Function no_na_message 

pub(crate) fn no_na_message(
    param: &str,
    ty: &Type,
    custom: Option<&str>,
) -> String

The condition message of a failed no_na check on parameter param of type ty: the author’s no_na(message = "...") verbatim (custom), else '<param>' must not be NA / '<param>' must not contain NA (no_na_requirement). The R guard builds the same text in R (.miniextendr_arg_error); the C wrapper’s post-conversion check passes this one to arg_check_condition_value.