rustc_hir_analysis/

delegation.rs

//! Support inheriting generic parameters and predicates for function delegation.
//!
//! For more information about delegation design, see the tracking issue #118212.

use std::assert_matches;

use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_hir::{DelegationSelfTyPropagationKind, PathSegment};
use rustc_middle::ty::{
    self, ConstKind, EarlyBinder, GenericArg, GenericArgKind, RegionKind, Ty, TyCtxt, TypeFoldable,
    TypeFolder, TypeSuperFoldable, TypeVisitableExt, Upcast,
};
use rustc_span::{ErrorGuaranteed, Span, kw};

use crate::collect::ItemCtxt;
use crate::hir_ty_lowering::HirTyLowerer;

type RemapTable = FxHashMap<u32, u32>;

struct ParamIndexRemapper<'tcx> {
    tcx: TyCtxt<'tcx>,
    remap_table: RemapTable,
    delegation_parent_consts: FxHashSet<ty::ParamConst>,
    arg_kinds: Vec<ty::GenericArgKind<'tcx>>,
}

impl<'tcx> ParamIndexRemapper<'tcx> {
    fn try_remap(
        &self,
        source_index: u32,
        expected_kind: ty::GenericArgKind<'tcx>,
    ) -> Result<u32, ()> {
        self.remap_table
            .get(&source_index)
            .copied()
            .filter(|&idx| {
                self.arg_kinds.get(idx as usize).is_some_and(|kind| {
                    std::mem::discriminant(&expected_kind) == std::mem::discriminant(kind)
                })
            })
            .ok_or(())
    }
}

impl<'tcx> ty::FallibleTypeFolder<TyCtxt<'tcx>> for ParamIndexRemapper<'tcx> {
    type Error = ();

    fn cx(&self) -> TyCtxt<'tcx> {
        self.tcx
    }

    fn try_fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
        if !ty.has_param() {
            return Ok(ty);
        }

        if let ty::Param(param) = ty.kind() {
            let index = self.try_remap(param.index, ty::GenericArgKind::Type(ty))?;
            return Ok(Ty::new_param(self.tcx, index, param.name));
        }

        ty.try_super_fold_with(self)
    }

    fn try_fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, Self::Error> {
        if let ty::ReEarlyParam(param) = r.kind() {
            let index = self.try_remap(param.index, ty::GenericArgKind::Lifetime(r))?;
            return Ok(ty::Region::new_early_param(
                self.tcx,
                ty::EarlyParamRegion { index, name: param.name },
            ));
        }
        Ok(r)
    }

    fn try_fold_const(&mut self, ct: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Self::Error> {
        if let ty::ConstKind::Param(param) = ct.kind() {
            let idx = self.try_remap(param.index, ty::GenericArgKind::Const(ct))?;
            let param = ty::ParamConst::new(idx, param.name);
            return Ok(ty::Const::new_param(self.tcx, param));
        }
        ct.try_super_fold_with(self)
    }
}

#[derive(Debug)]
enum SelfPositionKind {
    AfterLifetimes(Option<DelegationSelfTyPropagationKind>),
    Zero,
    None,
}

fn create_self_param_position_kind(
    tcx: TyCtxt<'_>,
    def_id: LocalDefId,
    sig_id: DefId,
) -> SelfPositionKind {
    match fn_kinds(tcx, def_id, sig_id) {
        (FnKind::Free, FnKind::AssocTrait) => {
            let kind = tcx.hir_delegation_info(def_id).self_ty_propagation_kind;
            SelfPositionKind::AfterLifetimes(kind)
        }

        (_, FnKind::AssocTraitImpl) => unreachable!(),

        (_, FnKind::AssocTrait) | (FnKind::AssocTrait, _) => SelfPositionKind::Zero,

        (FnKind::AssocTraitImpl, _) => unreachable!(),

        _ => SelfPositionKind::None,
    }
}

#[derive(Clone, Copy, Debug, PartialEq)]
enum FnKind {
    Free,
    AssocInherentImpl,
    AssocTrait,
    AssocTraitImpl,
}

fn fn_kind<'tcx>(tcx: TyCtxt<'tcx>, def_id: impl Into<DefId>) -> FnKind {
    let def_id = def_id.into();

    match tcx.def_kind(def_id) {
        DefKind::Fn => FnKind::Free,
        DefKind::AssocFn => match tcx.def_kind(tcx.parent(def_id)) {
            DefKind::Trait => FnKind::AssocTrait,
            DefKind::Impl { of_trait } => match of_trait {
                true => FnKind::AssocTraitImpl,
                false => FnKind::AssocInherentImpl,
            },
            _ => unreachable!("associated function can only be in trait or impl"),
        },
        _ => unreachable!("delegation/signature can be either free or associated function"),
    }
}

fn fn_kinds(tcx: TyCtxt<'_>, def_id: LocalDefId, sig_id: DefId) -> (FnKind, FnKind) {
    let kinds = (fn_kind(tcx, def_id), fn_kind(tcx, sig_id));

    // For trait impl's `sig_id` is always equal to the corresponding trait method.
    assert!(!matches!(kinds, (_, FnKind::AssocTraitImpl)));

    kinds
}

/// Given the current context(caller and callee `FnKind`), it specifies
/// the policy of predicates and generic parameters inheritance.
#[derive(Clone, Copy, Debug, PartialEq)]
enum InheritanceKind {
    /// Copying all predicates and parameters, including those of the parent
    /// container.
    ///
    /// Boolean value defines whether the `Self` parameter or `Self: Trait`
    /// predicate are copied. It's always equal to `false` except when
    /// delegating from a free function to a trait method.
    ///
    /// FIXME(fn_delegation): This often leads to type inference
    /// errors. Support providing generic arguments or restrict use sites.
    WithParent(bool),
    /// The trait implementation should be compatible with the original trait.
    /// Therefore, for trait implementations only the method's own parameters
    /// and predicates are copied.
    Own,
}

/// Maps sig generics into generic args of delegation. Delegation generics has the following pattern:
///
/// [SELF | maybe self in the beginning]
/// [PARENT | args of delegation parent]
/// [SIG PARENT LIFETIMES]
/// [SIG LIFETIMES]
/// [SELF | maybe self after lifetimes, when we reuse trait fn in free context]
/// [SIG PARENT TYPES/CONSTS]
/// [SIG TYPES/CONSTS]
fn create_mapping<'tcx>(
    tcx: TyCtxt<'tcx>,
    sig_id: DefId,
    def_id: LocalDefId,
) -> FxHashMap<u32, u32> {
    let mut mapping: FxHashMap<u32, u32> = Default::default();

    let self_pos_kind = create_self_param_position_kind(tcx, def_id, sig_id);
    let is_self_at_zero = matches!(self_pos_kind, SelfPositionKind::Zero);

    // Is self at zero? If so insert mapping, self in sig parent is always at 0.
    if is_self_at_zero {
        mapping.insert(0, 0);
    }

    let mut args_index = 0;

    args_index += is_self_at_zero as usize;
    args_index += get_delegation_parent_args_count_without_self(tcx, def_id, sig_id);

    let parent_kind = fn_kind(tcx, sig_id);
    let process_parent = matches!(parent_kind, FnKind::AssocTrait | FnKind::AssocInherentImpl);
    let parent_generics = process_parent.then(|| tcx.generics_of(tcx.parent(sig_id)));

    // In case of delegations to inherent impls indices of generic params which are passed
    // to ADT can be random numbers not from range 0..parent_params_count, so we need to
    // use original indices in mapping:
    // impl<'a, 'b, 'c, A: 'a, const C: usize> S<'a, A, C> {
    //      fn foo_static<'d: 'd, 'e, T, const B: bool>() {}
    //      fn foo_self<'d: 'd, 'e, T, const B: bool>(self) {}
    // },
    // 'a has index 0, A index 3, C index 4. If we encounter not a generic param as generic arg,
    // then we do not need to map it (i.e. consts like `1`, `2`, `3`; `'static`, etc.).
    let parent_params = match parent_kind {
        FnKind::AssocInherentImpl => {
            let ty::Adt(_, args) = tcx.type_of(tcx.parent(sig_id)).skip_binder().kind() else {
                unreachable!("parent of inherent function in delegation can be only struct or enum")
            };

            let opt_param_info = |arg: GenericArg<'_>| match arg.kind() {
                GenericArgKind::Lifetime(r) => (
                    match r.kind() {
                        RegionKind::ReEarlyParam(p) => Some(p.index),
                        _ => None,
                    },
                    false,
                ),
                GenericArgKind::Type(t) => (
                    match t.kind() {
                        ty::Param(p) => Some(p.index),
                        _ => None,
                    },
                    true,
                ),
                GenericArgKind::Const(c) => (
                    match c.kind() {
                        ConstKind::Param(p) => Some(p.index),
                        _ => None,
                    },
                    true,
                ),
            };

            args.iter().map(opt_param_info).collect::<Vec<_>>()
        }
        FnKind::AssocTrait => parent_generics
            .expect("trait must have generics")
            .own_params
            .iter()
            .map(|p| (Some(p.index as u32), p.kind.is_ty_or_const()))
            .collect::<Vec<_>>(),
        _ => vec![],
    };

    let has_self = match parent_kind {
        FnKind::AssocTrait => parent_generics.expect("trait must have generics").has_self,
        _ => false,
    };

    if process_parent {
        for i in (has_self as usize)..parent_params.len() {
            let (index, is_ty_or_const) = parent_params[i];
            if !is_ty_or_const {
                if let Some(index) = index {
                    mapping.insert(index, args_index as u32);
                }

                args_index += 1;
            }
        }
    }

    let child_generics = tcx.generics_of(sig_id);
    for param in &child_generics.own_params {
        if !param.kind.is_ty_or_const() {
            mapping.insert(param.index, args_index as u32);
            args_index += 1;
        }
    }

    // If self after lifetimes insert mapping, relying that self is at 0 in sig parent.
    // If self ty is propagated (meaning there is no generic param `Self`), the specified
    // self ty will be inserted in args in `create_generic_args`.
    if matches!(self_pos_kind, SelfPositionKind::AfterLifetimes { .. }) {
        mapping.insert(0, args_index as u32);
        args_index += 1;
    }

    if process_parent {
        for i in (has_self as usize)..parent_params.len() {
            let (index, is_ty_or_const) = parent_params[i];
            if is_ty_or_const {
                if let Some(index) = index {
                    mapping.insert(index, args_index as u32);
                }

                args_index += 1;
            }
        }
    }

    for param in &child_generics.own_params {
        if param.kind.is_ty_or_const() {
            mapping.insert(param.index, args_index as u32);
            args_index += 1;
        }
    }

    mapping
}

fn get_delegation_parent_args_count_without_self<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: LocalDefId,
    sig_id: DefId,
) -> usize {
    let kinds @ (def_kind, _) = fn_kinds(tcx, def_id, sig_id);

    match kinds {
        (FnKind::AssocTraitImpl, FnKind::AssocTrait) => 0,

        (FnKind::AssocTraitImpl, _) | (_, FnKind::AssocTraitImpl) => unreachable!(),

        (FnKind::Free, _) => 0,

        (_, _) => {
            let delegation_parent_args_count = tcx.generics_of(def_id).parent_count;
            let has_self = def_kind == FnKind::AssocTrait;

            delegation_parent_args_count - usize::from(has_self)
        }
    }
}

fn get_parent_and_inheritance_kind<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: LocalDefId,
    sig_id: DefId,
) -> (Option<DefId>, InheritanceKind) {
    let kinds @ (_, sig_kind) = fn_kinds(tcx, def_id, sig_id);

    match kinds {
        (FnKind::AssocTraitImpl, FnKind::AssocTrait) => {
            (Some(tcx.parent(def_id.to_def_id())), InheritanceKind::Own)
        }

        (FnKind::AssocTraitImpl, _) | (_, FnKind::AssocTraitImpl) => unreachable!(),

        (FnKind::Free, _) => {
            let copy_self_clauses = sig_kind == FnKind::AssocTrait;
            (None, InheritanceKind::WithParent(copy_self_clauses))
        }

        (_, _) => (Some(tcx.parent(def_id.to_def_id())), InheritanceKind::WithParent(false)),
    }
}

fn get_delegation_self_ty<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Option<Ty<'tcx>> {
    let sig_id = tcx.hir_opt_delegation_sig_id(def_id).expect("processing delegation");
    let (caller_kind, callee_kind) = fn_kinds(tcx, def_id, sig_id);

    match (caller_kind, callee_kind) {
        (FnKind::AssocTraitImpl, FnKind::AssocTrait) | (FnKind::AssocInherentImpl, _) => {
            Some(tcx.type_of(tcx.local_parent(def_id)).instantiate_identity().skip_norm_wip())
        }

        // For trait impl's `sig_id` is always equal to the corresponding trait method.
        (FnKind::AssocTraitImpl, _) | (_, FnKind::AssocTraitImpl) => unreachable!(),

        (_, _) => match create_self_param_position_kind(tcx, def_id, sig_id) {
            SelfPositionKind::None => None,
            SelfPositionKind::AfterLifetimes(propagation_kind) => Some(match propagation_kind {
                Some(kind) => match kind {
                    DelegationSelfTyPropagationKind::SelfTy(self_ty_id) => {
                        let ctx = ItemCtxt::new(tcx, def_id);
                        ctx.lower_ty(tcx.hir_node(self_ty_id).expect_ty())
                    }
                    DelegationSelfTyPropagationKind::SelfParam => {
                        let index = tcx.generics_of(def_id).own_counts().lifetimes;
                        Ty::new_param(tcx, index as u32, kw::SelfUpper)
                    }
                },
                None => Ty::new_error_with_message(
                    tcx,
                    tcx.def_span(def_id),
                    "self propagation kind must be specified for `AfterLifetimes` variant",
                ),
            }),
            SelfPositionKind::Zero => Some(Ty::new_param(tcx, 0, kw::SelfUpper)),
        },
    }
}

/// Creates generic arguments for further delegation signature and predicates instantiation.
/// Arguments can be user-specified (in this case they are in `parent_args` and `child_args`)
/// or propagated. User can specify either both `parent_args` and `child_args`, one of them or none,
/// that is why we firstly create generic arguments from generic params and then adjust them with
/// user-specified args.
///
/// The order of produced list is important, it must be of this pattern:
///
/// [SELF | maybe self in the beginning]
/// [PARENT | args of delegation parent]
/// [SIG PARENT LIFETIMES] <- `lifetimes_end_pos`
/// [SIG LIFETIMES]
/// [SELF | maybe self after lifetimes, when we reuse trait fn in free context]
/// [SIG PARENT TYPES/CONSTS]
/// [SIG TYPES/CONSTS]
fn create_generic_args<'tcx>(
    tcx: TyCtxt<'tcx>,
    sig_id: DefId,
    def_id: LocalDefId,
    mut parent_args: ty::GenericArgs<'tcx>,
    mut child_args: ty::GenericArgs<'tcx>,
) -> (Vec<ty::GenericArg<'tcx>>, ty::GenericArgs<'tcx>) {
    let delegation_generics = tcx.generics_of(def_id);
    let delegation_args = ty::GenericArgs::identity_for_item(tcx, def_id);

    let real_args_count = delegation_args.len() - delegation_generics.own_synthetic_params_count();
    let synth_args = &delegation_args.as_slice()[real_args_count..];

    let mut delegation_parent_args = tcx.mk_args(
        &delegation_args.as_slice()
            [delegation_generics.has_self as usize..delegation_generics.parent_count],
    );

    let delegation_args = &delegation_args.as_slice()[delegation_generics.parent_count..];

    let kinds @ (_, parent_kind) = fn_kinds(tcx, def_id, sig_id);
    if matches!(kinds, (FnKind::AssocTraitImpl, FnKind::AssocTrait)) {
        // Special case, as user specifies Trait args in trait impl header, we want to treat
        // them as parent args. We always generate a function whose generics match
        // child generics in trait.
        let parent = tcx.local_parent(def_id);

        parent_args =
            tcx.impl_trait_header(parent).trait_ref.instantiate_identity().skip_norm_wip().args;

        child_args = tcx.mk_args(
            &delegation_args[delegation_args.len() - delegation_generics.own_params.len()..],
        );

        delegation_parent_args = ty::GenericArgs::empty();
    }

    let self_type = get_delegation_self_ty(tcx, def_id).map(ty::GenericArg::from);

    if self_type.is_some() && !parent_args.is_empty() {
        parent_args = match parent_kind {
            FnKind::AssocInherentImpl => parent_args,
            // Remove `Self` from parent args (it is always at the `0th` index) as it is
            // added manually.
            FnKind::AssocTrait => tcx.mk_args(&parent_args.as_slice()[1..]),
            _ => unreachable!("if parent args are non-empty then the parent must exist"),
        }
    }

    let (zero_self, after_lifetimes_self) =
        match create_self_param_position_kind(tcx, def_id, sig_id) {
            SelfPositionKind::AfterLifetimes(_) => {
                assert!(self_type.is_some());
                (None, self_type)
            }
            SelfPositionKind::Zero => {
                assert!(self_type.is_some());
                (self_type, None)
            }
            SelfPositionKind::None => (None, None),
        };

    let zero_self = zero_self.as_ref().into_iter();
    let after_lifetimes_self = after_lifetimes_self.as_ref().into_iter();

    let args = zero_self
        .copied()
        .chain(delegation_parent_args.iter())
        .chain(parent_args.iter().filter(|a| a.as_region().is_some()))
        .chain(child_args.iter().filter(|a| a.as_region().is_some()))
        .chain(after_lifetimes_self.copied())
        .chain(parent_args.iter().filter(|a| a.as_region().is_none()))
        .chain(child_args.iter().filter(|a| a.as_region().is_none()))
        .chain(synth_args.iter().copied())
        .collect::<Vec<_>>();

    (args, delegation_parent_args)
}

pub(crate) fn inherit_clauses_for_delegation_item<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: LocalDefId,
    sig_id: DefId,
) -> ty::GenericClauses<'tcx> {
    struct ClausesCollector<'tcx> {
        tcx: TyCtxt<'tcx>,
        clauses: Vec<(ty::Clause<'tcx>, Span)>,
        args: Vec<ty::GenericArg<'tcx>>,
        folder: ParamIndexRemapper<'tcx>,
        filter_self_clauses: bool,
    }

    impl<'tcx> ClausesCollector<'tcx> {
        fn with_own_clauses(
            mut self,
            f: impl Fn(DefId) -> ty::GenericClauses<'tcx>,
            def_id: DefId,
        ) -> Result<Self, ()> {
            let clauses = f(def_id);
            let args = self.args.as_slice();

            for clause in clauses.clauses {
                // If self ty is specified then there will be no generic param `Self`,
                // so we do not need its clauses.
                if self.filter_self_clauses
                    && let Some(trait_clause) = clause.0.as_trait_clause()
                    // Rely that `Self` has zero index.
                    && trait_clause.self_ty().skip_binder().is_param(0)
                {
                    continue;
                }

                // If we have a constant in parent or child args that came from delegation
                // parent:
                // ```rust
                // trait Trait<T, const B: bool> { /* .. */}
                // impl<const N: usize> S<N> {
                //     reuse Trait::<S<N>, N>::foo;
                // }
                // ```
                // Then if we inherit const clause from `Trait` then we end up with
                // two `ConstArgHasType` for `N` constant:
                // 1) ConstArgHasType(N/#0, bool) from `Trait`
                // 2) ConstArgHasType(N/#0, usize) from delegation parent
                // So in case the constant came from delegation parent we will not inherit
                // ConstArgHasType from signature.
                // The check is so complicated because we build generic args for signature
                // and clauses inheritance, for the example above it will be
                // `args = [S<N/#0>, N/#0, S<N/#0>, N/#0]`, where
                // args[0] - Self type, args[1] - delegation parent const, args[2] - first
                // arg of callee path, args[3] - second arg of callee path.
                // When processing clause ConstArgHasType(B/#2, bool)
                // from delegation signature (`Trait::foo`), we need to map `B/#2` into some
                // arg from `args`. The mapping which is built by `create_mapping` function is:
                // `{0: 0, 2: 3, 1: 2}`, so as `B/#2` has index `2` it is mapped into third
                // arg from `args` - `N/#0`. After we obtained mapped const param, we check if
                // it came from delegation parent, and if so we do not process its `ConstArgHasType`
                // clause.
                // (Issue #158675).
                let new_clause = clause.0.try_fold_with(&mut self.folder)?;
                if let ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, _)) =
                    new_clause.as_predicate().kind().skip_binder()
                {
                    let unnorm_const = EarlyBinder::bind(self.tcx, ct).instantiate(self.tcx, args);
                    if let ty::ConstKind::Param(param) = unnorm_const.skip_norm_wip().kind()
                        && self.folder.delegation_parent_consts.contains(&param)
                    {
                        continue;
                    }
                }

                self.clauses.push((
                    EarlyBinder::bind(self.tcx, new_clause)
                        .instantiate(self.tcx, args)
                        .skip_norm_wip(),
                    clause.1,
                ));
            }

            Ok(self)
        }

        fn with_clauses(
            mut self,
            f: impl Fn(DefId) -> ty::GenericClauses<'tcx> + Copy,
            def_id: DefId,
        ) -> Result<Self, ()> {
            let preds = f(def_id);
            if let Some(parent_def_id) = preds.parent {
                self = self.with_own_clauses(f, parent_def_id)?;
            }

            self.with_own_clauses(f, def_id)
        }
    }

    let (parent_args, child_args) = tcx.delegation_user_specified_args(def_id);
    let (folder, args) = create_folder_and_args(tcx, def_id, sig_id, parent_args, child_args);
    let self_pos_kind = create_self_param_position_kind(tcx, def_id, sig_id);
    let filter_self_clauses = matches!(
        self_pos_kind,
        SelfPositionKind::AfterLifetimes(Some(DelegationSelfTyPropagationKind::SelfTy(..)))
    );

    let collector = ClausesCollector { tcx, clauses: vec![], args, folder, filter_self_clauses };
    let (parent, inh_kind) = get_parent_and_inheritance_kind(tcx, def_id, sig_id);

    // `explicit_clauses_of` is used here to avoid copying `Self: Trait` clause.
    // Note: `clauses_of` query can also add inferred outlives clauses, but that
    // is not the case here as `sig_id` is either a trait or a function.
    let collector = match inh_kind {
        InheritanceKind::WithParent(false) => {
            collector.with_clauses(|def_id| tcx.explicit_clauses_of(def_id), sig_id)
        }
        InheritanceKind::WithParent(true) => {
            collector.with_clauses(|def_id| tcx.clauses_of(def_id), sig_id)
        }
        InheritanceKind::Own => collector.with_own_clauses(|def_id| tcx.clauses_of(def_id), sig_id),
    };

    let clauses = match collector {
        Ok(collector) => collector.clauses,
        Err(()) => {
            tcx.dcx().emit_err(crate::diagnostics::DelegationInheritanceFailed {
                span: tcx.def_span(def_id),
                kind: "predicates",
                callee_span: tcx.def_span(sig_id),
            });

            // Even on error, the delegated item's const parameters need their
            // types in the parameter environment.
            tcx.generics_of(def_id)
                .own_params
                .iter()
                .filter_map(|param| {
                    if !matches!(param.kind, ty::GenericParamDefKind::Const { .. }) {
                        return None;
                    }

                    let ct =
                        ty::Const::new_param(tcx, ty::ParamConst::new(param.index, param.name));
                    let ct_ty = tcx.type_of(param.def_id).instantiate_identity().skip_norm_wip();

                    Some((
                        ty::ClauseKind::ConstArgHasType(ct, ct_ty).upcast(tcx),
                        tcx.def_span(param.def_id),
                    ))
                })
                .collect::<Vec<_>>()
        }
    };

    ty::GenericClauses { parent, clauses: tcx.arena.alloc_from_iter(clauses) }
}

fn create_folder_and_args<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: LocalDefId,
    sig_id: DefId,
    parent_args: ty::GenericArgs<'tcx>,
    child_args: ty::GenericArgs<'tcx>,
) -> (ParamIndexRemapper<'tcx>, Vec<ty::GenericArg<'tcx>>) {
    let (args, delegation_parent_args) =
        create_generic_args(tcx, sig_id, def_id, parent_args, child_args);

    let remap_table = create_mapping(tcx, sig_id, def_id);

    let delegation_parent_consts = delegation_parent_args
        .iter()
        .filter_map(|a| {
            a.as_const().and_then(|c| {
                if let ty::ConstKind::Param(param) = c.kind() { Some(param) } else { None }
            })
        })
        .collect();

    let arg_kinds = args.iter().map(|arg| arg.kind()).collect();
    (ParamIndexRemapper { tcx, remap_table, delegation_parent_consts, arg_kinds }, args)
}

fn check_constraints<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: LocalDefId,
    sig_id: DefId,
) -> Result<(), ErrorGuaranteed> {
    let mut ret = Ok(());

    let mut emit = |descr| {
        ret = Err(tcx.dcx().emit_err(crate::diagnostics::UnsupportedDelegation {
            span: tcx.def_span(def_id),
            descr,
            callee_span: tcx.def_span(sig_id),
        }));
    };

    if tcx.fn_sig(sig_id).skip_binder().skip_binder().c_variadic() {
        // See issue #127443 for explanation.
        emit("delegation to C-variadic functions is not allowed");
    }

    ret
}

fn error_sig_for_delegation<'tcx>(
    tcx: TyCtxt<'tcx>,
    sig_id: DefId,
    err: ErrorGuaranteed,
) -> &'tcx [Ty<'tcx>] {
    let sig_len = tcx.fn_sig(sig_id).instantiate_identity().skip_binder().inputs().len() + 1;
    let err_type = Ty::new_error(tcx, err);
    tcx.arena.alloc_from_iter((0..sig_len).map(|_| err_type))
}

pub(crate) fn inherit_sig_for_delegation_item<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: LocalDefId,
) -> &'tcx [Ty<'tcx>] {
    let sig_id = tcx.hir_opt_delegation_sig_id(def_id).expect("Delegation must have sig_id");
    let caller_sig = tcx.fn_sig(sig_id);
    if let Err(err) = check_constraints(tcx, def_id, sig_id) {
        return error_sig_for_delegation(tcx, sig_id, err);
    }

    let (parent_args, child_args) = tcx.delegation_user_specified_args(def_id);
    let (mut folder, args) = create_folder_and_args(tcx, def_id, sig_id, parent_args, child_args);
    let folded_sig = match caller_sig.skip_binder().try_fold_with(&mut folder) {
        Ok(sig) => sig,
        Err(()) => {
            let err = tcx.dcx().emit_err(crate::diagnostics::DelegationInheritanceFailed {
                span: tcx.def_span(def_id),
                kind: "signature",
                callee_span: tcx.def_span(sig_id),
            });
            return error_sig_for_delegation(tcx, sig_id, err);
        }
    };
    let caller_sig = EarlyBinder::bind(tcx, folded_sig);

    let sig = caller_sig.instantiate(tcx, args.as_slice()).skip_binder();
    let output = std::iter::once(sig.output());
    let mut sig = sig.inputs().iter().cloned().chain(output).collect::<Vec<_>>();

    adjust_sig_in_inherent_impl_cases(tcx, sig_id, def_id, parent_args, &mut sig);

    tcx.arena.alloc_from_iter(sig)
}

/// We need to replace `Self` type of the signature function parent with
/// either type of parent of delegation (which is either `Self` param in case of trait)
/// and other ADT in case of inherent impl. We do the same thing when delegating to trait,
/// in this case replacement happens during signature instantiation (as we can replace `Self`
/// generic param with other type from `args` when instantiating).
fn adjust_sig_in_inherent_impl_cases<'tcx>(
    tcx: TyCtxt<'tcx>,
    sig_id: DefId,
    def_id: LocalDefId,
    parent_args: ty::GenericArgs<'tcx>,
    sig: &mut [Ty<'tcx>],
) {
    if !tcx.is_method(sig_id) {
        return;
    }

    let kinds @ (def_kind, _) = fn_kinds(tcx, def_id, sig_id);
    if def_kind == FnKind::Free || !matches!(kinds, (_, FnKind::AssocInherentImpl)) {
        return;
    }

    let ty::Adt(def, _) = tcx.type_of(tcx.parent(sig_id)).skip_binder().kind() else {
        unreachable!("delegation is supported only to struct or enums")
    };

    for i in 0..sig.len() {
        let to_replace = Ty::new_adt(tcx, *def, parent_args);
        let replacement = match def_kind {
            FnKind::Free => unreachable!(),

            FnKind::AssocTrait => Ty::new_param(tcx, 0, kw::SelfUpper),
            _ => tcx.type_of(tcx.parent(def_id.to_def_id())).instantiate_identity().skip_norm_wip(),
        };

        struct Replacer<'tcx> {
            tcx: TyCtxt<'tcx>,
            to_replace: Ty<'tcx>,
            replacement: Ty<'tcx>,
        }

        impl<'tcx> TypeFolder<TyCtxt<'tcx>> for Replacer<'tcx> {
            fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
                if t == self.to_replace { self.replacement } else { t.super_fold_with(self) }
            }

            fn cx(&self) -> TyCtxt<'tcx> {
                self.tcx
            }
        }

        sig[i] = sig[i].fold_with(&mut Replacer { tcx, to_replace, replacement })
    }
}

// Creates user-specified generic arguments from delegation path,
// they will be used during delegation signature and predicates inheritance.
// Example: reuse Trait::<'static, i32, 1>::foo::<A, B>
// we want to extract [Self, 'static, i32, 1] for parent and [A, B] for child.
pub(crate) fn delegation_user_specified_args<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: LocalDefId,
) -> (ty::GenericArgs<'tcx>, ty::GenericArgs<'tcx>) {
    let info = tcx.hir_delegation_info(def_id);

    let get_segment = |hir_id| -> Option<(&'tcx PathSegment<'tcx>, DefId)> {
        let segment = tcx.hir_node(hir_id).expect_path_segment();
        segment.res.opt_def_id().map(|def_id| (segment, def_id))
    };

    let ctx = ItemCtxt::new_for_delegation(tcx, def_id);
    let lowerer = ctx.lowerer();

    let parent_args = info
        .parent_seg_id_for_sig
        .and_then(get_segment)
        .filter(|(_, def_id)| !matches!(tcx.def_kind(*def_id), DefKind::Mod))
        .map(|(segment, def_id)| {
            // After lowering parent segment can be resolved only to those variants (and `DefKind::Mod`),
            // which we do not process here.
            assert_matches!(tcx.def_kind(def_id), DefKind::Trait | DefKind::Struct | DefKind::Enum);

            let self_ty = (tcx.def_kind(def_id) == DefKind::Trait)
                .then(|| Ty::new_param(tcx, 0, kw::SelfUpper));

            tcx.mk_args(
                lowerer
                    .lower_generic_args_of_path(segment.ident.span, def_id, &[], segment, self_ty)
                    .0
                    .as_slice(),
            )
        });

    let child_args = info.child_seg_id_for_sig.and_then(get_segment).map(|(segment, def_id)| {
        assert_matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn);
        let parent = tcx.parent(def_id);

        let parent_args =
            if matches!(tcx.def_kind(parent), DefKind::Impl { of_trait: false } | DefKind::Trait) {
                ty::GenericArgs::identity_for_item(tcx, parent)
            } else if let Some(parent_args) = parent_args {
                parent_args
            } else {
                ty::GenericArgs::empty()
            };

        let args = lowerer
            .lower_generic_args_of_path(
                segment.ident.span,
                def_id,
                parent_args.as_slice(),
                segment,
                None,
            )
            .0;

        let synth_params_count = tcx.generics_of(def_id).own_synthetic_params_count();
        tcx.mk_args(&args.as_slice()[parent_args.len()..args.len() - synth_params_count])
    });

    (parent_args.unwrap_or_default(), child_args.unwrap_or_default())
}