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    /// Associates readiness events with [`event::Source`]s.
    ///
    /// `Token` is a wrapper around `usize` and is used as an argument to
    /// [`Registry::register`] and [`Registry::reregister`].
    ///
    /// See [`Poll`] for more documentation on polling.
    ///
    /// [`event::Source`]: ./event/trait.Source.html
    /// [`Poll`]: struct.Poll.html
    /// [`Registry::register`]: struct.Registry.html#method.register
    /// [`Registry::reregister`]: struct.Registry.html#method.reregister
    ///
    /// # Example
    ///
    /// Using `Token` to track which socket generated the event. In this example,
    /// `HashMap` is used, but usually something like [`slab`] is better.
    ///
    /// [`slab`]: https://crates.io/crates/slab
    ///
    #[cfg_attr(all(feature = "os-poll", features = "net"), doc = "```")]
    #[cfg_attr(not(all(feature = "os-poll", features = "net")), doc = "```ignore")]
    /// # use std::error::Error;
    /// # fn main() -> Result<(), Box<dyn Error>> {
    /// use mio::{Events, Interest, Poll, Token};
    /// use mio::net::TcpListener;
    ///
    /// use std::thread;
    /// use std::io::{self, Read};
    /// use std::collections::HashMap;
    ///
    /// // After this number of sockets is accepted, the server will shutdown.
    /// const MAX_SOCKETS: usize = 32;
    ///
    /// // Pick a token that will not be used by any other socket and use that one
    /// // for the listener.
    /// const LISTENER: Token = Token(1024);
    ///
    /// // Used to store the sockets.
    /// let mut sockets = HashMap::new();
    ///
    /// // This is used to generate a unique token for a socket
    /// let mut next_socket_index = 0;
    ///
    /// // The `Poll` instance
    /// let mut poll = Poll::new()?;
    ///
    /// // Tcp listener
    /// let mut listener = TcpListener::bind("127.0.0.1:0".parse()?)?;
    ///
    /// // Register the listener
    /// poll.registry().register(&mut listener, LISTENER, Interest::READABLE)?;
    ///
    /// // Spawn a thread that will connect a bunch of sockets then close them
    /// let addr = listener.local_addr()?;
    /// thread::spawn(move || {
    ///     use std::net::TcpStream;
    ///
    ///     // +1 here is to connect an extra socket to signal the socket to close
    ///     for _ in 0..(MAX_SOCKETS+1) {
    ///         // Connect then drop the socket
    ///         let _ = TcpStream::connect(addr).unwrap();
    ///     }
    /// });
    ///
    /// // Event storage
    /// let mut events = Events::with_capacity(1024);
    ///
    /// // Read buffer, this will never actually get filled
    /// let mut buf = [0; 256];
    ///
    /// // The main event loop
    /// loop {
    ///     // Wait for events
    ///     poll.poll(&mut events, None)?;
    ///
    ///     for event in &events {
    ///         match event.token() {
    ///             LISTENER => {
    ///                 // Perform operations in a loop until `WouldBlock` is
    ///                 // encountered.
    ///                 loop {
    ///                     match listener.accept() {
    ///                         Ok((mut socket, _)) => {
    ///                             // Shutdown the server
    ///                             if next_socket_index == MAX_SOCKETS {
    ///                                 return Ok(());
    ///                             }
    ///
    ///                             // Get the token for the socket
    ///                             let token = Token(next_socket_index);
    ///                             next_socket_index += 1;
    ///
    ///                             // Register the new socket w/ poll
    ///                             poll.registry().register(&mut socket, token, Interest::READABLE)?;
    ///
    ///                             // Store the socket
    ///                             sockets.insert(token, socket);
    ///                         }
    ///                         Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                             // Socket is not ready anymore, stop accepting
    ///                             break;
    ///                         }
    ///                         e => panic!("err={:?}", e), // Unexpected error
    ///                     }
    ///                 }
    ///             }
    ///             token => {
    ///                 // Always operate in a loop
    ///                 loop {
    ///                     match sockets.get_mut(&token).unwrap().read(&mut buf) {
    ///                         Ok(0) => {
    ///                             // Socket is closed, remove it from the map
    ///                             sockets.remove(&token);
    ///                             break;
    ///                         }
    ///                         // Data is not actually sent in this example
    ///                         Ok(_) => unreachable!(),
    ///                         Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
    ///                             // Socket is not ready anymore, stop reading
    ///                             break;
    ///                         }
    ///                         e => panic!("err={:?}", e), // Unexpected error
    ///                     }
    ///                 }
    ///             }
    ///         }
    ///     }
    /// }
    /// # }
    /// ```
    #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
    pub struct Token(pub usize);

    impl From<Token> for usize {
        fn from(val: Token) -> usize {
            val.0
        }
    }