time: move DelayQueue to tokio-util (#2897)

This change is intended to do the minimum to unblock 0.3; as such, for now, we
duplicate the internal `time::wheel` structures in tokio-util, rather than trying
to refactor things at this stage.

Co-authored-by: Bryan Donlan <bdonlan@amazon.com>

2 files changed, 0 insertions, 904 deletions

diff --git a/tokio/src/time/delay_queue.rs b/tokio/src/time/delay_queue.rs
deleted file mode 100644
index 910f75fb..00000000
--- a/tokio/src/time/delay_queue.rs
+++ /dev/null
@@ -1,897 +0,0 @@
-//! A queue of delayed elements.
-//!
-//! See [`DelayQueue`] for more details.
-//!
-//! [`DelayQueue`]: struct@DelayQueue
-
-use crate::time::wheel::{self, Wheel};
-use crate::time::{sleep_until, Delay, Duration, Error, Instant};
-
-use slab::Slab;
-use std::cmp;
-use std::future::Future;
-use std::marker::PhantomData;
-use std::pin::Pin;
-use std::task::{self, Poll};
-
-/// A queue of delayed elements.
-///
-/// Once an element is inserted into the `DelayQueue`, it is yielded once the
-/// specified deadline has been reached.
-///
-/// # Usage
-///
-/// Elements are inserted into `DelayQueue` using the [`insert`] or
-/// [`insert_at`] methods. A deadline is provided with the item and a [`Key`] is
-/// returned. The key is used to remove the entry or to change the deadline at
-/// which it should be yielded back.
-///
-/// Once delays have been configured, the `DelayQueue` is used via its
-/// [`Stream`] implementation. [`poll_expired`] is called. If an entry has reached its
-/// deadline, it is returned. If not, `Poll::Pending` indicating that the
-/// current task will be notified once the deadline has been reached.
-///
-/// # `Stream` implementation
-///
-/// Items are retrieved from the queue via [`DelayQueue::poll_expired`]. If no delays have
-/// expired, no items are returned. In this case, `NotReady` is returned and the
-/// current task is registered to be notified once the next item's delay has
-/// expired.
-///
-/// If no items are in the queue, i.e. `is_empty()` returns `true`, then `poll`
-/// returns `Ready(None)`. This indicates that the stream has reached an end.
-/// However, if a new item is inserted *after*, `poll` will once again start
-/// returning items or `NotReady.
-///
-/// Items are returned ordered by their expirations. Items that are configured
-/// to expire first will be returned first. There are no ordering guarantees
-/// for items configured to expire the same instant. Also note that delays are
-/// rounded to the closest millisecond.
-///
-/// # Implementation
-///
-/// The [`DelayQueue`] is backed by a separate instance of the same timer wheel used internally by
-/// Tokio's standalone timer utilities such as [`sleep`]. Because of this, it offers the same
-/// performance and scalability benefits.
-///
-/// State associated with each entry is stored in a [`slab`]. This amortizes the cost of allocation,
-/// and allows reuse of the memory allocated for expired entires.
-///
-/// Capacity can be checked using [`capacity`] and allocated preemptively by using
-/// the [`reserve`] method.
-///
-/// # Usage
-///
-/// Using `DelayQueue` to manage cache entries.
-///
-/// ```rust,no_run
-/// use tokio::time::{delay_queue, DelayQueue, Error};
-///
-/// use futures::ready;
-/// use std::collections::HashMap;
-/// use std::task::{Context, Poll};
-/// use std::time::Duration;
-/// # type CacheKey = String;
-/// # type Value = String;
-///
-/// struct Cache {
-///     entries: HashMap<CacheKey, (Value, delay_queue::Key)>,
-///     expirations: DelayQueue<CacheKey>,
-/// }
-///
-/// const TTL_SECS: u64 = 30;
-///
-/// impl Cache {
-///     fn insert(&mut self, key: CacheKey, value: Value) {
-///         let delay = self.expirations
-///             .insert(key.clone(), Duration::from_secs(TTL_SECS));
-///
-///         self.entries.insert(key, (value, delay));
-///     }
-///
-///     fn get(&self, key: &CacheKey) -> Option<&Value> {
-///         self.entries.get(key)
-///             .map(|&(ref v, _)| v)
-///     }
-///
-///     fn remove(&mut self, key: &CacheKey) {
-///         if let Some((_, cache_key)) = self.entries.remove(key) {
-///             self.expirations.remove(&cache_key);
-///         }
-///     }
-///
-///     fn poll_purge(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Error>> {
-///         while let Some(res) = ready!(self.expirations.poll_expired(cx)) {
-///             let entry = res?;
-///             self.entries.remove(entry.get_ref());
-///         }
-///
-///         Poll::Ready(Ok(()))
-///     }
-/// }
-/// ```
-///
-/// [`insert`]: method@Self::insert
-/// [`insert_at`]: method@Self::insert_at
-/// [`Key`]: struct@Key
-/// [`Stream`]: https://docs.rs/futures/0.1/futures/stream/trait.Stream.html
-/// [`poll_expired`]: method@Self::poll_expired
-/// [`Stream::poll_expired`]: method@Self::poll_expired
-/// [`DelayQueue`]: struct@DelayQueue
-/// [`sleep`]: fn@super::sleep
-/// [`slab`]: slab
-/// [`capacity`]: method@Self::capacity
-/// [`reserve`]: method@Self::reserve
-#[derive(Debug)]
-pub struct DelayQueue<T> {
-    /// Stores data associated with entries
-    slab: Slab<Data<T>>,
-
-    /// Lookup structure tracking all delays in the queue
-    wheel: Wheel<Stack<T>>,
-
-    /// Delays that were inserted when already expired. These cannot be stored
-    /// in the wheel
-    expired: Stack<T>,
-
-    /// Delay expiring when the *first* item in the queue expires
-    delay: Option<Delay>,
-
-    /// Wheel polling state
-    poll: wheel::Poll,
-
-    /// Instant at which the timer starts
-    start: Instant,
-}
-
-/// An entry in `DelayQueue` that has expired and removed.
-///
-/// Values are returned by [`DelayQueue::poll_expired`].
-///
-/// [`DelayQueue::poll_expired`]: method@DelayQueue::poll_expired
-#[derive(Debug)]
-pub struct Expired<T> {
-    /// The data stored in the queue
-    data: T,
-
-    /// The expiration time
-    deadline: Instant,
-
-    /// The key associated with the entry
-    key: Key,
-}
-
-/// Token to a value stored in a `DelayQueue`.
-///
-/// Instances of `Key` are returned by [`DelayQueue::insert`]. See [`DelayQueue`]
-/// documentation for more details.
-///
-/// [`DelayQueue`]: struct@DelayQueue
-/// [`DelayQueue::insert`]: method@DelayQueue::insert
-#[derive(Debug, Clone)]
-pub struct Key {
-    index: usize,
-}
-
-#[derive(Debug)]
-struct Stack<T> {
-    /// Head of the stack
-    head: Option<usize>,
-    _p: PhantomData<fn() -> T>,
-}
-
-#[derive(Debug)]
-struct Data<T> {
-    /// The data being stored in the queue and will be returned at the requested
-    /// instant.
-    inner: T,
-
-    /// The instant at which the item is returned.
-    when: u64,
-
-    /// Set to true when stored in the `expired` queue
-    expired: bool,
-
-    /// Next entry in the stack
-    next: Option<usize>,
-
-    /// Previous entry in the stack
-    prev: Option<usize>,
-}
-
-/// Maximum number of entries the queue can handle
-const MAX_ENTRIES: usize = (1 << 30) - 1;
-
-impl<T> DelayQueue<T> {
-    /// Creates a new, empty, `DelayQueue`
-    ///
-    /// The queue will not allocate storage until items are inserted into it.
-    ///
-    /// # Examples
-    ///
-    /// ```rust
-    /// # use tokio::time::DelayQueue;
-    /// let delay_queue: DelayQueue<u32> = DelayQueue::new();
-    /// ```
-    pub fn new() -> DelayQueue<T> {
-        DelayQueue::with_capacity(0)
-    }
-
-    /// Creates a new, empty, `DelayQueue` with the specified capacity.
-    ///
-    /// The queue will be able to hold at least `capacity` elements without
-    /// reallocating. If `capacity` is 0, the queue will not allocate for
-    /// storage.
-    ///
-    /// # Examples
-    ///
-    /// ```rust
-    /// # use tokio::time::DelayQueue;
-    /// # use std::time::Duration;
-    ///
-    /// # #[tokio::main]
-    /// # async fn main() {
-    ///     let mut delay_queue = DelayQueue::with_capacity(10);
-    ///
-    ///     // These insertions are done without further allocation
-    ///     for i in 0..10 {
-    ///         delay_queue.insert(i, Duration::from_secs(i));
-    ///     }
-    ///
-    ///     // This will make the queue allocate additional storage
-    ///     delay_queue.insert(11, Duration::from_secs(11));
-    /// # }
-    /// ```
-    pub fn with_capacity(capacity: usize) -> DelayQueue<T> {
-        DelayQueue {
-            wheel: Wheel::new(),
-            slab: Slab::with_capacity(capacity),
-            expired: Stack::default(),
-            delay: None,
-            poll: wheel::Poll::new(0),
-            start: Instant::now(),
-        }
-    }
-
-    /// Inserts `value` into the queue set to expire at a specific instant in
-    /// time.
-    ///
-    /// This function is identical to `insert`, but takes an `Instant` instead
-    /// of a `Duration`.
-    ///
-    /// `value` is stored in the queue until `when` is reached. At which point,
-    /// `value` will be returned from [`poll_expired`]. If `when` has already been
-    /// reached, then `value` is immediately made available to poll.
-    ///
-    /// The return value represents the insertion and is used at an argument to
-    /// [`remove`] and [`reset`]. Note that [`Key`] is token and is reused once
-    /// `value` is removed from the queue either by calling [`poll_expired`] after
-    /// `when` is reached or by calling [`remove`]. At this point, the caller
-    /// must take care to not use the returned [`Key`] again as it may reference
-    /// a different item in the queue.
-    ///
-    /// See [type] level documentation for more details.
-    ///
-    /// # Panics
-    ///
-    /// This function panics if `when` is too far in the future.
-    ///
-    /// # Examples
-    ///
-    /// Basic usage
-    ///
-    /// ```rust
-    /// use tokio::time::{DelayQueue, Duration, Instant};
-    ///
-    /// # #[tokio::main]
-    /// # async fn main() {
-    ///     let mut delay_queue = DelayQueue::new();
-    ///     let key = delay_queue.insert_at(
-    ///         "foo", Instant::now() + Duration::from_secs(5));
-    ///
-    ///     // Remove the entry
-    ///     let item = delay_queue.remove(&key);
-    ///     assert_eq!(*item.get_ref(), "foo");
-    /// # }
-    /// ```
-    ///
-    /// [`poll_expired`]: method@Self::poll_expired
-    /// [`remove`]: method@Self::remove
-    /// [`reset`]: method@Self::reset
-    /// [`Key`]: struct@Key
-    /// [type]: #
-    pub fn insert_at(&mut self, value: T, when: Instant) -> Key {
-        assert!(self.slab.len() < MAX_ENTRIES, "max entries exceeded");
-
-        // Normalize the deadline. Values cannot be set to expire in the past.
-        let when = self.normalize_deadline(when);
-
-        // Insert the value in the store
-        let key = self.slab.insert(Data {
-            inner: value,
-            when,
-            expired: false,
-            next: None,
-            prev: None,
-        });
-
-        self.insert_idx(when, key);
-
-        // Set a new delay if the current's deadline is later than the one of the new item
-        let should_set_delay = if let Some(ref delay) = self.delay {
-            let current_exp = self.normalize_deadline(delay.deadline());
-            current_exp > when
-        } else {
-            true
-        };
-
-        if should_set_delay {
-            let delay_time = self.start + Duration::from_millis(when);
-            if let Some(ref mut delay) = &mut self.delay {
-                delay.reset(delay_time);
-            } else {
-                self.delay = Some(sleep_until(delay_time));
-            }
-        }
-
-        Key::new(key)
-    }
-
-    /// Attempts to pull out the next value of the delay queue, registering the
-    /// current task for wakeup if the value is not yet available, and returning
-    /// None if the queue is exhausted.
-    pub fn poll_expired(
-        &mut self,
-        cx: &mut task::Context<'_>,
-    ) -> Poll<Option<Result<Expired<T>, Error>>> {
-        let item = ready!(self.poll_idx(cx));
-        Poll::Ready(item.map(|result| {
-            result.map(|idx| {
-                let data = self.slab.remove(idx);
-                debug_assert!(data.next.is_none());
-                debug_assert!(data.prev.is_none());
-
-                Expired {
-                    key: Key::new(idx),
-                    data: data.inner,
-                    deadline: self.start + Duration::from_millis(data.when),
-                }
-            })
-        }))
-    }
-
-    /// Inserts `value` into the queue set to expire after the requested duration
-    /// elapses.
-    ///
-    /// This function is identical to `insert_at`, but takes a `Duration`
-    /// instead of an `Instant`.
-    ///
-    /// `value` is stored in the queue until `timeout` duration has
-    /// elapsed after `insert` was called. At that point, `value` will
-    /// be returned from [`poll_expired`]. If `timeout` a Duration of
-    /// zero, then `value` is immediately made available to poll.
-    ///
-    /// The return value represents the insertion and is used as an
-    /// argument to [`remove`] and [`reset`]. Note that [`Key`] is a
-    /// token and is reused once `value` is removed from the queue
-    /// either by calling [`poll_expired`] after `timeout` has elapsed
-    /// or by calling [`remove`]. At this point, the caller must not
-    /// use the returned [`Key`] again as it may reference a different
-    /// item in the queue.
-    ///
-    /// See [type] level documentation for more details.
-    ///
-    /// # Panics
-    ///
-    /// This function panics if `timeout` is greater than the maximum
-    /// duration supported by the timer in the current `Runtime`.
-    ///
-    /// # Examples
-    ///
-    /// Basic usage
-    ///
-    /// ```rust
-    /// use tokio::time::DelayQueue;
-    /// use std::time::Duration;
-    ///
-    /// # #[tokio::main]
-    /// # async fn main() {
-    ///     let mut delay_queue = DelayQueue::new();
-    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
-    ///
-    ///     // Remove the entry
-    ///     let item = delay_queue.remove(&key);
-    ///     assert_eq!(*item.get_ref(), "foo");
-    /// # }
-    /// ```
-    ///
-    /// [`poll_expired`]: method@Self::poll_expired
-    /// [`remove`]: method@Self::remove
-    /// [`reset`]: method@Self::reset
-    /// [`Key`]: struct@Key
-    /// [type]: #
-    pub fn insert(&mut self, value: T, timeout: Duration) -> Key {
-        self.insert_at(value, Instant::now() + timeout)
-    }
-
-    fn insert_idx(&mut self, when: u64, key: usize) {
-        use self::wheel::{InsertError, Stack};
-
-        // Register the deadline with the timer wheel
-        match self.wheel.insert(when, key, &mut self.slab) {
-            Ok(_) => {}
-            Err((_, InsertError::Elapsed)) => {
-                self.slab[key].expired = true;
-                // The delay is already expired, store it in the expired queue
-                self.expired.push(key, &mut self.slab);
-            }
-            Err((_, err)) => panic!("invalid deadline; err={:?}", err),
-        }
-    }
-
-    /// Removes the key fom the expired queue or the timer wheel
-    /// depending on its expiration status
-    ///
-    /// # Panics
-    /// Panics if the key is not contained in the expired queue or the wheel
-    fn remove_key(&mut self, key: &Key) {
-        use crate::time::wheel::Stack;
-
-        // Special case the `expired` queue
-        if self.slab[key.index].expired {
-            self.expired.remove(&key.index, &mut self.slab);
-        } else {
-            self.wheel.remove(&key.index, &mut self.slab);
-        }
-    }
-
-    /// Removes the item associated with `key` from the queue.
-    ///
-    /// There must be an item associated with `key`. The function returns the
-    /// removed item as well as the `Instant` at which it will the delay will
-    /// have expired.
-    ///
-    /// # Panics
-    ///
-    /// The function panics if `key` is not contained by the queue.
-    ///
-    /// # Examples
-    ///
-    /// Basic usage
-    ///
-    /// ```rust
-    /// use tokio::time::DelayQueue;
-    /// use std::time::Duration;
-    ///
-    /// # #[tokio::main]
-    /// # async fn main() {
-    ///     let mut delay_queue = DelayQueue::new();
-    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
-    ///
-    ///     // Remove the entry
-    ///     let item = delay_queue.remove(&key);
-    ///     assert_eq!(*item.get_ref(), "foo");
-    /// # }
-    /// ```
-    pub fn remove(&mut self, key: &Key) -> Expired<T> {
-        self.remove_key(key);
-        let data = self.slab.remove(key.index);
-
-        Expired {
-            key: Key::new(key.index),
-            data: data.inner,
-            deadline: self.start + Duration::from_millis(data.when),
-        }
-    }
-
-    /// Sets the delay of the item associated with `key` to expire at `when`.
-    ///
-    /// This function is identical to `reset` but takes an `Instant` instead of
-    /// a `Duration`.
-    ///
-    /// The item remains in the queue but the delay is set to expire at `when`.
-    /// If `when` is in the past, then the item is immediately made available to
-    /// the caller.
-    ///
-    /// # Panics
-    ///
-    /// This function panics if `when` is too far in the future or if `key` is
-    /// not contained by the queue.
-    ///
-    /// # Examples
-    ///
-    /// Basic usage
-    ///
-    /// ```rust
-    /// use tokio::time::{DelayQueue, Duration, Instant};
-    ///
-    /// # #[tokio::main]
-    /// # async fn main() {
-    ///     let mut delay_queue = DelayQueue::new();
-    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
-    ///
-    ///     // "foo" is scheduled to be returned in 5 seconds
-    ///
-    ///     delay_queue.reset_at(&key, Instant::now() + Duration::from_secs(10));
-    ///
-    ///     // "foo"is now scheduled to be returned in 10 seconds
-    /// # }
-    /// ```
-    pub fn reset_at(&mut self, key: &Key, when: Instant) {
-        self.remove_key(key);
-
-        // Normalize the deadline. Values cannot be set to expire in the past.
-        let when = self.normalize_deadline(when);
-
-        self.slab[key.index].when = when;
-        self.insert_idx(when, key.index);
-
-        let next_deadline = self.next_deadline();
-        if let (Some(ref mut delay), Some(deadline)) = (&mut self.delay, next_deadline) {
-            delay.reset(deadline);
-        }
-    }
-
-    /// Returns the next time poll as determined by the wheel
-    fn next_deadline(&mut self) -> Option<Instant> {
-        self.wheel
-            .poll_at()
-            .map(|poll_at| self.start + Duration::from_millis(poll_at))
-    }
-
-    /// Sets the delay of the item associated with `key` to expire after
-    /// `timeout`.
-    ///
-    /// This function is identical to `reset_at` but takes a `Duration` instead
-    /// of an `Instant`.
-    ///
-    /// The item remains in the queue but the delay is set to expire after
-    /// `timeout`. If `timeout` is zero, then the item is immediately made
-    /// available to the caller.
-    ///
-    /// # Panics
-    ///
-    /// This function panics if `timeout` is greater than the maximum supported
-    /// duration or if `key` is not contained by the queue.
-    ///
-    /// # Examples
-    ///
-    /// Basic usage
-    ///
-    /// ```rust
-    /// use tokio::time::DelayQueue;
-    /// use std::time::Duration;
-    ///
-    /// # #[tokio::main]
-    /// # async fn main() {
-    ///     let mut delay_queue = DelayQueue::new();
-    ///     let key = delay_queue.insert("foo", Duration::from_secs(5));
-    ///
-    ///     // "foo" is scheduled to be returned in 5 seconds
-    ///
-    ///     delay_queue.reset(&key, Duration::from_secs(10));
-    ///
-    ///     // "foo"is now scheduled to be returned in 10 seconds
-    /// # }
-    /// ```
-    pub fn reset(&mut self, key: &Key, timeout: Duration) {
-        self.reset_at(key, Instant::now() + timeout);
-    }
-
-    /// Clears the queue, removing all items.
-    ///
-    /// After calling `clear`, [`poll_expired`] will return `Ok(Ready(None))`.
-    ///
-    /// Note that this method has no effect on the allocated capacity.
-    ///
-    /// [`poll_expired`]: method@Self::poll_expired
-    ///
-    /// # Examples
-    ///
-    /// ```rust
-    /// use tokio::time::DelayQueue;
-    /// use std::time::Duration;
-    ///
-    /// # #[tokio::main]
-    /// # async fn main() {
-    ///     let mut delay_queue = DelayQueue::new();
-    ///
-    ///     delay_queue.insert("foo", Duration::from_secs(5));
-    ///
-    ///     assert!(!delay_queue.is_empty());
-    ///
-    ///     delay_queue.clear();
-    ///
-    ///     assert!(delay_queue.is_empty());
-    /// # }
-    /// ```
-    pub fn clear(&mut self) {
-        self.slab.clear();
-        self.expired = Stack::default();
-        self.wheel = Wheel::new();
-        self.delay = None;
-    }
-
-    /// Returns the number of elements the queue can hold without reallocating.
-    ///
-    /// # Examples
-    ///
-    /// ```rust
-    /// use tokio::time::DelayQueue;
-    ///
-    /// let delay_queue: DelayQueue<i32> = DelayQueue::with_capacity(10);
-    /// assert_eq!(delay_queue.capacity(), 10);
-    /// ```
-    pub fn capacity(&self) -> usize {
-        self.slab.capacity()
-    }
-
-    /// Returns the number of elements currently in the queue.
-    ///
-    /// # Examples
-    ///
-    /// ```rust
-    /// use tokio::time::DelayQueue;
-    /// use std::time::Duration;
-    ///
-    /// # #[tokio::main]
-    /// # async fn main() {
-    ///     let mut delay_queue: DelayQueue<i32> = DelayQueue::with_capacity(10);
-    ///     assert_eq!(delay_queue.len(), 0);
-    ///     delay_queue.insert(3, Duration::from_secs(5));
-    ///     assert_eq!(delay_queue.len(), 1);
-    /// # }
-    /// ```
-    pub fn len(&self) -> usize {
-        self.slab.len()
-    }
-
-    /// Reserves capacity for at least `additional` more items to be queued
-    /// without allocating.
-    ///
-    /// `reserve` does nothing if the queue already has sufficient capacity for
-    /// `additional` more values. If more capacity is required, a new segment of
-    /// memory will be allocated and all existing values will be copied into it.
-    /// As such, if the queue is already very large, a call to `reserve` can end
-    /// up being expensive.
-    ///
-    /// The queue may reserve more than `additional` extra space in order to
-    /// avoid frequent reallocations.
-    ///
-    /// # Panics
-    ///
-    /// Panics if the new capacity exceeds the maximum number of entries the
-    /// queue can contain.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use tokio::time::DelayQueue;
-    /// use std::time::Duration;
-    ///
-    /// # #[tokio::main]
-    /// # async fn main() {
-    ///     let mut delay_queue = DelayQueue::new();
-    ///
-    ///     delay_queue.insert("hello", Duration::from_secs(10));
-    ///     delay_queue.reserve(10);
-    ///
-    ///     assert!(delay_queue.capacity() >= 11);
-    /// # }
-    /// ```
-    pub fn reserve(&mut self, additional: usize) {
-        self.slab.reserve(additional);
-    }
-
-    /// Returns `true` if there are no items in the queue.
-    ///
-    /// Note that this function returns `false` even if all items have not yet
-    /// expired and a call to `poll` will return `NotReady`.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use tokio::time::DelayQueue;
-    /// use std::time::Duration;
-    ///
-    /// # #[tokio::main]
-    /// # async fn main() {
-    ///     let mut delay_queue = DelayQueue::new();
-    ///     assert!(delay_queue.is_empty());
-    ///
-    ///     delay_queue.insert("hello", Duration::from_secs(5));
-    ///     assert!(!delay_queue.is_empty());
-    /// # }
-    /// ```
-    pub fn is_empty(&self) -> bool {
-        self.slab.is_empty()
-    }
-
-    /// Polls the queue, returning the index of the next slot in the slab that
-    /// should be returned.
-    ///
-    /// A slot should be returned when the associated deadline has been reached.
-    fn poll_idx(&mut self, cx: &mut task::Context<'_>) -> Poll<Option<Result<usize, Error>>> {
-        use self::wheel::Stack;
-
-        let expired = self.expired.pop(&mut self.slab);
-
-        if expired.is_some() {
-            return Poll::Ready(expired.map(Ok));
-        }
-
-        loop {
-            if let Some(ref mut delay) = self.delay {
-                if !delay.is_elapsed() {
-                    ready!(Pin::new(&mut *delay).poll(cx));
-                }
-
-                let now = crate::time::ms(delay.deadline() - self.start, crate::time::Round::Down);
-
-                self.poll = wheel::Poll::new(now);
-            }
-
-            // We poll the wheel to get the next value out before finding the next deadline.
-            let wheel_idx = self.wheel.poll(&mut self.poll, &mut self.slab);
-
-            self.delay = self.next_deadline().map(sleep_until);
-
-            if let Some(idx) = wheel_idx {
-                return Poll::Ready(Some(Ok(idx)));
-            }
-
-            if self.delay.is_none() {
-                return Poll::Ready(None);
-            }
-        }
-    }
-
-    fn normalize_deadline(&self, when: Instant) -> u64 {
-        let when = if when < self.start {
-            0
-        } else {
-            crate::time::ms(when - self.start, crate::time::Round::Up)
-        };
-
-        cmp::max(when, self.wheel.elapsed())
-    }
-}
-
-// We never put `T` in a `Pin`...
-impl<T> Unpin for DelayQueue<T> {}
-
-impl<T> Default for DelayQueue<T> {
-    fn default() -> DelayQueue<T> {
-        DelayQueue::new()
-    }
-}
-
-#[cfg(feature = "stream")]
-impl<T> futures_core::Stream for DelayQueue<T> {
-    // DelayQueue seems much more specific, where a user may care that it
-    // has reached capacity, so return those errors instead of panicking.
-    type Item = Result<Expired<T>, Error>;
-
-    fn poll_next(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Option<Self::Item>> {
-        DelayQueue::poll_expired(self.get_mut(), cx)
-    }
-}
-
-impl<T> wheel::Stack for Stack<T> {
-    type Owned = usize;
-    type Borrowed = usize;
-    type Store = Slab<Data<T>>;
-
-    fn is_empty(&self) -> bool {
-        self.head.is_none()
-    }
-
-    fn push(&mut self, item: Self::Owned, store: &mut Self::Store) {
-        // Ensure the entry is not already in a stack.
-        debug_assert!(store[item].next.is_none());
-        debug_assert!(store[item].prev.is_none());
-
-        // Remove the old head entry
-        let old = self.head.take();
-
-        if let Some(idx) = old {
-            store[idx].prev = Some(item);
-        }
-
-        store[item].next = old;
-        self.head = Some(item)
-    }
-
-    fn pop(&mut self, store: &mut Self::Store) -> Option<Self::Owned> {
-        if let Some(idx) = self.head {
-            self.head = store[idx].next;
-
-            if let Some(idx) = self.head {
-                store[idx].prev = None;
-            }
-
-            store[idx].next = None;
-            debug_assert!(store[idx].prev.is_none());
-
-            Some(idx)
-        } else {
-            None
-        }
-    }
-
-    fn remove(&mut self, item: &Self::Borrowed, store: &mut Self::Store) {
-        assert!(store.contains(*item));
-
-        // Ensure that the entry is in fact contained by the stack
-        debug_assert!({
-            // This walks the full linked list even if an entry is found.
-            let mut next = self.head;
-            let mut contains = false;
-
-            while let Some(idx) = next {
-                if idx == *item {
-                    debug_assert!(!contains);
-                    contains = true;
-                }
-
-                next = store[idx].next;
-            }
-
-            contains
-        });
-
-        if let Some(next) = store[*item].next {
-            store[next].prev = store[*item].prev;
-        }
-
-        if let Some(prev) = store[*item].prev {
-            store[prev].next = store[*item].next;
-        } else {
-            self.head = store[*item].next;
-        }
-
-        store[*item].next = None;
-        store[*item].prev = None;
-    }
-
-    fn when(item: &Self::Borrowed, store: &Self::Store) -> u64 {
-        store[*item].when
-    }
-}
-
-impl<T> Default for Stack<T> {
-    fn default() -> Stack<T> {
-        Stack {
-            head: None,
-            _p: PhantomData,
-        }
-    }
-}
-
-impl Key {
-    pub(crate) fn new(index: usize) -> Key {
-        Key { index }
-    }
-}
-
-impl<T> Expired<T> {
-    /// Returns a reference to the inner value.
-    pub fn get_ref(&self) -> &T {
-        &self.data
-    }
-
-    /// Returns a mutable reference to the inner value.
-    pub fn get_mut(&mut self) -> &mut T {
-        &mut self.data
-    }
-
-    /// Consumes `self` and returns the inner value.
-    pub fn into_inner(self) -> T {
-        self.data
-    }
-
-    /// Returns the deadline that the expiration was set to.
-    pub fn deadline(&self) -> Instant {
-        self.deadline
-    }
-}
diff --git a/tokio/src/time/mod.rs b/tokio/src/time/mod.rs
index c8c797d9..a68e11b5 100644
--- a/tokio/src/time/mod.rs
+++ b/tokio/src/time/mod.rs
@@ -14,9 +14,6 @@
 //!   of time it is allowed to execute. If the future or stream does not
 //!   complete in time, then it is canceled and an error is returned.
 //!
-//! * `DelayQueue`: A queue where items are returned once the requested delay
-//!   has expired.
-//!
 //! These types are sufficient for handling a large number of scenarios
 //! involving time.
 //!
@@ -96,10 +93,6 @@ pub(crate) use self::clock::Clock;
 #[cfg(feature = "test-util")]
 pub use clock::{advance, pause, resume};
 
-pub mod delay_queue;
-#[doc(inline)]
-pub use delay_queue::DelayQueue;
-
 mod delay;
 pub use delay::{sleep, sleep_until, Delay};