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    use internals::respan::respan;
    use proc_macro2::Span;
    use quote::ToTokens;
    use std::mem;
    use syn::punctuated::Punctuated;
    use syn::{
        parse_quote, Data, DeriveInput, Expr, ExprPath, GenericArgument, GenericParam, Generics, Macro,
        Path, PathArguments, QSelf, ReturnType, Type, TypeParamBound, TypePath, WherePredicate,
    };

    pub fn replace_receiver(input: &mut DeriveInput) {
        let self_ty = {
            let ident = &input.ident;
            let ty_generics = input.generics.split_for_impl().1;
            parse_quote!(#ident #ty_generics)
        };
        let mut visitor = ReplaceReceiver(&self_ty);
        visitor.visit_generics_mut(&mut input.generics);
        visitor.visit_data_mut(&mut input.data);
    }

    struct ReplaceReceiver<'a>(&'a TypePath);

    impl ReplaceReceiver<'_> {
        fn self_ty(&self, span: Span) -> TypePath {
            let tokens = self.0.to_token_stream();
            let respanned = respan(tokens, span);
            syn::parse2(respanned).unwrap()
        }

        fn self_to_qself(&self, qself: &mut Option<QSelf>, path: &mut Path) {
            if path.leading_colon.is_some() || path.segments[0].ident != "Self" {
                return;
            }

            if path.segments.len() == 1 {
                self.self_to_expr_path(path);
                return;
            }

            let span = path.segments[0].ident.span();
            *qself = Some(QSelf {
                lt_token: Token![<](span),
                ty: Box::new(Type::Path(self.self_ty(span))),
                position: 0,
                as_token: None,
                gt_token: Token![>](span),
            });

            path.leading_colon = Some(**path.segments.pairs().next().unwrap().punct().unwrap());

            let segments = mem::replace(&mut path.segments, Punctuated::new());
            path.segments = segments.into_pairs().skip(1).collect();
        }

        fn self_to_expr_path(&self, path: &mut Path) {
            let self_ty = self.self_ty(path.segments[0].ident.span());
            let variant = mem::replace(path, self_ty.path);
            for segment in &mut path.segments {
                if let PathArguments::AngleBracketed(bracketed) = &mut segment.arguments {
                    if bracketed.colon2_token.is_none() && !bracketed.args.is_empty() {
                        bracketed.colon2_token = Some(<Token![::]>::default());
                    }
                }
            }
            if variant.segments.len() > 1 {
                path.segments.push_punct(<Token![::]>::default());
                path.segments.extend(variant.segments.into_pairs().skip(1));
            }
        }
    }

    impl ReplaceReceiver<'_> {
        // `Self` -> `Receiver`
        fn visit_type_mut(&mut self, ty: &mut Type) {
            let span = if let Type::Path(node) = ty {
                if node.qself.is_none() && node.path.is_ident("Self") {
                    node.path.segments[0].ident.span()
                } else {
                    self.visit_type_path_mut(node);
                    return;
                }
            } else {
                self.visit_type_mut_impl(ty);
                return;
            };
            *ty = self.self_ty(span).into();
        }

        // `Self::Assoc` -> `<Receiver>::Assoc`
        fn visit_type_path_mut(&mut self, ty: &mut TypePath) {
            if ty.qself.is_none() {
                self.self_to_qself(&mut ty.qself, &mut ty.path);
            }
            self.visit_type_path_mut_impl(ty);
        }

        // `Self::method` -> `<Receiver>::method`
        fn visit_expr_path_mut(&mut self, expr: &mut ExprPath) {
            if expr.qself.is_none() {
                self.self_to_qself(&mut expr.qself, &mut expr.path);
            }
            self.visit_expr_path_mut_impl(expr);
        }

        // Everything below is simply traversing the syntax tree.

        fn visit_type_mut_impl(&mut self, ty: &mut Type) {
            match ty {
                Type::Array(ty) => {
                    self.visit_type_mut(&mut ty.elem);
                    self.visit_expr_mut(&mut ty.len);
                }
                Type::BareFn(ty) => {
                    for arg in &mut ty.inputs {
                        self.visit_type_mut(&mut arg.ty);
                    }
                    self.visit_return_type_mut(&mut ty.output);
                }
                Type::Group(ty) => self.visit_type_mut(&mut ty.elem),
                Type::ImplTrait(ty) => {
                    for bound in &mut ty.bounds {
                        self.visit_type_param_bound_mut(bound);
                    }
                }
                Type::Macro(ty) => self.visit_macro_mut(&mut ty.mac),
                Type::Paren(ty) => self.visit_type_mut(&mut ty.elem),
                Type::Path(ty) => {
                    if let Some(qself) = &mut ty.qself {
                        self.visit_type_mut(&mut qself.ty);
                    }
                    self.visit_path_mut(&mut ty.path);
                }
                Type::Ptr(ty) => self.visit_type_mut(&mut ty.elem),
                Type::Reference(ty) => self.visit_type_mut(&mut ty.elem),
                Type::Slice(ty) => self.visit_type_mut(&mut ty.elem),
                Type::TraitObject(ty) => {
                    for bound in &mut ty.bounds {
                        self.visit_type_param_bound_mut(bound);
                    }
                }
                Type::Tuple(ty) => {
                    for elem in &mut ty.elems {
                        self.visit_type_mut(elem);
                    }
                }

                Type::Infer(_) | Type::Never(_) | Type::Verbatim(_) => {}

                #[cfg_attr(all(test, exhaustive), deny(non_exhaustive_omitted_patterns))]
                _ => {}
            }
        }

        fn visit_type_path_mut_impl(&mut self, ty: &mut TypePath) {
            if let Some(qself) = &mut ty.qself {
                self.visit_type_mut(&mut qself.ty);
            }
            self.visit_path_mut(&mut ty.path);
        }

        fn visit_expr_path_mut_impl(&mut self, expr: &mut ExprPath) {
            if let Some(qself) = &mut expr.qself {
                self.visit_type_mut(&mut qself.ty);
            }
            self.visit_path_mut(&mut expr.path);
        }

        fn visit_path_mut(&mut self, path: &mut Path) {
            for segment in &mut path.segments {
                self.visit_path_arguments_mut(&mut segment.arguments);
            }
        }

        fn visit_path_arguments_mut(&mut self, arguments: &mut PathArguments) {
            match arguments {
                PathArguments::None => {}
                PathArguments::AngleBracketed(arguments) => {
                    for arg in &mut arguments.args {
                        match arg {
                            GenericArgument::Type(arg) => self.visit_type_mut(arg),
                            GenericArgument::Binding(arg) => self.visit_type_mut(&mut arg.ty),
                            GenericArgument::Lifetime(_)
                            | GenericArgument::Constraint(_)
                            | GenericArgument::Const(_) => {}
                        }
                    }
                }
                PathArguments::Parenthesized(arguments) => {
                    for argument in &mut arguments.inputs {
                        self.visit_type_mut(argument);
                    }
                    self.visit_return_type_mut(&mut arguments.output);
                }
            }
        }

        fn visit_return_type_mut(&mut self, return_type: &mut ReturnType) {
            match return_type {
                ReturnType::Default => {}
                ReturnType::Type(_, output) => self.visit_type_mut(output),
            }
        }

        fn visit_type_param_bound_mut(&mut self, bound: &mut TypeParamBound) {
            match bound {
                TypeParamBound::Trait(bound) => self.visit_path_mut(&mut bound.path),
                TypeParamBound::Lifetime(_) => {}
            }
        }

        fn visit_generics_mut(&mut self, generics: &mut Generics) {
            for param in &mut generics.params {
                match param {
                    GenericParam::Type(param) => {
                        for bound in &mut param.bounds {
                            self.visit_type_param_bound_mut(bound);
                        }
                    }
                    GenericParam::Lifetime(_) | GenericParam::Const(_) => {}
                }
            }
            if let Some(where_clause) = &mut generics.where_clause {
                for predicate in &mut where_clause.predicates {
                    match predicate {
                        WherePredicate::Type(predicate) => {
                            self.visit_type_mut(&mut predicate.bounded_ty);
                            for bound in &mut predicate.bounds {
                                self.visit_type_param_bound_mut(bound);
                            }
                        }
                        WherePredicate::Lifetime(_) | WherePredicate::Eq(_) => {}
                    }
                }
            }
        }

        fn visit_data_mut(&mut self, data: &mut Data) {
            match data {
                Data::Struct(data) => {
                    for field in &mut data.fields {
                        self.visit_type_mut(&mut field.ty);
                    }
                }
                Data::Enum(data) => {
                    for variant in &mut data.variants {
                        for field in &mut variant.fields {
                            self.visit_type_mut(&mut field.ty);
                        }
                    }
                }
                Data::Union(_) => {}
            }
        }

        fn visit_expr_mut(&mut self, expr: &mut Expr) {
            match expr {
                Expr::Binary(expr) => {
                    self.visit_expr_mut(&mut expr.left);
                    self.visit_expr_mut(&mut expr.right);
                }
                Expr::Call(expr) => {
                    self.visit_expr_mut(&mut expr.func);
                    for arg in &mut expr.args {
                        self.visit_expr_mut(arg);
                    }
                }
                Expr::Cast(expr) => {
                    self.visit_expr_mut(&mut expr.expr);
                    self.visit_type_mut(&mut expr.ty);
                }
                Expr::Field(expr) => self.visit_expr_mut(&mut expr.base),
                Expr::Index(expr) => {
                    self.visit_expr_mut(&mut expr.expr);
                    self.visit_expr_mut(&mut expr.index);
                }
                Expr::Paren(expr) => self.visit_expr_mut(&mut expr.expr),
                Expr::Path(expr) => self.visit_expr_path_mut(expr),
                Expr::Unary(expr) => self.visit_expr_mut(&mut expr.expr),
                _ => {}
            }
        }

        fn visit_macro_mut(&mut self, _mac: &mut Macro) {}
    }