Function refusal_message 

fn refusal_message<T: TypedExternal>(receiver: SEXP, refused: Refused) -> String

The message of a receiver refusal (#1920): the type’s own (TypedExternal::FIELD_ERROR_MESSAGE), else one naming receiver’s R class, which is what the user sees (the field methods dispatched on it), else, for a receiver without a class, one naming the Rust type.