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