/confirmation.rs

   1 //! Confirmation.
   2 //!
   3 //! Confirmation unifies the output type parameters of the trait
   4 //! with the values found in the obligation, possibly yielding a
   5 //! type error.  See the [rustc dev guide] for more details.
   6 //!
   7 //! [rustc dev guide]:
   8 //! https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation
   9 use rustc_data_structures::stack::ensure_sufficient_stack;
  10 use rustc_hir::lang_items::LangItem;
  11 use rustc_index::bit_set::GrowableBitSet;
  12 use rustc_infer::infer::InferOk;
  13 use rustc_infer::infer::LateBoundRegionConversionTime::HigherRankedType;
  14 use rustc_middle::ty::{
  15     self, GenericArg, GenericArgKind, GenericParamDefKind, InternalSubsts, SubstsRef,
  16     ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
  17 };
  18 use rustc_span::def_id::DefId;
  19
  20 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
  21 use crate::traits::util::{self, closure_trait_ref_and_return_type, predicate_for_trait_def};
  22 use crate::traits::{
  23     BuiltinDerivedObligation, ImplDerivedObligation, ImplDerivedObligationCause, ImplSource,
  24     ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
  25     ImplSourceConstDestructData, ImplSourceFnPointerData, ImplSourceFutureData,
  26     ImplSourceGeneratorData, ImplSourceObjectData, ImplSourceTraitAliasData,
  27     ImplSourceTraitUpcastingData, ImplSourceUserDefinedData, Normalized, ObjectCastObligation,
  28     Obligation, ObligationCause, OutputTypeParameterMismatch, PredicateObligation, Selection,
  29     SelectionError, TraitNotObjectSafe, TraitObligation, Unimplemented, VtblSegment,
  30 };
  31
  32 use super::BuiltinImplConditions;
  33 use super::SelectionCandidate::{self, *};
  34 use super::SelectionContext;
  35
  36 use std::iter;
  37 use std::ops::ControlFlow;
  38
  39 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
  40     #[instrument(level = "debug", skip(self))]
  41     pub(super) fn confirm_candidate(
  42         &mut self,
  43         obligation: &TraitObligation<'tcx>,
  44         candidate: SelectionCandidate<'tcx>,
  45     ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
  46         let mut impl_src = match candidate {
  47             BuiltinCandidate { has_nested } => {
  48                 let data = self.confirm_builtin_candidate(obligation, has_nested);
  49                 ImplSource::Builtin(data)
  50             }
  51
  52             TransmutabilityCandidate => {
  53                 let data = self.confirm_transmutability_candidate(obligation)?;
  54                 ImplSource::Builtin(data)
  55             }
  56
  57             ParamCandidate(param) => {
  58                 let obligations =
  59                     self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
  60                 ImplSource::Param(obligations, param.skip_binder().constness)
  61             }
  62
  63             ImplCandidate(impl_def_id) => {
  64                 ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id))
  65             }
  66
  67             AutoImplCandidate => {
  68                 let data = self.confirm_auto_impl_candidate(obligation);
  69                 ImplSource::AutoImpl(data)
  70             }
  71
  72             ProjectionCandidate(idx, constness) => {
  73                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
  74                 ImplSource::Param(obligations, constness)
  75             }
  76
  77             ObjectCandidate(idx) => {
  78                 let data = self.confirm_object_candidate(obligation, idx)?;
  79                 ImplSource::Object(data)
  80             }
  81
  82             ClosureCandidate => {
  83                 let vtable_closure = self.confirm_closure_candidate(obligation)?;
  84                 ImplSource::Closure(vtable_closure)
  85             }
  86
  87             GeneratorCandidate => {
  88                 let vtable_generator = self.confirm_generator_candidate(obligation)?;
  89                 ImplSource::Generator(vtable_generator)
  90             }
  91
  92             FutureCandidate => {
  93                 let vtable_future = self.confirm_future_candidate(obligation)?;
  94                 ImplSource::Future(vtable_future)
  95             }
  96
  97             FnPointerCandidate { .. } => {
  98                 let data = self.confirm_fn_pointer_candidate(obligation)?;
  99                 ImplSource::FnPointer(data)
 100             }
 101
 102             TraitAliasCandidate => {
 103                 let data = self.confirm_trait_alias_candidate(obligation);
 104                 ImplSource::TraitAlias(data)
 105             }
 106
 107             BuiltinObjectCandidate => {
 108                 // This indicates something like `Trait + Send: Send`. In this case, we know that
 109                 // this holds because that's what the object type is telling us, and there's really
 110                 // no additional obligations to prove and no types in particular to unify, etc.
 111                 ImplSource::Param(Vec::new(), ty::BoundConstness::NotConst)
 112             }
 113
 114             BuiltinUnsizeCandidate => {
 115                 let data = self.confirm_builtin_unsize_candidate(obligation)?;
 116                 ImplSource::Builtin(data)
 117             }
 118
 119             TraitUpcastingUnsizeCandidate(idx) => {
 120                 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
 121                 ImplSource::TraitUpcasting(data)
 122             }
 123
 124             ConstDestructCandidate(def_id) => {
 125                 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
 126                 ImplSource::ConstDestruct(data)
 127             }
 128         };
 129
 130         if !obligation.predicate.is_const_if_const() {
 131             // normalize nested predicates according to parent predicate's constness.
 132             impl_src = impl_src.map(|mut o| {
 133                 o.predicate = o.predicate.without_const(self.tcx());
 134                 o
 135             });
 136         }
 137
 138         Ok(impl_src)
 139     }
 140
 141     fn confirm_projection_candidate(
 142         &mut self,
 143         obligation: &TraitObligation<'tcx>,
 144         idx: usize,
 145     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 146         let tcx = self.tcx();
 147
 148         let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
 149         let placeholder_trait_predicate =
 150             self.infcx().replace_bound_vars_with_placeholders(trait_predicate).trait_ref;
 151         let placeholder_self_ty = placeholder_trait_predicate.self_ty();
 152         let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
 153         let (def_id, substs) = match *placeholder_self_ty.kind() {
 154             ty::Projection(proj) => (proj.item_def_id, proj.substs),
 155             ty::Opaque(def_id, substs) => (def_id, substs),
 156             _ => bug!("projection candidate for unexpected type: {:?}", placeholder_self_ty),
 157         };
 158
 159         let candidate_predicate =
 160             tcx.bound_item_bounds(def_id).map_bound(|i| i[idx]).subst(tcx, substs);
 161         let candidate = candidate_predicate
 162             .to_opt_poly_trait_pred()
 163             .expect("projection candidate is not a trait predicate")
 164             .map_bound(|t| t.trait_ref);
 165         let mut obligations = Vec::new();
 166         let candidate = normalize_with_depth_to(
 167             self,
 168             obligation.param_env,
 169             obligation.cause.clone(),
 170             obligation.recursion_depth + 1,
 171             candidate,
 172             &mut obligations,
 173         );
 174
 175         obligations.extend(self.infcx.commit_if_ok(|_| {
 176             self.infcx
 177                 .at(&obligation.cause, obligation.param_env)
 178                 .sup(placeholder_trait_predicate, candidate)
 179                 .map(|InferOk { obligations, .. }| obligations)
 180                 .map_err(|_| Unimplemented)
 181         })?);
 182
 183         if let ty::Projection(..) = placeholder_self_ty.kind() {
 184             let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates;
 185             debug!(?predicates, "projection predicates");
 186             for predicate in predicates {
 187                 let normalized = normalize_with_depth_to(
 188                     self,
 189                     obligation.param_env,
 190                     obligation.cause.clone(),
 191                     obligation.recursion_depth + 1,
 192                     predicate,
 193                     &mut obligations,
 194                 );
 195                 obligations.push(Obligation::with_depth(
 196                     self.tcx(),
 197                     obligation.cause.clone(),
 198                     obligation.recursion_depth + 1,
 199                     obligation.param_env,
 200                     normalized,
 201                 ));
 202             }
 203         }
 204
 205         Ok(obligations)
 206     }
 207
 208     fn confirm_param_candidate(
 209         &mut self,
 210         obligation: &TraitObligation<'tcx>,
 211         param: ty::PolyTraitRef<'tcx>,
 212     ) -> Vec<PredicateObligation<'tcx>> {
 213         debug!(?obligation, ?param, "confirm_param_candidate");
 214
 215         // During evaluation, we already checked that this
 216         // where-clause trait-ref could be unified with the obligation
 217         // trait-ref. Repeat that unification now without any
 218         // transactional boundary; it should not fail.
 219         match self.match_where_clause_trait_ref(obligation, param) {
 220             Ok(obligations) => obligations,
 221             Err(()) => {
 222                 bug!(
 223                     "Where clause `{:?}` was applicable to `{:?}` but now is not",
 224                     param,
 225                     obligation
 226                 );
 227             }
 228         }
 229     }
 230
 231     fn confirm_builtin_candidate(
 232         &mut self,
 233         obligation: &TraitObligation<'tcx>,
 234         has_nested: bool,
 235     ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
 236         debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
 237
 238         let lang_items = self.tcx().lang_items();
 239         let obligations = if has_nested {
 240             let trait_def = obligation.predicate.def_id();
 241             let conditions = if Some(trait_def) == lang_items.sized_trait() {
 242                 self.sized_conditions(obligation)
 243             } else if Some(trait_def) == lang_items.copy_trait() {
 244                 self.copy_clone_conditions(obligation)
 245             } else if Some(trait_def) == lang_items.clone_trait() {
 246                 self.copy_clone_conditions(obligation)
 247             } else {
 248                 bug!("unexpected builtin trait {:?}", trait_def)
 249             };
 250             let BuiltinImplConditions::Where(nested) = conditions else {
 251                 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
 252             };
 253
 254             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 255             ensure_sufficient_stack(|| {
 256                 self.collect_predicates_for_types(
 257                     obligation.param_env,
 258                     cause,
 259                     obligation.recursion_depth + 1,
 260                     trait_def,
 261                     nested,
 262                 )
 263             })
 264         } else {
 265             vec![]
 266         };
 267
 268         debug!(?obligations);
 269
 270         ImplSourceBuiltinData { nested: obligations }
 271     }
 272
 273     fn confirm_transmutability_candidate(
 274         &mut self,
 275         obligation: &TraitObligation<'tcx>,
 276     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 277         debug!(?obligation, "confirm_transmutability_candidate");
 278
 279         let predicate = obligation.predicate;
 280
 281         let type_at = |i| predicate.map_bound(|p| p.trait_ref.substs.type_at(i));
 282         let const_at = |i| predicate.skip_binder().trait_ref.substs.const_at(i);
 283
 284         let src_and_dst = predicate.map_bound(|p| rustc_transmute::Types {
 285             dst: p.trait_ref.substs.type_at(0),
 286             src: p.trait_ref.substs.type_at(1),
 287         });
 288
 289         let scope = type_at(2).skip_binder();
 290
 291         let Some(assume) =
 292             rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3)) else {
 293                 return Err(Unimplemented);
 294             };
 295
 296         let cause = obligation.cause.clone();
 297
 298         let mut transmute_env = rustc_transmute::TransmuteTypeEnv::new(self.infcx);
 299
 300         let maybe_transmutable = transmute_env.is_transmutable(cause, src_and_dst, scope, assume);
 301
 302         use rustc_transmute::Answer;
 303
 304         match maybe_transmutable {
 305             Answer::Yes => Ok(ImplSourceBuiltinData { nested: vec![] }),
 306             _ => Err(Unimplemented),
 307         }
 308     }
 309
 310     /// This handles the case where an `auto trait Foo` impl is being used.
 311     /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
 312     ///
 313     /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
 314     /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
 315     fn confirm_auto_impl_candidate(
 316         &mut self,
 317         obligation: &TraitObligation<'tcx>,
 318     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 319         debug!(?obligation, "confirm_auto_impl_candidate");
 320
 321         let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
 322         let types = self.constituent_types_for_ty(self_ty);
 323         self.vtable_auto_impl(obligation, obligation.predicate.def_id(), types)
 324     }
 325
 326     /// See `confirm_auto_impl_candidate`.
 327     fn vtable_auto_impl(
 328         &mut self,
 329         obligation: &TraitObligation<'tcx>,
 330         trait_def_id: DefId,
 331         nested: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
 332     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 333         debug!(?nested, "vtable_auto_impl");
 334         ensure_sufficient_stack(|| {
 335             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 336
 337             let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
 338             let trait_ref = self.infcx.replace_bound_vars_with_placeholders(poly_trait_ref);
 339             let trait_obligations: Vec<PredicateObligation<'_>> = self.impl_or_trait_obligations(
 340                 &cause,
 341                 obligation.recursion_depth + 1,
 342                 obligation.param_env,
 343                 trait_def_id,
 344                 &trait_ref.substs,
 345                 obligation.predicate,
 346             );
 347
 348             let mut obligations = self.collect_predicates_for_types(
 349                 obligation.param_env,
 350                 cause,
 351                 obligation.recursion_depth + 1,
 352                 trait_def_id,
 353                 nested,
 354             );
 355
 356             // Adds the predicates from the trait.  Note that this contains a `Self: Trait`
 357             // predicate as usual.  It won't have any effect since auto traits are coinductive.
 358             obligations.extend(trait_obligations);
 359
 360             debug!(?obligations, "vtable_auto_impl");
 361
 362             ImplSourceAutoImplData { trait_def_id, nested: obligations }
 363         })
 364     }
 365
 366     fn confirm_impl_candidate(
 367         &mut self,
 368         obligation: &TraitObligation<'tcx>,
 369         impl_def_id: DefId,
 370     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 371         debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");
 372
 373         // First, create the substitutions by matching the impl again,
 374         // this time not in a probe.
 375         let substs = self.rematch_impl(impl_def_id, obligation);
 376         debug!(?substs, "impl substs");
 377         ensure_sufficient_stack(|| {
 378             self.vtable_impl(
 379                 impl_def_id,
 380                 substs,
 381                 &obligation.cause,
 382                 obligation.recursion_depth + 1,
 383                 obligation.param_env,
 384                 obligation.predicate,
 385             )
 386         })
 387     }
 388
 389     fn vtable_impl(
 390         &mut self,
 391         impl_def_id: DefId,
 392         substs: Normalized<'tcx, SubstsRef<'tcx>>,
 393         cause: &ObligationCause<'tcx>,
 394         recursion_depth: usize,
 395         param_env: ty::ParamEnv<'tcx>,
 396         parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
 397     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 398         debug!(?impl_def_id, ?substs, ?recursion_depth, "vtable_impl");
 399
 400         let mut impl_obligations = self.impl_or_trait_obligations(
 401             cause,
 402             recursion_depth,
 403             param_env,
 404             impl_def_id,
 405             &substs.value,
 406             parent_trait_pred,
 407         );
 408
 409         debug!(?impl_obligations, "vtable_impl");
 410
 411         // Because of RFC447, the impl-trait-ref and obligations
 412         // are sufficient to determine the impl substs, without
 413         // relying on projections in the impl-trait-ref.
 414         //
 415         // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
 416         impl_obligations.extend(substs.obligations);
 417
 418         ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations }
 419     }
 420
 421     fn confirm_object_candidate(
 422         &mut self,
 423         obligation: &TraitObligation<'tcx>,
 424         index: usize,
 425     ) -> Result<ImplSourceObjectData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 426         let tcx = self.tcx();
 427         debug!(?obligation, ?index, "confirm_object_candidate");
 428
 429         let trait_predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
 430         let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
 431         let obligation_trait_ref = ty::Binder::dummy(trait_predicate.trait_ref);
 432         let ty::Dynamic(data, ..) = *self_ty.kind() else {
 433             span_bug!(obligation.cause.span, "object candidate with non-object");
 434         };
 435
 436         let object_trait_ref = data.principal().unwrap_or_else(|| {
 437             span_bug!(obligation.cause.span, "object candidate with no principal")
 438         });
 439         let object_trait_ref = self.infcx.replace_bound_vars_with_fresh_vars(
 440             obligation.cause.span,
 441             HigherRankedType,
 442             object_trait_ref,
 443         );
 444         let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);
 445
 446         let mut nested = vec![];
 447
 448         let mut supertraits = util::supertraits(tcx, ty::Binder::dummy(object_trait_ref));
 449         let unnormalized_upcast_trait_ref =
 450             supertraits.nth(index).expect("supertraits iterator no longer has as many elements");
 451
 452         let upcast_trait_ref = normalize_with_depth_to(
 453             self,
 454             obligation.param_env,
 455             obligation.cause.clone(),
 456             obligation.recursion_depth + 1,
 457             unnormalized_upcast_trait_ref,
 458             &mut nested,
 459         );
 460
 461         nested.extend(self.infcx.commit_if_ok(|_| {
 462             self.infcx
 463                 .at(&obligation.cause, obligation.param_env)
 464                 .sup(obligation_trait_ref, upcast_trait_ref)
 465                 .map(|InferOk { obligations, .. }| obligations)
 466                 .map_err(|_| Unimplemented)
 467         })?);
 468
 469         // Check supertraits hold. This is so that their associated type bounds
 470         // will be checked in the code below.
 471         for super_trait in tcx
 472             .super_predicates_of(trait_predicate.def_id())
 473             .instantiate(tcx, trait_predicate.trait_ref.substs)
 474             .predicates
 475             .into_iter()
 476         {
 477             let normalized_super_trait = normalize_with_depth_to(
 478                 self,
 479                 obligation.param_env,
 480                 obligation.cause.clone(),
 481                 obligation.recursion_depth + 1,
 482                 super_trait,
 483                 &mut nested,
 484             );
 485             nested.push(obligation.with(tcx, normalized_super_trait));
 486         }
 487
 488         let assoc_types: Vec<_> = tcx
 489             .associated_items(trait_predicate.def_id())
 490             .in_definition_order()
 491             .filter_map(
 492                 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
 493             )
 494             .collect();
 495
 496         for assoc_type in assoc_types {
 497             let defs: &ty::Generics = tcx.generics_of(assoc_type);
 498
 499             if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
 500                 tcx.sess.delay_span_bug(
 501                     obligation.cause.span,
 502                     "GATs in trait object shouldn't have been considered",
 503                 );
 504                 return Err(SelectionError::Unimplemented);
 505             }
 506
 507             // This maybe belongs in wf, but that can't (doesn't) handle
 508             // higher-ranked things.
 509             // Prevent, e.g., `dyn Iterator<Item = str>`.
 510             for bound in self.tcx().bound_item_bounds(assoc_type).transpose_iter() {
 511                 let subst_bound =
 512                     if defs.count() == 0 {
 513                         bound.subst(tcx, trait_predicate.trait_ref.substs)
 514                     } else {
 515                         let mut substs = smallvec::SmallVec::with_capacity(defs.count());
 516                         substs.extend(trait_predicate.trait_ref.substs.iter());
 517                         let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
 518                             smallvec::SmallVec::with_capacity(
 519                                 bound.0.kind().bound_vars().len() + defs.count(),
 520                             );
 521                         bound_vars.extend(bound.0.kind().bound_vars().into_iter());
 522                         InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
 523                             .kind
 524                         {
 525                             GenericParamDefKind::Type { .. } => {
 526                                 let kind = ty::BoundTyKind::Param(param.name);
 527                                 let bound_var = ty::BoundVariableKind::Ty(kind);
 528                                 bound_vars.push(bound_var);
 529                                 tcx.mk_ty(ty::Bound(
 530                                     ty::INNERMOST,
 531                                     ty::BoundTy {
 532                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 533                                         kind,
 534                                     },
 535                                 ))
 536                                 .into()
 537                             }
 538                             GenericParamDefKind::Lifetime => {
 539                                 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
 540                                 let bound_var = ty::BoundVariableKind::Region(kind);
 541                                 bound_vars.push(bound_var);
 542                                 tcx.mk_region(ty::ReLateBound(
 543                                     ty::INNERMOST,
 544                                     ty::BoundRegion {
 545                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 546                                         kind,
 547                                     },
 548                                 ))
 549                                 .into()
 550                             }
 551                             GenericParamDefKind::Const { .. } => {
 552                                 let bound_var = ty::BoundVariableKind::Const;
 553                                 bound_vars.push(bound_var);
 554                                 tcx.mk_const(
 555                                     ty::ConstKind::Bound(
 556                                         ty::INNERMOST,
 557                                         ty::BoundVar::from_usize(bound_vars.len() - 1),
 558                                     ),
 559                                     tcx.type_of(param.def_id),
 560                                 )
 561                                 .into()
 562                             }
 563                         });
 564                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 565                         let assoc_ty_substs = tcx.intern_substs(&substs);
 566
 567                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 568                         let bound =
 569                             bound.map_bound(|b| b.kind().skip_binder()).subst(tcx, assoc_ty_substs);
 570                         tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
 571                     };
 572                 let normalized_bound = normalize_with_depth_to(
 573                     self,
 574                     obligation.param_env,
 575                     obligation.cause.clone(),
 576                     obligation.recursion_depth + 1,
 577                     subst_bound,
 578                     &mut nested,
 579                 );
 580                 nested.push(obligation.with(tcx, normalized_bound));
 581             }
 582         }
 583
 584         debug!(?nested, "object nested obligations");
 585
 586         let vtable_base = super::super::vtable_trait_first_method_offset(
 587             tcx,
 588             (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
 589         );
 590
 591         Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
 592     }
 593
 594     fn confirm_fn_pointer_candidate(
 595         &mut self,
 596         obligation: &TraitObligation<'tcx>,
 597     ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 598     {
 599         debug!(?obligation, "confirm_fn_pointer_candidate");
 600
 601         let self_ty = self
 602             .infcx
 603             .shallow_resolve(obligation.self_ty().no_bound_vars())
 604             .expect("fn pointer should not capture bound vars from predicate");
 605         let sig = self_ty.fn_sig(self.tcx());
 606         let trait_ref = closure_trait_ref_and_return_type(
 607             self.tcx(),
 608             obligation.predicate.def_id(),
 609             self_ty,
 610             sig,
 611             util::TupleArgumentsFlag::Yes,
 612         )
 613         .map_bound(|(trait_ref, _)| trait_ref);
 614
 615         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 616
 617         // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
 618         let cause = obligation.derived_cause(BuiltinDerivedObligation);
 619         let output_ty = self.infcx.replace_bound_vars_with_placeholders(sig.output());
 620         let output_ty = normalize_with_depth_to(
 621             self,
 622             obligation.param_env,
 623             cause.clone(),
 624             obligation.recursion_depth,
 625             output_ty,
 626             &mut nested,
 627         );
 628         let tr =
 629             ty::Binder::dummy(self.tcx().at(cause.span).mk_trait_ref(LangItem::Sized, [output_ty]));
 630         nested.push(Obligation::new(
 631             self.infcx.tcx,
 632             cause,
 633             obligation.param_env,
 634             tr.to_poly_trait_predicate(),
 635         ));
 636
 637         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
 638     }
 639
 640     fn confirm_trait_alias_candidate(
 641         &mut self,
 642         obligation: &TraitObligation<'tcx>,
 643     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
 644         debug!(?obligation, "confirm_trait_alias_candidate");
 645
 646         let alias_def_id = obligation.predicate.def_id();
 647         let predicate = self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
 648         let trait_ref = predicate.trait_ref;
 649         let trait_def_id = trait_ref.def_id;
 650         let substs = trait_ref.substs;
 651
 652         let trait_obligations = self.impl_or_trait_obligations(
 653             &obligation.cause,
 654             obligation.recursion_depth,
 655             obligation.param_env,
 656             trait_def_id,
 657             &substs,
 658             obligation.predicate,
 659         );
 660
 661         debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
 662
 663         ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
 664     }
 665
 666     fn confirm_generator_candidate(
 667         &mut self,
 668         obligation: &TraitObligation<'tcx>,
 669     ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 670     {
 671         // Okay to skip binder because the substs on generator types never
 672         // touch bound regions, they just capture the in-scope
 673         // type/region parameters.
 674         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 675         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 676             bug!("closure candidate for non-closure {:?}", obligation);
 677         };
 678
 679         debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
 680
 681         let gen_sig = substs.as_generator().poly_sig();
 682
 683         // NOTE: The self-type is a generator type and hence is
 684         // in fact unparameterized (or at least does not reference any
 685         // regions bound in the obligation).
 686         let self_ty = obligation
 687             .predicate
 688             .self_ty()
 689             .no_bound_vars()
 690             .expect("unboxed closure type should not capture bound vars from the predicate");
 691
 692         let trait_ref = super::util::generator_trait_ref_and_outputs(
 693             self.tcx(),
 694             obligation.predicate.def_id(),
 695             self_ty,
 696             gen_sig,
 697         )
 698         .map_bound(|(trait_ref, ..)| trait_ref);
 699
 700         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 701         debug!(?trait_ref, ?nested, "generator candidate obligations");
 702
 703         Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
 704     }
 705
 706     fn confirm_future_candidate(
 707         &mut self,
 708         obligation: &TraitObligation<'tcx>,
 709     ) -> Result<ImplSourceFutureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 710         // Okay to skip binder because the substs on generator types never
 711         // touch bound regions, they just capture the in-scope
 712         // type/region parameters.
 713         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 714         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 715             bug!("closure candidate for non-closure {:?}", obligation);
 716         };
 717
 718         debug!(?obligation, ?generator_def_id, ?substs, "confirm_future_candidate");
 719
 720         let gen_sig = substs.as_generator().poly_sig();
 721
 722         let trait_ref = super::util::future_trait_ref_and_outputs(
 723             self.tcx(),
 724             obligation.predicate.def_id(),
 725             obligation.predicate.no_bound_vars().expect("future has no bound vars").self_ty(),
 726             gen_sig,
 727         )
 728         .map_bound(|(trait_ref, ..)| trait_ref);
 729
 730         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 731         debug!(?trait_ref, ?nested, "future candidate obligations");
 732
 733         Ok(ImplSourceFutureData { generator_def_id, substs, nested })
 734     }
 735
 736     #[instrument(skip(self), level = "debug")]
 737     fn confirm_closure_candidate(
 738         &mut self,
 739         obligation: &TraitObligation<'tcx>,
 740     ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 741         let kind = self
 742             .tcx()
 743             .fn_trait_kind_from_lang_item(obligation.predicate.def_id())
 744             .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
 745
 746         // Okay to skip binder because the substs on closure types never
 747         // touch bound regions, they just capture the in-scope
 748         // type/region parameters.
 749         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 750         let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
 751             bug!("closure candidate for non-closure {:?}", obligation);
 752         };
 753
 754         let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
 755         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 756
 757         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 758
 759         // FIXME: Chalk
 760
 761         if !self.tcx().sess.opts.unstable_opts.chalk {
 762             nested.push(obligation.with(
 763                 self.tcx(),
 764                 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind)),
 765             ));
 766         }
 767
 768         Ok(ImplSourceClosureData { closure_def_id, substs, nested })
 769     }
 770
 771     /// In the case of closure types and fn pointers,
 772     /// we currently treat the input type parameters on the trait as
 773     /// outputs. This means that when we have a match we have only
 774     /// considered the self type, so we have to go back and make sure
 775     /// to relate the argument types too. This is kind of wrong, but
 776     /// since we control the full set of impls, also not that wrong,
 777     /// and it DOES yield better error messages (since we don't report
 778     /// errors as if there is no applicable impl, but rather report
 779     /// errors are about mismatched argument types.
 780     ///
 781     /// Here is an example. Imagine we have a closure expression
 782     /// and we desugared it so that the type of the expression is
 783     /// `Closure`, and `Closure` expects `i32` as argument. Then it
 784     /// is "as if" the compiler generated this impl:
 785     /// ```ignore (illustrative)
 786     /// impl Fn(i32) for Closure { ... }
 787     /// ```
 788     /// Now imagine our obligation is `Closure: Fn(usize)`. So far
 789     /// we have matched the self type `Closure`. At this point we'll
 790     /// compare the `i32` to `usize` and generate an error.
 791     ///
 792     /// Note that this checking occurs *after* the impl has selected,
 793     /// because these output type parameters should not affect the
 794     /// selection of the impl. Therefore, if there is a mismatch, we
 795     /// report an error to the user.
 796     #[instrument(skip(self), level = "trace")]
 797     fn confirm_poly_trait_refs(
 798         &mut self,
 799         obligation: &TraitObligation<'tcx>,
 800         expected_trait_ref: ty::PolyTraitRef<'tcx>,
 801     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 802         let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
 803         // Normalize the obligation and expected trait refs together, because why not
 804         let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
 805             ensure_sufficient_stack(|| {
 806                 normalize_with_depth(
 807                     self,
 808                     obligation.param_env,
 809                     obligation.cause.clone(),
 810                     obligation.recursion_depth + 1,
 811                     (obligation_trait_ref, expected_trait_ref),
 812                 )
 813             });
 814
 815         self.infcx
 816             .at(&obligation.cause, obligation.param_env)
 817             .sup(obligation_trait_ref, expected_trait_ref)
 818             .map(|InferOk { mut obligations, .. }| {
 819                 obligations.extend(nested);
 820                 obligations
 821             })
 822             .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
 823     }
 824
 825     fn confirm_trait_upcasting_unsize_candidate(
 826         &mut self,
 827         obligation: &TraitObligation<'tcx>,
 828         idx: usize,
 829     ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 830     {
 831         let tcx = self.tcx();
 832
 833         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 834         // regions here. See the comment there for more details.
 835         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 836         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 837         let target = self.infcx.shallow_resolve(target);
 838
 839         debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
 840
 841         let mut nested = vec![];
 842         let source_trait_ref;
 843         let upcast_trait_ref;
 844         match (source.kind(), target.kind()) {
 845             // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
 846             (
 847                 &ty::Dynamic(ref data_a, r_a, repr_a @ ty::Dyn),
 848                 &ty::Dynamic(ref data_b, r_b, ty::Dyn),
 849             ) => {
 850                 // See `assemble_candidates_for_unsizing` for more info.
 851                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 852                 let principal_a = data_a.principal().unwrap();
 853                 source_trait_ref = principal_a.with_self_ty(tcx, source);
 854                 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
 855                 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
 856                 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
 857                     ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
 858                         tcx, trait_ref,
 859                     ))
 860                 });
 861                 let iter = Some(existential_predicate)
 862                     .into_iter()
 863                     .chain(
 864                         data_a
 865                             .projection_bounds()
 866                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 867                     )
 868                     .chain(
 869                         data_b
 870                             .auto_traits()
 871                             .map(ty::ExistentialPredicate::AutoTrait)
 872                             .map(ty::Binder::dummy),
 873                     );
 874                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 875                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_a);
 876
 877                 // Require that the traits involved in this upcast are **equal**;
 878                 // only the **lifetime bound** is changed.
 879                 let InferOk { obligations, .. } = self
 880                     .infcx
 881                     .at(&obligation.cause, obligation.param_env)
 882                     .sup(target, source_trait)
 883                     .map_err(|_| Unimplemented)?;
 884                 nested.extend(obligations);
 885
 886                 // Register one obligation for 'a: 'b.
 887                 let cause = ObligationCause::new(
 888                     obligation.cause.span,
 889                     obligation.cause.body_id,
 890                     ObjectCastObligation(source, target),
 891                 );
 892                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 893                 nested.push(Obligation::with_depth(
 894                     tcx,
 895                     cause,
 896                     obligation.recursion_depth + 1,
 897                     obligation.param_env,
 898                     obligation.predicate.rebind(outlives),
 899                 ));
 900             }
 901             _ => bug!(),
 902         };
 903
 904         let vtable_segment_callback = {
 905             let mut vptr_offset = 0;
 906             move |segment| {
 907                 match segment {
 908                     VtblSegment::MetadataDSA => {
 909                         vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
 910                     }
 911                     VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 912                         vptr_offset += util::count_own_vtable_entries(tcx, trait_ref);
 913                         if trait_ref == upcast_trait_ref {
 914                             if emit_vptr {
 915                                 return ControlFlow::Break(Some(vptr_offset));
 916                             } else {
 917                                 return ControlFlow::Break(None);
 918                             }
 919                         }
 920
 921                         if emit_vptr {
 922                             vptr_offset += 1;
 923                         }
 924                     }
 925                 }
 926                 ControlFlow::Continue(())
 927             }
 928         };
 929
 930         let vtable_vptr_slot =
 931             super::super::prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback)
 932                 .unwrap();
 933
 934         Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
 935     }
 936
 937     fn confirm_builtin_unsize_candidate(
 938         &mut self,
 939         obligation: &TraitObligation<'tcx>,
 940     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 941         let tcx = self.tcx();
 942
 943         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 944         // regions here. See the comment there for more details.
 945         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 946         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 947         let target = self.infcx.shallow_resolve(target);
 948
 949         debug!(?source, ?target, "confirm_builtin_unsize_candidate");
 950
 951         let mut nested = vec![];
 952         match (source.kind(), target.kind()) {
 953             // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
 954             (&ty::Dynamic(ref data_a, r_a, dyn_a), &ty::Dynamic(ref data_b, r_b, dyn_b))
 955                 if dyn_a == dyn_b =>
 956             {
 957                 // See `assemble_candidates_for_unsizing` for more info.
 958                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 959                 let iter = data_a
 960                     .principal()
 961                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
 962                     .into_iter()
 963                     .chain(
 964                         data_a
 965                             .projection_bounds()
 966                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 967                     )
 968                     .chain(
 969                         data_b
 970                             .auto_traits()
 971                             .map(ty::ExistentialPredicate::AutoTrait)
 972                             .map(ty::Binder::dummy),
 973                     );
 974                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 975                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, dyn_a);
 976
 977                 // Require that the traits involved in this upcast are **equal**;
 978                 // only the **lifetime bound** is changed.
 979                 let InferOk { obligations, .. } = self
 980                     .infcx
 981                     .at(&obligation.cause, obligation.param_env)
 982                     .sup(target, source_trait)
 983                     .map_err(|_| Unimplemented)?;
 984                 nested.extend(obligations);
 985
 986                 // Register one obligation for 'a: 'b.
 987                 let cause = ObligationCause::new(
 988                     obligation.cause.span,
 989                     obligation.cause.body_id,
 990                     ObjectCastObligation(source, target),
 991                 );
 992                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 993                 nested.push(Obligation::with_depth(
 994                     tcx,
 995                     cause,
 996                     obligation.recursion_depth + 1,
 997                     obligation.param_env,
 998                     obligation.predicate.rebind(outlives),
 999                 ));
1000             }
1001
1002             // `T` -> `Trait`
1003             (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
1004                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
1005                 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
1006                     return Err(TraitNotObjectSafe(did));
1007                 }
1008
1009                 let cause = ObligationCause::new(
1010                     obligation.cause.span,
1011                     obligation.cause.body_id,
1012                     ObjectCastObligation(source, target),
1013                 );
1014
1015                 let predicate_to_obligation = |predicate| {
1016                     Obligation::with_depth(
1017                         tcx,
1018                         cause.clone(),
1019                         obligation.recursion_depth + 1,
1020                         obligation.param_env,
1021                         predicate,
1022                     )
1023                 };
1024
1025                 // Create obligations:
1026                 //  - Casting `T` to `Trait`
1027                 //  - For all the various builtin bounds attached to the object cast. (In other
1028                 //  words, if the object type is `Foo + Send`, this would create an obligation for
1029                 //  the `Send` check.)
1030                 //  - Projection predicates
1031                 nested.extend(
1032                     data.iter().map(|predicate| {
1033                         predicate_to_obligation(predicate.with_self_ty(tcx, source))
1034                     }),
1035                 );
1036
1037                 // We can only make objects from sized types.
1038                 let tr =
1039                     ty::Binder::dummy(tcx.at(cause.span).mk_trait_ref(LangItem::Sized, [source]));
1040                 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
1041
1042                 // If the type is `Foo + 'a`, ensure that the type
1043                 // being cast to `Foo + 'a` outlives `'a`:
1044                 let outlives = ty::OutlivesPredicate(source, r);
1045                 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
1046             }
1047
1048             // `[T; n]` -> `[T]`
1049             (&ty::Array(a, _), &ty::Slice(b)) => {
1050                 let InferOk { obligations, .. } = self
1051                     .infcx
1052                     .at(&obligation.cause, obligation.param_env)
1053                     .eq(b, a)
1054                     .map_err(|_| Unimplemented)?;
1055                 nested.extend(obligations);
1056             }
1057
1058             // `Struct<T>` -> `Struct<U>`
1059             (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
1060                 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
1061                     GenericArgKind::Type(ty) => match ty.kind() {
1062                         ty::Param(p) => Some(p.index),
1063                         _ => None,
1064                     },
1065
1066                     // Lifetimes aren't allowed to change during unsizing.
1067                     GenericArgKind::Lifetime(_) => None,
1068
1069                     GenericArgKind::Const(ct) => match ct.kind() {
1070                         ty::ConstKind::Param(p) => Some(p.index),
1071                         _ => None,
1072                     },
1073                 };
1074
1075                 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1076                 // by putting it in a query; it would only need the `DefId` as it
1077                 // looks at declared field types, not anything substituted.
1078
1079                 // The last field of the structure has to exist and contain type/const parameters.
1080                 let (tail_field, prefix_fields) =
1081                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1082                 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1083
1084                 let mut unsizing_params = GrowableBitSet::new_empty();
1085                 for arg in tail_field_ty.0.walk() {
1086                     if let Some(i) = maybe_unsizing_param_idx(arg) {
1087                         unsizing_params.insert(i);
1088                     }
1089                 }
1090
1091                 // Ensure none of the other fields mention the parameters used
1092                 // in unsizing.
1093                 for field in prefix_fields {
1094                     for arg in tcx.type_of(field.did).walk() {
1095                         if let Some(i) = maybe_unsizing_param_idx(arg) {
1096                             unsizing_params.remove(i);
1097                         }
1098                     }
1099                 }
1100
1101                 if unsizing_params.is_empty() {
1102                     return Err(Unimplemented);
1103                 }
1104
1105                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
1106                 // normalizing in the process, since `type_of` returns something directly from
1107                 // astconv (which means it's un-normalized).
1108                 let source_tail = normalize_with_depth_to(
1109                     self,
1110                     obligation.param_env,
1111                     obligation.cause.clone(),
1112                     obligation.recursion_depth + 1,
1113                     tail_field_ty.subst(tcx, substs_a),
1114                     &mut nested,
1115                 );
1116                 let target_tail = normalize_with_depth_to(
1117                     self,
1118                     obligation.param_env,
1119                     obligation.cause.clone(),
1120                     obligation.recursion_depth + 1,
1121                     tail_field_ty.subst(tcx, substs_b),
1122                     &mut nested,
1123                 );
1124
1125                 // Check that the source struct with the target's
1126                 // unsizing parameters is equal to the target.
1127                 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1128                     if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1129                 }));
1130                 let new_struct = tcx.mk_adt(def, substs);
1131                 let InferOk { obligations, .. } = self
1132                     .infcx
1133                     .at(&obligation.cause, obligation.param_env)
1134                     .eq(target, new_struct)
1135                     .map_err(|_| Unimplemented)?;
1136                 nested.extend(obligations);
1137
1138                 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1139                 nested.push(predicate_for_trait_def(
1140                     tcx,
1141                     obligation.param_env,
1142                     obligation.cause.clone(),
1143                     obligation.predicate.def_id(),
1144                     obligation.recursion_depth + 1,
1145                     [source_tail, target_tail],
1146                 ));
1147             }
1148
1149             // `(.., T)` -> `(.., U)`
1150             (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1151                 assert_eq!(tys_a.len(), tys_b.len());
1152
1153                 // The last field of the tuple has to exist.
1154                 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1155                 let &b_last = tys_b.last().unwrap();
1156
1157                 // Check that the source tuple with the target's
1158                 // last element is equal to the target.
1159                 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1160                 let InferOk { obligations, .. } = self
1161                     .infcx
1162                     .at(&obligation.cause, obligation.param_env)
1163                     .eq(target, new_tuple)
1164                     .map_err(|_| Unimplemented)?;
1165                 nested.extend(obligations);
1166
1167                 // Construct the nested `T: Unsize<U>` predicate.
1168                 nested.push(ensure_sufficient_stack(|| {
1169                     predicate_for_trait_def(
1170                         tcx,
1171                         obligation.param_env,
1172                         obligation.cause.clone(),
1173                         obligation.predicate.def_id(),
1174                         obligation.recursion_depth + 1,
1175                         [a_last, b_last],
1176                     )
1177                 }));
1178             }
1179
1180             _ => bug!("source: {source}, target: {target}"),
1181         };
1182
1183         Ok(ImplSourceBuiltinData { nested })
1184     }
1185
1186     fn confirm_const_destruct_candidate(
1187         &mut self,
1188         obligation: &TraitObligation<'tcx>,
1189         impl_def_id: Option<DefId>,
1190     ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1191         // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1192         if !obligation.is_const() {
1193             return Ok(ImplSourceConstDestructData { nested: vec![] });
1194         }
1195
1196         let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1197
1198         let tcx = self.tcx();
1199         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1200
1201         let mut nested = vec![];
1202         let cause = obligation.derived_cause(BuiltinDerivedObligation);
1203
1204         // If we have a custom `impl const Drop`, then
1205         // first check it like a regular impl candidate.
1206         // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1207         if let Some(impl_def_id) = impl_def_id {
1208             let mut new_obligation = obligation.clone();
1209             new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1210                 trait_pred.trait_ref.def_id = drop_trait;
1211                 trait_pred
1212             });
1213             let substs = self.rematch_impl(impl_def_id, &new_obligation);
1214             debug!(?substs, "impl substs");
1215
1216             let cause = obligation.derived_cause(|derived| {
1217                 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1218                     derived,
1219                     impl_def_id,
1220                     span: obligation.cause.span,
1221                 }))
1222             });
1223             let obligations = ensure_sufficient_stack(|| {
1224                 self.vtable_impl(
1225                     impl_def_id,
1226                     substs,
1227                     &cause,
1228                     new_obligation.recursion_depth + 1,
1229                     new_obligation.param_env,
1230                     obligation.predicate,
1231                 )
1232             });
1233             nested.extend(obligations.nested);
1234         }
1235
1236         // We want to confirm the ADT's fields if we have an ADT
1237         let mut stack = match *self_ty.skip_binder().kind() {
1238             ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1239             _ => vec![self_ty.skip_binder()],
1240         };
1241
1242         while let Some(nested_ty) = stack.pop() {
1243             match *nested_ty.kind() {
1244                 // We know these types are trivially drop
1245                 ty::Bool
1246                 | ty::Char
1247                 | ty::Int(_)
1248                 | ty::Uint(_)
1249                 | ty::Float(_)
1250                 | ty::Infer(ty::IntVar(_))
1251                 | ty::Infer(ty::FloatVar(_))
1252                 | ty::Str
1253                 | ty::RawPtr(_)
1254                 | ty::Ref(..)
1255                 | ty::FnDef(..)
1256                 | ty::FnPtr(_)
1257                 | ty::Never
1258                 | ty::Foreign(_) => {}
1259
1260                 // `ManuallyDrop` is trivially drop
1261                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
1262
1263                 // These types are built-in, so we can fast-track by registering
1264                 // nested predicates for their constituent type(s)
1265                 ty::Array(ty, _) | ty::Slice(ty) => {
1266                     stack.push(ty);
1267                 }
1268                 ty::Tuple(tys) => {
1269                     stack.extend(tys.iter());
1270                 }
1271                 ty::Closure(_, substs) => {
1272                     stack.push(substs.as_closure().tupled_upvars_ty());
1273                 }
1274                 ty::Generator(_, substs, _) => {
1275                     let generator = substs.as_generator();
1276                     stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1277                 }
1278                 ty::GeneratorWitness(tys) => {
1279                     stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1280                 }
1281
1282                 // If we have a projection type, make sure to normalize it so we replace it
1283                 // with a fresh infer variable
1284                 ty::Projection(..) => {
1285                     let predicate = normalize_with_depth_to(
1286                         self,
1287                         obligation.param_env,
1288                         cause.clone(),
1289                         obligation.recursion_depth + 1,
1290                         self_ty.rebind(ty::TraitPredicate {
1291                             trait_ref: self
1292                                 .tcx()
1293                                 .at(cause.span)
1294                                 .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1295                             constness: ty::BoundConstness::ConstIfConst,
1296                             polarity: ty::ImplPolarity::Positive,
1297                         }),
1298                         &mut nested,
1299                     );
1300
1301                     nested.push(Obligation::with_depth(
1302                         tcx,
1303                         cause.clone(),
1304                         obligation.recursion_depth + 1,
1305                         obligation.param_env,
1306                         predicate,
1307                     ));
1308                 }
1309
1310                 // If we have any other type (e.g. an ADT), just register a nested obligation
1311                 // since it's either not `const Drop` (and we raise an error during selection),
1312                 // or it's an ADT (and we need to check for a custom impl during selection)
1313                 _ => {
1314                     let predicate = self_ty.rebind(ty::TraitPredicate {
1315                         trait_ref: self
1316                             .tcx()
1317                             .at(cause.span)
1318                             .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1319                         constness: ty::BoundConstness::ConstIfConst,
1320                         polarity: ty::ImplPolarity::Positive,
1321                     });
1322
1323                     nested.push(Obligation::with_depth(
1324                         tcx,
1325                         cause.clone(),
1326                         obligation.recursion_depth + 1,
1327                         obligation.param_env,
1328                         predicate,
1329                     ));
1330                 }
1331             }
1332         }
1333
1334         Ok(ImplSourceConstDestructData { nested })
1335     }
1336 }