in src/fields/enum_variant.rs [142:151]
fn default_fn_serializer(&self, default_fn_ident: syn::Ident) -> proc_macro2::TokenStream {
quote! {
{
// Call user defined fn to provide a variant that exists in target version.
let new_variant = self.#default_fn_ident(version)?;
// The new_variant will serialize its index and data.
new_variant.serialize(writer, version_map, app_version)?;
},
}
}