confirmation.rs

//! Confirmation.
//!
//! Confirmation unifies the output type parameters of the trait
//! with the values found in the obligation, possibly yielding a
//! type error. See the [rustc dev guide] for more details.
//!
//! [rustc dev guide]:
//! https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation
use rustc_ast::Mutability;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_hir::lang_items::LangItem;
use rustc_infer::infer::HigherRankedType;
use rustc_infer::infer::{DefineOpaqueTypes, InferOk};
use rustc_middle::traits::{BuiltinImplSource, SignatureMismatchData};
use rustc_middle::ty::{
    self, GenericArgs, GenericArgsRef, GenericParamDefKind, ToPolyTraitRef, ToPredicate,
    TraitPredicate, Ty, TyCtxt, TypeVisitableExt,
};
use rustc_span::def_id::DefId;

use crate::traits::normalize::{normalize_with_depth, normalize_with_depth_to};
use crate::traits::util::{self, closure_trait_ref_and_return_type};
use crate::traits::vtable::{
    count_own_vtable_entries, prepare_vtable_segments, vtable_trait_first_method_offset,
    VtblSegment,
};
use crate::traits::{
    BuiltinDerivedObligation, ImplDerivedObligation, ImplDerivedObligationCause, ImplSource,
    ImplSourceUserDefinedData, Normalized, Obligation, ObligationCause, PolyTraitObligation,
    PredicateObligation, Selection, SelectionError, SignatureMismatch, TraitNotObjectSafe,
    Unimplemented,
};

use super::BuiltinImplConditions;
use super::SelectionCandidate::{self, *};
use super::SelectionContext;

use std::iter;
use std::ops::ControlFlow;

impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
    #[instrument(level = "debug", skip(self))]
    pub(super) fn confirm_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
        candidate: SelectionCandidate<'tcx>,
    ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
        let mut impl_src = match candidate {
            BuiltinCandidate { has_nested } => {
                let data = self.confirm_builtin_candidate(obligation, has_nested);
                ImplSource::Builtin(BuiltinImplSource::Misc, data)
            }

            TransmutabilityCandidate => {
                let data = self.confirm_transmutability_candidate(obligation)?;
                ImplSource::Builtin(BuiltinImplSource::Misc, data)
            }

            ParamCandidate(param) => {
                let obligations =
                    self.confirm_param_candidate(obligation, param.map_bound(|t| t.trait_ref));
                ImplSource::Param(obligations)
            }

            ImplCandidate(impl_def_id) => {
                ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id))
            }

            AutoImplCandidate => {
                let data = self.confirm_auto_impl_candidate(obligation)?;
                ImplSource::Builtin(BuiltinImplSource::Misc, data)
            }

            ProjectionCandidate(idx) => {
                let obligations = self.confirm_projection_candidate(obligation, idx)?;
                ImplSource::Param(obligations)
            }

            ObjectCandidate(idx) => self.confirm_object_candidate(obligation, idx)?,

            ClosureCandidate { .. } => {
                let vtable_closure = self.confirm_closure_candidate(obligation)?;
                ImplSource::Builtin(BuiltinImplSource::Misc, vtable_closure)
            }

            AsyncClosureCandidate => {
                let vtable_closure = self.confirm_async_closure_candidate(obligation)?;
                ImplSource::Builtin(BuiltinImplSource::Misc, vtable_closure)
            }

            // No nested obligations or confirmation process. The checks that we do in
            // candidate assembly are sufficient.
            AsyncFnKindHelperCandidate => ImplSource::Builtin(BuiltinImplSource::Misc, vec![]),

            CoroutineCandidate => {
                let vtable_coroutine = self.confirm_coroutine_candidate(obligation)?;
                ImplSource::Builtin(BuiltinImplSource::Misc, vtable_coroutine)
            }

            FutureCandidate => {
                let vtable_future = self.confirm_future_candidate(obligation)?;
                ImplSource::Builtin(BuiltinImplSource::Misc, vtable_future)
            }

            IteratorCandidate => {
                let vtable_iterator = self.confirm_iterator_candidate(obligation)?;
                ImplSource::Builtin(BuiltinImplSource::Misc, vtable_iterator)
            }

            AsyncIteratorCandidate => {
                let vtable_iterator = self.confirm_async_iterator_candidate(obligation)?;
                ImplSource::Builtin(BuiltinImplSource::Misc, vtable_iterator)
            }

            FnPointerCandidate { fn_host_effect } => {
                let data = self.confirm_fn_pointer_candidate(obligation, fn_host_effect)?;
                ImplSource::Builtin(BuiltinImplSource::Misc, data)
            }

            TraitAliasCandidate => {
                let data = self.confirm_trait_alias_candidate(obligation);
                ImplSource::Builtin(BuiltinImplSource::Misc, data)
            }

            BuiltinObjectCandidate => {
                // This indicates something like `Trait + Send: Send`. In this case, we know that
                // this holds because that's what the object type is telling us, and there's really
                // no additional obligations to prove and no types in particular to unify, etc.
                ImplSource::Builtin(BuiltinImplSource::Misc, Vec::new())
            }

            BuiltinUnsizeCandidate => self.confirm_builtin_unsize_candidate(obligation)?,

            TraitUpcastingUnsizeCandidate(idx) => {
                self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?
            }

            ConstDestructCandidate(def_id) => {
                let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
                ImplSource::Builtin(BuiltinImplSource::Misc, data)
            }
        };

        // The obligations returned by confirmation are recursively evaluated
        // so we need to make sure they have the correct depth.
        for subobligation in impl_src.borrow_nested_obligations_mut() {
            subobligation.set_depth_from_parent(obligation.recursion_depth);
        }

        Ok(impl_src)
    }

    fn confirm_projection_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
        idx: usize,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        let tcx = self.tcx();

        let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
        let placeholder_trait_predicate =
            self.infcx.enter_forall_and_leak_universe(trait_predicate).trait_ref;
        let placeholder_self_ty = placeholder_trait_predicate.self_ty();
        let candidate_predicate = self
            .for_each_item_bound(
                placeholder_self_ty,
                |_, clause, clause_idx| {
                    if clause_idx == idx {
                        ControlFlow::Break(clause)
                    } else {
                        ControlFlow::Continue(())
                    }
                },
                || unreachable!(),
            )
            .break_value()
            .expect("expected to index into clause that exists");
        let candidate = candidate_predicate
            .as_trait_clause()
            .expect("projection candidate is not a trait predicate")
            .map_bound(|t| t.trait_ref);

        let candidate = self.infcx.instantiate_binder_with_fresh_vars(
            obligation.cause.span,
            HigherRankedType,
            candidate,
        );
        let mut obligations = Vec::new();
        let candidate = normalize_with_depth_to(
            self,
            obligation.param_env,
            obligation.cause.clone(),
            obligation.recursion_depth + 1,
            candidate,
            &mut obligations,
        );

        obligations.extend(
            self.infcx
                .at(&obligation.cause, obligation.param_env)
                .eq(DefineOpaqueTypes::No, placeholder_trait_predicate, candidate)
                .map(|InferOk { obligations, .. }| obligations)
                .map_err(|_| Unimplemented)?,
        );

        // FIXME(compiler-errors): I don't think this is needed.
        if let ty::Alias(ty::Projection, alias_ty) = placeholder_self_ty.kind() {
            let predicates = tcx.predicates_of(alias_ty.def_id).instantiate_own(tcx, alias_ty.args);
            for (predicate, _) in predicates {
                let normalized = normalize_with_depth_to(
                    self,
                    obligation.param_env,
                    obligation.cause.clone(),
                    obligation.recursion_depth + 1,
                    predicate,
                    &mut obligations,
                );
                obligations.push(Obligation::with_depth(
                    self.tcx(),
                    obligation.cause.clone(),
                    obligation.recursion_depth + 1,
                    obligation.param_env,
                    normalized,
                ));
            }
        }

        Ok(obligations)
    }

    fn confirm_param_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
        param: ty::PolyTraitRef<'tcx>,
    ) -> Vec<PredicateObligation<'tcx>> {
        debug!(?obligation, ?param, "confirm_param_candidate");

        // During evaluation, we already checked that this
        // where-clause trait-ref could be unified with the obligation
        // trait-ref. Repeat that unification now without any
        // transactional boundary; it should not fail.
        match self.match_where_clause_trait_ref(obligation, param) {
            Ok(obligations) => obligations,
            Err(()) => {
                bug!(
                    "Where clause `{:?}` was applicable to `{:?}` but now is not",
                    param,
                    obligation
                );
            }
        }
    }

    fn confirm_builtin_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
        has_nested: bool,
    ) -> Vec<PredicateObligation<'tcx>> {
        debug!(?obligation, ?has_nested, "confirm_builtin_candidate");

        let lang_items = self.tcx().lang_items();
        let obligations = if has_nested {
            let trait_def = obligation.predicate.def_id();
            let conditions = if Some(trait_def) == lang_items.sized_trait() {
                self.sized_conditions(obligation)
            } else if Some(trait_def) == lang_items.copy_trait() {
                self.copy_clone_conditions(obligation)
            } else if Some(trait_def) == lang_items.clone_trait() {
                self.copy_clone_conditions(obligation)
            } else {
                bug!("unexpected builtin trait {:?}", trait_def)
            };
            let BuiltinImplConditions::Where(nested) = conditions else {
                bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
            };

            let cause = obligation.derived_cause(BuiltinDerivedObligation);
            self.collect_predicates_for_types(
                obligation.param_env,
                cause,
                obligation.recursion_depth + 1,
                trait_def,
                nested,
            )
        } else {
            vec![]
        };

        debug!(?obligations);

        obligations
    }

    #[instrument(level = "debug", skip(self))]
    fn confirm_transmutability_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        use rustc_transmute::{Answer, Condition};
        #[instrument(level = "debug", skip(tcx, obligation, predicate))]
        fn flatten_answer_tree<'tcx>(
            tcx: TyCtxt<'tcx>,
            obligation: &PolyTraitObligation<'tcx>,
            predicate: TraitPredicate<'tcx>,
            cond: Condition<rustc_transmute::layout::rustc::Ref<'tcx>>,
        ) -> Vec<PredicateObligation<'tcx>> {
            match cond {
                // FIXME(bryangarza): Add separate `IfAny` case, instead of treating as `IfAll`
                // Not possible until the trait solver supports disjunctions of obligations
                Condition::IfAll(conds) | Condition::IfAny(conds) => conds
                    .into_iter()
                    .flat_map(|cond| flatten_answer_tree(tcx, obligation, predicate, cond))
                    .collect(),
                Condition::IfTransmutable { src, dst } => {
                    let trait_def_id = obligation.predicate.def_id();
                    let assume_const = predicate.trait_ref.args.const_at(2);
                    let make_obl = |from_ty, to_ty| {
                        let trait_ref1 = ty::TraitRef::new(
                            tcx,
                            trait_def_id,
                            [
                                ty::GenericArg::from(to_ty),
                                ty::GenericArg::from(from_ty),
                                ty::GenericArg::from(assume_const),
                            ],
                        );
                        Obligation::with_depth(
                            tcx,
                            obligation.cause.clone(),
                            obligation.recursion_depth + 1,
                            obligation.param_env,
                            trait_ref1,
                        )
                    };

                    // If Dst is mutable, check bidirectionally.
                    // For example, transmuting bool -> u8 is OK as long as you can't update that u8
                    // to be > 1, because you could later transmute the u8 back to a bool and get UB.
                    match dst.mutability {
                        Mutability::Not => vec![make_obl(src.ty, dst.ty)],
                        Mutability::Mut => vec![make_obl(src.ty, dst.ty), make_obl(dst.ty, src.ty)],
                    }
                }
            }
        }

        // We erase regions here because transmutability calls layout queries,
        // which does not handle inference regions and doesn't particularly
        // care about other regions. Erasing late-bound regions is equivalent
        // to instantiating the binder with placeholders then erasing those
        // placeholder regions.
        let predicate = self
            .tcx()
            .erase_regions(self.tcx().instantiate_bound_regions_with_erased(obligation.predicate));

        let Some(assume) = rustc_transmute::Assume::from_const(
            self.infcx.tcx,
            obligation.param_env,
            predicate.trait_ref.args.const_at(2),
        ) else {
            return Err(Unimplemented);
        };

        let dst = predicate.trait_ref.args.type_at(0);
        let src = predicate.trait_ref.args.type_at(1);
        debug!(?src, ?dst);
        let mut transmute_env = rustc_transmute::TransmuteTypeEnv::new(self.infcx);
        let maybe_transmutable = transmute_env.is_transmutable(
            obligation.cause.clone(),
            rustc_transmute::Types { dst, src },
            assume,
        );

        let fully_flattened = match maybe_transmutable {
            Answer::No(_) => Err(Unimplemented)?,
            Answer::If(cond) => flatten_answer_tree(self.tcx(), obligation, predicate, cond),
            Answer::Yes => vec![],
        };

        debug!(?fully_flattened);
        Ok(fully_flattened)
    }

    /// This handles the case where an `auto trait Foo` impl is being used.
    /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
    ///
    /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
    /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
    fn confirm_auto_impl_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        debug!(?obligation, "confirm_auto_impl_candidate");

        let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
        let types = self.constituent_types_for_ty(self_ty)?;
        Ok(self.vtable_auto_impl(obligation, obligation.predicate.def_id(), types))
    }

    /// See `confirm_auto_impl_candidate`.
    fn vtable_auto_impl(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
        trait_def_id: DefId,
        nested: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
    ) -> Vec<PredicateObligation<'tcx>> {
        debug!(?nested, "vtable_auto_impl");
        ensure_sufficient_stack(|| {
            let cause = obligation.derived_cause(BuiltinDerivedObligation);

            let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
            let trait_ref = self.infcx.enter_forall_and_leak_universe(poly_trait_ref);
            let trait_obligations: Vec<PredicateObligation<'_>> = self.impl_or_trait_obligations(
                &cause,
                obligation.recursion_depth + 1,
                obligation.param_env,
                trait_def_id,
                trait_ref.args,
                obligation.predicate,
            );

            let mut obligations = self.collect_predicates_for_types(
                obligation.param_env,
                cause,
                obligation.recursion_depth + 1,
                trait_def_id,
                nested,
            );

            // Adds the predicates from the trait. Note that this contains a `Self: Trait`
            // predicate as usual. It won't have any effect since auto traits are coinductive.
            obligations.extend(trait_obligations);

            debug!(?obligations, "vtable_auto_impl");

            obligations
        })
    }

    fn confirm_impl_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
        impl_def_id: DefId,
    ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
        debug!(?obligation, ?impl_def_id, "confirm_impl_candidate");

        // First, create the generic parameters by matching the impl again,
        // this time not in a probe.
        let args = self.rematch_impl(impl_def_id, obligation);
        debug!(?args, "impl args");
        ensure_sufficient_stack(|| {
            self.vtable_impl(
                impl_def_id,
                args,
                &obligation.cause,
                obligation.recursion_depth + 1,
                obligation.param_env,
                obligation.predicate,
            )
        })
    }

    fn vtable_impl(
        &mut self,
        impl_def_id: DefId,
        args: Normalized<'tcx, GenericArgsRef<'tcx>>,
        cause: &ObligationCause<'tcx>,
        recursion_depth: usize,
        param_env: ty::ParamEnv<'tcx>,
        parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
    ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
        debug!(?impl_def_id, ?args, ?recursion_depth, "vtable_impl");

        let mut impl_obligations = self.impl_or_trait_obligations(
            cause,
            recursion_depth,
            param_env,
            impl_def_id,
            args.value,
            parent_trait_pred,
        );

        debug!(?impl_obligations, "vtable_impl");

        // Because of RFC447, the impl-trait-ref and obligations
        // are sufficient to determine the impl args, without
        // relying on projections in the impl-trait-ref.
        //
        // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
        impl_obligations.extend(args.obligations);

        ImplSourceUserDefinedData { impl_def_id, args: args.value, nested: impl_obligations }
    }

    fn confirm_object_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
        index: usize,
    ) -> Result<ImplSource<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        let tcx = self.tcx();
        debug!(?obligation, ?index, "confirm_object_candidate");

        let trait_predicate = self.infcx.enter_forall_and_leak_universe(obligation.predicate);
        let self_ty = self.infcx.shallow_resolve(trait_predicate.self_ty());
        let ty::Dynamic(data, ..) = *self_ty.kind() else {
            span_bug!(obligation.cause.span, "object candidate with non-object");
        };

        let object_trait_ref = data.principal().unwrap_or_else(|| {
            span_bug!(obligation.cause.span, "object candidate with no principal")
        });
        let object_trait_ref = self.infcx.instantiate_binder_with_fresh_vars(
            obligation.cause.span,
            HigherRankedType,
            object_trait_ref,
        );
        let object_trait_ref = object_trait_ref.with_self_ty(self.tcx(), self_ty);

        let mut nested = vec![];

        let mut supertraits = util::supertraits(tcx, ty::Binder::dummy(object_trait_ref));
        let unnormalized_upcast_trait_ref =
            supertraits.nth(index).expect("supertraits iterator no longer has as many elements");

        let upcast_trait_ref = self.infcx.instantiate_binder_with_fresh_vars(
            obligation.cause.span,
            HigherRankedType,
            unnormalized_upcast_trait_ref,
        );
        let upcast_trait_ref = normalize_with_depth_to(
            self,
            obligation.param_env,
            obligation.cause.clone(),
            obligation.recursion_depth + 1,
            upcast_trait_ref,
            &mut nested,
        );

        nested.extend(
            self.infcx
                .at(&obligation.cause, obligation.param_env)
                .eq(DefineOpaqueTypes::No, trait_predicate.trait_ref, upcast_trait_ref)
                .map(|InferOk { obligations, .. }| obligations)
                .map_err(|_| Unimplemented)?,
        );

        // Check supertraits hold. This is so that their associated type bounds
        // will be checked in the code below.
        for super_trait in tcx
            .super_predicates_of(trait_predicate.def_id())
            .instantiate(tcx, trait_predicate.trait_ref.args)
            .predicates
            .into_iter()
        {
            let normalized_super_trait = normalize_with_depth_to(
                self,
                obligation.param_env,
                obligation.cause.clone(),
                obligation.recursion_depth + 1,
                super_trait,
                &mut nested,
            );
            nested.push(obligation.with(tcx, normalized_super_trait));
        }

        let assoc_types: Vec<_> = tcx
            .associated_items(trait_predicate.def_id())
            .in_definition_order()
            // Associated types that require `Self: Sized` do not show up in the built-in
            // implementation of `Trait for dyn Trait`, and can be dropped here.
            .filter(|item| !tcx.generics_require_sized_self(item.def_id))
            .filter_map(
                |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
            )
            .collect();

        for assoc_type in assoc_types {
            let defs: &ty::Generics = tcx.generics_of(assoc_type);

            if !defs.params.is_empty() && !tcx.features().generic_associated_types_extended {
                tcx.dcx().span_delayed_bug(
                    obligation.cause.span,
                    "GATs in trait object shouldn't have been considered",
                );
                return Err(SelectionError::TraitNotObjectSafe(trait_predicate.trait_ref.def_id));
            }

            // This maybe belongs in wf, but that can't (doesn't) handle
            // higher-ranked things.
            // Prevent, e.g., `dyn Iterator<Item = str>`.
            for bound in self.tcx().item_bounds(assoc_type).transpose_iter() {
                let arg_bound = if defs.count() == 0 {
                    bound.instantiate(tcx, trait_predicate.trait_ref.args)
                } else {
                    let mut args = smallvec::SmallVec::with_capacity(defs.count());
                    args.extend(trait_predicate.trait_ref.args.iter());
                    let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
                        smallvec::SmallVec::with_capacity(
                            bound.skip_binder().kind().bound_vars().len() + defs.count(),
                        );
                    bound_vars.extend(bound.skip_binder().kind().bound_vars().into_iter());
                    GenericArgs::fill_single(&mut args, defs, &mut |param, _| match param.kind {
                        GenericParamDefKind::Type { .. } => {
                            let kind = ty::BoundTyKind::Param(param.def_id, param.name);
                            let bound_var = ty::BoundVariableKind::Ty(kind);
                            bound_vars.push(bound_var);
                            Ty::new_bound(
                                tcx,
                                ty::INNERMOST,
                                ty::BoundTy {
                                    var: ty::BoundVar::from_usize(bound_vars.len() - 1),
                                    kind,
                                },
                            )
                            .into()
                        }
                        GenericParamDefKind::Lifetime => {
                            let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
                            let bound_var = ty::BoundVariableKind::Region(kind);
                            bound_vars.push(bound_var);
                            ty::Region::new_bound(
                                tcx,
                                ty::INNERMOST,
                                ty::BoundRegion {
                                    var: ty::BoundVar::from_usize(bound_vars.len() - 1),
                                    kind,
                                },
                            )
                            .into()
                        }
                        GenericParamDefKind::Const { .. } => {
                            let bound_var = ty::BoundVariableKind::Const;
                            bound_vars.push(bound_var);
                            ty::Const::new_bound(
                                tcx,
                                ty::INNERMOST,
                                ty::BoundVar::from_usize(bound_vars.len() - 1),
                                tcx.type_of(param.def_id)
                                    .no_bound_vars()
                                    .expect("const parameter types cannot be generic"),
                            )
                            .into()
                        }
                    });
                    let bound_vars = tcx.mk_bound_variable_kinds(&bound_vars);
                    let assoc_ty_args = tcx.mk_args(&args);
                    let bound =
                        bound.map_bound(|b| b.kind().skip_binder()).instantiate(tcx, assoc_ty_args);
                    ty::Binder::bind_with_vars(bound, bound_vars).to_predicate(tcx)
                };
                let normalized_bound = normalize_with_depth_to(
                    self,
                    obligation.param_env,
                    obligation.cause.clone(),
                    obligation.recursion_depth + 1,
                    arg_bound,
                    &mut nested,
                );
                nested.push(obligation.with(tcx, normalized_bound));
            }
        }

        debug!(?nested, "object nested obligations");

        let vtable_base = vtable_trait_first_method_offset(
            tcx,
            (unnormalized_upcast_trait_ref, ty::Binder::dummy(object_trait_ref)),
        );

        Ok(ImplSource::Builtin(BuiltinImplSource::Object { vtable_base: vtable_base }, nested))
    }

    fn confirm_fn_pointer_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
        fn_host_effect: ty::Const<'tcx>,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        debug!(?obligation, "confirm_fn_pointer_candidate");

        let tcx = self.tcx();

        let Some(self_ty) = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars()) else {
            // FIXME: Ideally we'd support `for<'a> fn(&'a ()): Fn(&'a ())`,
            // but we do not currently. Luckily, such a bound is not
            // particularly useful, so we don't expect users to write
            // them often.
            return Err(SelectionError::Unimplemented);
        };

        let sig = self_ty.fn_sig(tcx);
        let trait_ref = closure_trait_ref_and_return_type(
            tcx,
            obligation.predicate.def_id(),
            self_ty,
            sig,
            util::TupleArgumentsFlag::Yes,
            fn_host_effect,
        )
        .map_bound(|(trait_ref, _)| trait_ref);

        let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
        let cause = obligation.derived_cause(BuiltinDerivedObligation);

        // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
        let output_ty = self.infcx.enter_forall_and_leak_universe(sig.output());
        let output_ty = normalize_with_depth_to(
            self,
            obligation.param_env,
            cause.clone(),
            obligation.recursion_depth,
            output_ty,
            &mut nested,
        );
        let tr = ty::TraitRef::from_lang_item(self.tcx(), LangItem::Sized, cause.span, [output_ty]);
        nested.push(Obligation::new(self.infcx.tcx, cause, obligation.param_env, tr));

        Ok(nested)
    }

    fn confirm_trait_alias_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
    ) -> Vec<PredicateObligation<'tcx>> {
        debug!(?obligation, "confirm_trait_alias_candidate");

        let predicate = self.infcx.enter_forall_and_leak_universe(obligation.predicate);
        let trait_ref = predicate.trait_ref;
        let trait_def_id = trait_ref.def_id;
        let args = trait_ref.args;

        let trait_obligations = self.impl_or_trait_obligations(
            &obligation.cause,
            obligation.recursion_depth,
            obligation.param_env,
            trait_def_id,
            args,
            obligation.predicate,
        );

        debug!(?trait_def_id, ?trait_obligations, "trait alias obligations");

        trait_obligations
    }

    fn confirm_coroutine_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        // Okay to skip binder because the args on coroutine types never
        // touch bound regions, they just capture the in-scope
        // type/region parameters.
        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
        let ty::Coroutine(coroutine_def_id, args) = *self_ty.kind() else {
            bug!("closure candidate for non-closure {:?}", obligation);
        };

        debug!(?obligation, ?coroutine_def_id, ?args, "confirm_coroutine_candidate");

        let coroutine_sig = args.as_coroutine().sig();

        // NOTE: The self-type is a coroutine type and hence is
        // in fact unparameterized (or at least does not reference any
        // regions bound in the obligation).
        let self_ty = obligation
            .predicate
            .self_ty()
            .no_bound_vars()
            .expect("unboxed closure type should not capture bound vars from the predicate");

        let (trait_ref, _, _) = super::util::coroutine_trait_ref_and_outputs(
            self.tcx(),
            obligation.predicate.def_id(),
            self_ty,
            coroutine_sig,
        );

        let nested = self.confirm_poly_trait_refs(obligation, ty::Binder::dummy(trait_ref))?;
        debug!(?trait_ref, ?nested, "coroutine candidate obligations");

        Ok(nested)
    }

    fn confirm_future_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        // Okay to skip binder because the args on coroutine types never
        // touch bound regions, they just capture the in-scope
        // type/region parameters.
        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
        let ty::Coroutine(coroutine_def_id, args) = *self_ty.kind() else {
            bug!("closure candidate for non-closure {:?}", obligation);
        };

        debug!(?obligation, ?coroutine_def_id, ?args, "confirm_future_candidate");

        let coroutine_sig = args.as_coroutine().sig();

        let (trait_ref, _) = super::util::future_trait_ref_and_outputs(
            self.tcx(),
            obligation.predicate.def_id(),
            obligation.predicate.no_bound_vars().expect("future has no bound vars").self_ty(),
            coroutine_sig,
        );

        let nested = self.confirm_poly_trait_refs(obligation, ty::Binder::dummy(trait_ref))?;
        debug!(?trait_ref, ?nested, "future candidate obligations");

        Ok(nested)
    }

    fn confirm_iterator_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        // Okay to skip binder because the args on coroutine types never
        // touch bound regions, they just capture the in-scope
        // type/region parameters.
        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
        let ty::Coroutine(coroutine_def_id, args) = *self_ty.kind() else {
            bug!("closure candidate for non-closure {:?}", obligation);
        };

        debug!(?obligation, ?coroutine_def_id, ?args, "confirm_iterator_candidate");

        let gen_sig = args.as_coroutine().sig();

        let (trait_ref, _) = super::util::iterator_trait_ref_and_outputs(
            self.tcx(),
            obligation.predicate.def_id(),
            obligation.predicate.no_bound_vars().expect("iterator has no bound vars").self_ty(),
            gen_sig,
        );

        let nested = self.confirm_poly_trait_refs(obligation, ty::Binder::dummy(trait_ref))?;
        debug!(?trait_ref, ?nested, "iterator candidate obligations");

        Ok(nested)
    }

    fn confirm_async_iterator_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        // Okay to skip binder because the args on coroutine types never
        // touch bound regions, they just capture the in-scope
        // type/region parameters.
        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
        let ty::Coroutine(coroutine_def_id, args) = *self_ty.kind() else {
            bug!("closure candidate for non-closure {:?}", obligation);
        };

        debug!(?obligation, ?coroutine_def_id, ?args, "confirm_async_iterator_candidate");

        let gen_sig = args.as_coroutine().sig();

        let (trait_ref, _) = super::util::async_iterator_trait_ref_and_outputs(
            self.tcx(),
            obligation.predicate.def_id(),
            obligation.predicate.no_bound_vars().expect("iterator has no bound vars").self_ty(),
            gen_sig,
        );

        let nested = self.confirm_poly_trait_refs(obligation, ty::Binder::dummy(trait_ref))?;
        debug!(?trait_ref, ?nested, "iterator candidate obligations");

        Ok(nested)
    }

    #[instrument(skip(self), level = "debug")]
    fn confirm_closure_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        // Okay to skip binder because the args on closure types never
        // touch bound regions, they just capture the in-scope
        // type/region parameters.
        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
        let trait_ref = match *self_ty.kind() {
            ty::Closure(_, args) => {
                self.closure_trait_ref_unnormalized(obligation, args, self.tcx().consts.true_)
            }
            ty::CoroutineClosure(_, args) => {
                args.as_coroutine_closure().coroutine_closure_sig().map_bound(|sig| {
                    ty::TraitRef::new(
                        self.tcx(),
                        obligation.predicate.def_id(),
                        [self_ty, sig.tupled_inputs_ty],
                    )
                })
            }
            _ => {
                bug!("closure candidate for non-closure {:?}", obligation);
            }
        };

        self.confirm_poly_trait_refs(obligation, trait_ref)
    }

    #[instrument(skip(self), level = "debug")]
    fn confirm_async_closure_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        let tcx = self.tcx();
        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());

        let mut nested = vec![];
        let (trait_ref, kind_ty) = match *self_ty.kind() {
            ty::CoroutineClosure(_, args) => {
                let args = args.as_coroutine_closure();
                let trait_ref = args.coroutine_closure_sig().map_bound(|sig| {
                    ty::TraitRef::new(
                        self.tcx(),
                        obligation.predicate.def_id(),
                        [self_ty, sig.tupled_inputs_ty],
                    )
                });
                (trait_ref, args.kind_ty())
            }
            ty::FnDef(..) | ty::FnPtr(..) => {
                let sig = self_ty.fn_sig(tcx);
                let trait_ref = sig.map_bound(|sig| {
                    ty::TraitRef::new(
                        self.tcx(),
                        obligation.predicate.def_id(),
                        [self_ty, Ty::new_tup(tcx, sig.inputs())],
                    )
                });

                // We must additionally check that the return type impls `Future`.

                // FIXME(async_closures): Investigate this before stabilization.
                // We instantiate this binder eagerly because the `confirm_future_candidate`
                // method doesn't support higher-ranked futures, which the `AsyncFn`
                // traits expressly allow the user to write. To fix this correctly,
                // we'd need to instantiate trait bounds before we get to selection,
                // like the new trait solver does.
                let future_trait_def_id = tcx.require_lang_item(LangItem::Future, None);
                let placeholder_output_ty = self.infcx.enter_forall_and_leak_universe(sig.output());
                nested.push(obligation.with(
                    tcx,
                    ty::TraitRef::new(tcx, future_trait_def_id, [placeholder_output_ty]),
                ));

                (trait_ref, Ty::from_closure_kind(tcx, ty::ClosureKind::Fn))
            }
            ty::Closure(_, args) => {
                let args = args.as_closure();
                let sig = args.sig();
                let trait_ref = sig.map_bound(|sig| {
                    ty::TraitRef::new(
                        self.tcx(),
                        obligation.predicate.def_id(),
                        [self_ty, sig.inputs()[0]],
                    )
                });

                // We must additionally check that the return type impls `Future`.
                // See FIXME in last branch for why we instantiate the binder eagerly.
                let future_trait_def_id = tcx.require_lang_item(LangItem::Future, None);
                let placeholder_output_ty = self.infcx.enter_forall_and_leak_universe(sig.output());
                nested.push(obligation.with(
                    tcx,
                    ty::TraitRef::new(tcx, future_trait_def_id, [placeholder_output_ty]),
                ));

                (trait_ref, args.kind_ty())
            }
            _ => bug!("expected callable type for AsyncFn candidate"),
        };

        nested.extend(self.confirm_poly_trait_refs(obligation, trait_ref)?);

        let goal_kind =
            self.tcx().async_fn_trait_kind_from_def_id(obligation.predicate.def_id()).unwrap();

        // If we have not yet determiend the `ClosureKind` of the closure or coroutine-closure,
        // then additionally register an `AsyncFnKindHelper` goal which will fail if the kind
        // is constrained to an insufficient type later on.
        if let Some(closure_kind) = self.infcx.shallow_resolve(kind_ty).to_opt_closure_kind() {
            if !closure_kind.extends(goal_kind) {
                return Err(SelectionError::Unimplemented);
            }
        } else {
            nested.push(obligation.with(
                self.tcx(),
                ty::TraitRef::from_lang_item(
                    self.tcx(),
                    LangItem::AsyncFnKindHelper,
                    obligation.cause.span,
                    [kind_ty, Ty::from_closure_kind(self.tcx(), goal_kind)],
                ),
            ));
        }

        Ok(nested)
    }

    /// In the case of closure types and fn pointers,
    /// we currently treat the input type parameters on the trait as
    /// outputs. This means that when we have a match we have only
    /// considered the self type, so we have to go back and make sure
    /// to relate the argument types too. This is kind of wrong, but
    /// since we control the full set of impls, also not that wrong,
    /// and it DOES yield better error messages (since we don't report
    /// errors as if there is no applicable impl, but rather report
    /// errors are about mismatched argument types.
    ///
    /// Here is an example. Imagine we have a closure expression
    /// and we desugared it so that the type of the expression is
    /// `Closure`, and `Closure` expects `i32` as argument. Then it
    /// is "as if" the compiler generated this impl:
    /// ```ignore (illustrative)
    /// impl Fn(i32) for Closure { ... }
    /// ```
    /// Now imagine our obligation is `Closure: Fn(usize)`. So far
    /// we have matched the self type `Closure`. At this point we'll
    /// compare the `i32` to `usize` and generate an error.
    ///
    /// Note that this checking occurs *after* the impl has selected,
    /// because these output type parameters should not affect the
    /// selection of the impl. Therefore, if there is a mismatch, we
    /// report an error to the user.
    #[instrument(skip(self), level = "trace")]
    fn confirm_poly_trait_refs(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
        self_ty_trait_ref: ty::PolyTraitRef<'tcx>,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        let obligation_trait_ref =
            self.infcx.enter_forall_and_leak_universe(obligation.predicate.to_poly_trait_ref());
        let self_ty_trait_ref = self.infcx.instantiate_binder_with_fresh_vars(
            obligation.cause.span,
            HigherRankedType,
            self_ty_trait_ref,
        );
        // Normalize the obligation and expected trait refs together, because why not
        let Normalized { obligations: nested, value: (obligation_trait_ref, expected_trait_ref) } =
            ensure_sufficient_stack(|| {
                normalize_with_depth(
                    self,
                    obligation.param_env,
                    obligation.cause.clone(),
                    obligation.recursion_depth + 1,
                    (obligation_trait_ref, self_ty_trait_ref),
                )
            });

        // needed to define opaque types for tests/ui/type-alias-impl-trait/assoc-projection-ice.rs
        self.infcx
            .at(&obligation.cause, obligation.param_env)
            .eq(DefineOpaqueTypes::Yes, obligation_trait_ref, expected_trait_ref)
            .map(|InferOk { mut obligations, .. }| {
                obligations.extend(nested);
                obligations
            })
            .map_err(|terr| {
                SignatureMismatch(Box::new(SignatureMismatchData {
                    expected_trait_ref: ty::Binder::dummy(obligation_trait_ref),
                    found_trait_ref: ty::Binder::dummy(expected_trait_ref),
                    terr,
                }))
            })
    }

    fn confirm_trait_upcasting_unsize_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
        idx: usize,
    ) -> Result<ImplSource<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        let tcx = self.tcx();

        // `assemble_candidates_for_unsizing` should ensure there are no late-bound
        // regions here. See the comment there for more details.
        let predicate = obligation.predicate.no_bound_vars().unwrap();
        let a_ty = self.infcx.shallow_resolve(predicate.self_ty());
        let b_ty = self.infcx.shallow_resolve(predicate.trait_ref.args.type_at(1));

        let ty::Dynamic(a_data, a_region, ty::Dyn) = *a_ty.kind() else {
            bug!("expected `dyn` type in `confirm_trait_upcasting_unsize_candidate`")
        };
        let ty::Dynamic(b_data, b_region, ty::Dyn) = *b_ty.kind() else {
            bug!("expected `dyn` type in `confirm_trait_upcasting_unsize_candidate`")
        };

        let source_principal = a_data.principal().unwrap().with_self_ty(tcx, a_ty);
        let unnormalized_upcast_principal =
            util::supertraits(tcx, source_principal).nth(idx).unwrap();

        let nested = self
            .match_upcast_principal(
                obligation,
                unnormalized_upcast_principal,
                a_data,
                b_data,
                a_region,
                b_region,
            )?
            .expect("did not expect ambiguity during confirmation");

        let vtable_segment_callback = {
            let mut vptr_offset = 0;
            move |segment| {
                match segment {
                    VtblSegment::MetadataDSA => {
                        vptr_offset += TyCtxt::COMMON_VTABLE_ENTRIES.len();
                    }
                    VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
                        vptr_offset += count_own_vtable_entries(tcx, trait_ref);
                        if trait_ref == unnormalized_upcast_principal {
                            if emit_vptr {
                                return ControlFlow::Break(Some(vptr_offset));
                            } else {
                                return ControlFlow::Break(None);
                            }
                        }

                        if emit_vptr {
                            vptr_offset += 1;
                        }
                    }
                }
                ControlFlow::Continue(())
            }
        };

        let vtable_vptr_slot =
            prepare_vtable_segments(tcx, source_principal, vtable_segment_callback).unwrap();

        Ok(ImplSource::Builtin(BuiltinImplSource::TraitUpcasting { vtable_vptr_slot }, nested))
    }

    fn confirm_builtin_unsize_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
    ) -> Result<ImplSource<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        let tcx = self.tcx();

        // `assemble_candidates_for_unsizing` should ensure there are no late-bound
        // regions here. See the comment there for more details.
        let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
        let target = obligation.predicate.skip_binder().trait_ref.args.type_at(1);
        let target = self.infcx.shallow_resolve(target);
        debug!(?source, ?target, "confirm_builtin_unsize_candidate");

        Ok(match (source.kind(), target.kind()) {
            // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
            (&ty::Dynamic(data_a, r_a, dyn_a), &ty::Dynamic(data_b, r_b, dyn_b))
                if dyn_a == dyn_b =>
            {
                // See `assemble_candidates_for_unsizing` for more info.
                // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
                let iter = data_a
                    .principal()
                    .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
                    .into_iter()
                    .chain(
                        data_a
                            .projection_bounds()
                            .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
                    )
                    .chain(
                        data_b
                            .auto_traits()
                            .map(ty::ExistentialPredicate::AutoTrait)
                            .map(ty::Binder::dummy),
                    );
                let existential_predicates = tcx.mk_poly_existential_predicates_from_iter(iter);
                let source_trait = Ty::new_dynamic(tcx, existential_predicates, r_b, dyn_a);

                // Require that the traits involved in this upcast are **equal**;
                // only the **lifetime bound** is changed.
                let InferOk { mut obligations, .. } = self
                    .infcx
                    .at(&obligation.cause, obligation.param_env)
                    .sup(DefineOpaqueTypes::No, target, source_trait)
                    .map_err(|_| Unimplemented)?;

                // Register one obligation for 'a: 'b.
                let outlives = ty::OutlivesPredicate(r_a, r_b);
                obligations.push(Obligation::with_depth(
                    tcx,
                    obligation.cause.clone(),
                    obligation.recursion_depth + 1,
                    obligation.param_env,
                    obligation.predicate.rebind(outlives),
                ));

                ImplSource::Builtin(BuiltinImplSource::Misc, obligations)
            }

            // `T` -> `Trait`
            (_, &ty::Dynamic(data, r, ty::Dyn)) => {
                let mut object_dids = data.auto_traits().chain(data.principal_def_id());
                if let Some(did) = object_dids.find(|did| !tcx.check_is_object_safe(*did)) {
                    return Err(TraitNotObjectSafe(did));
                }

                let predicate_to_obligation = |predicate| {
                    Obligation::with_depth(
                        tcx,
                        obligation.cause.clone(),
                        obligation.recursion_depth + 1,
                        obligation.param_env,
                        predicate,
                    )
                };

                // Create obligations:
                //  - Casting `T` to `Trait`
                //  - For all the various builtin bounds attached to the object cast. (In other
                //  words, if the object type is `Foo + Send`, this would create an obligation for
                //  the `Send` check.)
                //  - Projection predicates
                let mut nested: Vec<_> = data
                    .iter()
                    .map(|predicate| predicate_to_obligation(predicate.with_self_ty(tcx, source)))
                    .collect();

                // We can only make objects from sized types.
                let tr = ty::TraitRef::from_lang_item(
                    tcx,
                    LangItem::Sized,
                    obligation.cause.span,
                    [source],
                );
                nested.push(predicate_to_obligation(tr.to_predicate(tcx)));

                // If the type is `Foo + 'a`, ensure that the type
                // being cast to `Foo + 'a` outlives `'a`:
                let outlives = ty::OutlivesPredicate(source, r);
                nested.push(predicate_to_obligation(
                    ty::Binder::dummy(ty::ClauseKind::TypeOutlives(outlives)).to_predicate(tcx),
                ));

                ImplSource::Builtin(BuiltinImplSource::Misc, nested)
            }

            // `[T; n]` -> `[T]`
            (&ty::Array(a, _), &ty::Slice(b)) => {
                let InferOk { obligations, .. } = self
                    .infcx
                    .at(&obligation.cause, obligation.param_env)
                    .eq(DefineOpaqueTypes::No, b, a)
                    .map_err(|_| Unimplemented)?;

                ImplSource::Builtin(BuiltinImplSource::Misc, obligations)
            }

            // `Struct<T>` -> `Struct<U>`
            (&ty::Adt(def, args_a), &ty::Adt(_, args_b)) => {
                let unsizing_params = tcx.unsizing_params_for_adt(def.did());
                if unsizing_params.is_empty() {
                    return Err(Unimplemented);
                }

                let tail_field = def.non_enum_variant().tail();
                let tail_field_ty = tcx.type_of(tail_field.did);

                let mut nested = vec![];

                // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
                // normalizing in the process, since `type_of` returns something directly from
                // astconv (which means it's un-normalized).
                let source_tail = normalize_with_depth_to(
                    self,
                    obligation.param_env,
                    obligation.cause.clone(),
                    obligation.recursion_depth + 1,
                    tail_field_ty.instantiate(tcx, args_a),
                    &mut nested,
                );
                let target_tail = normalize_with_depth_to(
                    self,
                    obligation.param_env,
                    obligation.cause.clone(),
                    obligation.recursion_depth + 1,
                    tail_field_ty.instantiate(tcx, args_b),
                    &mut nested,
                );

                // Check that the source struct with the target's
                // unsizing parameters is equal to the target.
                let args =
                    tcx.mk_args_from_iter(args_a.iter().enumerate().map(|(i, k)| {
                        if unsizing_params.contains(i as u32) { args_b[i] } else { k }
                    }));
                let new_struct = Ty::new_adt(tcx, def, args);
                let InferOk { obligations, .. } = self
                    .infcx
                    .at(&obligation.cause, obligation.param_env)
                    .eq(DefineOpaqueTypes::No, target, new_struct)
                    .map_err(|_| Unimplemented)?;
                nested.extend(obligations);

                // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
                let tail_unsize_obligation = obligation.with(
                    tcx,
                    ty::TraitRef::new(
                        tcx,
                        obligation.predicate.def_id(),
                        [source_tail, target_tail],
                    ),
                );
                nested.push(tail_unsize_obligation);

                ImplSource::Builtin(BuiltinImplSource::Misc, nested)
            }

            // `(.., T)` -> `(.., U)`
            (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
                assert_eq!(tys_a.len(), tys_b.len());

                // The last field of the tuple has to exist.
                let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
                let &b_last = tys_b.last().unwrap();

                // Check that the source tuple with the target's
                // last element is equal to the target.
                let new_tuple =
                    Ty::new_tup_from_iter(tcx, a_mid.iter().copied().chain(iter::once(b_last)));
                let InferOk { mut obligations, .. } = self
                    .infcx
                    .at(&obligation.cause, obligation.param_env)
                    .eq(DefineOpaqueTypes::No, target, new_tuple)
                    .map_err(|_| Unimplemented)?;

                // Add a nested `T: Unsize<U>` predicate.
                let last_unsize_obligation = obligation.with(
                    tcx,
                    ty::TraitRef::new(tcx, obligation.predicate.def_id(), [a_last, b_last]),
                );
                obligations.push(last_unsize_obligation);

                ImplSource::Builtin(BuiltinImplSource::TupleUnsizing, obligations)
            }

            _ => bug!("source: {source}, target: {target}"),
        })
    }

    fn confirm_const_destruct_candidate(
        &mut self,
        obligation: &PolyTraitObligation<'tcx>,
        impl_def_id: Option<DefId>,
    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
        let Some(host_effect_index) =
            self.tcx().generics_of(obligation.predicate.def_id()).host_effect_index
        else {
            bug!()
        };
        let host_effect_param: ty::GenericArg<'tcx> =
            obligation.predicate.skip_binder().trait_ref.args.const_at(host_effect_index).into();

        let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);

        let tcx = self.tcx();
        let self_ty = self.infcx.shallow_resolve(obligation.self_ty());

        let mut nested = vec![];
        let cause = obligation.derived_cause(BuiltinDerivedObligation);

        // If we have a custom `impl const Drop`, then
        // first check it like a regular impl candidate.
        // This is copied from confirm_impl_candidate but remaps the predicate to `~const Drop` beforehand.
        if let Some(impl_def_id) = impl_def_id {
            let mut new_obligation = obligation.clone();
            new_obligation.predicate = new_obligation.predicate.map_bound(|mut trait_pred| {
                trait_pred.trait_ref.def_id = drop_trait;
                trait_pred
            });
            let args = self.rematch_impl(impl_def_id, &new_obligation);
            debug!(?args, "impl args");

            let cause = obligation.derived_cause(|derived| {
                ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
                    derived,
                    impl_or_alias_def_id: impl_def_id,
                    impl_def_predicate_index: None,
                    span: obligation.cause.span,
                }))
            });
            let obligations = ensure_sufficient_stack(|| {
                self.vtable_impl(
                    impl_def_id,
                    args,
                    &cause,
                    new_obligation.recursion_depth + 1,
                    new_obligation.param_env,
                    obligation.predicate,
                )
            });
            nested.extend(obligations.nested);
        }

        // We want to confirm the ADT's fields if we have an ADT
        let mut stack = match *self_ty.skip_binder().kind() {
            ty::Adt(def, args) => def.all_fields().map(|f| f.ty(tcx, args)).collect(),
            _ => vec![self_ty.skip_binder()],
        };

        while let Some(nested_ty) = stack.pop() {
            match *nested_ty.kind() {
                // We know these types are trivially drop
                ty::Bool
                | ty::Char
                | ty::Int(_)
                | ty::Uint(_)
                | ty::Float(_)
                | ty::Infer(ty::IntVar(_))
                | ty::Infer(ty::FloatVar(_))
                | ty::Str
                | ty::RawPtr(_)
                | ty::Ref(..)
                | ty::FnDef(..)
                | ty::FnPtr(_)
                | ty::Never
                | ty::Foreign(_) => {}

                // `ManuallyDrop` is trivially drop
                ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}

                // These types are built-in, so we can fast-track by registering
                // nested predicates for their constituent type(s)
                ty::Array(ty, _) | ty::Slice(ty) => {
                    stack.push(ty);
                }
                ty::Tuple(tys) => {
                    stack.extend(tys.iter());
                }
                ty::Closure(_, args) => {
                    stack.push(args.as_closure().tupled_upvars_ty());
                }
                ty::Coroutine(_, args) => {
                    let coroutine = args.as_coroutine();
                    stack.extend([coroutine.tupled_upvars_ty(), coroutine.witness()]);
                }
                ty::CoroutineWitness(def_id, args) => {
                    let tcx = self.tcx();
                    stack.extend(tcx.coroutine_hidden_types(def_id).map(|bty| {
                        let ty = bty.instantiate(tcx, args);
                        debug_assert!(!ty.has_bound_regions());
                        ty
                    }))
                }

                // If we have a projection type, make sure to normalize it so we replace it
                // with a fresh infer variable
                ty::Alias(ty::Projection | ty::Inherent, ..) => {
                    let predicate = normalize_with_depth_to(
                        self,
                        obligation.param_env,
                        cause.clone(),
                        obligation.recursion_depth + 1,
                        self_ty.rebind(ty::TraitPredicate {
                            trait_ref: ty::TraitRef::from_lang_item(
                                self.tcx(),
                                LangItem::Destruct,
                                cause.span,
                                [nested_ty.into(), host_effect_param],
                            ),
                            polarity: ty::ImplPolarity::Positive,
                        }),
                        &mut nested,
                    );

                    nested.push(Obligation::with_depth(
                        tcx,
                        cause.clone(),
                        obligation.recursion_depth + 1,
                        obligation.param_env,
                        predicate,
                    ));
                }

                // If we have any other type (e.g. an ADT), just register a nested obligation
                // since it's either not `const Drop` (and we raise an error during selection),
                // or it's an ADT (and we need to check for a custom impl during selection)
                _ => {
                    let predicate = self_ty.rebind(ty::TraitPredicate {
                        trait_ref: ty::TraitRef::from_lang_item(
                            self.tcx(),
                            LangItem::Destruct,
                            cause.span,
                            [nested_ty.into(), host_effect_param],
                        ),
                        polarity: ty::ImplPolarity::Positive,
                    });

                    nested.push(Obligation::with_depth(
                        tcx,
                        cause.clone(),
                        obligation.recursion_depth + 1,
                        obligation.param_env,
                        predicate,
                    ));
                }
            }
        }

        Ok(nested)
    }
}