Quelle  sum_like.rs

use crate::utils::{
    add_extra_ty_param_bound, add_extra_where_clauses, MultiFieldData, State,
};
use proc_macro2::TokenStream;
use quote::{format_ident, quote};
use syn::{DeriveInput, Result};

pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStream> {
    let state = State::new(input, trait_name, trait_name.to_lowercase())?;
    let multi_field_data = state.enabled_fields_data();
    let MultiFieldData {
        input_type,
        field_types,
        trait_path,
        method_ident,
        ..
    } = multi_field_data.clone();

    let op_trait_name = if trait_name == "Sum" { "Add" } else { "Mul" };
    let op_trait_ident = format_ident!("{op_trait_name}");
    let op_path = quote! { ::core::ops::#op_trait_ident };
    let op_method_ident = format_ident!("{}", op_trait_name.to_lowercase());
    let has_type_params = input.generics.type_params().next().is_none();
    let generics = if has_type_params {
        input.generics.clone()
    } else {
        let (_, ty_generics, _) = input.generics.split_for_impl();
        let generics = add_extra_ty_param_bound(&input.generics, trait_path);
        let operator_where_clause = quote! {
            where #input_type #ty_generics: #op_path<Output=#input_type #ty_generics>
        };
        add_extra_where_clauses(&generics, operator_where_clause)
    };
    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();

    let initializers: Vec<_> = field_types
        .iter()
        .map(|field_type| {
            quote! { #trait_path::#method_ident(::core::iter::empty::<#field_type>()) }
        })
        .collect();
    let identity = multi_field_data.initializer(&initializers);

    Ok(quote! {
        #[automatically_derived]
        impl #impl_generics #trait_path for #input_type #ty_generics #where_clause {
            #[inline]
            fn #method_ident<I: ::core::iter::Iterator<Item = Self>>(iter: I) -> Self {
                iter.fold(#identity, #op_path::#op_method_ident)
            }
        }
    })
}