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

   1 //! Confirmation.
   2 //!
   3 //! Confirmation unifies the output type parameters of the trait
   4 //! with the values found in the obligation, possibly yielding a
   5 //! type error.  See the [rustc dev guide] for more details.
   6 //!
   7 //! [rustc dev guide]:
   8 //! https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation
   9 use rustc_data_structures::stack::ensure_sufficient_stack;
  10 use rustc_hir::lang_items::LangItem;
  11 use rustc_index::bit_set::GrowableBitSet;
  12 use rustc_infer::infer::InferOk;
  13 use rustc_infer::infer::LateBoundRegionConversionTime::HigherRankedType;
  14 use rustc_middle::ty::{
  15     self, GenericArg, GenericArgKind, GenericParamDefKind, InternalSubsts, SubstsRef,
  16     ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
  17 };
  18 use rustc_span::def_id::DefId;
  19
  20 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
  21 use crate::traits::util::{self, closure_trait_ref_and_return_type, predicate_for_trait_def};
  22 use crate::traits::{
  23     BuiltinDerivedObligation, ImplDerivedObligation, ImplDerivedObligationCause, ImplSource,
  24     ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
  25     ImplSourceConstDestructData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
  26     ImplSourceGeneratorData, ImplSourceObjectData, ImplSourcePointeeData, ImplSourceTraitAliasData,
  27     ImplSourceTraitUpcastingData, ImplSourceUserDefinedData, Normalized, ObjectCastObligation,
  28     Obligation, ObligationCause, OutputTypeParameterMismatch, PredicateObligation, Selection,
  29     SelectionError, TraitNotObjectSafe, TraitObligation, Unimplemented, VtblSegment,
  30 };
  31
  32 use super::BuiltinImplConditions;
  33 use super::SelectionCandidate::{self, *};
  34 use super::SelectionContext;
  35
  36 use std::iter;
  37 use std::ops::ControlFlow;
  38
  39 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
  40     #[instrument(level = "debug", skip(self))]
  41     pub(super) fn confirm_candidate(
  42         &mut self,
  43         obligation: &TraitObligation<'tcx>,
  44         candidate: SelectionCandidate<'tcx>,
  45     ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
  46         let mut impl_src = match candidate {
  47             BuiltinCandidate { has_nested } => {
  48                 let data = self.confirm_builtin_candidate(obligation, has_nested);
  49                 ImplSource::Builtin(data)
  50             }
  51
  52             TransmutabilityCandidate => {
  53                 let data = self.confirm_transmutability_candidate(obligation)?;
  54                 ImplSource::Builtin(data)
  55             }
  56
  57             ParamCandidate(param) => {
  58                 let obligations =
  59                     self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
  60                 ImplSource::Param(obligations, param.skip_binder().constness)
  61             }
  62
  63             ImplCandidate(impl_def_id) => {
  64                 ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id))
  65             }
  66
  67             AutoImplCandidate => {
  68                 let data = self.confirm_auto_impl_candidate(obligation);
  69                 ImplSource::AutoImpl(data)
  70             }
  71
  72             ProjectionCandidate(idx, constness) => {
  73                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
  74                 ImplSource::Param(obligations, constness)
  75             }
  76
  77             ObjectCandidate(idx) => {
  78                 let data = self.confirm_object_candidate(obligation, idx)?;
  79                 ImplSource::Object(data)
  80             }
  81
  82             ClosureCandidate => {
  83                 let vtable_closure = self.confirm_closure_candidate(obligation)?;
  84                 ImplSource::Closure(vtable_closure)
  85             }
  86
  87             GeneratorCandidate => {
  88                 let vtable_generator = self.confirm_generator_candidate(obligation)?;
  89                 ImplSource::Generator(vtable_generator)
  90             }
  91
  92             FnPointerCandidate { .. } => {
  93                 let data = self.confirm_fn_pointer_candidate(obligation)?;
  94                 ImplSource::FnPointer(data)
  95             }
  96
  97             DiscriminantKindCandidate => {
  98                 ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
  99             }
 100
 101             PointeeCandidate => ImplSource::Pointee(ImplSourcePointeeData),
 102
 103             TraitAliasCandidate => {
 104                 let data = self.confirm_trait_alias_candidate(obligation);
 105                 ImplSource::TraitAlias(data)
 106             }
 107
 108             BuiltinObjectCandidate => {
 109                 // This indicates something like `Trait + Send: Send`. In this case, we know that
 110                 // this holds because that's what the object type is telling us, and there's really
 111                 // no additional obligations to prove and no types in particular to unify, etc.
 112                 ImplSource::Param(Vec::new(), ty::BoundConstness::NotConst)
 113             }
 114
 115             BuiltinUnsizeCandidate => {
 116                 let data = self.confirm_builtin_unsize_candidate(obligation)?;
 117                 ImplSource::Builtin(data)
 118             }
 119
 120             TraitUpcastingUnsizeCandidate(idx) => {
 121                 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
 122                 ImplSource::TraitUpcasting(data)
 123             }
 124
 125             ConstDestructCandidate(def_id) => {
 126                 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
 127                 ImplSource::ConstDestruct(data)
 128             }
 129         };
 130
 131         if !obligation.predicate.is_const_if_const() {
 132             // normalize nested predicates according to parent predicate's constness.
 133             impl_src = impl_src.map(|mut o| {
 134                 o.predicate = o.predicate.without_const(self.tcx());
 135                 o
 136             });
 137         }
 138
 139         Ok(impl_src)
 140     }
 141
 142     fn confirm_projection_candidate(
 143         &mut self,
 144         obligation: &TraitObligation<'tcx>,
 145         idx: usize,
 146     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 147         let tcx = self.tcx();
 148
 149         let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
 150         let placeholder_trait_predicate =
 151             self.infcx().replace_bound_vars_with_placeholders(trait_predicate).trait_ref;
 152         let placeholder_self_ty = placeholder_trait_predicate.self_ty();
 153         let placeholder_trait_predicate = ty::Binder::dummy(placeholder_trait_predicate);
 154         let (def_id, substs) = match *placeholder_self_ty.kind() {
 155             ty::Projection(proj) => (proj.item_def_id, proj.substs),
 156             ty::Opaque(def_id, substs) => (def_id, substs),
 157             _ => bug!("projection candidate for unexpected type: {:?}", placeholder_self_ty),
 158         };
 159
 160         let candidate_predicate =
 161             tcx.bound_item_bounds(def_id).map_bound(|i| i[idx]).subst(tcx, substs);
 162         let candidate = candidate_predicate
 163             .to_opt_poly_trait_pred()
 164             .expect("projection candidate is not a trait predicate")
 165             .map_bound(|t| t.trait_ref);
 166         let mut obligations = Vec::new();
 167         let candidate = normalize_with_depth_to(
 168             self,
 169             obligation.param_env,
 170             obligation.cause.clone(),
 171             obligation.recursion_depth + 1,
 172             candidate,
 173             &mut obligations,
 174         );
 175
 176         obligations.extend(self.infcx.commit_if_ok(|_| {
 177             self.infcx
 178                 .at(&obligation.cause, obligation.param_env)
 179                 .sup(placeholder_trait_predicate, candidate)
 180                 .map(|InferOk { obligations, .. }| obligations)
 181                 .map_err(|_| Unimplemented)
 182         })?);
 183
 184         if let ty::Projection(..) = placeholder_self_ty.kind() {
 185             let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates;
 186             debug!(?predicates, "projection predicates");
 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.replace_bound_vars_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.replace_bound_vars_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.replace_bound_vars_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().bound_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.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 = super::super::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     ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 599     {
 600         debug!(?obligation, "confirm_fn_pointer_candidate");
 601
 602         // Okay to skip binder; it is reintroduced below.
 603         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 604         let sig = self_ty.fn_sig(self.tcx());
 605         let trait_ref = closure_trait_ref_and_return_type(
 606             self.tcx(),
 607             obligation.predicate.def_id(),
 608             self_ty,
 609             sig,
 610             util::TupleArgumentsFlag::Yes,
 611         )
 612         .map_bound(|(trait_ref, _)| trait_ref);
 613
 614         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 615
 616         // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
 617         let cause = obligation.derived_cause(BuiltinDerivedObligation);
 618         // The binder on the Fn obligation is "less" important than the one on
 619         // the signature, as evidenced by how we treat it during projection.
 620         // The safe thing to do here is to liberate it, though, which should
 621         // have no worse effect than skipping the binder here.
 622         let liberated_fn_ty =
 623             self.infcx.replace_bound_vars_with_placeholders(obligation.predicate.rebind(self_ty));
 624         let output_ty = self
 625             .infcx
 626             .replace_bound_vars_with_placeholders(liberated_fn_ty.fn_sig(self.tcx()).output());
 627         let output_ty = normalize_with_depth_to(
 628             self,
 629             obligation.param_env,
 630             cause.clone(),
 631             obligation.recursion_depth,
 632             output_ty,
 633             &mut nested,
 634         );
 635         let tr = ty::Binder::dummy(self.tcx().mk_trait_ref(
 636             self.tcx().require_lang_item(LangItem::Sized, None),
 637             output_ty,
 638             [],
 639         ));
 640         nested.push(Obligation::new(
 641             self.infcx.tcx,
 642             cause,
 643             obligation.param_env,
 644             tr.to_poly_trait_predicate(),
 645         ));
 646
 647         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
 648     }
 649
 650     fn confirm_trait_alias_candidate(
 651         &mut self,
 652         obligation: &TraitObligation<'tcx>,
 653     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
 654         debug!(?obligation, "confirm_trait_alias_candidate");
 655
 656         let alias_def_id = obligation.predicate.def_id();
 657         let predicate = self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
 658         let trait_ref = predicate.trait_ref;
 659         let trait_def_id = trait_ref.def_id;
 660         let substs = trait_ref.substs;
 661
 662         let trait_obligations = self.impl_or_trait_obligations(
 663             &obligation.cause,
 664             obligation.recursion_depth,
 665             obligation.param_env,
 666             trait_def_id,
 667             &substs,
 668             obligation.predicate,
 669         );
 670
 671         debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");
 672
 673         ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations }
 674     }
 675
 676     fn confirm_generator_candidate(
 677         &mut self,
 678         obligation: &TraitObligation<'tcx>,
 679     ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 680     {
 681         // Okay to skip binder because the substs on generator types never
 682         // touch bound regions, they just capture the in-scope
 683         // type/region parameters.
 684         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 685         let ty::Generator(generator_def_id, substs, _) = *self_ty.kind() else {
 686             bug!("closure candidate for non-closure {:?}", obligation);
 687         };
 688
 689         debug!(?obligation, ?generator_def_id, ?substs, "confirm_generator_candidate");
 690
 691         let trait_ref = self.generator_trait_ref_unnormalized(obligation, substs);
 692
 693         let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 694         debug!(?trait_ref, ?nested, "generator candidate obligations");
 695
 696         Ok(ImplSourceGeneratorData { generator_def_id, substs, nested })
 697     }
 698
 699     #[instrument(skip(self), level = "debug")]
 700     fn confirm_closure_candidate(
 701         &mut self,
 702         obligation: &TraitObligation<'tcx>,
 703     ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 704         let kind = self
 705             .tcx()
 706             .fn_trait_kind_from_lang_item(obligation.predicate.def_id())
 707             .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
 708
 709         // Okay to skip binder because the substs on closure types never
 710         // touch bound regions, they just capture the in-scope
 711         // type/region parameters.
 712         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 713         let ty::Closure(closure_def_id, substs) = *self_ty.kind() else {
 714             bug!("closure candidate for non-closure {:?}", obligation);
 715         };
 716
 717         let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
 718         let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 719
 720         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 721
 722         // FIXME: Chalk
 723
 724         if !self.tcx().sess.opts.unstable_opts.chalk {
 725             nested.push(obligation.with(
 726                 self.tcx(),
 727                 ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, substs, kind)),
 728             ));
 729         }
 730
 731         Ok(ImplSourceClosureData { closure_def_id, substs, nested })
 732     }
 733
 734     /// In the case of closure types and fn pointers,
 735     /// we currently treat the input type parameters on the trait as
 736     /// outputs. This means that when we have a match we have only
 737     /// considered the self type, so we have to go back and make sure
 738     /// to relate the argument types too. This is kind of wrong, but
 739     /// since we control the full set of impls, also not that wrong,
 740     /// and it DOES yield better error messages (since we don't report
 741     /// errors as if there is no applicable impl, but rather report
 742     /// errors are about mismatched argument types.
 743     ///
 744     /// Here is an example. Imagine we have a closure expression
 745     /// and we desugared it so that the type of the expression is
 746     /// `Closure`, and `Closure` expects `i32` as argument. Then it
 747     /// is "as if" the compiler generated this impl:
 748     /// ```ignore (illustrative)
 749     /// impl Fn(i32) for Closure { ... }
 750     /// ```
 751     /// Now imagine our obligation is `Closure: Fn(usize)`. So far
 752     /// we have matched the self type `Closure`. At this point we'll
 753     /// compare the `i32` to `usize` and generate an error.
 754     ///
 755     /// Note that this checking occurs *after* the impl has selected,
 756     /// because these output type parameters should not affect the
 757     /// selection of the impl. Therefore, if there is a mismatch, we
 758     /// report an error to the user.
 759     #[instrument(skip(self), level = "trace")]
 760     fn confirm_poly_trait_refs(
 761         &mut self,
 762         obligation: &TraitObligation<'tcx>,
 763         expected_trait_ref: ty::PolyTraitRef<'tcx>,
 764     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 765         let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
 766         // Normalize the obligation and expected trait refs together, because why not
 767         let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
 768             ensure_sufficient_stack(|| {
 769                 normalize_with_depth(
 770                     self,
 771                     obligation.param_env,
 772                     obligation.cause.clone(),
 773                     obligation.recursion_depth + 1,
 774                     (obligation_trait_ref, expected_trait_ref),
 775                 )
 776             });
 777
 778         self.infcx
 779             .at(&obligation.cause, obligation.param_env)
 780             .sup(obligation_trait_ref, expected_trait_ref)
 781             .map(|InferOk { mut obligations, .. }| {
 782                 obligations.extend(nested);
 783                 obligations
 784             })
 785             .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
 786     }
 787
 788     fn confirm_trait_upcasting_unsize_candidate(
 789         &mut self,
 790         obligation: &TraitObligation<'tcx>,
 791         idx: usize,
 792     ) -> Result<ImplSourceTraitUpcastingData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
 793     {
 794         let tcx = self.tcx();
 795
 796         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 797         // regions here. See the comment there for more details.
 798         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 799         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 800         let target = self.infcx.shallow_resolve(target);
 801
 802         debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
 803
 804         let mut nested = vec![];
 805         let source_trait_ref;
 806         let upcast_trait_ref;
 807         match (source.kind(), target.kind()) {
 808             // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
 809             (&ty::Dynamic(ref data_a, r_a, repr_a), &ty::Dynamic(ref data_b, r_b, repr_b))
 810                 if repr_a == repr_b =>
 811             {
 812                 // See `assemble_candidates_for_unsizing` for more info.
 813                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 814                 let principal_a = data_a.principal().unwrap();
 815                 source_trait_ref = principal_a.with_self_ty(tcx, source);
 816                 upcast_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
 817                 assert_eq!(data_b.principal_def_id(), Some(upcast_trait_ref.def_id()));
 818                 let existential_predicate = upcast_trait_ref.map_bound(|trait_ref| {
 819                     ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
 820                         tcx, trait_ref,
 821                     ))
 822                 });
 823                 let iter = Some(existential_predicate)
 824                     .into_iter()
 825                     .chain(
 826                         data_a
 827                             .projection_bounds()
 828                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 829                     )
 830                     .chain(
 831                         data_b
 832                             .auto_traits()
 833                             .map(ty::ExistentialPredicate::AutoTrait)
 834                             .map(ty::Binder::dummy),
 835                     );
 836                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 837                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_b);
 838
 839                 // Require that the traits involved in this upcast are **equal**;
 840                 // only the **lifetime bound** is changed.
 841                 let InferOk { obligations, .. } = self
 842                     .infcx
 843                     .at(&obligation.cause, obligation.param_env)
 844                     .sup(target, source_trait)
 845                     .map_err(|_| Unimplemented)?;
 846                 nested.extend(obligations);
 847
 848                 // Register one obligation for 'a: 'b.
 849                 let cause = ObligationCause::new(
 850                     obligation.cause.span,
 851                     obligation.cause.body_id,
 852                     ObjectCastObligation(source, target),
 853                 );
 854                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 855                 nested.push(Obligation::with_depth(
 856                     tcx,
 857                     cause,
 858                     obligation.recursion_depth + 1,
 859                     obligation.param_env,
 860                     obligation.predicate.rebind(outlives),
 861                 ));
 862             }
 863             _ => bug!(),
 864         };
 865
 866         let vtable_segment_callback = {
 867             let mut vptr_offset = 0;
 868             move |segment| {
 869                 match segment {
 870                     VtblSegment::MetadataDSA => {
 871                         vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
 872                     }
 873                     VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
 874                         vptr_offset += util::count_own_vtable_entries(tcx, trait_ref);
 875                         if trait_ref == upcast_trait_ref {
 876                             if emit_vptr {
 877                                 return ControlFlow::Break(Some(vptr_offset));
 878                             } else {
 879                                 return ControlFlow::Break(None);
 880                             }
 881                         }
 882
 883                         if emit_vptr {
 884                             vptr_offset += 1;
 885                         }
 886                     }
 887                 }
 888                 ControlFlow::Continue(())
 889             }
 890         };
 891
 892         let vtable_vptr_slot =
 893             super::super::prepare_vtable_segments(tcx, source_trait_ref, vtable_segment_callback)
 894                 .unwrap();
 895
 896         Ok(ImplSourceTraitUpcastingData { upcast_trait_ref, vtable_vptr_slot, nested })
 897     }
 898
 899     fn confirm_builtin_unsize_candidate(
 900         &mut self,
 901         obligation: &TraitObligation<'tcx>,
 902     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
 903         let tcx = self.tcx();
 904
 905         // `assemble_candidates_for_unsizing` should ensure there are no late-bound
 906         // regions here. See the comment there for more details.
 907         let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
 908         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 909         let target = self.infcx.shallow_resolve(target);
 910
 911         debug!(?source, ?target, "confirm_builtin_unsize_candidate");
 912
 913         let mut nested = vec![];
 914         match (source.kind(), target.kind()) {
 915             // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
 916             (&ty::Dynamic(ref data_a, r_a, dyn_a), &ty::Dynamic(ref data_b, r_b, dyn_b))
 917                 if dyn_a == dyn_b =>
 918             {
 919                 // See `assemble_candidates_for_unsizing` for more info.
 920                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
 921                 let iter = data_a
 922                     .principal()
 923                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
 924                     .into_iter()
 925                     .chain(
 926                         data_a
 927                             .projection_bounds()
 928                             .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
 929                     )
 930                     .chain(
 931                         data_b
 932                             .auto_traits()
 933                             .map(ty::ExistentialPredicate::AutoTrait)
 934                             .map(ty::Binder::dummy),
 935                     );
 936                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
 937                 let source_trait = tcx.mk_dynamic(existential_predicates, r_b, dyn_a);
 938
 939                 // Require that the traits involved in this upcast are **equal**;
 940                 // only the **lifetime bound** is changed.
 941                 let InferOk { obligations, .. } = self
 942                     .infcx
 943                     .at(&obligation.cause, obligation.param_env)
 944                     .sup(target, source_trait)
 945                     .map_err(|_| Unimplemented)?;
 946                 nested.extend(obligations);
 947
 948                 // Register one obligation for 'a: 'b.
 949                 let cause = ObligationCause::new(
 950                     obligation.cause.span,
 951                     obligation.cause.body_id,
 952                     ObjectCastObligation(source, target),
 953                 );
 954                 let outlives = ty::OutlivesPredicate(r_a, r_b);
 955                 nested.push(Obligation::with_depth(
 956                     tcx,
 957                     cause,
 958                     obligation.recursion_depth + 1,
 959                     obligation.param_env,
 960                     obligation.predicate.rebind(outlives),
 961                 ));
 962             }
 963
 964             // `T` -> `Trait`
 965             (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
 966                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
 967                 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
 968                     return Err(TraitNotObjectSafe(did));
 969                 }
 970
 971                 let cause = ObligationCause::new(
 972                     obligation.cause.span,
 973                     obligation.cause.body_id,
 974                     ObjectCastObligation(source, target),
 975                 );
 976
 977                 let predicate_to_obligation = |predicate| {
 978                     Obligation::with_depth(
 979                         tcx,
 980                         cause.clone(),
 981                         obligation.recursion_depth + 1,
 982                         obligation.param_env,
 983                         predicate,
 984                     )
 985                 };
 986
 987                 // Create obligations:
 988                 //  - Casting `T` to `Trait`
 989                 //  - For all the various builtin bounds attached to the object cast. (In other
 990                 //  words, if the object type is `Foo + Send`, this would create an obligation for
 991                 //  the `Send` check.)
 992                 //  - Projection predicates
 993                 nested.extend(
 994                     data.iter().map(|predicate| {
 995                         predicate_to_obligation(predicate.with_self_ty(tcx, source))
 996                     }),
 997                 );
 998
 999                 // We can only make objects from sized types.
1000                 let tr = ty::Binder::dummy(tcx.mk_trait_ref(
1001                     tcx.require_lang_item(LangItem::Sized, None),
1002                     source,
1003                     [],
1004                 ));
1005                 nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
1006
1007                 // If the type is `Foo + 'a`, ensure that the type
1008                 // being cast to `Foo + 'a` outlives `'a`:
1009                 let outlives = ty::OutlivesPredicate(source, r);
1010                 nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
1011             }
1012
1013             // `[T; n]` -> `[T]`
1014             (&ty::Array(a, _), &ty::Slice(b)) => {
1015                 let InferOk { obligations, .. } = self
1016                     .infcx
1017                     .at(&obligation.cause, obligation.param_env)
1018                     .eq(b, a)
1019                     .map_err(|_| Unimplemented)?;
1020                 nested.extend(obligations);
1021             }
1022
1023             // `Struct<T>` -> `Struct<U>`
1024             (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
1025                 let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
1026                     GenericArgKind::Type(ty) => match ty.kind() {
1027                         ty::Param(p) => Some(p.index),
1028                         _ => None,
1029                     },
1030
1031                     // Lifetimes aren't allowed to change during unsizing.
1032                     GenericArgKind::Lifetime(_) => None,
1033
1034                     GenericArgKind::Const(ct) => match ct.kind() {
1035                         ty::ConstKind::Param(p) => Some(p.index),
1036                         _ => None,
1037                     },
1038                 };
1039
1040                 // FIXME(eddyb) cache this (including computing `unsizing_params`)
1041                 // by putting it in a query; it would only need the `DefId` as it
1042                 // looks at declared field types, not anything substituted.
1043
1044                 // The last field of the structure has to exist and contain type/const parameters.
1045                 let (tail_field, prefix_fields) =
1046                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
1047                 let tail_field_ty = tcx.bound_type_of(tail_field.did);
1048
1049                 let mut unsizing_params = GrowableBitSet::new_empty();
1050                 for arg in tail_field_ty.0.walk() {
1051                     if let Some(i) = maybe_unsizing_param_idx(arg) {
1052                         unsizing_params.insert(i);
1053                     }
1054                 }
1055
1056                 // Ensure none of the other fields mention the parameters used
1057                 // in unsizing.
1058                 for field in prefix_fields {
1059                     for arg in tcx.type_of(field.did).walk() {
1060                         if let Some(i) = maybe_unsizing_param_idx(arg) {
1061                             unsizing_params.remove(i);
1062                         }
1063                     }
1064                 }
1065
1066                 if unsizing_params.is_empty() {
1067                     return Err(Unimplemented);
1068                 }
1069
1070                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
1071                 // normalizing in the process, since `type_of` returns something directly from
1072                 // astconv (which means it's un-normalized).
1073                 let source_tail = normalize_with_depth_to(
1074                     self,
1075                     obligation.param_env,
1076                     obligation.cause.clone(),
1077                     obligation.recursion_depth + 1,
1078                     tail_field_ty.subst(tcx, substs_a),
1079                     &mut nested,
1080                 );
1081                 let target_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_b),
1087                     &mut nested,
1088                 );
1089
1090                 // Check that the source struct with the target's
1091                 // unsizing parameters is equal to the target.
1092                 let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
1093                     if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
1094                 }));
1095                 let new_struct = tcx.mk_adt(def, substs);
1096                 let InferOk { obligations, .. } = self
1097                     .infcx
1098                     .at(&obligation.cause, obligation.param_env)
1099                     .eq(target, new_struct)
1100                     .map_err(|_| Unimplemented)?;
1101                 nested.extend(obligations);
1102
1103                 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
1104                 nested.push(predicate_for_trait_def(
1105                     tcx,
1106                     obligation.param_env,
1107                     obligation.cause.clone(),
1108                     obligation.predicate.def_id(),
1109                     obligation.recursion_depth + 1,
1110                     source_tail,
1111                     [target_tail.into()],
1112                 ));
1113             }
1114
1115             // `(.., T)` -> `(.., U)`
1116             (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
1117                 assert_eq!(tys_a.len(), tys_b.len());
1118
1119                 // The last field of the tuple has to exist.
1120                 let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
1121                 let &b_last = tys_b.last().unwrap();
1122
1123                 // Check that the source tuple with the target's
1124                 // last element is equal to the target.
1125                 let new_tuple = tcx.mk_tup(a_mid.iter().copied().chain(iter::once(b_last)));
1126                 let InferOk { obligations, .. } = self
1127                     .infcx
1128                     .at(&obligation.cause, obligation.param_env)
1129                     .eq(target, new_tuple)
1130                     .map_err(|_| Unimplemented)?;
1131                 nested.extend(obligations);
1132
1133                 // Construct the nested `T: Unsize<U>` predicate.
1134                 nested.push(ensure_sufficient_stack(|| {
1135                     predicate_for_trait_def(
1136                         tcx,
1137                         obligation.param_env,
1138                         obligation.cause.clone(),
1139                         obligation.predicate.def_id(),
1140                         obligation.recursion_depth + 1,
1141                         a_last,
1142                         [b_last.into()],
1143                     )
1144                 }));
1145             }
1146
1147             _ => bug!("source: {source}, target: {target}"),
1148         };
1149
1150         Ok(ImplSourceBuiltinData { nested })
1151     }
1152
1153     fn confirm_const_destruct_candidate(
1154         &mut self,
1155         obligation: &TraitObligation<'tcx>,
1156         impl_def_id: Option<DefId>,
1157     ) -> Result<ImplSourceConstDestructData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
1158         // `~const Destruct` in a non-const environment is always trivially true, since our type is `Drop`
1159         if !obligation.is_const() {
1160             return Ok(ImplSourceConstDestructData { nested: vec![] });
1161         }
1162
1163         let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
1164
1165         let tcx = self.tcx();
1166         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
1167
1168         let mut nested = vec![];
1169         let cause = obligation.derived_cause(BuiltinDerivedObligation);
1170
1171         // If we have a custom `impl const Drop`, then
1172         // first check it like a regular impl candidate.
1173         // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
1174         if let Some(impl_def_id) = impl_def_id {
1175             let mut new_obligation = obligation.clone();
1176             new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
1177                 trait_pred.trait_ref.def_id = drop_trait;
1178                 trait_pred
1179             });
1180             let substs = self.rematch_impl(impl_def_id, &new_obligation);
1181             debug!(?substs, "impl substs");
1182
1183             let cause = obligation.derived_cause(|derived| {
1184                 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
1185                     derived,
1186                     impl_def_id,
1187                     span: obligation.cause.span,
1188                 }))
1189             });
1190             let obligations = ensure_sufficient_stack(|| {
1191                 self.vtable_impl(
1192                     impl_def_id,
1193                     substs,
1194                     &cause,
1195                     new_obligation.recursion_depth + 1,
1196                     new_obligation.param_env,
1197                     obligation.predicate,
1198                 )
1199             });
1200             nested.extend(obligations.nested);
1201         }
1202
1203         // We want to confirm the ADT's fields if we have an ADT
1204         let mut stack = match *self_ty.skip_binder().kind() {
1205             ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(tcx, substs)).collect(),
1206             _ => vec![self_ty.skip_binder()],
1207         };
1208
1209         while let Some(nested_ty) = stack.pop() {
1210             match *nested_ty.kind() {
1211                 // We know these types are trivially drop
1212                 ty::Bool
1213                 | ty::Char
1214                 | ty::Int(_)
1215                 | ty::Uint(_)
1216                 | ty::Float(_)
1217                 | ty::Infer(ty::IntVar(_))
1218                 | ty::Infer(ty::FloatVar(_))
1219                 | ty::Str
1220                 | ty::RawPtr(_)
1221                 | ty::Ref(..)
1222                 | ty::FnDef(..)
1223                 | ty::FnPtr(_)
1224                 | ty::Never
1225                 | ty::Foreign(_) => {}
1226
1227                 // `ManuallyDrop` is trivially drop
1228                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
1229
1230                 // These types are built-in, so we can fast-track by registering
1231                 // nested predicates for their constituent type(s)
1232                 ty::Array(ty, _) | ty::Slice(ty) => {
1233                     stack.push(ty);
1234                 }
1235                 ty::Tuple(tys) => {
1236                     stack.extend(tys.iter());
1237                 }
1238                 ty::Closure(_, substs) => {
1239                     stack.push(substs.as_closure().tupled_upvars_ty());
1240                 }
1241                 ty::Generator(_, substs, _) => {
1242                     let generator = substs.as_generator();
1243                     stack.extend([generator.tupled_upvars_ty(), generator.witness()]);
1244                 }
1245                 ty::GeneratorWitness(tys) => {
1246                     stack.extend(tcx.erase_late_bound_regions(tys).to_vec());
1247                 }
1248
1249                 // If we have a projection type, make sure to normalize it so we replace it
1250                 // with a fresh infer variable
1251                 ty::Projection(..) => {
1252                     let predicate = normalize_with_depth_to(
1253                         self,
1254                         obligation.param_env,
1255                         cause.clone(),
1256                         obligation.recursion_depth + 1,
1257                         self_ty.rebind(ty::TraitPredicate {
1258                             trait_ref: self.tcx().mk_trait_ref(
1259                                 self.tcx().require_lang_item(LangItem::Destruct, None),
1260                                 nested_ty,
1261                                 [],
1262                             ),
1263                             constness: ty::BoundConstness::ConstIfConst,
1264                             polarity: ty::ImplPolarity::Positive,
1265                         }),
1266                         &mut nested,
1267                     );
1268
1269                     nested.push(Obligation::with_depth(
1270                         tcx,
1271                         cause.clone(),
1272                         obligation.recursion_depth + 1,
1273                         obligation.param_env,
1274                         predicate,
1275                     ));
1276                 }
1277
1278                 // If we have any other type (e.g. an ADT), just register a nested obligation
1279                 // since it's either not `const Drop` (and we raise an error during selection),
1280                 // or it's an ADT (and we need to check for a custom impl during selection)
1281                 _ => {
1282                     let predicate = self_ty.rebind(ty::TraitPredicate {
1283                         trait_ref: self.tcx().mk_trait_ref(
1284                             self.tcx().require_lang_item(LangItem::Destruct, None),
1285                             nested_ty,
1286                             [],
1287                         ),
1288                         constness: ty::BoundConstness::ConstIfConst,
1289                         polarity: ty::ImplPolarity::Positive,
1290                     });
1291
1292                     nested.push(Obligation::with_depth(
1293                         tcx,
1294                         cause.clone(),
1295                         obligation.recursion_depth + 1,
1296                         obligation.param_env,
1297                         predicate,
1298                     ));
1299                 }
1300             }
1301         }
1302
1303         Ok(ImplSourceConstDestructData { nested })
1304     }
1305 }