compiler/rustc_ast_lowering/src/delegation/mod.rs - rust-lang/rust - Git at Google

 //! This module implements expansion of delegation items with early resolved paths.
 //! It includes a delegation to a free functions:
 //!
 //! ```ignore (illustrative)
 //! reuse module::name { target_expr_template }
 //! ```
 //!
 //! And delegation to a trait methods:
 //!
 //! ```ignore (illustrative)
 //! reuse <Type as Trait>::name { target_expr_template }
 //! ```
 //!
 //! After expansion for both cases we get:
 //!
 //! ```ignore (illustrative)
 //! fn name(
 //!     arg0: InferDelegation(sig_id, Input(0)),
 //!     arg1: InferDelegation(sig_id, Input(1)),
 //!     ...,
 //!     argN: InferDelegation(sig_id, Input(N)),
 //! ) -> InferDelegation(sig_id, Output) {
 //!     callee_path(target_expr_template(arg0), arg1, ..., argN)
 //! }
 //! ```
 //!
 //! Where `callee_path` is a path in delegation item e.g. `<Type as Trait>::name`.
 //! `sig_id` is a id of item from which the signature is inherited. It may be a delegation
 //! item id (`item_id`) in case of impl trait or path resolution id (`path_id`) otherwise.
 //!
 //! Since we do not have a proper way to obtain function type information by path resolution
 //! in AST, we mark each function parameter type as `InferDelegation` and inherit it during
 //! HIR ty lowering.
 //!
 //! Similarly generics, predicates and header are set to the "default" values.
 //! In case of discrepancy with callee function the `UnsupportedDelegation` error will
 //! also be emitted during HIR ty lowering.

 use std::iter;

 use ast::visit::Visitor;
 use generics::GenericsGenerationResult;
 use hir::HirId;
 use hir::def::Res;
 use rustc_abi::ExternAbi;
 use rustc_ast as ast;
 use rustc_ast::*;
 use rustc_hir::def::DefKind;
 use rustc_hir::{self as hir, FnDeclFlags};
 use rustc_middle::ty::Asyncness;
 use rustc_span::def_id::DefId;
 use rustc_span::symbol::kw;
 use rustc_span::{Ident, Span, Symbol, sym};

 use crate::delegation::generics::{GenericsGenerationResults, GenericsPosition};
 use crate::delegation::resolution::resolver::DelegationResolver;
 use crate::delegation::resolution::{DelegationResolution, ParamInfo};
 use crate::{
     AllowReturnTypeNotation, ImplTraitContext, ImplTraitPosition, LoweringContext, ParamMode,
 };

 mod attributes;
 mod generics;
 mod resolution;

 pub(crate) struct DelegationResults<'hir> {
     pub body_id: hir::BodyId,
     pub sig: hir::FnSig<'hir>,
     pub ident: Ident,
     pub generics: &'hir hir::Generics<'hir>,
 }

 impl<'hir> LoweringContext<'_, 'hir> {
     pub(crate) fn lower_delegation(&mut self, delegation: &Delegation) -> DelegationResults<'hir> {
         let span = self.lower_span(delegation.last_segment_span());

         let resolver = DelegationResolver::new(self);
         let Ok((res, mut generics)) = resolver.resolve_delegation(delegation, span) else {
             return self.generate_delegation_error(span, delegation);
         };

         self.add_attrs_if_needed(&res);

         let (body_id, call_expr_id, unused_target_expr) =
             self.lower_delegation_body(delegation, &res, &mut generics);

         let decl = self.lower_delegation_decl(&res, &generics, call_expr_id, unused_target_expr);

         let sig = self.lower_delegation_sig(res.sig_id, decl, span);

         let ident = self.lower_ident(delegation.ident);

         let generics = self.arena.alloc(hir::Generics {
             has_where_clause_predicates: false,
             params: self.arena.alloc_from_iter(generics.all_params()),
             predicates: self.arena.alloc_from_iter(generics.all_predicates()),
             span,
             where_clause_span: span,
         });

         DelegationResults { body_id, sig, ident, generics }
     }

     fn lower_delegation_decl(
         &mut self,
         res: &DelegationResolution,
         generics: &GenericsGenerationResults<'hir>,
         call_expr_id: HirId,
         unused_target_expr: bool,
     ) -> &'hir hir::FnDecl<'hir> {
         let &DelegationResolution { source, call_path_res, span, sig_id, .. } = res;
         let ParamInfo { param_count, c_variadic, splatted } = res.param_info;

         // The last parameter in C variadic functions is skipped in the signature,
         // like during regular lowering.
         let decl_param_count = param_count - c_variadic as usize;
         let inputs = self.arena.alloc_from_iter((0..decl_param_count).map(|arg| hir::Ty {
             hir_id: self.next_id(),
             kind: hir::TyKind::InferDelegation(hir::InferDelegation::Sig(
                 sig_id,
                 hir::InferDelegationSig::Input(arg),
             )),
             span,
         }));

         let output = self.arena.alloc(hir::Ty {
             hir_id: self.next_id(),
             kind: hir::TyKind::InferDelegation(hir::InferDelegation::Sig(
                 sig_id,
                 hir::InferDelegationSig::Output(self.arena.alloc(hir::DelegationInfo {
                     call_expr_id,
                     call_path_res,
                     arguments_to_map: res.sig_mapping.arguments_to_map.clone(),
                     child_seg_id: generics.child.args_segment_id,
                     child_seg_id_for_sig: generics.child.segment_id_for_sig(),
                     parent_seg_id_for_sig: generics.parent.segment_id_for_sig(),
                     self_ty_propagation_kind: generics.self_ty_propagation_kind,
                     group_id: {
                         let id = match source {
                             DelegationSource::Single => None,
                             DelegationSource::List(expn_id) => Some(expn_id),
                             DelegationSource::Glob => {
                                 Some(self.tcx.expn_that_defined(self.owner.def_id).expect_local())
                             }
                         };

                         id.map(|id| (id, unused_target_expr))
                     },
                 })),
             )),
             span,
         });

         self.arena.alloc(hir::FnDecl {
             inputs,
             output: hir::FnRetTy::Return(output),
             fn_decl_kind: FnDeclFlags::default()
                 .set_lifetime_elision_allowed(true)
                 .set_c_variadic(c_variadic)
                 .set_splatted(splatted, inputs.len())
                 .unwrap(),
         })
     }

     fn lower_delegation_sig(
         &mut self,
         sig_id: DefId,
         decl: &'hir hir::FnDecl<'hir>,
         span: Span,
     ) -> hir::FnSig<'hir> {
         let sig = self.tcx.fn_sig(sig_id).skip_binder().skip_binder();
         let asyncness = match self.tcx.asyncness(sig_id) {
             Asyncness::Yes => hir::IsAsync::Async(span),
             Asyncness::No => hir::IsAsync::NotAsync,
         };

         let header = hir::FnHeader {
             safety: if self.tcx.codegen_fn_attrs(sig_id).safe_target_features {
                 hir::HeaderSafety::SafeTargetFeatures
             } else {
                 hir::HeaderSafety::Normal(sig.safety())
             },
             constness: self.tcx.constness(sig_id),
             asyncness,
             abi: sig.abi(),
         };

         hir::FnSig { decl, header, span }
     }

     fn generate_param(
         &mut self,
         is_method: bool,
         idx: usize,
         span: Span,
     ) -> (hir::Param<'hir>, NodeId) {
         let pat_node_id = self.next_node_id();
         let pat_id = self.lower_node_id(pat_node_id);
         // FIXME(cjgillot) AssocItem currently relies on self parameter being exactly named `self`.
         let name = if is_method && idx == 0 {
             kw::SelfLower
         } else {
             Symbol::intern(&format!("arg{idx}"))
         };
         let ident = Ident::with_dummy_span(name);
         let pat = self.arena.alloc(hir::Pat {
             hir_id: pat_id,
             kind: hir::PatKind::Binding(hir::BindingMode::NONE, pat_id, ident, None),
             span,
             default_binding_modes: false,
         });

         (hir::Param { hir_id: self.next_id(), pat, ty_span: span, span }, pat_node_id)
     }

     fn generate_arg(
         &mut self,
         is_method: bool,
         idx: usize,
         param_id: HirId,
         span: Span,
     ) -> hir::Expr<'hir> {
         // FIXME(cjgillot) AssocItem currently relies on self parameter being exactly named `self`.
         let name = if is_method && idx == 0 {
             kw::SelfLower
         } else {
             Symbol::intern(&format!("arg{idx}"))
         };

         let segments = self.arena.alloc_from_iter(iter::once(hir::PathSegment {
             ident: Ident::with_dummy_span(name),
             hir_id: self.next_id(),
             res: Res::Local(param_id),
             args: None,
             infer_args: false,
             delegation_child_segment: false,
         }));

         let path = self.arena.alloc(hir::Path { span, res: Res::Local(param_id), segments });
         self.mk_expr(hir::ExprKind::Path(hir::QPath::Resolved(None, path)), span)
     }

     fn lower_delegation_body(
         &mut self,
         delegation: &Delegation,
         res: &DelegationResolution,
         generics: &mut GenericsGenerationResults<'hir>,
     ) -> (hir::BodyId, HirId, bool) {
         let block = delegation.body.as_deref();
         let mut call_expr_id = HirId::INVALID;
         let mut unused_target_expr = false;

         let block_id = self.lower_body(|this| {
             let &DelegationResolution { param_info, span, is_method, .. } = res;
             let ParamInfo { param_count, .. } = param_info;
             let arguments_to_map = &res.sig_mapping.arguments_to_map;

             let mut parameters: Vec<hir::Param<'_>> = Vec::with_capacity(param_count);
             let mut args: Vec<hir::Expr<'_>> = Vec::with_capacity(param_count);
             let mut stmts = vec![];

             // Consider non-specified target expression as generated,
             // as we do not want to emit error when target expression is
             // not specified.
             unused_target_expr =
                 block.is_some() && (param_count == 0 || arguments_to_map.is_empty());

             for idx in 0..param_count {
                 let (param, pat_node_id) = this.generate_param(is_method, idx, span);
                 parameters.push(param);

                 let generate_arg =
                     |this: &mut Self| this.generate_arg(is_method, idx, param.pat.hir_id, span);

                 let arg = block
                     .filter(|_| arguments_to_map.contains(&idx))
                     .and_then(|block| {
                         let block = this.lower_block_maybe_more_than_once(
                             block,
                             pat_node_id,
                             param.pat.hir_id.local_id,
                             delegation.id,
                         );

                         stmts.push(block.stmts);

                         // The behavior of the delegation's target expression differs from the
                         // behavior of the usual block, where if there is no final expression
                         // the `()` is returned. In case of the similar situation in delegation
                         // (no final expression) we propagate first argument instead of replacing
                         // it with `()`.
                         block.expr.copied()
                     })
                     .unwrap_or_else(|| generate_arg(this));

                 args.push(arg);
             }

             let (final_expr, hir_id) = this.finalize_body_lowering(
                 delegation,
                 this.arena.alloc_from_iter(stmts.into_iter().flatten().copied()),
                 args,
                 res,
                 generics,
                 span,
             );

             call_expr_id = hir_id;

             (this.arena.alloc_from_iter(parameters), final_expr)
         });

         debug_assert_ne!(call_expr_id, HirId::INVALID);

         (block_id, call_expr_id, unused_target_expr)
     }

     fn lower_block_maybe_more_than_once(
         &mut self,
         block: &Block,
         pat_node_id: NodeId,
         param_local_id: hir::ItemLocalId,
         delegation_id: NodeId,
     ) -> hir::Block<'hir> {
         let mut self_resolver = SelfResolver {
             ctxt: self,
             path_id: delegation_id,
             self_param_id: pat_node_id,
             overwrites: vec![],
         };

         self_resolver.visit_block(block);

         let overwrites = self_resolver.overwrites;

         // Target expr needs to lower `self` path.
         self.ident_and_label_to_local_id.insert(pat_node_id, param_local_id);

         let block = cfg_select! {
             debug_assertions => {
                 crate::re_lowering::ReloweringChecker::allow_relowering(self, |this| {
                     this.lower_block_noalloc(HirId::INVALID, block, false)
                 })
             },
             _ => self.lower_block_noalloc(HirId::INVALID, block, false)
         };

         // Remove node ids for which we overwrote resolution to generated param
         // before block lowering as block can be relowered. We need to do it because
         // check in `SelfResolver` uses `get_partial_res` to decide whether to overwrite
         // resolution, and if it is already overwritten from previous block lowering this
         // check will not pass.
         for id in overwrites {
             self.partial_res_overrides.remove(&id);
         }

         block
     }

     fn finalize_body_lowering(
         &mut self,
         delegation: &Delegation,
         stmts: &'hir [hir::Stmt<'hir>],
         args: Vec<hir::Expr<'hir>>,
         res: &DelegationResolution,
         generics: &mut GenericsGenerationResults<'hir>,
         span: Span,
     ) -> (hir::Expr<'hir>, HirId) {
         let path = self.lower_qpath(
             delegation.id,
             &delegation.qself,
             &delegation.path,
             ParamMode::Optional,
             AllowReturnTypeNotation::No,
             ImplTraitContext::Disallowed(ImplTraitPosition::Path),
             None,
         );

         let new_path = match path {
             hir::QPath::Resolved(ty, path) => {
                 let mut new_path = path.clone();
                 let len = new_path.segments.len();

                 new_path.segments = self.arena.alloc_from_iter(
                     new_path.segments.iter().enumerate().map(|(idx, segment)| {
                         if idx + 2 == len {
                             self.process_segment(span, segment, &mut generics.parent)
                         } else if idx + 1 == len {
                             self.process_segment(span, segment, &mut generics.child)
                         } else {
                             segment.clone()
                         }
                     }),
                 );

                 // Explicitly create `Self` self-type in case of infers or static
                 // free-to-trait reuses.
                 let ty = match generics.self_ty_propagation_kind {
                     Some(hir::DelegationSelfTyPropagationKind::SelfParam) => {
                         let self_param = generics.parent.generics.find_self_param();
                         let path = self.create_generic_arg_path(self_param);
                         let kind = hir::TyKind::Path(path);

                         let ty = match ty {
                             Some(ty) => hir::Ty { kind, ..ty.clone() },
                             None => hir::Ty { kind, hir_id: self.next_id(), span },
                         };

                         Some(&*self.arena.alloc(ty))
                     }
                     _ => ty,
                 };

                 hir::QPath::Resolved(ty, self.arena.alloc(new_path))
             }
             hir::QPath::TypeRelative(..) => unreachable!("until inherent methods are supported"),
         };

         if let Some(hir::DelegationSelfTyPropagationKind::SelfTy(id)) =
             generics.self_ty_propagation_kind.as_mut()
         {
             *id = match new_path {
                 hir::QPath::Resolved(ty, _) => {
                     ty.expect("must contain self type as `SelfTy` propagation kind is specified")
                 }
                 hir::QPath::TypeRelative(ty, _) => ty,
             }
             .hir_id;
         }

         let callee_path = self.arena.alloc(self.mk_expr(hir::ExprKind::Path(new_path), span));
         let args = self.arena.alloc_from_iter(args);
         let call = self.mk_expr(hir::ExprKind::Call(callee_path, args), span);

         let expr = if res.sig_mapping.map_return {
             let res = Res::SelfTyAlias {
                 alias_to: res.parent.to_def_id(),
                 is_trait_impl: self.tcx.def_kind(res.parent) == DefKind::Impl { of_trait: true },
             };

             let ident = Ident::new(kw::SelfUpper, span);
             let path = self.create_resolved_path(res, ident, span);

             // FIXME(fn_delegation): add default `..` for all other fields.
             let initializer = hir::ExprKind::Struct(
                 self.arena.alloc(path),
                 self.arena.alloc_slice(&[hir::ExprField {
                     hir_id: self.next_id(),
                     is_shorthand: false,
                     ident: Ident::new(sym::integer(0), span),
                     expr: self.arena.alloc(call),
                     span,
                 }]),
                 hir::StructTailExpr::None,
             );

             self.arena.alloc(self.mk_expr(initializer, span))
         } else {
             self.arena.alloc(call)
         };

         let block = self.arena.alloc(hir::Block {
             stmts,
             expr: Some(expr),
             hir_id: self.next_id(),
             rules: hir::BlockCheckMode::DefaultBlock,
             span,
             targeted_by_break: false,
         });

         (self.mk_expr(hir::ExprKind::Block(block, None), span), call.hir_id)
     }

     fn process_segment(
         &mut self,
         span: Span,
         segment: &hir::PathSegment<'hir>,
         result: &mut GenericsGenerationResult<'hir>,
     ) -> hir::PathSegment<'hir> {
         let infer_indices = result.generics.infer_indices();
         result.generics.into_hir_generics(self, span);

         let mut segment = segment.clone();
         let mut args_iter = result.generics.create_args_iterator();

         let new_args = segment
             .args
             .filter(|args| !args.is_empty())
             .map(|args| {
                 self.arena.alloc_from_iter(args.args.iter().enumerate().map(|(idx, arg)| {
                     if infer_indices.contains(&idx) {
                         args_iter.next(self, |_| arg.hir_id()).expect("arg must exist for infer")
                     } else {
                         *arg
                     }
                 }))
             })
             .unwrap_or_else(|| self.arena.alloc_from_iter(args_iter.consume_all(self)));

         // Do not omit constraints as there might be some and they must be present in HIR (#158812).
         let has_constraints = segment.args.is_some_and(|a| !a.constraints.is_empty());

         // Needed for better error messages (`trait-impl-wrong-args-count.rs` test).
         segment.args = (has_constraints || !new_args.is_empty()).then(|| {
             &*self.arena.alloc(hir::GenericArgs {
                 args: new_args,
                 constraints: segment.args.map(|a| a.constraints).unwrap_or(&[]),
                 parenthesized: hir::GenericArgsParentheses::No,
                 span_ext: segment.args.map_or(span, |args| args.span_ext),
             })
         });

         result.args_segment_id = segment.hir_id;
         result.use_for_sig_inheritance = !result.generics.is_trait_impl();

         segment.delegation_child_segment = result.generics.pos() == GenericsPosition::Child;

         segment
     }

     fn generate_delegation_error(
         &mut self,
         span: Span,
         delegation: &Delegation,
     ) -> DelegationResults<'hir> {
         let decl = self.arena.alloc(hir::FnDecl::dummy(span));

         let header = self.generate_header_error();
         let sig = hir::FnSig { decl, header, span };

         let ident = self.lower_ident(delegation.ident);

         let body_id = self.lower_body(|this| {
             let path = this.lower_qpath(
                 delegation.id,
                 &delegation.qself,
                 &delegation.path,
                 ParamMode::Optional,
                 AllowReturnTypeNotation::No,
                 ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                 None,
             );

             let callee_path = this.arena.alloc(this.mk_expr(hir::ExprKind::Path(path), span));
             let args = if let Some(block) = &delegation.body {
                 this.arena.alloc_slice(&[this.lower_block_expr(block)])
             } else {
                 &mut []
             };

             let call = this.arena.alloc(this.mk_expr(hir::ExprKind::Call(callee_path, args), span));

             let block = this.arena.alloc(hir::Block {
                 stmts: &[],
                 expr: Some(call),
                 hir_id: this.next_id(),
                 rules: hir::BlockCheckMode::DefaultBlock,
                 span,
                 targeted_by_break: false,
             });

             (&[], this.mk_expr(hir::ExprKind::Block(block, None), span))
         });

         let generics = hir::Generics::empty();
         DelegationResults { ident, generics, body_id, sig }
     }

     fn generate_header_error(&self) -> hir::FnHeader {
         hir::FnHeader {
             safety: hir::Safety::Safe.into(),
             constness: hir::Constness::NotConst,
             asyncness: hir::IsAsync::NotAsync,
             abi: ExternAbi::Rust,
         }
     }

     #[inline]
     fn mk_expr(&mut self, kind: hir::ExprKind<'hir>, span: Span) -> hir::Expr<'hir> {
         hir::Expr { hir_id: self.next_id(), kind, span }
     }
 }

 struct SelfResolver<'a, 'b, 'hir> {
     ctxt: &'a mut LoweringContext<'b, 'hir>,
     path_id: NodeId,
     self_param_id: NodeId,
     overwrites: Vec<NodeId>,
 }

 impl SelfResolver<'_, '_, '_> {
     fn try_replace_id(&mut self, id: NodeId) {
         if let Some(res) = self.ctxt.get_partial_res(id)
             && let Some(Res::Local(sig_id)) = res.full_res()
             && sig_id == self.path_id
         {
             self.overwrites.push(id);
             self.ctxt.partial_res_overrides.insert(id, self.self_param_id);
         }
     }
 }

 impl<'ast> Visitor<'ast> for SelfResolver<'_, '_, '_> {
     fn visit_id(&mut self, id: NodeId) {
         self.try_replace_id(id);
     }
 }
	//! This module implements expansion of delegation items with early resolved paths.
	//! It includes a delegation to a free functions:
	//!
	//! ```ignore (illustrative)
	//! reuse module::name { target_expr_template }
	//! ```
	//!
	//! And delegation to a trait methods:
	//!
	//! ```ignore (illustrative)
	//! reuse <Type as Trait>::name { target_expr_template }
	//! ```
	//!
	//! After expansion for both cases we get:
	//!
	//! ```ignore (illustrative)
	//! fn name(
	//! arg0: InferDelegation(sig_id, Input(0)),
	//! arg1: InferDelegation(sig_id, Input(1)),
	//! ...,
	//! argN: InferDelegation(sig_id, Input(N)),
	//! ) -> InferDelegation(sig_id, Output) {
	//! callee_path(target_expr_template(arg0), arg1, ..., argN)
	//! }
	//! ```
	//!
	//! Where `callee_path` is a path in delegation item e.g. `<Type as Trait>::name`.
	//! `sig_id` is a id of item from which the signature is inherited. It may be a delegation
	//! item id (`item_id`) in case of impl trait or path resolution id (`path_id`) otherwise.
	//!
	//! Since we do not have a proper way to obtain function type information by path resolution
	//! in AST, we mark each function parameter type as `InferDelegation` and inherit it during
	//! HIR ty lowering.
	//!
	//! Similarly generics, predicates and header are set to the "default" values.
	//! In case of discrepancy with callee function the `UnsupportedDelegation` error will
	//! also be emitted during HIR ty lowering.

	use std::iter;

	use ast::visit::Visitor;
	use generics::GenericsGenerationResult;
	use hir::HirId;
	use hir::def::Res;
	use rustc_abi::ExternAbi;
	use rustc_ast as ast;
	use rustc_ast::*;
	use rustc_hir::def::DefKind;
	use rustc_hir::{self as hir, FnDeclFlags};
	use rustc_middle::ty::Asyncness;
	use rustc_span::def_id::DefId;
	use rustc_span::symbol::kw;
	use rustc_span::{Ident, Span, Symbol, sym};

	use crate::delegation::generics::{GenericsGenerationResults, GenericsPosition};
	use crate::delegation::resolution::resolver::DelegationResolver;
	use crate::delegation::resolution::{DelegationResolution, ParamInfo};
	use crate::{
	AllowReturnTypeNotation, ImplTraitContext, ImplTraitPosition, LoweringContext, ParamMode,
	};

	mod attributes;
	mod generics;
	mod resolution;

	pub(crate) struct DelegationResults<'hir> {
	pub body_id: hir::BodyId,
	pub sig: hir::FnSig<'hir>,
	pub ident: Ident,
	pub generics: &'hir hir::Generics<'hir>,
	}

	impl<'hir> LoweringContext<'_, 'hir> {
	pub(crate) fn lower_delegation(&mut self, delegation: &Delegation) -> DelegationResults<'hir> {
	let span = self.lower_span(delegation.last_segment_span());

	let resolver = DelegationResolver::new(self);
	let Ok((res, mut generics)) = resolver.resolve_delegation(delegation, span) else {
	return self.generate_delegation_error(span, delegation);
	};

	self.add_attrs_if_needed(&res);

	let (body_id, call_expr_id, unused_target_expr) =
	self.lower_delegation_body(delegation, &res, &mut generics);

	let decl = self.lower_delegation_decl(&res, &generics, call_expr_id, unused_target_expr);

	let sig = self.lower_delegation_sig(res.sig_id, decl, span);

	let ident = self.lower_ident(delegation.ident);

	let generics = self.arena.alloc(hir::Generics {
	has_where_clause_predicates: false,
	params: self.arena.alloc_from_iter(generics.all_params()),
	predicates: self.arena.alloc_from_iter(generics.all_predicates()),
	span,
	where_clause_span: span,
	});

	DelegationResults { body_id, sig, ident, generics }
	}

	fn lower_delegation_decl(
	&mut self,
	res: &DelegationResolution,
	generics: &GenericsGenerationResults<'hir>,
	call_expr_id: HirId,
	unused_target_expr: bool,
	) -> &'hir hir::FnDecl<'hir> {
	let &DelegationResolution { source, call_path_res, span, sig_id, .. } = res;
	let ParamInfo { param_count, c_variadic, splatted } = res.param_info;

	// The last parameter in C variadic functions is skipped in the signature,
	// like during regular lowering.
	let decl_param_count = param_count - c_variadic as usize;
	let inputs = self.arena.alloc_from_iter((0..decl_param_count).map(\|arg\| hir::Ty {
	hir_id: self.next_id(),
	kind: hir::TyKind::InferDelegation(hir::InferDelegation::Sig(
	sig_id,
	hir::InferDelegationSig::Input(arg),
	)),
	span,
	}));

	let output = self.arena.alloc(hir::Ty {
	hir_id: self.next_id(),
	kind: hir::TyKind::InferDelegation(hir::InferDelegation::Sig(
	sig_id,
	hir::InferDelegationSig::Output(self.arena.alloc(hir::DelegationInfo {
	call_expr_id,
	call_path_res,
	arguments_to_map: res.sig_mapping.arguments_to_map.clone(),
	child_seg_id: generics.child.args_segment_id,
	child_seg_id_for_sig: generics.child.segment_id_for_sig(),
	parent_seg_id_for_sig: generics.parent.segment_id_for_sig(),
	self_ty_propagation_kind: generics.self_ty_propagation_kind,
	group_id: {
	let id = match source {
	DelegationSource::Single => None,
	DelegationSource::List(expn_id) => Some(expn_id),
	DelegationSource::Glob => {
	Some(self.tcx.expn_that_defined(self.owner.def_id).expect_local())
	}
	};

	id.map(\|id\| (id, unused_target_expr))
	},
	})),
	)),
	span,
	});

	self.arena.alloc(hir::FnDecl {
	inputs,
	output: hir::FnRetTy::Return(output),
	fn_decl_kind: FnDeclFlags::default()
	.set_lifetime_elision_allowed(true)
	.set_c_variadic(c_variadic)
	.set_splatted(splatted, inputs.len())
	.unwrap(),
	})
	}

	fn lower_delegation_sig(
	&mut self,
	sig_id: DefId,
	decl: &'hir hir::FnDecl<'hir>,
	span: Span,
	) -> hir::FnSig<'hir> {
	let sig = self.tcx.fn_sig(sig_id).skip_binder().skip_binder();
	let asyncness = match self.tcx.asyncness(sig_id) {
	Asyncness::Yes => hir::IsAsync::Async(span),
	Asyncness::No => hir::IsAsync::NotAsync,
	};

	let header = hir::FnHeader {
	safety: if self.tcx.codegen_fn_attrs(sig_id).safe_target_features {
	hir::HeaderSafety::SafeTargetFeatures
	} else {
	hir::HeaderSafety::Normal(sig.safety())
	},
	constness: self.tcx.constness(sig_id),
	asyncness,
	abi: sig.abi(),
	};

	hir::FnSig { decl, header, span }
	}

	fn generate_param(
	&mut self,
	is_method: bool,
	idx: usize,
	span: Span,
	) -> (hir::Param<'hir>, NodeId) {
	let pat_node_id = self.next_node_id();
	let pat_id = self.lower_node_id(pat_node_id);
	// FIXME(cjgillot) AssocItem currently relies on self parameter being exactly named `self`.
	let name = if is_method && idx == 0 {
	kw::SelfLower
	} else {
	Symbol::intern(&format!("arg{idx}"))
	};
	let ident = Ident::with_dummy_span(name);
	let pat = self.arena.alloc(hir::Pat {
	hir_id: pat_id,
	kind: hir::PatKind::Binding(hir::BindingMode::NONE, pat_id, ident, None),
	span,
	default_binding_modes: false,
	});

	(hir::Param { hir_id: self.next_id(), pat, ty_span: span, span }, pat_node_id)
	}

	fn generate_arg(
	&mut self,
	is_method: bool,
	idx: usize,
	param_id: HirId,
	span: Span,
	) -> hir::Expr<'hir> {
	// FIXME(cjgillot) AssocItem currently relies on self parameter being exactly named `self`.
	let name = if is_method && idx == 0 {
	kw::SelfLower
	} else {
	Symbol::intern(&format!("arg{idx}"))
	};

	let segments = self.arena.alloc_from_iter(iter::once(hir::PathSegment {
	ident: Ident::with_dummy_span(name),
	hir_id: self.next_id(),
	res: Res::Local(param_id),
	args: None,
	infer_args: false,
	delegation_child_segment: false,
	}));

	let path = self.arena.alloc(hir::Path { span, res: Res::Local(param_id), segments });
	self.mk_expr(hir::ExprKind::Path(hir::QPath::Resolved(None, path)), span)
	}

	fn lower_delegation_body(
	&mut self,
	delegation: &Delegation,
	res: &DelegationResolution,
	generics: &mut GenericsGenerationResults<'hir>,
	) -> (hir::BodyId, HirId, bool) {
	let block = delegation.body.as_deref();
	let mut call_expr_id = HirId::INVALID;
	let mut unused_target_expr = false;

	let block_id = self.lower_body(\|this\| {
	let &DelegationResolution { param_info, span, is_method, .. } = res;
	let ParamInfo { param_count, .. } = param_info;
	let arguments_to_map = &res.sig_mapping.arguments_to_map;

	let mut parameters: Vec<hir::Param<'_>> = Vec::with_capacity(param_count);
	let mut args: Vec<hir::Expr<'_>> = Vec::with_capacity(param_count);
	let mut stmts = vec![];

	// Consider non-specified target expression as generated,
	// as we do not want to emit error when target expression is
	// not specified.
	unused_target_expr =
	block.is_some() && (param_count == 0 \|\| arguments_to_map.is_empty());

	for idx in 0..param_count {
	let (param, pat_node_id) = this.generate_param(is_method, idx, span);
	parameters.push(param);

	let generate_arg =
	\|this: &mut Self\| this.generate_arg(is_method, idx, param.pat.hir_id, span);

	let arg = block
	.filter(\|_\| arguments_to_map.contains(&idx))
	.and_then(\|block\| {
	let block = this.lower_block_maybe_more_than_once(
	block,
	pat_node_id,
	param.pat.hir_id.local_id,
	delegation.id,
	);

	stmts.push(block.stmts);

	// The behavior of the delegation's target expression differs from the
	// behavior of the usual block, where if there is no final expression
	// the `()` is returned. In case of the similar situation in delegation
	// (no final expression) we propagate first argument instead of replacing
	// it with `()`.
	block.expr.copied()
	})
	.unwrap_or_else(\|\| generate_arg(this));

	args.push(arg);
	}

	let (final_expr, hir_id) = this.finalize_body_lowering(
	delegation,
	this.arena.alloc_from_iter(stmts.into_iter().flatten().copied()),
	args,
	res,
	generics,
	span,
	);

	call_expr_id = hir_id;

	(this.arena.alloc_from_iter(parameters), final_expr)
	});

	debug_assert_ne!(call_expr_id, HirId::INVALID);

	(block_id, call_expr_id, unused_target_expr)
	}

	fn lower_block_maybe_more_than_once(
	&mut self,
	block: &Block,
	pat_node_id: NodeId,
	param_local_id: hir::ItemLocalId,
	delegation_id: NodeId,
	) -> hir::Block<'hir> {
	let mut self_resolver = SelfResolver {
	ctxt: self,
	path_id: delegation_id,
	self_param_id: pat_node_id,
	overwrites: vec![],
	};

	self_resolver.visit_block(block);

	let overwrites = self_resolver.overwrites;

	// Target expr needs to lower `self` path.
	self.ident_and_label_to_local_id.insert(pat_node_id, param_local_id);

	let block = cfg_select! {
	debug_assertions => {
	crate::re_lowering::ReloweringChecker::allow_relowering(self, \|this\| {
	this.lower_block_noalloc(HirId::INVALID, block, false)
	})
	},
	_ => self.lower_block_noalloc(HirId::INVALID, block, false)
	};

	// Remove node ids for which we overwrote resolution to generated param
	// before block lowering as block can be relowered. We need to do it because
	// check in `SelfResolver` uses `get_partial_res` to decide whether to overwrite
	// resolution, and if it is already overwritten from previous block lowering this
	// check will not pass.
	for id in overwrites {
	self.partial_res_overrides.remove(&id);
	}

	block
	}

	fn finalize_body_lowering(
	&mut self,
	delegation: &Delegation,
	stmts: &'hir [hir::Stmt<'hir>],
	args: Vec<hir::Expr<'hir>>,
	res: &DelegationResolution,
	generics: &mut GenericsGenerationResults<'hir>,
	span: Span,
	) -> (hir::Expr<'hir>, HirId) {
	let path = self.lower_qpath(
	delegation.id,
	&delegation.qself,
	&delegation.path,
	ParamMode::Optional,
	AllowReturnTypeNotation::No,
	ImplTraitContext::Disallowed(ImplTraitPosition::Path),
	None,
	);

	let new_path = match path {
	hir::QPath::Resolved(ty, path) => {
	let mut new_path = path.clone();
	let len = new_path.segments.len();

	new_path.segments = self.arena.alloc_from_iter(
	new_path.segments.iter().enumerate().map(\|(idx, segment)\| {
	if idx + 2 == len {
	self.process_segment(span, segment, &mut generics.parent)
	} else if idx + 1 == len {
	self.process_segment(span, segment, &mut generics.child)
	} else {
	segment.clone()
	}
	}),
	);

	// Explicitly create `Self` self-type in case of infers or static
	// free-to-trait reuses.
	let ty = match generics.self_ty_propagation_kind {
	Some(hir::DelegationSelfTyPropagationKind::SelfParam) => {
	let self_param = generics.parent.generics.find_self_param();
	let path = self.create_generic_arg_path(self_param);
	let kind = hir::TyKind::Path(path);

	let ty = match ty {
	Some(ty) => hir::Ty { kind, ..ty.clone() },
	None => hir::Ty { kind, hir_id: self.next_id(), span },
	};

	Some(&*self.arena.alloc(ty))
	}
	_ => ty,
	};

	hir::QPath::Resolved(ty, self.arena.alloc(new_path))
	}
	hir::QPath::TypeRelative(..) => unreachable!("until inherent methods are supported"),
	};

	if let Some(hir::DelegationSelfTyPropagationKind::SelfTy(id)) =
	generics.self_ty_propagation_kind.as_mut()
	{
	*id = match new_path {
	hir::QPath::Resolved(ty, _) => {
	ty.expect("must contain self type as `SelfTy` propagation kind is specified")
	}
	hir::QPath::TypeRelative(ty, _) => ty,
	}
	.hir_id;
	}

	let callee_path = self.arena.alloc(self.mk_expr(hir::ExprKind::Path(new_path), span));
	let args = self.arena.alloc_from_iter(args);
	let call = self.mk_expr(hir::ExprKind::Call(callee_path, args), span);

	let expr = if res.sig_mapping.map_return {
	let res = Res::SelfTyAlias {
	alias_to: res.parent.to_def_id(),
	is_trait_impl: self.tcx.def_kind(res.parent) == DefKind::Impl { of_trait: true },
	};

	let ident = Ident::new(kw::SelfUpper, span);
	let path = self.create_resolved_path(res, ident, span);

	// FIXME(fn_delegation): add default `..` for all other fields.
	let initializer = hir::ExprKind::Struct(
	self.arena.alloc(path),
	self.arena.alloc_slice(&[hir::ExprField {
	hir_id: self.next_id(),
	is_shorthand: false,
	ident: Ident::new(sym::integer(0), span),
	expr: self.arena.alloc(call),
	span,
	}]),
	hir::StructTailExpr::None,
	);

	self.arena.alloc(self.mk_expr(initializer, span))
	} else {
	self.arena.alloc(call)
	};

	let block = self.arena.alloc(hir::Block {
	stmts,
	expr: Some(expr),
	hir_id: self.next_id(),
	rules: hir::BlockCheckMode::DefaultBlock,
	span,
	targeted_by_break: false,
	});

	(self.mk_expr(hir::ExprKind::Block(block, None), span), call.hir_id)
	}

	fn process_segment(
	&mut self,
	span: Span,
	segment: &hir::PathSegment<'hir>,
	result: &mut GenericsGenerationResult<'hir>,
	) -> hir::PathSegment<'hir> {
	let infer_indices = result.generics.infer_indices();
	result.generics.into_hir_generics(self, span);

	let mut segment = segment.clone();
	let mut args_iter = result.generics.create_args_iterator();

	let new_args = segment
	.args
	.filter(\|args\| !args.is_empty())
	.map(\|args\| {
	self.arena.alloc_from_iter(args.args.iter().enumerate().map(\|(idx, arg)\| {
	if infer_indices.contains(&idx) {
	args_iter.next(self, \|_\| arg.hir_id()).expect("arg must exist for infer")
	} else {
	*arg
	}
	}))
	})
	.unwrap_or_else(\|\| self.arena.alloc_from_iter(args_iter.consume_all(self)));

	// Do not omit constraints as there might be some and they must be present in HIR (#158812).
	let has_constraints = segment.args.is_some_and(\|a\| !a.constraints.is_empty());

	// Needed for better error messages (`trait-impl-wrong-args-count.rs` test).
	segment.args = (has_constraints \|\| !new_args.is_empty()).then(\|\| {
	&*self.arena.alloc(hir::GenericArgs {
	args: new_args,
	constraints: segment.args.map(\|a\| a.constraints).unwrap_or(&[]),
	parenthesized: hir::GenericArgsParentheses::No,
	span_ext: segment.args.map_or(span, \|args\| args.span_ext),
	})
	});

	result.args_segment_id = segment.hir_id;
	result.use_for_sig_inheritance = !result.generics.is_trait_impl();

	segment.delegation_child_segment = result.generics.pos() == GenericsPosition::Child;

	segment
	}

	fn generate_delegation_error(
	&mut self,
	span: Span,
	delegation: &Delegation,
	) -> DelegationResults<'hir> {
	let decl = self.arena.alloc(hir::FnDecl::dummy(span));

	let header = self.generate_header_error();
	let sig = hir::FnSig { decl, header, span };

	let ident = self.lower_ident(delegation.ident);

	let body_id = self.lower_body(\|this\| {
	let path = this.lower_qpath(
	delegation.id,
	&delegation.qself,
	&delegation.path,
	ParamMode::Optional,
	AllowReturnTypeNotation::No,
	ImplTraitContext::Disallowed(ImplTraitPosition::Path),
	None,
	);

	let callee_path = this.arena.alloc(this.mk_expr(hir::ExprKind::Path(path), span));
	let args = if let Some(block) = &delegation.body {
	this.arena.alloc_slice(&[this.lower_block_expr(block)])
	} else {
	&mut []
	};

	let call = this.arena.alloc(this.mk_expr(hir::ExprKind::Call(callee_path, args), span));

	let block = this.arena.alloc(hir::Block {
	stmts: &[],
	expr: Some(call),
	hir_id: this.next_id(),
	rules: hir::BlockCheckMode::DefaultBlock,
	span,
	targeted_by_break: false,
	});

	(&[], this.mk_expr(hir::ExprKind::Block(block, None), span))
	});

	let generics = hir::Generics::empty();
	DelegationResults { ident, generics, body_id, sig }
	}

	fn generate_header_error(&self) -> hir::FnHeader {
	hir::FnHeader {
	safety: hir::Safety::Safe.into(),
	constness: hir::Constness::NotConst,
	asyncness: hir::IsAsync::NotAsync,
	abi: ExternAbi::Rust,
	}
	}

	#[inline]
	fn mk_expr(&mut self, kind: hir::ExprKind<'hir>, span: Span) -> hir::Expr<'hir> {
	hir::Expr { hir_id: self.next_id(), kind, span }
	}
	}

	struct SelfResolver<'a, 'b, 'hir> {
	ctxt: &'a mut LoweringContext<'b, 'hir>,
	path_id: NodeId,
	self_param_id: NodeId,
	overwrites: Vec<NodeId>,
	}

	impl SelfResolver<'_, '_, '_> {
	fn try_replace_id(&mut self, id: NodeId) {
	if let Some(res) = self.ctxt.get_partial_res(id)
	&& let Some(Res::Local(sig_id)) = res.full_res()
	&& sig_id == self.path_id
	{
	self.overwrites.push(id);
	self.ctxt.partial_res_overrides.insert(id, self.self_param_id);
	}
	}
	}

	impl<'ast> Visitor<'ast> for SelfResolver<'_, '_, '_> {
	fn visit_id(&mut self, id: NodeId) {
	self.try_replace_id(id);
	}
	}