//! Watch error types. use std::fmt; #[derive(Debug, Eq, PartialEq)]pub struct NoReceiverError; impl fmt::Display for NoReceiverError {    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {        write!(fmt, "all receivers were dropped")    }} impl std::error::Error for NoReceiverError {} #[derive(Debug, Eq, PartialEq)]pub struct NoSenderError; impl fmt::Display for NoSenderError {    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {        write!(fmt, "sender was dropped")    }} impl std::error::Error for NoSenderError {}