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 =
 164             tcx.bound_item_bounds(def_id).map_bound(|i| i[idx]).subst(tcx, substs);
 165         let candidate = candidate_predicate
 166             .to_opt_poly_trait_pred()
 167             .expect("projection candidate is not a trait predicate")
 168             .map_bound(|t| t.trait_ref);
 169         let mut obligations = Vec::new();
 170         let candidate = normalize_with_depth_to(
 171             self,
 172             obligation.param_env,
 173             obligation.cause.clone(),
 174             obligation.recursion_depth + 1,
 175             candidate,
 176             &mut obligations,
 177         );
 178
 179         obligations.extend(self.infcx.commit_if_ok(|_| {
 180             self.infcx
 181                 .at(&obligation.cause, obligation.param_env)
 182                 .sup(placeholder_trait_predicate, candidate)
 183                 .map(|InferOk { obligations, .. }| obligations)
 184                 .map_err(|_| Unimplemented)
 185         })?);
 186
 187         if let ty::Alias(ty::Projection, ..) = placeholder_self_ty.kind() {
 188             let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates;
 189             debug!(?predicates, "projection predicates");
 190             for predicate in predicates {
 191                 let normalized = normalize_with_depth_to(
 192                     self,
 193                     obligation.param_env,
 194                     obligation.cause.clone(),
 195                     obligation.recursion_depth + 1,
 196                     predicate,
 197                     &mut obligations,
 198                 );
 199                 obligations.push(Obligation::with_depth(
 200                     self.tcx(),
 201                     obligation.cause.clone(),
 202                     obligation.recursion_depth + 1,
 203                     obligation.param_env,
 204                     normalized,
 205                 ));
 206             }
 207         }
 208
 209         Ok(obligations)
 210     }
 211
 212     fn confirm_param_candidate(
 213         &mut self,
 214         obligation: &TraitObligation<'tcx>,
 215         param: ty::PolyTraitRef<'tcx>,
 216     ) -> Vec<PredicateObligation<'tcx>> {
 217         debug!(?obligation, ?param, "confirm_param_candidate");
 218
 219         // During evaluation, we already checked that this
 220         // where-clause trait-ref could be unified with the obligation
 221         // trait-ref. Repeat that unification now without any
 222         // transactional boundary; it should not fail.
 223         match self.match_where_clause_trait_ref(obligation, param) {
 224             Ok(obligations) => obligations,
 225             Err(()) => {
 226                 bug!(
 227                     "Where clause `{:?}` was applicable to `{:?}` but now is not",
 228                     param,
 229                     obligation
 230                 );
 231             }
 232         }
 233     }
 234
 235     fn confirm_builtin_candidate(
 236         &mut self,
 237         obligation: &TraitObligation<'tcx>,
 238         has_nested: bool,
 239     ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> {
 240         debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
 241
 242         let lang_items = self.tcx().lang_items();
 243         let obligations = if has_nested {
 244             let trait_def = obligation.predicate.def_id();
 245             let conditions = if Some(trait_def) == lang_items.sized_trait() {
 246                 self.sized_conditions(obligation)
 247             } else if Some(trait_def) == lang_items.copy_trait() {
 248                 self.copy_clone_conditions(obligation)
 249             } else if Some(trait_def) == lang_items.clone_trait() {
 250                 self.copy_clone_conditions(obligation)
 251             } else {
 252                 bug!("unexpected builtin trait {:?}", trait_def)
 253             };
 254             let BuiltinImplConditions::Where(nested) = conditions else {
 255                 bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
 256             };
 257
 258             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 259             ensure_sufficient_stack(|| {
 260                 self.collect_predicates_for_types(
 261                     obligation.param_env,
 262                     cause,
 263                     obligation.recursion_depth + 1,
 264                     trait_def,
 265                     nested,
 266                 )
 267             })
 268         } else {
 269             vec![]
 270         };
 271
 272         debug!(?obligations);
 273
 274         ImplSourceBuiltinData { nested: obligations }
 275     }
 276
 277     fn confirm_transmutability_candidate(
 278         &mut self,
 279         obligation: &TraitObligation<'tcx>,
 280     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 281         debug!(?obligation, "confirm_transmutability_candidate");
 282
 283         let predicate = obligation.predicate;
 284
 285         let type_at = |i| predicate.map_bound(|p| p.trait_ref.substs.type_at(i));
 286         let const_at = |i| predicate.skip_binder().trait_ref.substs.const_at(i);
 287
 288         let src_and_dst = predicate.map_bound(|p| rustc_transmute::Types {
 289             dst: p.trait_ref.substs.type_at(0),
 290             src: p.trait_ref.substs.type_at(1),
 291         });
 292
 293         let scope = type_at(2).skip_binder();
 294
 295         let Some(assume) =
 296             rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3)) else {
 297                 return Err(Unimplemented);
 298             };
 299
 300         let cause = obligation.cause.clone();
 301
 302         let mut transmute_env = rustc_transmute::TransmuteTypeEnv::new(self.infcx);
 303
 304         let maybe_transmutable = transmute_env.is_transmutable(cause, src_and_dst, scope, assume);
 305
 306         use rustc_transmute::Answer;
 307
 308         match maybe_transmutable {
 309             Answer::Yes => Ok(ImplSourceBuiltinData { nested: vec![] }),
 310             _ => Err(Unimplemented),
 311         }
 312     }
 313
 314     /// This handles the case where an `auto trait Foo` impl is being used.
 315     /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
 316     ///
 317     /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
 318     /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
 319     fn confirm_auto_impl_candidate(
 320         &mut self,
 321         obligation: &TraitObligation<'tcx>,
 322     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 323         debug!(?obligation, "confirm_auto_impl_candidate");
 324
 325         let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
 326         let types = self.constituent_types_for_ty(self_ty);
 327         self.vtable_auto_impl(obligation, obligation.predicate.def_id(), types)
 328     }
 329
 330     /// See `confirm_auto_impl_candidate`.
 331     fn vtable_auto_impl(
 332         &mut self,
 333         obligation: &TraitObligation<'tcx>,
 334         trait_def_id: DefId,
 335         nested: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
 336     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
 337         debug!(?nested, "vtable_auto_impl");
 338         ensure_sufficient_stack(|| {
 339             let cause = obligation.derived_cause(BuiltinDerivedObligation);
 340
 341             let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
 342             let trait_ref = self.infcx.replace_bound_vars_with_placeholders(poly_trait_ref);
 343             let trait_obligations: Vec<PredicateObligation<'_>> = self.impl_or_trait_obligations(
 344                 &cause,
 345                 obligation.recursion_depth + 1,
 346                 obligation.param_env,
 347                 trait_def_id,
 348                 &trait_ref.substs,
 349                 obligation.predicate,
 350             );
 351
 352             let mut obligations = self.collect_predicates_for_types(
 353                 obligation.param_env,
 354                 cause,
 355                 obligation.recursion_depth + 1,
 356                 trait_def_id,
 357                 nested,
 358             );
 359
 360             // Adds the predicates from the trait.  Note that this contains a `Self: Trait`
 361             // predicate as usual.  It won't have any effect since auto traits are coinductive.
 362             obligations.extend(trait_obligations);
 363
 364             debug!(?obligations, "vtable_auto_impl");
 365
 366             ImplSourceAutoImplData { trait_def_id, nested: obligations }
 367         })
 368     }
 369
 370     fn confirm_impl_candidate(
 371         &mut self,
 372         obligation: &TraitObligation<'tcx>,
 373         impl_def_id: DefId,
 374     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 375         debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");
 376
 377         // First, create the substitutions by matching the impl again,
 378         // this time not in a probe.
 379         let substs = self.rematch_impl(impl_def_id, obligation);
 380         debug!(?substs, "impl substs");
 381         ensure_sufficient_stack(|| {
 382             self.vtable_impl(
 383                 impl_def_id,
 384                 substs,
 385                 &obligation.cause,
 386                 obligation.recursion_depth + 1,
 387                 obligation.param_env,
 388                 obligation.predicate,
 389             )
 390         })
 391     }
 392
 393     fn vtable_impl(
 394         &mut self,
 395         impl_def_id: DefId,
 396         substs: Normalized<'tcx, SubstsRef<'tcx>>,
 397         cause: &ObligationCause<'tcx>,
 398         recursion_depth: usize,
 399         param_env: ty::ParamEnv<'tcx>,
 400         parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
 401     ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
 402         debug!(?impl_def_id, ?substs, ?recursion_depth, "vtable_impl");
 403
 404         let mut impl_obligations = self.impl_or_trait_obligations(
 405             cause,
 406             recursion_depth,
 407             param_env,
 408             impl_def_id,
 409             &substs.value,
 410             parent_trait_pred,
 411         );
 412
 413         debug!(?impl_obligations, "vtable_impl");
 414
 415         // Because of RFC447, the impl-trait-ref and obligations
 416         // are sufficient to determine the impl substs, without
 417         // relying on projections in the impl-trait-ref.
 418         //
 419         // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
 420         impl_obligations.extend(substs.obligations);
 421
 422         ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations }
 423     }
 424
 425     fn confirm_object_candidate(
 426         &mut self,
 427         obligation: &TraitObligation<'tcx>,
 428         index: usize,
 429     ) -> Result<ImplSourceObjectData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 430         let tcx = self.tcx();
 431         debug!(?obligation, ?index, "confirm_object_candidate");
 432
 433         let trait_predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
 434         let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
 435         let obligation_trait_ref = ty::Binder::dummy(trait_predicate.trait_ref);
 436         let ty::Dynamic(data, ..) = *self_ty.kind() else {
 437             span_bug!(obligation.cause.span, "object candidate with non-object");
 438         };
 439
 440         let object_trait_ref = data.principal().unwrap_or_else(|| {
 441             span_bug!(obligation.cause.span, "object candidate with no principal")
 442         });
 443         let object_trait_ref = self.infcx.replace_bound_vars_with_fresh_vars(
 444             obligation.cause.span,
 445             HigherRankedType,
 446             object_trait_ref,
 447         );
 448         let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);
 449
 450         let mut nested = vec![];
 451
 452         let mut supertraits = util::supertraits(tcx, ty::Binder::dummy(object_trait_ref));
 453         let unnormalized_upcast_trait_ref =
 454             supertraits.nth(index).expect("supertraits iterator no longer has as many elements");
 455
 456         let upcast_trait_ref = normalize_with_depth_to(
 457             self,
 458             obligation.param_env,
 459             obligation.cause.clone(),
 460             obligation.recursion_depth + 1,
 461             unnormalized_upcast_trait_ref,
 462             &mut nested,
 463         );
 464
 465         nested.extend(self.infcx.commit_if_ok(|_| {
 466             self.infcx
 467                 .at(&obligation.cause, obligation.param_env)
 468                 .sup(obligation_trait_ref, upcast_trait_ref)
 469                 .map(|InferOk { obligations, .. }| obligations)
 470                 .map_err(|_| Unimplemented)
 471         })?);
 472
 473         // Check supertraits hold. This is so that their associated type bounds
 474         // will be checked in the code below.
 475         for super_trait in tcx
 476             .super_predicates_of(trait_predicate.def_id())
 477             .instantiate(tcx, trait_predicate.trait_ref.substs)
 478             .predicates
 479             .into_iter()
 480         {
 481             let normalized_super_trait = normalize_with_depth_to(
 482                 self,
 483                 obligation.param_env,
 484                 obligation.cause.clone(),
 485                 obligation.recursion_depth + 1,
 486                 super_trait,
 487                 &mut nested,
 488             );
 489             nested.push(obligation.with(tcx, normalized_super_trait));
 490         }
 491
 492         let assoc_types: Vec<_> = tcx
 493             .associated_items(trait_predicate.def_id())
 494             .in_definition_order()
 495             .filter_map(
 496                 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
 497             )
 498             .collect();
 499
 500         for assoc_type in assoc_types {
 501             let defs: &ty::Generics = tcx.generics_of(assoc_type);
 502
 503             if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
 504                 tcx.sess.delay_span_bug(
 505                     obligation.cause.span,
 506                     "GATs in trait object shouldn't have been considered",
 507                 );
 508                 return Err(SelectionError::Unimplemented);
 509             }
 510
 511             // This maybe belongs in wf, but that can't (doesn't) handle
 512             // higher-ranked things.
 513             // Prevent, e.g., `dyn Iterator<Item = str>`.
 514             for bound in self.tcx().bound_item_bounds(assoc_type).transpose_iter() {
 515                 let subst_bound =
 516                     if defs.count() == 0 {
 517                         bound.subst(tcx, trait_predicate.trait_ref.substs)
 518                     } else {
 519                         let mut substs = smallvec::SmallVec::with_capacity(defs.count());
 520                         substs.extend(trait_predicate.trait_ref.substs.iter());
 521                         let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
 522                             smallvec::SmallVec::with_capacity(
 523                                 bound.0.kind().bound_vars().len() + defs.count(),
 524                             );
 525                         bound_vars.extend(bound.0.kind().bound_vars().into_iter());
 526                         InternalSubsts::fill_single(&mut substs, defs, &mut |param, _| match param
 527                             .kind
 528                         {
 529                             GenericParamDefKind::Type { .. } => {
 530                                 let kind = ty::BoundTyKind::Param(param.name);
 531                                 let bound_var = ty::BoundVariableKind::Ty(kind);
 532                                 bound_vars.push(bound_var);
 533                                 tcx.mk_ty(ty::Bound(
 534                                     ty::INNERMOST,
 535                                     ty::BoundTy {
 536                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 537                                         kind,
 538                                     },
 539                                 ))
 540                                 .into()
 541                             }
 542                             GenericParamDefKind::Lifetime => {
 543                                 let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
 544                                 let bound_var = ty::BoundVariableKind::Region(kind);
 545                                 bound_vars.push(bound_var);
 546                                 tcx.mk_region(ty::ReLateBound(
 547                                     ty::INNERMOST,
 548                                     ty::BoundRegion {
 549                                         var: ty::BoundVar::from_usize(bound_vars.len() - 1),
 550                                         kind,
 551                                     },
 552                                 ))
 553                                 .into()
 554                             }
 555                             GenericParamDefKind::Const { .. } => {
 556                                 let bound_var = ty::BoundVariableKind::Const;
 557                                 bound_vars.push(bound_var);
 558                                 tcx.mk_const(
 559                                     ty::ConstKind::Bound(
 560                                         ty::INNERMOST,
 561                                         ty::BoundVar::from_usize(bound_vars.len() - 1),
 562                                     ),
 563                                     tcx.type_of(param.def_id),
 564                                 )
 565                                 .into()
 566                             }
 567                         });
 568                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 569                         let assoc_ty_substs = tcx.intern_substs(&substs);
 570
 571                         let bound_vars = tcx.mk_bound_variable_kinds(bound_vars.into_iter());
 572                         let bound =
 573                             bound.map_bound(|b| b.kind().skip_binder()).subst(tcx, assoc_ty_substs);
 574                         tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))
 575                     };
 576                 let normalized_bound = normalize_with_depth_to(
 577                     self,
 578                     obligation.param_env,
 579                     obligation.cause.clone(),
 580                     obligation.recursion_depth + 1,
 581                     subst_bound,
 582                     &mut nested,
 583                 );
 584                 nested.push(obligation.with(tcx, normalized_bound));
 585             }
 586         }
 587
 588         debug!(?nested, "object nested obligations");
 589
 590         let vtable_base = vtable_trait_first_method_offset(
 591             tcx,
 592             (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
 593         );
 594
 595         Ok(ImplSourceObjectData { upcast_trait_ref, vtable_base, nested })
 596     }
 597
 598     fn confirm_fn_pointer_candidate(
 599         &mut self,
 600         obligation: &TraitObligation<'tcx>,
 601         is_const: bool,
 602     ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 603     {
 604         debug!(?obligation, "confirm_fn_pointer_candidate");
 605
 606         let tcx = self.tcx();
 607         let self_ty = self
 608             .infcx
 609             .shallow_resolve(obligation.self_ty().no_bound_vars())
 610             .expect("fn pointer should not capture bound vars from predicate");
 611         let sig = self_ty.fn_sig(tcx);
 612         let trait_ref = closure_trait_ref_and_return_type(
 613             tcx,
 614             obligation.predicate.def_id(),
 615             self_ty,
 616             sig,
 617             util::TupleArgumentsFlag::Yes,
 618         )
 619         .map_bound(|(trait_ref, _)| trait_ref);
 620
 621         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 622         let cause = obligation.derived_cause(BuiltinDerivedObligation);
 623
 624         if obligation.is_const() && !is_const {
 625             // function is a trait method
 626             if let ty::FnDef(def_id, substs) = self_ty.kind() && let Some(trait_id) = tcx.trait_of_item(*def_id) {
 627                 let trait_ref = TraitRef::from_method(tcx, trait_id, *substs);
 628                 let poly_trait_pred = Binder::dummy(trait_ref).with_constness(ty::BoundConstness::ConstIfConst);
 629                 let obligation = Obligation::new(tcx, cause.clone(), obligation.param_env, poly_trait_pred);
 630                 nested.push(obligation);
 631             }
 632         }
 633
 634         // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
 635         let output_ty = self.infcx.replace_bound_vars_with_placeholders(sig.output());
 636         let output_ty = normalize_with_depth_to(
 637             self,
 638             obligation.param_env,
 639             cause.clone(),
 640             obligation.recursion_depth,
 641             output_ty,
 642             &mut nested,
 643         );
 644         let tr =
 645             ty::Binder::dummy(self.tcx().at(cause.span).mk_trait_ref(LangItem::Sized, [output_ty]));
 646         nested.push(Obligation::new(self.infcx.tcx, cause, obligation.param_env, tr));
 647
 648         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
 649     }
 650
 651     fn confirm_trait_alias_candidate(
 652         &mut self,
 653         obligation: &TraitObligation<'tcx>,
 654     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
 655         debug!(?obligation, "confirm_trait_alias_candidate");
 656
 657         let alias_def_id = obligation.predicate.def_id();
 658         let predicate = self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
 659         let trait_ref = predicate.trait_ref;
 660         let trait_def_id = trait_ref.def_id;
 661         let substs = trait_ref.substs;
 662
 663         let trait_obligations = self.impl_or_trait_obligations(
 664             &obligation.cause,
 665             obligation.recursion_depth,
 666             obligation.param_env,
 667             trait_def_id,
 668             &substs,
 669             obligation.predicate,
 670         );
 671
 672         debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
 673
 674         ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
 675     }
 676
 677     fn confirm_generator_candidate(
 678         &mut self,
 679         obligation: &TraitObligation<'tcx>,
 680     ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 681     {
 682         // Okay to skip binder because the substs on generator types never
 683         // touch bound regions, they just capture the in-scope
 684         // type/region parameters.
 685         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 686         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 687             bug!("closure candidate for non-closure {:?}", obligation);
 688         };
 689
 690         debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
 691
 692         let gen_sig = substs.as_generator().poly_sig();
 693
 694         // NOTE: The self-type is a generator type and hence is
 695         // in fact unparameterized (or at least does not reference any
 696         // regions bound in the obligation).
 697         let self_ty = obligation
 698             .predicate
 699             .self_ty()
 700             .no_bound_vars()
 701             .expect("unboxed closure type should not capture bound vars from the predicate");
 702
 703         let trait_ref = super::util::generator_trait_ref_and_outputs(
 704             self.tcx(),
 705             obligation.predicate.def_id(),
 706             self_ty,
 707             gen_sig,
 708         )
 709         .map_bound(|(trait_ref, ..)| trait_ref);
 710
 711         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 712         debug!(?trait_ref, ?nested, "generator candidate obligations");
 713
 714         Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
 715     }
 716
 717     fn confirm_future_candidate(
 718         &mut self,
 719         obligation: &TraitObligation<'tcx>,
 720     ) -> Result<ImplSourceFutureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 721         // Okay to skip binder because the substs on generator types never
 722         // touch bound regions, they just capture the in-scope
 723         // type/region parameters.
 724         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 725         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 726             bug!("closure candidate for non-closure {:?}", obligation);
 727         };
 728
 729         debug!(?obligation, ?generator_def_id, ?substs, "confirm_future_candidate");
 730
 731         let gen_sig = substs.as_generator().poly_sig();
 732
 733         let trait_ref = super::util::future_trait_ref_and_outputs(
 734             self.tcx(),
 735             obligation.predicate.def_id(),
 736             obligation.predicate.no_bound_vars().expect("future has no bound vars").self_ty(),
 737             gen_sig,
 738         )
 739         .map_bound(|(trait_ref, ..)| trait_ref);
 740
 741         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 742         debug!(?trait_ref, ?nested, "future candidate obligations");
 743
 744         Ok(ImplSourceFutureData { generator_def_id, substs, nested })
 745     }
 746
 747     #[instrument(skip(self), level = "debug")]
 748     fn confirm_closure_candidate(
 749         &mut self,
 750         obligation: &TraitObligation<'tcx>,
 751     ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 752         let kind = self
 753             .tcx()
 754             .fn_trait_kind_from_def_id(obligation.predicate.def_id())
 755             .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
 756
 757         // Okay to skip binder because the substs on closure types never
 758         // touch bound regions, they just capture the in-scope
 759         // type/region parameters.
 760         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 761         let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
 762             bug!("closure candidate for non-closure {:?}", obligation);
 763         };
 764
 765         let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
 766         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 767
 768         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 769
 770         // FIXME: Chalk
 771         if self.tcx().sess.opts.unstable_opts.trait_solver != TraitSolver::Chalk {
 772             nested.push(obligation.with(
 773                 self.tcx(),
 774                 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind)),
 775             ));
 776         }
 777
 778         Ok(ImplSourceClosureData { closure_def_id, substs, nested })
 779     }
 780
 781     /// In the case of closure types and fn pointers,
 782     /// we currently treat the input type parameters on the trait as
 783     /// outputs. This means that when we have a match we have only
 784     /// considered the self type, so we have to go back and make sure
 785     /// to relate the argument types too. This is kind of wrong, but
 786     /// since we control the full set of impls, also not that wrong,
 787     /// and it DOES yield better error messages (since we don't report
 788     /// errors as if there is no applicable impl, but rather report
 789     /// errors are about mismatched argument types.
 790     ///
 791     /// Here is an example. Imagine we have a closure expression
 792     /// and we desugared it so that the type of the expression is
 793     /// `Closure`, and `Closure` expects `i32` as argument. Then it
 794     /// is "as if" the compiler generated this impl:
 795     /// ```ignore (illustrative)
 796     /// impl Fn(i32) for Closure { ... }
 797     /// ```
 798     /// Now imagine our obligation is `Closure: Fn(usize)`. So far
 799     /// we have matched the self type `Closure`. At this point we'll
 800     /// compare the `i32` to `usize` and generate an error.
 801     ///
 802     /// Note that this checking occurs *after* the impl has selected,
 803     /// because these output type parameters should not affect the
 804     /// selection of the impl. Therefore, if there is a mismatch, we
 805     /// report an error to the user.
 806     #[instrument(skip(self), level = "trace")]
 807     fn confirm_poly_trait_refs(
 808         &mut self,
 809         obligation: &TraitObligation<'tcx>,
 810         expected_trait_ref: ty::PolyTraitRef<'tcx>,
 811     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 812         let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
 813         // Normalize the obligation and expected trait refs together, because why not
 814         let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
 815             ensure_sufficient_stack(|| {
 816                 normalize_with_depth(
 817                     self,
 818                     obligation.param_env,
 819                     obligation.cause.clone(),
 820                     obligation.recursion_depth + 1,
 821                     (obligation_trait_ref, expected_trait_ref),
 822                 )
 823             });
 824
 825         self.infcx
 826             .at(&obligation.cause, obligation.param_env)
 827             .sup(obligation_trait_ref, expected_trait_ref)
 828             .map(|InferOk { mut obligations, .. }| {
 829                 obligations.extend(nested);
 830                 obligations
 831             })
 832             .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
 833     }
 834
 835     fn confirm_trait_upcasting_unsize_candidate(
 836         &mut self,
 837         obligation: &TraitObligation<'tcx>,
 838         idx: usize,
 839     ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 840     {
 841         let tcx = self.tcx();
 842
 843         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 844         // regions here. See the comment there for more details.
 845         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 846         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 847         let target = self.infcx.shallow_resolve(target);
 848
 849         debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
 850
 851         let mut nested = vec![];
 852         let source_trait_ref;
 853         let upcast_trait_ref;
 854         match (source.kind(), target.kind()) {
 855             // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
 856             (
 857                 &ty::Dynamic(ref data_a, r_a, repr_a @ ty::Dyn),
 858                 &ty::Dynamic(ref data_b, r_b, ty::Dyn),
 859             ) => {
 860                 // See `assemble_candidates_for_unsizing` for more info.
 861                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 862                 let principal_a = data_a.principal().unwrap();
 863                 source_trait_ref = principal_a.with_self_ty(tcx, source);
 864                 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
 865                 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
 866                 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
 867                     ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
 868                         tcx, trait_ref,
 869                     ))
 870                 });
 871                 let iter = Some(existential_predicate)
 872                     .into_iter()
 873                     .chain(
 874                         data_a
 875                             .projection_bounds()
 876                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 877                     )
 878                     .chain(
 879                         data_b
 880                             .auto_traits()
 881                             .map(ty::ExistentialPredicate::AutoTrait)
 882                             .map(ty::Binder::dummy),
 883                     );
 884                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 885                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_a);
 886
 887                 // Require that the traits involved in this upcast are **equal**;
 888                 // only the **lifetime bound** is changed.
 889                 let InferOk { obligations, .. } = self
 890                     .infcx
 891                     .at(&obligation.cause, obligation.param_env)
 892                     .sup(target, source_trait)
 893                     .map_err(|_| Unimplemented)?;
 894                 nested.extend(obligations);
 895
 896                 // Register one obligation for 'a: 'b.
 897                 let cause = ObligationCause::new(
 898                     obligation.cause.span,
 899                     obligation.cause.body_id,
 900                     ObjectCastObligation(source, target),
 901                 );
 902                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 903                 nested.push(Obligation::with_depth(
 904                     tcx,
 905                     cause,
 906                     obligation.recursion_depth + 1,
 907                     obligation.param_env,
 908                     obligation.predicate.rebind(outlives),
 909                 ));
 910             }
 911             _ => bug!(),
 912         };
 913
 914         let vtable_segment_callback = {
 915             let mut vptr_offset = 0;
 916             move |segment| {
 917                 match segment {
 918                     VtblSegment::MetadataDSA => {
 919                         vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
 920                     }
 921                     VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 922                         vptr_offset += count_own_vtable_entries(tcx, trait_ref);
 923                         if trait_ref == upcast_trait_ref {
 924                             if emit_vptr {
 925                                 return ControlFlow::Break(Some(vptr_offset));
 926                             } else {
 927                                 return ControlFlow::Break(None);
 928                             }
 929                         }
 930
 931                         if emit_vptr {
 932                             vptr_offset += 1;
 933                         }
 934                     }
 935                 }
 936                 ControlFlow::Continue(())
 937             }
 938         };
 939
 940         let vtable_vptr_slot =
 941             prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback).unwrap();
 942
 943         Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
 944     }
 945
 946     fn confirm_builtin_unsize_candidate(
 947         &mut self,
 948         obligation: &TraitObligation<'tcx>,
 949     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 950         let tcx = self.tcx();
 951
 952         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 953         // regions here. See the comment there for more details.
 954         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 955         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 956         let target = self.infcx.shallow_resolve(target);
 957
 958         debug!(?source, ?target, "confirm_builtin_unsize_candidate");
 959
 960         let mut nested = vec![];
 961         match (source.kind(), target.kind()) {
 962             // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
 963             (&ty::Dynamic(ref data_a, r_a, dyn_a), &ty::Dynamic(ref data_b, r_b, dyn_b))
 964                 if dyn_a == dyn_b =>
 965             {
 966                 // See `assemble_candidates_for_unsizing` for more info.
 967                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 968                 let iter = data_a
 969                     .principal()
 970                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
 971                     .into_iter()
 972                     .chain(
 973                         data_a
 974                             .projection_bounds()
 975                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 976                     )
 977                     .chain(
 978                         data_b
 979                             .auto_traits()
 980                             .map(ty::ExistentialPredicate::AutoTrait)
 981                             .map(ty::Binder::dummy),
 982                     );
 983                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 984                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, dyn_a);
 985
 986                 // Require that the traits involved in this upcast are **equal**;
 987                 // only the **lifetime bound** is changed.
 988                 let InferOk { obligations, .. } = self
 989                     .infcx
 990                     .at(&obligation.cause, obligation.param_env)
 991                     .sup(target, source_trait)
 992                     .map_err(|_| Unimplemented)?;
 993                 nested.extend(obligations);
 994
 995                 // Register one obligation for 'a: 'b.
 996                 let cause = ObligationCause::new(
 997                     obligation.cause.span,
 998                     obligation.cause.body_id,
 999                     ObjectCastObligation(source, target),
1000                 );
1001                 let outlives = ty::OutlivesPredicate(r_a, r_b);
1002                 nested.push(Obligation::with_depth(
1003                     tcx,
1004                     cause,
1005                     obligation.recursion_depth + 1,
1006                     obligation.param_env,
1007                     obligation.predicate.rebind(outlives),
1008                 ));
1009             }
1010
1011             // `T` -> `Trait`
1012             (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
1013                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
1014                 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
1015                     return Err(TraitNotObjectSafe(did));
1016                 }
1017
1018                 let cause = ObligationCause::new(
1019                     obligation.cause.span,
1020                     obligation.cause.body_id,
1021                     ObjectCastObligation(source, target),
1022                 );
1023
1024                 let predicate_to_obligation = |predicate| {
1025                     Obligation::with_depth(
1026                         tcx,
1027                         cause.clone(),
1028                         obligation.recursion_depth + 1,
1029                         obligation.param_env,
1030                         predicate,
1031                     )
1032                 };
1033
1034                 // Create obligations:
1035                 //  - Casting `T` to `Trait`
1036                 //  - For all the various builtin bounds attached to the object cast. (In other
1037                 //  words, if the object type is `Foo + Send`, this would create an obligation for
1038                 //  the `Send` check.)
1039                 //  - Projection predicates
1040                 nested.extend(
1041                     data.iter().map(|predicate| {
1042                         predicate_to_obligation(predicate.with_self_ty(tcx, source))
1043                     }),
1044                 );
1045
1046                 // We can only make objects from sized types.
1047                 let tr =
1048                     ty::Binder::dummy(tcx.at(cause.span).mk_trait_ref(LangItem::Sized, [source]));
1049                 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
1050
1051                 // If the type is `Foo + 'a`, ensure that the type
1052                 // being cast to `Foo + 'a` outlives `'a`:
1053                 let outlives = ty::OutlivesPredicate(source, r);
1054                 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
1055             }
1056
1057             // `[T; n]` -> `[T]`
1058             (&ty::Array(a, _), &ty::Slice(b)) => {
1059                 let InferOk { obligations, .. } = self
1060                     .infcx
1061                     .at(&obligation.cause, obligation.param_env)
1062                     .eq(b, a)
1063                     .map_err(|_| Unimplemented)?;
1064                 nested.extend(obligations);
1065             }
1066
1067             // `Struct<T>` -> `Struct<U>`
1068             (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
1069                 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
1070                     GenericArgKind::Type(ty) => match ty.kind() {
1071                         ty::Param(p) => Some(p.index),
1072                         _ => None,
1073                     },
1074
1075                     // Lifetimes aren't allowed to change during unsizing.
1076                     GenericArgKind::Lifetime(_) => None,
1077
1078                     GenericArgKind::Const(ct) => match ct.kind() {
1079                         ty::ConstKind::Param(p) => Some(p.index),
1080                         _ => None,
1081                     },
1082                 };
1083
1084                 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1085                 // by putting it in a query; it would only need the `DefId` as it
1086                 // looks at declared field types, not anything substituted.
1087
1088                 // The last field of the structure has to exist and contain type/const parameters.
1089                 let (tail_field, prefix_fields) =
1090                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1091                 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1092
1093                 let mut unsizing_params = GrowableBitSet::new_empty();
1094                 for arg in tail_field_ty.0.walk() {
1095                     if let Some(i) = maybe_unsizing_param_idx(arg) {
1096                         unsizing_params.insert(i);
1097                     }
1098                 }
1099
1100                 // Ensure none of the other fields mention the parameters used
1101                 // in unsizing.
1102                 for field in prefix_fields {
1103                     for arg in tcx.type_of(field.did).walk() {
1104                         if let Some(i) = maybe_unsizing_param_idx(arg) {
1105                             unsizing_params.remove(i);
1106                         }
1107                     }
1108                 }
1109
1110                 if unsizing_params.is_empty() {
1111                     return Err(Unimplemented);
1112                 }
1113
1114                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
1115                 // normalizing in the process, since `type_of` returns something directly from
1116                 // astconv (which means it's un-normalized).
1117                 let source_tail = normalize_with_depth_to(
1118                     self,
1119                     obligation.param_env,
1120                     obligation.cause.clone(),
1121                     obligation.recursion_depth + 1,
1122                     tail_field_ty.subst(tcx, substs_a),
1123                     &mut nested,
1124                 );
1125                 let target_tail = normalize_with_depth_to(
1126                     self,
1127                     obligation.param_env,
1128                     obligation.cause.clone(),
1129                     obligation.recursion_depth + 1,
1130                     tail_field_ty.subst(tcx, substs_b),
1131                     &mut nested,
1132                 );
1133
1134                 // Check that the source struct with the target's
1135                 // unsizing parameters is equal to the target.
1136                 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1137                     if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1138                 }));
1139                 let new_struct = tcx.mk_adt(def, substs);
1140                 let InferOk { obligations, .. } = self
1141                     .infcx
1142                     .at(&obligation.cause, obligation.param_env)
1143                     .eq(target, new_struct)
1144                     .map_err(|_| Unimplemented)?;
1145                 nested.extend(obligations);
1146
1147                 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1148                 nested.push(predicate_for_trait_def(
1149                     tcx,
1150                     obligation.param_env,
1151                     obligation.cause.clone(),
1152                     obligation.predicate.def_id(),
1153                     obligation.recursion_depth + 1,
1154                     [source_tail, target_tail],
1155                 ));
1156             }
1157
1158             // `(.., T)` -> `(.., U)`
1159             (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1160                 assert_eq!(tys_a.len(), tys_b.len());
1161
1162                 // The last field of the tuple has to exist.
1163                 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1164                 let &b_last = tys_b.last().unwrap();
1165
1166                 // Check that the source tuple with the target's
1167                 // last element is equal to the target.
1168                 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1169                 let InferOk { obligations, .. } = self
1170                     .infcx
1171                     .at(&obligation.cause, obligation.param_env)
1172                     .eq(target, new_tuple)
1173                     .map_err(|_| Unimplemented)?;
1174                 nested.extend(obligations);
1175
1176                 // Construct the nested `T: Unsize<U>` predicate.
1177                 nested.push(ensure_sufficient_stack(|| {
1178                     predicate_for_trait_def(
1179                         tcx,
1180                         obligation.param_env,
1181                         obligation.cause.clone(),
1182                         obligation.predicate.def_id(),
1183                         obligation.recursion_depth + 1,
1184                         [a_last, b_last],
1185                     )
1186                 }));
1187             }
1188
1189             _ => bug!("source: {source}, target: {target}"),
1190         };
1191
1192         Ok(ImplSourceBuiltinData { nested })
1193     }
1194
1195     fn confirm_const_destruct_candidate(
1196         &mut self,
1197         obligation: &TraitObligation<'tcx>,
1198         impl_def_id: Option<DefId>,
1199     ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1200         // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1201         if !obligation.is_const() {
1202             return Ok(ImplSourceConstDestructData { nested: vec![] });
1203         }
1204
1205         let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1206
1207         let tcx = self.tcx();
1208         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1209
1210         let mut nested = vec![];
1211         let cause = obligation.derived_cause(BuiltinDerivedObligation);
1212
1213         // If we have a custom `impl const Drop`, then
1214         // first check it like a regular impl candidate.
1215         // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1216         if let Some(impl_def_id) = impl_def_id {
1217             let mut new_obligation = obligation.clone();
1218             new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1219                 trait_pred.trait_ref.def_id = drop_trait;
1220                 trait_pred
1221             });
1222             let substs = self.rematch_impl(impl_def_id, &new_obligation);
1223             debug!(?substs, "impl substs");
1224
1225             let cause = obligation.derived_cause(|derived| {
1226                 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1227                     derived,
1228                     impl_def_id,
1229                     span: obligation.cause.span,
1230                 }))
1231             });
1232             let obligations = ensure_sufficient_stack(|| {
1233                 self.vtable_impl(
1234                     impl_def_id,
1235                     substs,
1236                     &cause,
1237                     new_obligation.recursion_depth + 1,
1238                     new_obligation.param_env,
1239                     obligation.predicate,
1240                 )
1241             });
1242             nested.extend(obligations.nested);
1243         }
1244
1245         // We want to confirm the ADT's fields if we have an ADT
1246         let mut stack = match *self_ty.skip_binder().kind() {
1247             ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1248             _ => vec![self_ty.skip_binder()],
1249         };
1250
1251         while let Some(nested_ty) = stack.pop() {
1252             match *nested_ty.kind() {
1253                 // We know these types are trivially drop
1254                 ty::Bool
1255                 | ty::Char
1256                 | ty::Int(_)
1257                 | ty::Uint(_)
1258                 | ty::Float(_)
1259                 | ty::Infer(ty::IntVar(_))
1260                 | ty::Infer(ty::FloatVar(_))
1261                 | ty::Str
1262                 | ty::RawPtr(_)
1263                 | ty::Ref(..)
1264                 | ty::FnDef(..)
1265                 | ty::FnPtr(_)
1266                 | ty::Never
1267                 | ty::Foreign(_) => {}
1268
1269                 // `ManuallyDrop` is trivially drop
1270                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
1271
1272                 // These types are built-in, so we can fast-track by registering
1273                 // nested predicates for their constituent type(s)
1274                 ty::Array(ty, _) | ty::Slice(ty) => {
1275                     stack.push(ty);
1276                 }
1277                 ty::Tuple(tys) => {
1278                     stack.extend(tys.iter());
1279                 }
1280                 ty::Closure(_, substs) => {
1281                     stack.push(substs.as_closure().tupled_upvars_ty());
1282                 }
1283                 ty::Generator(_, substs, _) => {
1284                     let generator = substs.as_generator();
1285                     stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1286                 }
1287                 ty::GeneratorWitness(tys) => {
1288                     stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1289                 }
1290
1291                 // If we have a projection type, make sure to normalize it so we replace it
1292                 // with a fresh infer variable
1293                 ty::Alias(ty::Projection, ..) => {
1294                     let predicate = normalize_with_depth_to(
1295                         self,
1296                         obligation.param_env,
1297                         cause.clone(),
1298                         obligation.recursion_depth + 1,
1299                         self_ty.rebind(ty::TraitPredicate {
1300                             trait_ref: self
1301                                 .tcx()
1302                                 .at(cause.span)
1303                                 .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1304                             constness: ty::BoundConstness::ConstIfConst,
1305                             polarity: ty::ImplPolarity::Positive,
1306                         }),
1307                         &mut nested,
1308                     );
1309
1310                     nested.push(Obligation::with_depth(
1311                         tcx,
1312                         cause.clone(),
1313                         obligation.recursion_depth + 1,
1314                         obligation.param_env,
1315                         predicate,
1316                     ));
1317                 }
1318
1319                 // If we have any other type (e.g. an ADT), just register a nested obligation
1320                 // since it's either not `const Drop` (and we raise an error during selection),
1321                 // or it's an ADT (and we need to check for a custom impl during selection)
1322                 _ => {
1323                     let predicate = self_ty.rebind(ty::TraitPredicate {
1324                         trait_ref: self
1325                             .tcx()
1326                             .at(cause.span)
1327                             .mk_trait_ref(LangItem::Destruct, [nested_ty]),
1328                         constness: ty::BoundConstness::ConstIfConst,
1329                         polarity: ty::ImplPolarity::Positive,
1330                     });
1331
1332                     nested.push(Obligation::with_depth(
1333                         tcx,
1334                         cause.clone(),
1335                         obligation.recursion_depth + 1,
1336                         obligation.param_env,
1337                         predicate,
1338                     ));
1339                 }
1340             }
1341         }
1342
1343         Ok(ImplSourceConstDestructData { nested })
1344     }
1345 }