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    // Copyright 2018 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    #![unstable(feature = "futures_api",
                reason = "futures in libcore are unstable",
                issue = "50547")]

    //! Types and Traits for working with asynchronous tasks.

    use fmt;
    use ptr::NonNull;
    use future::Future;
    use mem::PinMut;

    /// Indicates whether a value is available or if the current task has been
    /// scheduled to receive a wakeup instead.
    #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
    pub enum Poll<T> {
        /// Represents that a value is immediately ready.
        Ready(T),

        /// Represents that a value is not ready yet.
        ///
        /// When a function returns `Pending`, the function *must* also
        /// ensure that the current task is scheduled to be awoken when
        /// progress can be made.
        Pending,
    }

    impl<T> Poll<T> {
        /// Change the ready value of this `Poll` with the closure provided
        pub fn map<U, F>(self, f: F) -> Poll<U>
            where F: FnOnce(T) -> U
        {
            match self {
                Poll::Ready(t) => Poll::Ready(f(t)),
                Poll::Pending => Poll::Pending,
            }
        }

        /// Returns whether this is `Poll::Ready`
        pub fn is_ready(&self) -> bool {
            match *self {
                Poll::Ready(_) => true,
                Poll::Pending => false,
            }
        }

        /// Returns whether this is `Poll::Pending`
        pub fn is_pending(&self) -> bool {
            !self.is_ready()
        }
    }

    impl<T, E> Poll<Result<T, E>> {
        /// Change the success value of this `Poll` with the closure provided
        pub fn map_ok<U, F>(self, f: F) -> Poll<Result<U, E>>
            where F: FnOnce(T) -> U
        {
            match self {
                Poll::Ready(Ok(t)) => Poll::Ready(Ok(f(t))),
                Poll::Ready(Err(e)) => Poll::Ready(Err(e)),
                Poll::Pending => Poll::Pending,
            }
        }

        /// Change the error value of this `Poll` with the closure provided
        pub fn map_err<U, F>(self, f: F) -> Poll<Result<T, U>>
            where F: FnOnce(E) -> U
        {
            match self {
                Poll::Ready(Ok(t)) => Poll::Ready(Ok(t)),
                Poll::Ready(Err(e)) => Poll::Ready(Err(f(e))),
                Poll::Pending => Poll::Pending,
            }
        }
    }

    impl<T> From<T> for Poll<T> {
        fn from(t: T) -> Poll<T> {
            Poll::Ready(t)
        }
    }

    /// A `Waker` is a handle for waking up a task by notifying its executor that it
    /// is ready to be run.
    ///
    /// This handle contains a trait object pointing to an instance of the `UnsafeWake`
    /// trait, allowing notifications to get routed through it.
    #[repr(transparent)]
    pub struct Waker {
        inner: NonNull<UnsafeWake>,
    }

    unsafe impl Send for Waker {}
    unsafe impl Sync for Waker {}

    impl Waker {
        /// Constructs a new `Waker` directly.
        ///
        /// Note that most code will not need to call this. Implementers of the
        /// `UnsafeWake` trait will typically provide a wrapper that calls this
        /// but you otherwise shouldn't call it directly.
        ///
        /// If you're working with the standard library then it's recommended to
        /// use the `Waker::from` function instead which works with the safe
        /// `Arc` type and the safe `Wake` trait.
        #[inline]
        pub unsafe fn new(inner: NonNull<UnsafeWake>) -> Self {
            Waker { inner: inner }
        }

        /// Wake up the task associated with this `Waker`.
        #[inline]
        pub fn wake(&self) {
            unsafe { self.inner.as_ref().wake() }
        }

        /// Returns whether or not this `Waker` and `other` awaken the same task.
        ///
        /// This function works on a best-effort basis, and may return false even
        /// when the `Waker`s would awaken the same task. However, if this function
        /// returns true, it is guaranteed that the `Waker`s will awaken the same
        /// task.
        ///
        /// This function is primarily used for optimization purposes.
        #[inline]
        pub fn will_wake(&self, other: &Waker) -> bool {
            self.inner == other.inner
        }
    }

    impl Clone for Waker {
        #[inline]
        fn clone(&self) -> Self {
            unsafe {
                self.inner.as_ref().clone_raw()
            }
        }
    }

    impl fmt::Debug for Waker {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Waker")
                .finish()
        }
    }

    impl Drop for Waker {
        #[inline]
        fn drop(&mut self) {
            unsafe {
                self.inner.as_ref().drop_raw()
            }
        }
    }

    /// A `LocalWaker` is a handle for waking up a task by notifying its executor that it
    /// is ready to be run.
    ///
    /// This is similar to the `Waker` type, but cannot be sent across threads.
    /// Task executors can use this type to implement more optimized singlethreaded wakeup
    /// behavior.
    #[repr(transparent)]
    pub struct LocalWaker {
        inner: NonNull<UnsafeWake>,
    }

    impl !Send for LocalWaker {}
    impl !Sync for LocalWaker {}

    impl LocalWaker {
        /// Constructs a new `LocalWaker` directly.
        ///
        /// Note that most code will not need to call this. Implementers of the
        /// `UnsafeWake` trait will typically provide a wrapper that calls this
        /// but you otherwise shouldn't call it directly.
        ///
        /// If you're working with the standard library then it's recommended to
        /// use the `LocalWaker::from` function instead which works with the safe
        /// `Rc` type and the safe `LocalWake` trait.
        ///
        /// For this function to be used safely, it must be sound to call `inner.wake_local()`
        /// on the current thread.
        #[inline]
        pub unsafe fn new(inner: NonNull<UnsafeWake>) -> Self {
            LocalWaker { inner: inner }
        }

        /// Wake up the task associated with this `LocalWaker`.
        #[inline]
        pub fn wake(&self) {
            unsafe { self.inner.as_ref().wake_local() }
        }

        /// Returns whether or not this `LocalWaker` and `other` `LocalWaker` awaken the same task.
        ///
        /// This function works on a best-effort basis, and may return false even
        /// when the `LocalWaker`s would awaken the same task. However, if this function
        /// returns true, it is guaranteed that the `LocalWaker`s will awaken the same
        /// task.
        ///
        /// This function is primarily used for optimization purposes.
        #[inline]
        pub fn will_wake(&self, other: &LocalWaker) -> bool {
            self.inner == other.inner
        }

        /// Returns whether or not this `LocalWaker` and `other` `Waker` awaken the same task.
        ///
        /// This function works on a best-effort basis, and may return false even
        /// when the `Waker`s would awaken the same task. However, if this function
        /// returns true, it is guaranteed that the `LocalWaker`s will awaken the same
        /// task.
        ///
        /// This function is primarily used for optimization purposes.
        #[inline]
        pub fn will_wake_nonlocal(&self, other: &Waker) -> bool {
            self.inner == other.inner
        }
    }

    impl From<LocalWaker> for Waker {
        #[inline]
        fn from(local_waker: LocalWaker) -> Self {
            Waker { inner: local_waker.inner }
        }
    }

    impl Clone for LocalWaker {
        #[inline]
        fn clone(&self) -> Self {
            unsafe {
                LocalWaker { inner: self.inner.as_ref().clone_raw().inner }
            }
        }
    }

    impl fmt::Debug for LocalWaker {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Waker")
                .finish()
        }
    }

    impl Drop for LocalWaker {
        #[inline]
        fn drop(&mut self) {
            unsafe {
                self.inner.as_ref().drop_raw()
            }
        }
    }

    /// An unsafe trait for implementing custom memory management for a `Waker` or `LocalWaker`.
    ///
    /// A `Waker` conceptually is a cloneable trait object for `Wake`, and is
    /// most often essentially just `Arc<dyn Wake>`. However, in some contexts
    /// (particularly `no_std`), it's desirable to avoid `Arc` in favor of some
    /// custom memory management strategy. This trait is designed to allow for such
    /// customization.
    ///
    /// When using `std`, a default implementation of the `UnsafeWake` trait is provided for
    /// `Arc<T>` where `T: Wake` and `Rc<T>` where `T: LocalWake`.
    ///
    /// Although the methods on `UnsafeWake` take pointers rather than references,
    pub unsafe trait UnsafeWake: Send + Sync {
        /// Creates a clone of this `UnsafeWake` and stores it behind a `Waker`.
        ///
        /// This function will create a new uniquely owned handle that under the
        /// hood references the same notification instance. In other words calls
        /// to `wake` on the returned handle should be equivalent to calls to
        /// `wake` on this handle.
        ///
        /// # Unsafety
        ///
        /// This function is unsafe to call because it's asserting the `UnsafeWake`
        /// value is in a consistent state, i.e. hasn't been dropped.
        unsafe fn clone_raw(&self) -> Waker;

        /// Drops this instance of `UnsafeWake`, deallocating resources
        /// associated with it.
        ///
        /// FIXME(cramertj)
        /// This method is intended to have a signature such as:
        ///
        /// ```ignore (not-a-doctest)
        /// fn drop_raw(self: *mut Self);
        /// ```
        ///
        /// Unfortunately in Rust today that signature is not object safe.
        /// Nevertheless it's recommended to implement this function *as if* that
        /// were its signature. As such it is not safe to call on an invalid
        /// pointer, nor is the validity of the pointer guaranteed after this
        /// function returns.
        ///
        /// # Unsafety
        ///
        /// This function is unsafe to call because it's asserting the `UnsafeWake`
        /// value is in a consistent state, i.e. hasn't been dropped.
        unsafe fn drop_raw(&self);

        /// Indicates that the associated task is ready to make progress and should
        /// be `poll`ed.
        ///
        /// Executors generally maintain a queue of "ready" tasks; `wake` should place
        /// the associated task onto this queue.
        ///
        /// # Panics
        ///
        /// Implementations should avoid panicking, but clients should also be prepared
        /// for panics.
        ///
        /// # Unsafety
        ///
        /// This function is unsafe to call because it's asserting the `UnsafeWake`
        /// value is in a consistent state, i.e. hasn't been dropped.
        unsafe fn wake(&self);

        /// Indicates that the associated task is ready to make progress and should
        /// be `poll`ed. This function is the same as `wake`, but can only be called
        /// from the thread that this `UnsafeWake` is "local" to. This allows for
        /// implementors to provide specialized wakeup behavior specific to the current
        /// thread. This function is called by `LocalWaker::wake`.
        ///
        /// Executors generally maintain a queue of "ready" tasks; `wake_local` should place
        /// the associated task onto this queue.
        ///
        /// # Panics
        ///
        /// Implementations should avoid panicking, but clients should also be prepared
        /// for panics.
        ///
        /// # Unsafety
        ///
        /// This function is unsafe to call because it's asserting the `UnsafeWake`
        /// value is in a consistent state, i.e. hasn't been dropped, and that the
        /// `UnsafeWake` hasn't moved from the thread on which it was created.
        unsafe fn wake_local(&self) {
            self.wake()
        }
    }

    /// Information about the currently-running task.
    ///
    /// Contexts are always tied to the stack, since they are set up specifically
    /// when performing a single `poll` step on a task.
    pub struct Context<'a> {
        local_waker: &'a LocalWaker,
        executor: &'a mut Executor,
    }

    impl<'a> fmt::Debug for Context<'a> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Context")
                .finish()
        }
    }

    impl<'a> Context<'a> {
        /// Create a new task `Context` with the provided `local_waker`, `waker`, and `executor`.
        #[inline]
        pub fn new(local_waker: &'a LocalWaker, executor: &'a mut Executor) -> Context<'a> {
            Context {
                local_waker,
                executor,
            }
        }

        /// Get the `LocalWaker` associated with the current task.
        #[inline]
        pub fn local_waker(&self) -> &'a LocalWaker {
            self.local_waker
        }

        /// Get the `Waker` associated with the current task.
        #[inline]
        pub fn waker(&self) -> &'a Waker {
            unsafe { &*(self.local_waker as *const LocalWaker as *const Waker) }
        }

        /// Get the default executor associated with this task.
        ///
        /// This method is useful primarily if you want to explicitly handle
        /// spawn failures.
        #[inline]
        pub fn executor(&mut self) -> &mut Executor {
            self.executor
        }

        /// Produce a context like the current one, but using the given waker instead.
        ///
        /// This advanced method is primarily used when building "internal
        /// schedulers" within a task, where you want to provide some customized
        /// wakeup logic.
        #[inline]
        pub fn with_waker<'b>(&'b mut self, local_waker: &'b LocalWaker) -> Context<'b> {
            Context {
                local_waker,
                executor: self.executor,
            }
        }

        /// Produce a context like the current one, but using the given executor
        /// instead.
        ///
        /// This advanced method is primarily used when building "internal
        /// schedulers" within a task.
        #[inline]
        pub fn with_executor<'b, E>(&'b mut self, executor: &'b mut E) -> Context<'b>
            where E: Executor
        {
            Context {
                local_waker: self.local_waker,
                executor: executor,
            }
        }
    }

    /// A task executor.
    ///
    /// A *task* is a `()`-producing async value that runs at the top level, and will
    /// be `poll`ed until completion. It's also the unit at which wake-up
    /// notifications occur. Executors, such as thread pools, allow tasks to be
    /// spawned and are responsible for putting tasks onto ready queues when
    /// they are woken up, and polling them when they are ready.
    pub trait Executor {
        /// Spawn the given task, polling it until completion.
        ///
        /// # Errors
        ///
        /// The executor may be unable to spawn tasks, either because it has
        /// been shut down or is resource-constrained.
        fn spawn_obj(&mut self, task: TaskObj) -> Result<(), SpawnObjError>;

        /// Determine whether the executor is able to spawn new tasks.
        ///
        /// # Returns
        ///
        /// An `Ok` return means the executor is *likely* (but not guaranteed)
        /// to accept a subsequent spawn attempt. Likewise, an `Err` return
        /// means that `spawn` is likely, but not guaranteed, to yield an error.
        #[inline]
        fn status(&self) -> Result<(), SpawnErrorKind> {
            Ok(())
        }
    }

    /// A custom trait object for polling tasks, roughly akin to
    /// `Box<Future<Output = ()> + Send>`.
    pub struct TaskObj {
        ptr: *mut (),
        poll_fn: unsafe fn(*mut (), &mut Context) -> Poll<()>,
        drop_fn: unsafe fn(*mut ()),
    }

    impl fmt::Debug for TaskObj {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("TaskObj")
                .finish()
        }
    }

    unsafe impl Send for TaskObj {}

    /// A custom implementation of a task trait object for `TaskObj`, providing
    /// a hand-rolled vtable.
    ///
    /// This custom representation is typically used only in `no_std` contexts,
    /// where the default `Box`-based implementation is not available.
    ///
    /// The implementor must guarantee that it is safe to call `poll` repeatedly (in
    /// a non-concurrent fashion) with the result of `into_raw` until `drop` is
    /// called.
    pub unsafe trait UnsafeTask: Send + 'static {
        /// Convert a owned instance into a (conceptually owned) void pointer.
        fn into_raw(self) -> *mut ();

        /// Poll the task represented by the given void pointer.
        ///
        /// # Safety
        ///
        /// The trait implementor must guarantee that it is safe to repeatedly call
        /// `poll` with the result of `into_raw` until `drop` is called; such calls
        /// are not, however, allowed to race with each other or with calls to `drop`.
        unsafe fn poll(task: *mut (), cx: &mut Context) -> Poll<()>;

        /// Drops the task represented by the given void pointer.
        ///
        /// # Safety
        ///
        /// The trait implementor must guarantee that it is safe to call this
        /// function once per `into_raw` invocation; that call cannot race with
        /// other calls to `drop` or `poll`.
        unsafe fn drop(task: *mut ());
    }

    impl TaskObj {
        /// Create a `TaskObj` from a custom trait object representation.
        #[inline]
        pub fn new<T: UnsafeTask>(t: T) -> TaskObj {
            TaskObj {
                ptr: t.into_raw(),
                poll_fn: T::poll,
                drop_fn: T::drop,
            }
        }
    }

    impl Future for TaskObj {
        type Output = ();

        #[inline]
        fn poll(self: PinMut<Self>, cx: &mut Context) -> Poll<()> {
            unsafe {
                (self.poll_fn)(self.ptr, cx)
            }
        }
    }

    impl Drop for TaskObj {
        fn drop(&mut self) {
            unsafe {
                (self.drop_fn)(self.ptr)
            }
        }
    }

    /// Provides the reason that an executor was unable to spawn.
    pub struct SpawnErrorKind {
        _hidden: (),
    }

    impl fmt::Debug for SpawnErrorKind {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("SpawnErrorKind")
                .field(&"shutdown")
                .finish()
        }
    }

    impl SpawnErrorKind {
        /// Spawning is failing because the executor has been shut down.
        pub fn shutdown() -> SpawnErrorKind {
            SpawnErrorKind { _hidden: () }
        }

        /// Check whether this error is the `shutdown` error.
        pub fn is_shutdown(&self) -> bool {
            true
        }
    }

    /// The result of a failed spawn
    #[derive(Debug)]
    pub struct SpawnObjError {
        /// The kind of error
        pub kind: SpawnErrorKind,

        /// The task for which spawning was attempted
        pub task: TaskObj,
    }

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