Function set_check_message 

fn set_check_message(
    slot: &mut Option<String>,
    value: &LitStr,
    check: &str,
) -> Result<()>

Record the message = "..." of an inherits / not_inherits / no_na check: the condition message of a failure, used verbatim. It is given once, is not empty, and holds no NUL (an R string cannot).