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    //! Time error types.

    use self::Kind::*;
    use std::error;
    use std::fmt;

    /// Errors encountered by the timer implementation.
    ///
    /// Currently, there are two different errors that can occur:
    ///
    /// * `shutdown` occurs when a timer operation is attempted, but the timer
    ///   instance has been dropped. In this case, the operation will never be able
    ///   to complete and the `shutdown` error is returned. This is a permanent
    ///   error, i.e., once this error is observed, timer operations will never
    ///   succeed in the future.
    ///
    /// * `at_capacity` occurs when a timer operation is attempted, but the timer
    ///   instance is currently handling its maximum number of outstanding sleep instances.
    ///   In this case, the operation is not able to be performed at the current
    ///   moment, and `at_capacity` is returned. This is a transient error, i.e., at
    ///   some point in the future, if the operation is attempted again, it might
    ///   succeed. Callers that observe this error should attempt to [shed load]. One
    ///   way to do this would be dropping the future that issued the timer operation.
    ///
    /// [shed load]: https://en.wikipedia.org/wiki/Load_Shedding
    #[derive(Debug, Copy, Clone)]
    pub struct Error(Kind);

    #[derive(Debug, Clone, Copy, Eq, PartialEq)]
    #[repr(u8)]
    pub(crate) enum Kind {
        Shutdown = 1,
        AtCapacity = 2,
        Invalid = 3,
    }

    impl From<Kind> for Error {
        fn from(k: Kind) -> Self {
            Error(k)
        }
    }

    /// Error returned by `Timeout`.
    #[derive(Debug, PartialEq)]
    pub struct Elapsed(());

    #[derive(Debug)]
    pub(crate) enum InsertError {
        Elapsed,
    }

    // ===== impl Error =====

    impl Error {
        /// Creates an error representing a shutdown timer.
        pub fn shutdown() -> Error {
            Error(Shutdown)
        }

        /// Returns `true` if the error was caused by the timer being shutdown.
        pub fn is_shutdown(&self) -> bool {
            matches!(self.0, Kind::Shutdown)
        }

        /// Creates an error representing a timer at capacity.
        pub fn at_capacity() -> Error {
            Error(AtCapacity)
        }

        /// Returns `true` if the error was caused by the timer being at capacity.
        pub fn is_at_capacity(&self) -> bool {
            matches!(self.0, Kind::AtCapacity)
        }

        /// Create an error representing a misconfigured timer.
        pub fn invalid() -> Error {
            Error(Invalid)
        }

        /// Returns `true` if the error was caused by the timer being misconfigured.
        pub fn is_invalid(&self) -> bool {
            matches!(self.0, Kind::Invalid)
        }
    }

    impl error::Error for Error {}

    impl fmt::Display for Error {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            use self::Kind::*;
            let descr = match self.0 {
                Shutdown => "the timer is shutdown, must be called from the context of Tokio runtime",
                AtCapacity => "timer is at capacity and cannot create a new entry",
                Invalid => "timer duration exceeds maximum duration",
            };
            write!(fmt, "{}", descr)
        }
    }

    // ===== impl Elapsed =====

    impl Elapsed {
        pub(crate) fn new() -> Self {
            Elapsed(())
        }
    }

    impl fmt::Display for Elapsed {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            "deadline has elapsed".fmt(fmt)
        }
    }

    impl std::error::Error for Elapsed {}

    impl From<Elapsed> for std::io::Error {
        fn from(_err: Elapsed) -> std::io::Error {
            std::io::ErrorKind::TimedOut.into()
        }
    }