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    use std::num::NonZeroU8;
    use std::{fmt, ops};

    /// Interest used in registering.
    ///
    /// Interest are used in [registering] [`event::Source`]s with [`Poll`], they
    /// indicate what readiness should be monitored for. For example if a socket is
    /// registered with [readable] interests and the socket becomes writable, no
    /// event will be returned from a call to [`poll`].
    ///
    /// [registering]: struct.Registry.html#method.register
    /// [`event::Source`]: ./event/trait.Source.html
    /// [`Poll`]: struct.Poll.html
    /// [readable]: struct.Interest.html#associatedconstant.READABLE
    /// [`poll`]: struct.Poll.html#method.poll
    #[derive(Copy, PartialEq, Eq, Clone, PartialOrd, Ord)]
    pub struct Interest(NonZeroU8);

    // These must be unique.
    const READABLE: u8 = 0b0_001;
    const WRITABLE: u8 = 0b0_010;
    // The following are not available on all platforms.
    #[cfg_attr(
        not(any(
            target_os = "dragonfly",
            target_os = "freebsd",
            target_os = "ios",
            target_os = "macos"
        )),
        allow(dead_code)
    )]
    const AIO: u8 = 0b0_100;
    #[cfg_attr(not(target_os = "freebsd"), allow(dead_code))]
    const LIO: u8 = 0b1_000;

    impl Interest {
        /// Returns a `Interest` set representing readable interests.
        pub const READABLE: Interest = Interest(unsafe { NonZeroU8::new_unchecked(READABLE) });

        /// Returns a `Interest` set representing writable interests.
        pub const WRITABLE: Interest = Interest(unsafe { NonZeroU8::new_unchecked(WRITABLE) });

        /// Returns a `Interest` set representing AIO completion interests.
        #[cfg(any(
            target_os = "dragonfly",
            target_os = "freebsd",
            target_os = "ios",
            target_os = "macos"
        ))]
        pub const AIO: Interest = Interest(unsafe { NonZeroU8::new_unchecked(AIO) });

        /// Returns a `Interest` set representing LIO completion interests.
        #[cfg(target_os = "freebsd")]
        pub const LIO: Interest = Interest(unsafe { NonZeroU8::new_unchecked(LIO) });

        /// Add together two `Interest`.
        ///
        /// This does the same thing as the `BitOr` implementation, but is a
        /// constant function.
        ///
        /// ```
        /// use mio::Interest;
        ///
        /// const INTERESTS: Interest = Interest::READABLE.add(Interest::WRITABLE);
        /// # fn silent_dead_code_warning(_: Interest) { }
        /// # silent_dead_code_warning(INTERESTS)
        /// ```
        #[allow(clippy::should_implement_trait)]
        pub const fn add(self, other: Interest) -> Interest {
            Interest(unsafe { NonZeroU8::new_unchecked(self.0.get() | other.0.get()) })
        }

        /// Removes `other` `Interest` from `self`.
        ///
        /// Returns `None` if the set would be empty after removing `other`.
        ///
        /// ```
        /// use mio::Interest;
        ///
        /// const RW_INTERESTS: Interest = Interest::READABLE.add(Interest::WRITABLE);
        ///
        /// // As long a one interest remain this will return `Some`.
        /// let w_interest = RW_INTERESTS.remove(Interest::READABLE).unwrap();
        /// assert!(!w_interest.is_readable());
        /// assert!(w_interest.is_writable());
        ///
        /// // Removing all interests from the set will return `None`.
        /// assert_eq!(w_interest.remove(Interest::WRITABLE), None);
        ///
        /// // Its also possible to remove multiple interests at once.
        /// assert_eq!(RW_INTERESTS.remove(RW_INTERESTS), None);
        /// ```
        pub fn remove(self, other: Interest) -> Option<Interest> {
            NonZeroU8::new(self.0.get() & !other.0.get()).map(Interest)
        }

        /// Returns true if the value includes readable readiness.
        pub const fn is_readable(self) -> bool {
            (self.0.get() & READABLE) != 0
        }

        /// Returns true if the value includes writable readiness.
        pub const fn is_writable(self) -> bool {
            (self.0.get() & WRITABLE) != 0
        }

        /// Returns true if `Interest` contains AIO readiness
        pub const fn is_aio(self) -> bool {
            (self.0.get() & AIO) != 0
        }

        /// Returns true if `Interest` contains LIO readiness
        pub const fn is_lio(self) -> bool {
            (self.0.get() & LIO) != 0
        }
    }

    impl ops::BitOr for Interest {
        type Output = Self;

        #[inline]
        fn bitor(self, other: Self) -> Self {
            self.add(other)
        }
    }

    impl ops::BitOrAssign for Interest {
        #[inline]
        fn bitor_assign(&mut self, other: Self) {
            self.0 = (*self | other).0;
        }
    }

    impl fmt::Debug for Interest {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            let mut one = false;
            if self.is_readable() {
                if one {
                    write!(fmt, " | ")?
                }
                write!(fmt, "READABLE")?;
                one = true
            }
            if self.is_writable() {
                if one {
                    write!(fmt, " | ")?
                }
                write!(fmt, "WRITABLE")?;
                one = true
            }
            #[cfg(any(
                target_os = "dragonfly",
                target_os = "freebsd",
                target_os = "ios",
                target_os = "macos"
            ))]
            {
                if self.is_aio() {
                    if one {
                        write!(fmt, " | ")?
                    }
                    write!(fmt, "AIO")?;
                    one = true
                }
            }
            #[cfg(any(target_os = "freebsd"))]
            {
                if self.is_lio() {
                    if one {
                        write!(fmt, " | ")?
                    }
                    write!(fmt, "LIO")?;
                    one = true
                }
            }
            debug_assert!(one, "printing empty interests");
            Ok(())
        }
    }