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_session::config::TraitSolver;
  19 use rustc_span::def_id::DefId;
  20
  21 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
  22 use crate::traits::util::{self, closure_trait_ref_and_return_type, predicate_for_trait_def};
  23 use crate::traits::vtable::{
  24     count_own_vtable_entries, prepare_vtable_segments, vtable_trait_first_method_offset,
  25     VtblSegment,
  26 };
  27 use crate::traits::{
  28     BuiltinDerivedObligation, ImplDerivedObligation, ImplDerivedObligationCause, ImplSource,
  29     ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
  30     ImplSourceConstDestructData, ImplSourceFnPointerData, ImplSourceFutureData,
  31     ImplSourceGeneratorData, ImplSourceObjectData, ImplSourceTraitAliasData,
  32     ImplSourceTraitUpcastingData, ImplSourceUserDefinedData, Normalized, ObjectCastObligation,
  33     Obligation, ObligationCause, OutputTypeParameterMismatch, PredicateObligation, Selection,
  34     SelectionError, TraitNotObjectSafe, TraitObligation, Unimplemented,
  35 };
  36
  37 use super::BuiltinImplConditions;
  38 use super::SelectionCandidate::{self, *};
  39 use super::SelectionContext;
  40
  41 use std::iter;
  42 use std::ops::ControlFlow;
  43
  44 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
  45     #[instrument(level = "debug", skip(self))]
  46     pub(super) fn confirm_candidate(
  47         &mut self,
  48         obligation: &TraitObligation<'tcx>,
  49         candidate: SelectionCandidate<'tcx>,
  50     ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
  51         let mut impl_src = match candidate {
  52             BuiltinCandidate { has_nested } => {
  53                 let data = self.confirm_builtin_candidate(obligation, has_nested);
  54                 ImplSource::Builtin(data)
  55             }
  56
  57             TransmutabilityCandidate => {
  58                 let data = self.confirm_transmutability_candidate(obligation)?;
  59                 ImplSource::Builtin(data)
  60             }
  61
  62             ParamCandidate(param) => {
  63                 let obligations =
  64                     self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
  65                 ImplSource::Param(obligations, param.skip_binder().constness)
  66             }
  67
  68             ImplCandidate(impl_def_id) => {
  69                 ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id))
  70             }
  71
  72             AutoImplCandidate => {
  73                 let data = self.confirm_auto_impl_candidate(obligation);
  74                 ImplSource::AutoImpl(data)
  75             }
  76
  77             ProjectionCandidate(idx, constness) => {
  78                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
  79                 ImplSource::Param(obligations, constness)
  80             }
  81
  82             ObjectCandidate(idx) => {
  83                 let data = self.confirm_object_candidate(obligation, idx)?;
  84                 ImplSource::Object(data)
  85             }
  86
  87             ClosureCandidate { .. } => {
  88                 let vtable_closure = self.confirm_closure_candidate(obligation)?;
  89                 ImplSource::Closure(vtable_closure)
  90             }
  91
  92             GeneratorCandidate => {
  93                 let vtable_generator = self.confirm_generator_candidate(obligation)?;
  94                 ImplSource::Generator(vtable_generator)
  95             }
  96
  97             FutureCandidate => {
  98                 let vtable_future = self.confirm_future_candidate(obligation)?;
  99                 ImplSource::Future(vtable_future)
 100             }
 101
 102             FnPointerCandidate { is_const } => {
 103                 let data = self.confirm_fn_pointer_candidate(obligation, is_const)?;
 104                 ImplSource::FnPointer(data)
 105             }
 106
 107             TraitAliasCandidate => {
 108                 let data = self.confirm_trait_alias_candidate(obligation);
 109                 ImplSource::TraitAlias(data)
 110             }
 111
 112             BuiltinObjectCandidate => {
 113                 // This indicates something like `Trait + Send: Send`. In this case, we know that
 114                 // this holds because that's what the object type is telling us, and there's really
 115                 // no additional obligations to prove and no types in particular to unify, etc.
 116                 ImplSource::Param(Vec::new(), ty::BoundConstness::NotConst)
 117             }
 118
 119             BuiltinUnsizeCandidate => {
 120                 let data = self.confirm_builtin_unsize_candidate(obligation)?;
 121                 ImplSource::Builtin(data)
 122             }
 123
 124             TraitUpcastingUnsizeCandidate(idx) => {
 125                 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
 126                 ImplSource::TraitUpcasting(data)
 127             }
 128
 129             ConstDestructCandidate(def_id) => {
 130                 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
 131                 ImplSource::ConstDestruct(data)
 132             }
 133         };
 134
 135         if !obligation.predicate.is_const_if_const() {
 136             // normalize nested predicates according to parent predicate's constness.
 137             impl_src = impl_src.map(|mut o| {
 138                 o.predicate = o.predicate.without_const(self.tcx());
 139                 o
 140             });
 141         }
 142
 143         Ok(impl_src)
 144     }
 145
 146     fn confirm_projection_candidate(
 147         &mut self,
 148         obligation: &TraitObligation<'tcx>,
 149         idx: usize,
 150     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 151         let tcx = self.tcx();
 152
 153         let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
 154         let placeholder_trait_predicate =
 155             self.infcx.replace_bound_vars_with_placeholders(trait_predicate).trait_ref;
 156         let placeholder_self_ty = placeholder_trait_predicate.self_ty();
 157         let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
 158         let (def_id, substs) = match *placeholder_self_ty.kind() {
 159             ty::Alias(_, ty::AliasTy { def_id, substs, .. }) => (def_id, substs),
 160             _ => bug!("projection candidate for unexpected type: {:?}", placeholder_self_ty),
 161         };
 162
 163         let candidate_predicate = tcx.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);
 188             for (predicate, _) in predicates {
 189                 let normalized = normalize_with_depth_to(
 190                     self,
 191                     obligation.param_env,
 192                     obligation.cause.clone(),
 193                     obligation.recursion_depth + 1,
 194                     predicate,
 195                     &mut obligations,
 196                 );
 197                 obligations.push(Obligation::with_depth(
 198                     self.tcx(),
 199                     obligation.cause.clone(),
 200                     obligation.recursion_depth + 1,
 201                     obligation.param_env,
 202                     normalized,
 203                 ));
 204             }
 205         }
 206
 207         Ok(obligations)
 208     }
 209
 210     fn confirm_param_candidate(
 211         &mut self,
 212         obligation: &TraitObligation<'tcx>,
 213         param: ty::PolyTraitRef<'tcx>,
 214     ) -> Vec<PredicateObligation<'tcx>> {
 215         debug!(?obligation, ?param, "confirm_param_candidate");
 216
 217         // During evaluation, we already checked that this
 218         // where-clause trait-ref could be unified with the obligation
 219         // trait-ref. Repeat that unification now without any
 220         // transactional boundary; it should not fail.
 221         match self.match_where_clause_trait_ref(obligation, param) {
 222             Ok(obligations) => obligations,
 223             Err(()) => {
 224                 bug!(
 225                     "Where clause `{:?}` was applicable to `{:?}` but now is not",
 226                     param,
 227                     obligation
 228                 );
 229             }
 230         }
 231     }
 232
 233     fn confirm_builtin_candidate(
 234         &mut self,
 235         obligation: &TraitObligation<'tcx>,
 236         has_nested: bool,
 237     ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
 238         debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
 239
 240         let lang_items = self.tcx().lang_items();
 241         let obligations = if has_nested {
 242             let trait_def = obligation.predicate.def_id();
 243             let conditions = if Some(trait_def) == lang_items.sized_trait() {
 244                 self.sized_conditions(obligation)
 245             } else if Some(trait_def) == lang_items.copy_trait() {
 246                 self.copy_clone_conditions(obligation)
 247             } else if Some(trait_def) == lang_items.clone_trait() {
 248                 self.copy_clone_conditions(obligation)
 249             } else {
 250                 bug!("unexpected builtin trait {:?}", trait_def)
 251             };
 252             let BuiltinImplConditions::Where(nested) = conditions else {
 253                 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
 254             };
 255
 256             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 257             ensure_sufficient_stack(|| {
 258                 self.collect_predicates_for_types(
 259                     obligation.param_env,
 260                     cause,
 261                     obligation.recursion_depth + 1,
 262                     trait_def,
 263                     nested,
 264                 )
 265             })
 266         } else {
 267             vec![]
 268         };
 269
 270         debug!(?obligations);
 271
 272         ImplSourceBuiltinData { nested: obligations }
 273     }
 274
 275     fn confirm_transmutability_candidate(
 276         &mut self,
 277         obligation: &TraitObligation<'tcx>,
 278     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 279         debug!(?obligation, "confirm_transmutability_candidate");
 280
 281         let predicate = obligation.predicate;
 282
 283         let type_at = |i| predicate.map_bound(|p| p.trait_ref.substs.type_at(i));
 284         let const_at = |i| predicate.skip_binder().trait_ref.substs.const_at(i);
 285
 286         let src_and_dst = predicate.map_bound(|p| rustc_transmute::Types {
 287             dst: p.trait_ref.substs.type_at(0),
 288             src: p.trait_ref.substs.type_at(1),
 289         });
 290
 291         let scope = type_at(2).skip_binder();
 292
 293         let Some(assume) =
 294             rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3)) else {
 295                 return Err(Unimplemented);
 296             };
 297
 298         let cause = obligation.cause.clone();
 299
 300         let mut transmute_env = rustc_transmute::TransmuteTypeEnv::new(self.infcx);
 301
 302         let maybe_transmutable = transmute_env.is_transmutable(cause, src_and_dst, scope, assume);
 303
 304         use rustc_transmute::Answer;
 305
 306         match maybe_transmutable {
 307             Answer::Yes => Ok(ImplSourceBuiltinData { nested: vec![] }),
 308             _ => Err(Unimplemented),
 309         }
 310     }
 311
 312     /// This handles the case where an `auto trait Foo` impl is being used.
 313     /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
 314     ///
 315     /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
 316     /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
 317     fn confirm_auto_impl_candidate(
 318         &mut self,
 319         obligation: &TraitObligation<'tcx>,
 320     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 321         debug!(?obligation, "confirm_auto_impl_candidate");
 322
 323         let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
 324         let types = self.constituent_types_for_ty(self_ty);
 325         self.vtable_auto_impl(obligation, obligation.predicate.def_id(), types)
 326     }
 327
 328     /// See `confirm_auto_impl_candidate`.
 329     fn vtable_auto_impl(
 330         &mut self,
 331         obligation: &TraitObligation<'tcx>,
 332         trait_def_id: DefId,
 333         nested: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
 334     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 335         debug!(?nested, "vtable_auto_impl");
 336         ensure_sufficient_stack(|| {
 337             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 338
 339             let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
 340             let trait_ref = self.infcx.replace_bound_vars_with_placeholders(poly_trait_ref);
 341             let trait_obligations: Vec<PredicateObligation<'_>> = self.impl_or_trait_obligations(
 342                 &cause,
 343                 obligation.recursion_depth + 1,
 344                 obligation.param_env,
 345                 trait_def_id,
 346                 &trait_ref.substs,
 347                 obligation.predicate,
 348             );
 349
 350             let mut obligations = self.collect_predicates_for_types(
 351                 obligation.param_env,
 352                 cause,
 353                 obligation.recursion_depth + 1,
 354                 trait_def_id,
 355                 nested,
 356             );
 357
 358             // Adds the predicates from the trait. Note that this contains a `Self: Trait`
 359             // predicate as usual. It won't have any effect since auto traits are coinductive.
 360             obligations.extend(trait_obligations);
 361
 362             debug!(?obligations, "vtable_auto_impl");
 363
 364             ImplSourceAutoImplData { trait_def_id, nested: obligations }
 365         })
 366     }
 367
 368     fn confirm_impl_candidate(
 369         &mut self,
 370         obligation: &TraitObligation<'tcx>,
 371         impl_def_id: DefId,
 372     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 373         debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");
 374
 375         // First, create the substitutions by matching the impl again,
 376         // this time not in a probe.
 377         let substs = self.rematch_impl(impl_def_id, obligation);
 378         debug!(?substs, "impl substs");
 379         ensure_sufficient_stack(|| {
 380             self.vtable_impl(
 381                 impl_def_id,
 382                 substs,
 383                 &obligation.cause,
 384                 obligation.recursion_depth + 1,
 385                 obligation.param_env,
 386                 obligation.predicate,
 387             )
 388         })
 389     }
 390
 391     fn vtable_impl(
 392         &mut self,
 393         impl_def_id: DefId,
 394         substs: Normalized<'tcx, SubstsRef<'tcx>>,
 395         cause: &ObligationCause<'tcx>,
 396         recursion_depth: usize,
 397         param_env: ty::ParamEnv<'tcx>,
 398         parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
 399     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 400         debug!(?impl_def_id, ?substs, ?recursion_depth, "vtable_impl");
 401
 402         let mut impl_obligations = self.impl_or_trait_obligations(
 403             cause,
 404             recursion_depth,
 405             param_env,
 406             impl_def_id,
 407             &substs.value,
 408             parent_trait_pred,
 409         );
 410
 411         debug!(?impl_obligations, "vtable_impl");
 412
 413         // Because of RFC447, the impl-trait-ref and obligations
 414         // are sufficient to determine the impl substs, without
 415         // relying on projections in the impl-trait-ref.
 416         //
 417         // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
 418         impl_obligations.extend(substs.obligations);
 419
 420         ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations }
 421     }
 422
 423     fn confirm_object_candidate(
 424         &mut self,
 425         obligation: &TraitObligation<'tcx>,
 426         index: usize,
 427     ) -> Result<ImplSourceObjectData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 428         let tcx = self.tcx();
 429         debug!(?obligation, ?index, "confirm_object_candidate");
 430
 431         let trait_predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
 432         let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
 433         let obligation_trait_ref = ty::Binder::dummy(trait_predicate.trait_ref);
 434         let ty::Dynamic(data, ..) = *self_ty.kind() else {
 435             span_bug!(obligation.cause.span, "object candidate with non-object");
 436         };
 437
 438         let object_trait_ref = data.principal().unwrap_or_else(|| {
 439             span_bug!(obligation.cause.span, "object candidate with no principal")
 440         });
 441         let object_trait_ref = self.infcx.replace_bound_vars_with_fresh_vars(
 442             obligation.cause.span,
 443             HigherRankedType,
 444             object_trait_ref,
 445         );
 446         let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);
 447
 448         let mut nested = vec![];
 449
 450         let mut supertraits = util::supertraits(tcx, ty::Binder::dummy(object_trait_ref));
 451         let unnormalized_upcast_trait_ref =
 452             supertraits.nth(index).expect("supertraits iterator no longer has as many elements");
 453
 454         let upcast_trait_ref = normalize_with_depth_to(
 455             self,
 456             obligation.param_env,
 457             obligation.cause.clone(),
 458             obligation.recursion_depth + 1,
 459             unnormalized_upcast_trait_ref,
 460             &mut nested,
 461         );
 462
 463         nested.extend(self.infcx.commit_if_ok(|_| {
 464             self.infcx
 465                 .at(&obligation.cause, obligation.param_env)
 466                 .sup(obligation_trait_ref, upcast_trait_ref)
 467                 .map(|InferOk { obligations, .. }| obligations)
 468                 .map_err(|_| Unimplemented)
 469         })?);
 470
 471         // Check supertraits hold. This is so that their associated type bounds
 472         // will be checked in the code below.
 473         for super_trait in tcx
 474             .super_predicates_of(trait_predicate.def_id())
 475             .instantiate(tcx, trait_predicate.trait_ref.substs)
 476             .predicates
 477             .into_iter()
 478         {
 479             let normalized_super_trait = normalize_with_depth_to(
 480                 self,
 481                 obligation.param_env,
 482                 obligation.cause.clone(),
 483                 obligation.recursion_depth + 1,
 484                 super_trait,
 485                 &mut nested,
 486             );
 487             nested.push(obligation.with(tcx, normalized_super_trait));
 488         }
 489
 490         let assoc_types: Vec<_> = tcx
 491             .associated_items(trait_predicate.def_id())
 492             .in_definition_order()
 493             .filter_map(
 494                 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
 495             )
 496             .collect();
 497
 498         for assoc_type in assoc_types {
 499             let defs: &ty::Generics = tcx.generics_of(assoc_type);
 500
 501             if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
 502                 tcx.sess.delay_span_bug(
 503                     obligation.cause.span,
 504                     "GATs in trait object shouldn't have been considered",
 505                 );
 506                 return Err(SelectionError::Unimplemented);
 507             }
 508
 509             // This maybe belongs in wf, but that can't (doesn't) handle
 510             // higher-ranked things.
 511             // Prevent, e.g., `dyn Iterator<Item = str>`.
 512             for bound in self.tcx().item_bounds(assoc_type).transpose_iter() {
 513                 let subst_bound =
 514                     if defs.count() == 0 {
 515                         bound.subst(tcx, trait_predicate.trait_ref.substs)
 516                     } else {
 517                         let mut substs = smallvec::SmallVec::with_capacity(defs.count());
 518                         substs.extend(trait_predicate.trait_ref.substs.iter());
 519                         let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
 520                             smallvec::SmallVec::with_capacity(
 521                                 bound.0.kind().bound_vars().len() + defs.count(),
 522                             );
 523                         bound_vars.extend(bound.0.kind().bound_vars().into_iter());
 524                         InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
 525                             .kind
 526                         {
 527                             GenericParamDefKind::Type { .. } => {
 528                                 let kind = ty::BoundTyKind::Param(param.name);
 529                                 let bound_var = ty::BoundVariableKind::Ty(kind);
 530                                 bound_vars.push(bound_var);
 531                                 tcx.mk_ty(ty::Bound(
 532                                     ty::INNERMOST,
 533                                     ty::BoundTy {
 534                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 535                                         kind,
 536                                     },
 537                                 ))
 538                                 .into()
 539                             }
 540                             GenericParamDefKind::Lifetime => {
 541                                 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
 542                                 let bound_var = ty::BoundVariableKind::Region(kind);
 543                                 bound_vars.push(bound_var);
 544                                 tcx.mk_region(ty::ReLateBound(
 545                                     ty::INNERMOST,
 546                                     ty::BoundRegion {
 547                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 548                                         kind,
 549                                     },
 550                                 ))
 551                                 .into()
 552                             }
 553                             GenericParamDefKind::Const { .. } => {
 554                                 let bound_var = ty::BoundVariableKind::Const;
 555                                 bound_vars.push(bound_var);
 556                                 tcx.mk_const(
 557                                     ty::ConstKind::Bound(
 558                                         ty::INNERMOST,
 559                                         ty::BoundVar::from_usize(bound_vars.len() - 1),
 560                                     ),
 561                                     tcx.type_of(param.def_id),
 562                                 )
 563                                 .into()
 564                             }
 565                         });
 566                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 567                         let assoc_ty_substs = tcx.intern_substs(&substs);
 568
 569                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 570                         let bound =
 571                             bound.map_bound(|b| b.kind().skip_binder()).subst(tcx, assoc_ty_substs);
 572                         tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
 573                     };
 574                 let normalized_bound = normalize_with_depth_to(
 575                     self,
 576                     obligation.param_env,
 577                     obligation.cause.clone(),
 578                     obligation.recursion_depth + 1,
 579                     subst_bound,
 580                     &mut nested,
 581                 );
 582                 nested.push(obligation.with(tcx, normalized_bound));
 583             }
 584         }
 585
 586         debug!(?nested, "object nested obligations");
 587
 588         let vtable_base = vtable_trait_first_method_offset(
 589             tcx,
 590             (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
 591         );
 592
 593         Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
 594     }
 595
 596     fn confirm_fn_pointer_candidate(
 597         &mut self,
 598         obligation: &TraitObligation<'tcx>,
 599         is_const: bool,
 600     ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 601     {
 602         debug!(?obligation, "confirm_fn_pointer_candidate");
 603
 604         let tcx = self.tcx();
 605         let self_ty = self
 606             .infcx
 607             .shallow_resolve(obligation.self_ty().no_bound_vars())
 608             .expect("fn pointer should not capture bound vars from predicate");
 609         let sig = self_ty.fn_sig(tcx);
 610         let trait_ref = closure_trait_ref_and_return_type(
 611             tcx,
 612             obligation.predicate.def_id(),
 613             self_ty,
 614             sig,
 615             util::TupleArgumentsFlag::Yes,
 616         )
 617         .map_bound(|(trait_ref, _)| trait_ref);
 618
 619         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 620         let cause = obligation.derived_cause(BuiltinDerivedObligation);
 621
 622         if obligation.is_const() && !is_const {
 623             // function is a trait method
 624             if let ty::FnDef(def_id, substs) = self_ty.kind() && let Some(trait_id) = tcx.trait_of_item(*def_id) {
 625                 let trait_ref = TraitRef::from_method(tcx, trait_id, *substs);
 626                 let poly_trait_pred = Binder::dummy(trait_ref).with_constness(ty::BoundConstness::ConstIfConst);
 627                 let obligation = Obligation::new(tcx, cause.clone(), obligation.param_env, poly_trait_pred);
 628                 nested.push(obligation);
 629             }
 630         }
 631
 632         // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
 633         let output_ty = self.infcx.replace_bound_vars_with_placeholders(sig.output());
 634         let output_ty = normalize_with_depth_to(
 635             self,
 636             obligation.param_env,
 637             cause.clone(),
 638             obligation.recursion_depth,
 639             output_ty,
 640             &mut nested,
 641         );
 642         let tr =
 643             ty::Binder::dummy(self.tcx().at(cause.span).mk_trait_ref(LangItem::Sized, [output_ty]));
 644         nested.push(Obligation::new(self.infcx.tcx, cause, obligation.param_env, tr));
 645
 646         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
 647     }
 648
 649     fn confirm_trait_alias_candidate(
 650         &mut self,
 651         obligation: &TraitObligation<'tcx>,
 652     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
 653         debug!(?obligation, "confirm_trait_alias_candidate");
 654
 655         let alias_def_id = obligation.predicate.def_id();
 656         let predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
 657         let trait_ref = predicate.trait_ref;
 658         let trait_def_id = trait_ref.def_id;
 659         let substs = trait_ref.substs;
 660
 661         let trait_obligations = self.impl_or_trait_obligations(
 662             &obligation.cause,
 663             obligation.recursion_depth,
 664             obligation.param_env,
 665             trait_def_id,
 666             &substs,
 667             obligation.predicate,
 668         );
 669
 670         debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
 671
 672         ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
 673     }
 674
 675     fn confirm_generator_candidate(
 676         &mut self,
 677         obligation: &TraitObligation<'tcx>,
 678     ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 679     {
 680         // Okay to skip binder because the substs on generator types never
 681         // touch bound regions, they just capture the in-scope
 682         // type/region parameters.
 683         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 684         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 685             bug!("closure candidate for non-closure {:?}", obligation);
 686         };
 687
 688         debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
 689
 690         let gen_sig = substs.as_generator().poly_sig();
 691
 692         // NOTE: The self-type is a generator type and hence is
 693         // in fact unparameterized (or at least does not reference any
 694         // regions bound in the obligation).
 695         let self_ty = obligation
 696             .predicate
 697             .self_ty()
 698             .no_bound_vars()
 699             .expect("unboxed closure type should not capture bound vars from the predicate");
 700
 701         let trait_ref = super::util::generator_trait_ref_and_outputs(
 702             self.tcx(),
 703             obligation.predicate.def_id(),
 704             self_ty,
 705             gen_sig,
 706         )
 707         .map_bound(|(trait_ref, ..)| trait_ref);
 708
 709         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 710         debug!(?trait_ref, ?nested, "generator candidate obligations");
 711
 712         Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
 713     }
 714
 715     fn confirm_future_candidate(
 716         &mut self,
 717         obligation: &TraitObligation<'tcx>,
 718     ) -> Result<ImplSourceFutureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 719         // Okay to skip binder because the substs on generator types never
 720         // touch bound regions, they just capture the in-scope
 721         // type/region parameters.
 722         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 723         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 724             bug!("closure candidate for non-closure {:?}", obligation);
 725         };
 726
 727         debug!(?obligation, ?generator_def_id, ?substs, "confirm_future_candidate");
 728
 729         let gen_sig = substs.as_generator().poly_sig();
 730
 731         let trait_ref = super::util::future_trait_ref_and_outputs(
 732             self.tcx(),
 733             obligation.predicate.def_id(),
 734             obligation.predicate.no_bound_vars().expect("future has no bound vars").self_ty(),
 735             gen_sig,
 736         )
 737         .map_bound(|(trait_ref, ..)| trait_ref);
 738
 739         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 740         debug!(?trait_ref, ?nested, "future candidate obligations");
 741
 742         Ok(ImplSourceFutureData { generator_def_id, substs, nested })
 743     }
 744
 745     #[instrument(skip(self), level = "debug")]
 746     fn confirm_closure_candidate(
 747         &mut self,
 748         obligation: &TraitObligation<'tcx>,
 749     ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 750         let kind = self
 751             .tcx()
 752             .fn_trait_kind_from_def_id(obligation.predicate.def_id())
 753             .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
 754
 755         // Okay to skip binder because the substs on closure types never
 756         // touch bound regions, they just capture the in-scope
 757         // type/region parameters.
 758         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 759         let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
 760             bug!("closure candidate for non-closure {:?}", obligation);
 761         };
 762
 763         let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
 764         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 765
 766         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 767
 768         // FIXME: Chalk
 769         if self.tcx().sess.opts.unstable_opts.trait_solver != TraitSolver::Chalk {
 770             nested.push(obligation.with(
 771                 self.tcx(),
 772                 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind)),
 773             ));
 774         }
 775
 776         Ok(ImplSourceClosureData { closure_def_id, substs, nested })
 777     }
 778
 779     /// In the case of closure types and fn pointers,
 780     /// we currently treat the input type parameters on the trait as
 781     /// outputs. This means that when we have a match we have only
 782     /// considered the self type, so we have to go back and make sure
 783     /// to relate the argument types too. This is kind of wrong, but
 784     /// since we control the full set of impls, also not that wrong,
 785     /// and it DOES yield better error messages (since we don't report
 786     /// errors as if there is no applicable impl, but rather report
 787     /// errors are about mismatched argument types.
 788     ///
 789     /// Here is an example. Imagine we have a closure expression
 790     /// and we desugared it so that the type of the expression is
 791     /// `Closure`, and `Closure` expects `i32` as argument. Then it
 792     /// is "as if" the compiler generated this impl:
 793     /// ```ignore (illustrative)
 794     /// impl Fn(i32) for Closure { ... }
 795     /// ```
 796     /// Now imagine our obligation is `Closure: Fn(usize)`. So far
 797     /// we have matched the self type `Closure`. At this point we'll
 798     /// compare the `i32` to `usize` and generate an error.
 799     ///
 800     /// Note that this checking occurs *after* the impl has selected,
 801     /// because these output type parameters should not affect the
 802     /// selection of the impl. Therefore, if there is a mismatch, we
 803     /// report an error to the user.
 804     #[instrument(skip(self), level = "trace")]
 805     fn confirm_poly_trait_refs(
 806         &mut self,
 807         obligation: &TraitObligation<'tcx>,
 808         expected_trait_ref: ty::PolyTraitRef<'tcx>,
 809     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 810         let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
 811         // Normalize the obligation and expected trait refs together, because why not
 812         let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
 813             ensure_sufficient_stack(|| {
 814                 normalize_with_depth(
 815                     self,
 816                     obligation.param_env,
 817                     obligation.cause.clone(),
 818                     obligation.recursion_depth + 1,
 819                     (obligation_trait_ref, expected_trait_ref),
 820                 )
 821             });
 822
 823         self.infcx
 824             .at(&obligation.cause, obligation.param_env)
 825             .sup(obligation_trait_ref, expected_trait_ref)
 826             .map(|InferOk { mut obligations, .. }| {
 827                 obligations.extend(nested);
 828                 obligations
 829             })
 830             .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
 831     }
 832
 833     fn confirm_trait_upcasting_unsize_candidate(
 834         &mut self,
 835         obligation: &TraitObligation<'tcx>,
 836         idx: usize,
 837     ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 838     {
 839         let tcx = self.tcx();
 840
 841         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 842         // regions here. See the comment there for more details.
 843         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 844         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 845         let target = self.infcx.shallow_resolve(target);
 846
 847         debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
 848
 849         let mut nested = vec![];
 850         let source_trait_ref;
 851         let upcast_trait_ref;
 852         match (source.kind(), target.kind()) {
 853             // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
 854             (
 855                 &ty::Dynamic(ref data_a, r_a, repr_a @ ty::Dyn),
 856                 &ty::Dynamic(ref data_b, r_b, ty::Dyn),
 857             ) => {
 858                 // See `assemble_candidates_for_unsizing` for more info.
 859                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 860                 let principal_a = data_a.principal().unwrap();
 861                 source_trait_ref = principal_a.with_self_ty(tcx, source);
 862                 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
 863                 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
 864                 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
 865                     ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
 866                         tcx, trait_ref,
 867                     ))
 868                 });
 869                 let iter = Some(existential_predicate)
 870                     .into_iter()
 871                     .chain(
 872                         data_a
 873                             .projection_bounds()
 874                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 875                     )
 876                     .chain(
 877                         data_b
 878                             .auto_traits()
 879                             .map(ty::ExistentialPredicate::AutoTrait)
 880                             .map(ty::Binder::dummy),
 881                     );
 882                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 883                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_a);
 884
 885                 // Require that the traits involved in this upcast are **equal**;
 886                 // only the **lifetime bound** is changed.
 887                 let InferOk { obligations, .. } = self
 888                     .infcx
 889                     .at(&obligation.cause, obligation.param_env)
 890                     .sup(target, source_trait)
 891                     .map_err(|_| Unimplemented)?;
 892                 nested.extend(obligations);
 893
 894                 // Register one obligation for 'a: 'b.
 895                 let cause = ObligationCause::new(
 896                     obligation.cause.span,
 897                     obligation.cause.body_id,
 898                     ObjectCastObligation(source, target),
 899                 );
 900                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 901                 nested.push(Obligation::with_depth(
 902                     tcx,
 903                     cause,
 904                     obligation.recursion_depth + 1,
 905                     obligation.param_env,
 906                     obligation.predicate.rebind(outlives),
 907                 ));
 908             }
 909             _ => bug!(),
 910         };
 911
 912         let vtable_segment_callback = {
 913             let mut vptr_offset = 0;
 914             move |segment| {
 915                 match segment {
 916                     VtblSegment::MetadataDSA => {
 917                         vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
 918                     }
 919                     VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 920                         vptr_offset += count_own_vtable_entries(tcx, trait_ref);
 921                         if trait_ref == upcast_trait_ref {
 922                             if emit_vptr {
 923                                 return ControlFlow::Break(Some(vptr_offset));
 924                             } else {
 925                                 return ControlFlow::Break(None);
 926                             }
 927                         }
 928
 929                         if emit_vptr {
 930                             vptr_offset += 1;
 931                         }
 932                     }
 933                 }
 934                 ControlFlow::Continue(())
 935             }
 936         };
 937
 938         let vtable_vptr_slot =
 939             prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback).unwrap();
 940
 941         Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
 942     }
 943
 944     fn confirm_builtin_unsize_candidate(
 945         &mut self,
 946         obligation: &TraitObligation<'tcx>,
 947     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 948         let tcx = self.tcx();
 949
 950         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 951         // regions here. See the comment there for more details.
 952         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 953         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 954         let target = self.infcx.shallow_resolve(target);
 955
 956         debug!(?source, ?target, "confirm_builtin_unsize_candidate");
 957
 958         let mut nested = vec![];
 959         match (source.kind(), target.kind()) {
 960             // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
 961             (&ty::Dynamic(ref data_a, r_a, dyn_a), &ty::Dynamic(ref data_b, r_b, dyn_b))
 962                 if dyn_a == dyn_b =>
 963             {
 964                 // See `assemble_candidates_for_unsizing` for more info.
 965                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 966                 let iter = data_a
 967                     .principal()
 968                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
 969                     .into_iter()
 970                     .chain(
 971                         data_a
 972                             .projection_bounds()
 973                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 974                     )
 975                     .chain(
 976                         data_b
 977                             .auto_traits()
 978                             .map(ty::ExistentialPredicate::AutoTrait)
 979                             .map(ty::Binder::dummy),
 980                     );
 981                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 982                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, dyn_a);
 983
 984                 // Require that the traits involved in this upcast are **equal**;
 985                 // only the **lifetime bound** is changed.
 986                 let InferOk { obligations, .. } = self
 987                     .infcx
 988                     .at(&obligation.cause, obligation.param_env)
 989                     .sup(target, source_trait)
 990                     .map_err(|_| Unimplemented)?;
 991                 nested.extend(obligations);
 992
 993                 // Register one obligation for 'a: 'b.
 994                 let cause = ObligationCause::new(
 995                     obligation.cause.span,
 996                     obligation.cause.body_id,
 997                     ObjectCastObligation(source, target),
 998                 );
 999                 let outlives = ty::OutlivesPredicate(r_a, r_b);
1000                 nested.push(Obligation::with_depth(
1001                     tcx,
1002                     cause,
1003                     obligation.recursion_depth + 1,
1004                     obligation.param_env,
1005                     obligation.predicate.rebind(outlives),
1006                 ));
1007             }
1008
1009             // `T` -> `Trait`
1010             (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
1011                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
1012                 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
1013                     return Err(TraitNotObjectSafe(did));
1014                 }
1015
1016                 let cause = ObligationCause::new(
1017                     obligation.cause.span,
1018                     obligation.cause.body_id,
1019                     ObjectCastObligation(source, target),
1020                 );
1021
1022                 let predicate_to_obligation = |predicate| {
1023                     Obligation::with_depth(
1024                         tcx,
1025                         cause.clone(),
1026                         obligation.recursion_depth + 1,
1027                         obligation.param_env,
1028                         predicate,
1029                     )
1030                 };
1031
1032                 // Create obligations:
1033                 //  - Casting `T` to `Trait`
1034                 //  - For all the various builtin bounds attached to the object cast. (In other
1035                 //  words, if the object type is `Foo + Send`, this would create an obligation for
1036                 //  the `Send` check.)
1037                 //  - Projection predicates
1038                 nested.extend(
1039                     data.iter().map(|predicate| {
1040                         predicate_to_obligation(predicate.with_self_ty(tcx, source))
1041                     }),
1042                 );
1043
1044                 // We can only make objects from sized types.
1045                 let tr =
1046                     ty::Binder::dummy(tcx.at(cause.span).mk_trait_ref(LangItem::Sized, [source]));
1047                 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
1048
1049                 // If the type is `Foo + 'a`, ensure that the type
1050                 // being cast to `Foo + 'a` outlives `'a`:
1051                 let outlives = ty::OutlivesPredicate(source, r);
1052                 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
1053             }
1054
1055             // `[T; n]` -> `[T]`
1056             (&ty::Array(a, _), &ty::Slice(b)) => {
1057                 let InferOk { obligations, .. } = self
1058                     .infcx
1059                     .at(&obligation.cause, obligation.param_env)
1060                     .eq(b, a)
1061                     .map_err(|_| Unimplemented)?;
1062                 nested.extend(obligations);
1063             }
1064
1065             // `Struct<T>` -> `Struct<U>`
1066             (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
1067                 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
1068                     GenericArgKind::Type(ty) => match ty.kind() {
1069                         ty::Param(p) => Some(p.index),
1070                         _ => None,
1071                     },
1072
1073                     // Lifetimes aren't allowed to change during unsizing.
1074                     GenericArgKind::Lifetime(_) => None,
1075
1076                     GenericArgKind::Const(ct) => match ct.kind() {
1077                         ty::ConstKind::Param(p) => Some(p.index),
1078                         _ => None,
1079                     },
1080                 };
1081
1082                 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1083                 // by putting it in a query; it would only need the `DefId` as it
1084                 // looks at declared field types, not anything substituted.
1085
1086                 // The last field of the structure has to exist and contain type/const parameters.
1087                 let (tail_field, prefix_fields) =
1088                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1089                 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1090
1091                 let mut unsizing_params = GrowableBitSet::new_empty();
1092                 for arg in tail_field_ty.0.walk() {
1093                     if let Some(i) = maybe_unsizing_param_idx(arg) {
1094                         unsizing_params.insert(i);
1095                     }
1096                 }
1097
1098                 // Ensure none of the other fields mention the parameters used
1099                 // in unsizing.
1100                 for field in prefix_fields {
1101                     for arg in tcx.type_of(field.did).walk() {
1102                         if let Some(i) = maybe_unsizing_param_idx(arg) {
1103                             unsizing_params.remove(i);
1104                         }
1105                     }
1106                 }
1107
1108                 if unsizing_params.is_empty() {
1109                     return Err(Unimplemented);
1110                 }
1111
1112                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
1113                 // normalizing in the process, since `type_of` returns something directly from
1114                 // astconv (which means it's un-normalized).
1115                 let source_tail = normalize_with_depth_to(
1116                     self,
1117                     obligation.param_env,
1118                     obligation.cause.clone(),
1119                     obligation.recursion_depth + 1,
1120                     tail_field_ty.subst(tcx, substs_a),
1121                     &mut nested,
1122                 );
1123                 let target_tail = normalize_with_depth_to(
1124                     self,
1125                     obligation.param_env,
1126                     obligation.cause.clone(),
1127                     obligation.recursion_depth + 1,
1128                     tail_field_ty.subst(tcx, substs_b),
1129                     &mut nested,
1130                 );
1131
1132                 // Check that the source struct with the target's
1133                 // unsizing parameters is equal to the target.
1134                 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1135                     if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1136                 }));
1137                 let new_struct = tcx.mk_adt(def, substs);
1138                 let InferOk { obligations, .. } = self
1139                     .infcx
1140                     .at(&obligation.cause, obligation.param_env)
1141                     .eq(target, new_struct)
1142                     .map_err(|_| Unimplemented)?;
1143                 nested.extend(obligations);
1144
1145                 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1146                 nested.push(predicate_for_trait_def(
1147                     tcx,
1148                     obligation.param_env,
1149                     obligation.cause.clone(),
1150                     obligation.predicate.def_id(),
1151                     obligation.recursion_depth + 1,
1152                     [source_tail, target_tail],
1153                 ));
1154             }
1155
1156             // `(.., T)` -> `(.., U)`
1157             (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1158                 assert_eq!(tys_a.len(), tys_b.len());
1159
1160                 // The last field of the tuple has to exist.
1161                 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1162                 let &b_last = tys_b.last().unwrap();
1163
1164                 // Check that the source tuple with the target's
1165                 // last element is equal to the target.
1166                 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1167                 let InferOk { obligations, .. } = self
1168                     .infcx
1169                     .at(&obligation.cause, obligation.param_env)
1170                     .eq(target, new_tuple)
1171                     .map_err(|_| Unimplemented)?;
1172                 nested.extend(obligations);
1173
1174                 // Construct the nested `T: Unsize<U>` predicate.
1175                 nested.push(ensure_sufficient_stack(|| {
1176                     predicate_for_trait_def(
1177                         tcx,
1178                         obligation.param_env,
1179                         obligation.cause.clone(),
1180                         obligation.predicate.def_id(),
1181                         obligation.recursion_depth + 1,
1182                         [a_last, b_last],
1183                     )
1184                 }));
1185             }
1186
1187             _ => bug!("source: {source}, target: {target}"),
1188         };
1189
1190         Ok(ImplSourceBuiltinData { nested })
1191     }
1192
1193     fn confirm_const_destruct_candidate(
1194         &mut self,
1195         obligation: &TraitObligation<'tcx>,
1196         impl_def_id: Option<DefId>,
1197     ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1198         // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1199         if !obligation.is_const() {
1200             return Ok(ImplSourceConstDestructData { nested: vec![] });
1201         }
1202
1203         let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1204
1205         let tcx = self.tcx();
1206         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1207
1208         let mut nested = vec![];
1209         let cause = obligation.derived_cause(BuiltinDerivedObligation);
1210
1211         // If we have a custom `impl const Drop`, then
1212         // first check it like a regular impl candidate.
1213         // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1214         if let Some(impl_def_id) = impl_def_id {
1215             let mut new_obligation = obligation.clone();
1216             new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1217                 trait_pred.trait_ref.def_id = drop_trait;
1218                 trait_pred
1219             });
1220             let substs = self.rematch_impl(impl_def_id, &new_obligation);
1221             debug!(?substs, "impl substs");
1222
1223             let cause = obligation.derived_cause(|derived| {
1224                 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1225                     derived,
1226                     impl_def_id,
1227                     span: obligation.cause.span,
1228                 }))
1229             });
1230             let obligations = ensure_sufficient_stack(|| {
1231                 self.vtable_impl(
1232                     impl_def_id,
1233                     substs,
1234                     &cause,
1235                     new_obligation.recursion_depth + 1,
1236                     new_obligation.param_env,
1237                     obligation.predicate,
1238                 )
1239             });
1240             nested.extend(obligations.nested);
1241         }
1242
1243         // We want to confirm the ADT's fields if we have an ADT
1244         let mut stack = match *self_ty.skip_binder().kind() {
1245             ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1246             _ => vec![self_ty.skip_binder()],
1247         };
1248
1249         while let Some(nested_ty) = stack.pop() {
1250             match *nested_ty.kind() {
1251                 // We know these types are trivially drop
1252                 ty::Bool
1253                 | ty::Char
1254                 | ty::Int(_)
1255                 | ty::Uint(_)
1256                 | ty::Float(_)
1257                 | ty::Infer(ty::IntVar(_))
1258                 | ty::Infer(ty::FloatVar(_))
1259                 | ty::Str
1260                 | ty::RawPtr(_)
1261                 | ty::Ref(..)
1262                 | ty::FnDef(..)
1263                 | ty::FnPtr(_)
1264                 | ty::Never
1265                 | ty::Foreign(_) => {}
1266
1267                 // `ManuallyDrop` is trivially drop
1268                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
1269
1270                 // These types are built-in, so we can fast-track by registering
1271                 // nested predicates for their constituent type(s)
1272                 ty::Array(ty, _) | ty::Slice(ty) => {
1273                     stack.push(ty);
1274                 }
1275                 ty::Tuple(tys) => {
1276                     stack.extend(tys.iter());
1277                 }
1278                 ty::Closure(_, substs) => {
1279                     stack.push(substs.as_closure().tupled_upvars_ty());
1280                 }
1281                 ty::Generator(_, substs, _) => {
1282                     let generator = substs.as_generator();
1283                     stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1284                 }
1285                 ty::GeneratorWitness(tys) => {
1286                     stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1287                 }
1288
1289                 // If we have a projection type, make sure to normalize it so we replace it
1290                 // with a fresh infer variable
1291                 ty::Alias(ty::Projection, ..) => {
1292                     let predicate = normalize_with_depth_to(
1293                         self,
1294                         obligation.param_env,
1295                         cause.clone(),
1296                         obligation.recursion_depth + 1,
1297                         self_ty.rebind(ty::TraitPredicate {
1298                             trait_ref: self
1299                                 .tcx()
1300                                 .at(cause.span)
1301                                 .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1302                             constness: ty::BoundConstness::ConstIfConst,
1303                             polarity: ty::ImplPolarity::Positive,
1304                         }),
1305                         &mut nested,
1306                     );
1307
1308                     nested.push(Obligation::with_depth(
1309                         tcx,
1310                         cause.clone(),
1311                         obligation.recursion_depth + 1,
1312                         obligation.param_env,
1313                         predicate,
1314                     ));
1315                 }
1316
1317                 // If we have any other type (e.g. an ADT), just register a nested obligation
1318                 // since it's either not `const Drop` (and we raise an error during selection),
1319                 // or it's an ADT (and we need to check for a custom impl during selection)
1320                 _ => {
1321                     let predicate = self_ty.rebind(ty::TraitPredicate {
1322                         trait_ref: self
1323                             .tcx()
1324                             .at(cause.span)
1325                             .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1326                         constness: ty::BoundConstness::ConstIfConst,
1327                         polarity: ty::ImplPolarity::Positive,
1328                     });
1329
1330                     nested.push(Obligation::with_depth(
1331                         tcx,
1332                         cause.clone(),
1333                         obligation.recursion_depth + 1,
1334                         obligation.param_env,
1335                         predicate,
1336                     ));
1337                 }
1338             }
1339         }
1340
1341         Ok(ImplSourceConstDestructData { nested })
1342     }
1343 }