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