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use proc_macro::TokenStream; use quote::quote; use syn::{parse_macro_input, ItemStruct}; #[proc_macro_derive(FieldCount)] pub fn derive_field_count(input: TokenStream) -> TokenStream { let input = parse_macro_input!(input as ItemStruct); let name = &input.ident; let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl(); let field_count = input.fields.iter().count(); let output = quote! { impl #impl_generics FieldCount for #name #ty_generics #where_clause { fn field_count() -> usize { #field_count } } }; TokenStream::from(output) }