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#![stable(feature = "futures_api", since = "1.36.0")]
use crate::any::Any;
use crate::marker::PhantomData;
use crate::mem::{transmute, ManuallyDrop};
use crate::panic::AssertUnwindSafe;
use crate::{fmt, ptr};
/// A `RawWaker` allows the implementor of a task executor to create a [`Waker`]
/// or a [`LocalWaker`] which provides customized wakeup behavior.
///
/// It consists of a data pointer and a [virtual function pointer table (vtable)][vtable]
/// that customizes the behavior of the `RawWaker`.
///
/// `RawWaker`s are unsafe to use.
/// Implementing the [`Wake`] trait is a safe alternative that requires memory allocation.
///
/// [vtable]: https://en.wikipedia.org/wiki/Virtual_method_table
/// [`Wake`]: ../../alloc/task/trait.Wake.html
#[derive(PartialEq, Debug)]
#[stable(feature = "futures_api", since = "1.36.0")]
pub struct RawWaker {
/// A data pointer, which can be used to store arbitrary data as required
/// by the executor. This could be e.g. a type-erased pointer to an `Arc`
/// that is associated with the task.
/// The value of this field gets passed to all functions that are part of
/// the vtable as the first parameter.
data: *const (),
/// Virtual function pointer table that customizes the behavior of this waker.
vtable: &'static RawWakerVTable,
}
impl RawWaker {
/// Creates a new `RawWaker` from the provided `data` pointer and `vtable`.
///
/// The `data` pointer can be used to store arbitrary data as required
/// by the executor. This could be e.g. a type-erased pointer to an `Arc`
/// that is associated with the task.
/// The value of this pointer will get passed to all functions that are part
/// of the `vtable` as the first parameter.
///
/// It is important to consider that the `data` pointer must point to a
/// thread safe type such as an `[Arc]<T: Send + Sync>`
/// when used to construct a [`Waker`]. This restriction is lifted when
/// constructing a [`LocalWaker`], which allows using types that do not implement
/// <code>[Send] + [Sync]</code> like `[Rc]<T>`.
///
/// The `vtable` customizes the behavior of a `Waker` which gets created
/// from a `RawWaker`. For each operation on the `Waker`, the associated
/// function in the `vtable` of the underlying `RawWaker` will be called.
///
/// [`Arc`]: std::sync::Arc
/// [`Rc`]: std::rc::Rc
#[inline]
#[rustc_promotable]
#[stable(feature = "futures_api", since = "1.36.0")]
#[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
#[must_use]
pub const fn new(data: *const (), vtable: &'static RawWakerVTable) -> RawWaker {
RawWaker { data, vtable }
}
/// Gets the `data` pointer used to create this `RawWaker`.
#[inline]
#[must_use]
#[unstable(feature = "waker_getters", issue = "96992")]
pub fn data(&self) -> *const () {
self.data
}
/// Gets the `vtable` pointer used to create this `RawWaker`.
#[inline]
#[must_use]
#[unstable(feature = "waker_getters", issue = "96992")]
pub fn vtable(&self) -> &'static RawWakerVTable {
self.vtable
}
#[unstable(feature = "noop_waker", issue = "98286")]
const NOOP: RawWaker = {
const VTABLE: RawWakerVTable = RawWakerVTable::new(
// Cloning just returns a new no-op raw waker
|_| RawWaker::NOOP,
// `wake` does nothing
|_| {},
// `wake_by_ref` does nothing
|_| {},
// Dropping does nothing as we don't allocate anything
|_| {},
);
RawWaker::new(ptr::null(), &VTABLE)
};
}
/// A virtual function pointer table (vtable) that specifies the behavior
/// of a [`RawWaker`].
///
/// The pointer passed to all functions inside the vtable is the `data` pointer
/// from the enclosing [`RawWaker`] object.
///
/// The functions inside this struct are only intended to be called on the `data`
/// pointer of a properly constructed [`RawWaker`] object from inside the
/// [`RawWaker`] implementation. Calling one of the contained functions using
/// any other `data` pointer will cause undefined behavior.
///
/// Note that while this type implements `PartialEq`, comparing function pointers, and hence
/// comparing structs like this that contain function pointers, is unreliable: pointers to the same
/// function can compare inequal (because functions are duplicated in multiple codegen units), and
/// pointers to *different* functions can compare equal (since identical functions can be
/// deduplicated within a codegen unit).
///
/// # Thread safety
/// If the [`RawWaker`] will be used to construct a [`Waker`] then
/// these functions must all be thread-safe (even though [`RawWaker`] is
/// <code>\![Send] + \![Sync]</code>). This is because [`Waker`] is <code>[Send] + [Sync]</code>,
/// and it may be moved to arbitrary threads or invoked by `&` reference. For example,
/// this means that if the `clone` and `drop` functions manage a reference count,
/// they must do so atomically.
///
/// However, if the [`RawWaker`] will be used to construct a [`LocalWaker`] instead, then
/// these functions don't need to be thread safe. This means that <code>\![Send] + \![Sync]</code>
/// data can be stored in the data pointer, and reference counting does not need any atomic
/// synchronization. This is because [`LocalWaker`] is not thread safe itself, so it cannot
/// be sent across threads.
#[stable(feature = "futures_api", since = "1.36.0")]
#[derive(PartialEq, Copy, Clone, Debug)]
pub struct RawWakerVTable {
/// This function will be called when the [`RawWaker`] gets cloned, e.g. when
/// the [`Waker`] in which the [`RawWaker`] is stored gets cloned.
///
/// The implementation of this function must retain all resources that are
/// required for this additional instance of a [`RawWaker`] and associated
/// task. Calling `wake` on the resulting [`RawWaker`] should result in a wakeup
/// of the same task that would have been awoken by the original [`RawWaker`].
clone: unsafe fn(*const ()) -> RawWaker,
/// This function will be called when `wake` is called on the [`Waker`].
/// It must wake up the task associated with this [`RawWaker`].
///
/// The implementation of this function must make sure to release any
/// resources that are associated with this instance of a [`RawWaker`] and
/// associated task.
wake: unsafe fn(*const ()),
/// This function will be called when `wake_by_ref` is called on the [`Waker`].
/// It must wake up the task associated with this [`RawWaker`].
///
/// This function is similar to `wake`, but must not consume the provided data
/// pointer.
wake_by_ref: unsafe fn(*const ()),
/// This function will be called when a [`Waker`] gets dropped.
///
/// The implementation of this function must make sure to release any
/// resources that are associated with this instance of a [`RawWaker`] and
/// associated task.
drop: unsafe fn(*const ()),
}
impl RawWakerVTable {
/// Creates a new `RawWakerVTable` from the provided `clone`, `wake`,
/// `wake_by_ref`, and `drop` functions.
///
/// If the [`RawWaker`] will be used to construct a [`Waker`] then
/// these functions must all be thread-safe (even though [`RawWaker`] is
/// <code>\![Send] + \![Sync]</code>). This is because [`Waker`] is <code>[Send] + [Sync]</code>,
/// and it may be moved to arbitrary threads or invoked by `&` reference. For example,
/// this means that if the `clone` and `drop` functions manage a reference count,
/// they must do so atomically.
///
/// However, if the [`RawWaker`] will be used to construct a [`LocalWaker`] instead, then
/// these functions don't need to be thread safe. This means that <code>\![Send] + \![Sync]</code>
/// data can be stored in the data pointer, and reference counting does not need any atomic
/// synchronization. This is because [`LocalWaker`] is not thread safe itself, so it cannot
/// be sent across threads.
/// # `clone`
///
/// This function will be called when the [`RawWaker`] gets cloned, e.g. when
/// the [`Waker`]/[`LocalWaker`] in which the [`RawWaker`] is stored gets cloned.
///
/// The implementation of this function must retain all resources that are
/// required for this additional instance of a [`RawWaker`] and associated
/// task. Calling `wake` on the resulting [`RawWaker`] should result in a wakeup
/// of the same task that would have been awoken by the original [`RawWaker`].
///
/// # `wake`
///
/// This function will be called when `wake` is called on the [`Waker`].
/// It must wake up the task associated with this [`RawWaker`].
///
/// The implementation of this function must make sure to release any
/// resources that are associated with this instance of a [`RawWaker`] and
/// associated task.
///
/// # `wake_by_ref`
///
/// This function will be called when `wake_by_ref` is called on the [`Waker`].
/// It must wake up the task associated with this [`RawWaker`].
///
/// This function is similar to `wake`, but must not consume the provided data
/// pointer.
///
/// # `drop`
///
/// This function will be called when a [`Waker`]/[`LocalWaker`] gets
/// dropped.
///
/// The implementation of this function must make sure to release any
/// resources that are associated with this instance of a [`RawWaker`] and
/// associated task.
#[rustc_promotable]
#[stable(feature = "futures_api", since = "1.36.0")]
#[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
pub const fn new(
clone: unsafe fn(*const ()) -> RawWaker,
wake: unsafe fn(*const ()),
wake_by_ref: unsafe fn(*const ()),
drop: unsafe fn(*const ()),
) -> Self {
Self { clone, wake, wake_by_ref, drop }
}
}
#[derive(Debug)]
enum ExtData<'a> {
Some(&'a mut dyn Any),
None(()),
}
/// The context of an asynchronous task.
///
/// Currently, `Context` only serves to provide access to a [`&Waker`](Waker)
/// which can be used to wake the current task.
#[stable(feature = "futures_api", since = "1.36.0")]
#[lang = "Context"]
pub struct Context<'a> {
waker: &'a Waker,
local_waker: &'a LocalWaker,
ext: AssertUnwindSafe<ExtData<'a>>,
// Ensure we future-proof against variance changes by forcing
// the lifetime to be invariant (argument-position lifetimes
// are contravariant while return-position lifetimes are
// covariant).
_marker: PhantomData<fn(&'a ()) -> &'a ()>,
// Ensure `Context` is `!Send` and `!Sync` in order to allow
// for future `!Send` and / or `!Sync` fields.
_marker2: PhantomData<*mut ()>,
}
impl<'a> Context<'a> {
/// Creates a new `Context` from a [`&Waker`](Waker).
#[stable(feature = "futures_api", since = "1.36.0")]
#[rustc_const_stable(feature = "const_waker", since = "1.82.0")]
#[must_use]
#[inline]
pub const fn from_waker(waker: &'a Waker) -> Self {
ContextBuilder::from_waker(waker).build()
}
/// Returns a reference to the [`Waker`] for the current task.
#[inline]
#[must_use]
#[stable(feature = "futures_api", since = "1.36.0")]
#[rustc_const_stable(feature = "const_waker", since = "1.82.0")]
pub const fn waker(&self) -> &'a Waker {
&self.waker
}
/// Returns a reference to the [`LocalWaker`] for the current task.
#[inline]
#[unstable(feature = "local_waker", issue = "118959")]
#[rustc_const_unstable(feature = "local_waker", issue = "118959")]
pub const fn local_waker(&self) -> &'a LocalWaker {
&self.local_waker
}
/// Returns a reference to the extension data for the current task.
#[inline]
#[unstable(feature = "context_ext", issue = "123392")]
#[rustc_const_unstable(feature = "context_ext", issue = "123392")]
pub const fn ext(&mut self) -> &mut dyn Any {
// FIXME: this field makes Context extra-weird about unwind safety
// can we justify AssertUnwindSafe if we stabilize this? do we care?
match &mut self.ext.0 {
ExtData::Some(data) => *data,
ExtData::None(unit) => unit,
}
}
}
#[stable(feature = "futures_api", since = "1.36.0")]
impl fmt::Debug for Context<'_> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Context").field("waker", &self.waker).finish()
}
}
/// A Builder used to construct a `Context` instance
/// with support for `LocalWaker`.
///
/// # Examples
/// ```
/// #![feature(local_waker)]
/// #![feature(noop_waker)]
/// use std::task::{ContextBuilder, LocalWaker, Waker, Poll};
/// use std::future::Future;
///
/// let local_waker = LocalWaker::noop();
/// let waker = Waker::noop();
///
/// let mut cx = ContextBuilder::from_waker(&waker)
/// .local_waker(&local_waker)
/// .build();
///
/// let mut future = std::pin::pin!(async { 20 });
/// let poll = future.as_mut().poll(&mut cx);
/// assert_eq!(poll, Poll::Ready(20));
///
/// ```
#[unstable(feature = "local_waker", issue = "118959")]
#[derive(Debug)]
pub struct ContextBuilder<'a> {
waker: &'a Waker,
local_waker: &'a LocalWaker,
ext: ExtData<'a>,
// Ensure we future-proof against variance changes by forcing
// the lifetime to be invariant (argument-position lifetimes
// are contravariant while return-position lifetimes are
// covariant).
_marker: PhantomData<fn(&'a ()) -> &'a ()>,
// Ensure `Context` is `!Send` and `!Sync` in order to allow
// for future `!Send` and / or `!Sync` fields.
_marker2: PhantomData<*mut ()>,
}
impl<'a> ContextBuilder<'a> {
/// Creates a ContextBuilder from a Waker.
#[inline]
#[unstable(feature = "local_waker", issue = "118959")]
#[rustc_const_stable(feature = "const_waker", since = "1.82.0")]
pub const fn from_waker(waker: &'a Waker) -> Self {
// SAFETY: LocalWaker is just Waker without thread safety
let local_waker = unsafe { transmute(waker) };
Self {
waker: waker,
local_waker,
ext: ExtData::None(()),
_marker: PhantomData,
_marker2: PhantomData,
}
}
/// Creates a ContextBuilder from an existing Context.
#[inline]
#[unstable(feature = "context_ext", issue = "123392")]
#[rustc_const_unstable(feature = "context_ext", issue = "123392")]
pub const fn from(cx: &'a mut Context<'_>) -> Self {
let ext = match &mut cx.ext.0 {
ExtData::Some(ext) => ExtData::Some(*ext),
ExtData::None(()) => ExtData::None(()),
};
Self {
waker: cx.waker,
local_waker: cx.local_waker,
ext,
_marker: PhantomData,
_marker2: PhantomData,
}
}
/// Sets the value for the waker on `Context`.
#[inline]
#[unstable(feature = "context_ext", issue = "123392")]
#[rustc_const_unstable(feature = "context_ext", issue = "123392")]
pub const fn waker(self, waker: &'a Waker) -> Self {
Self { waker, ..self }
}
/// Sets the value for the local waker on `Context`.
#[inline]
#[unstable(feature = "local_waker", issue = "118959")]
#[rustc_const_unstable(feature = "local_waker", issue = "118959")]
pub const fn local_waker(self, local_waker: &'a LocalWaker) -> Self {
Self { local_waker, ..self }
}
/// Sets the value for the extension data on `Context`.
#[inline]
#[unstable(feature = "context_ext", issue = "123392")]
#[rustc_const_unstable(feature = "context_ext", issue = "123392")]
pub const fn ext(self, data: &'a mut dyn Any) -> Self {
Self { ext: ExtData::Some(data), ..self }
}
/// Builds the `Context`.
#[inline]
#[unstable(feature = "local_waker", issue = "118959")]
#[rustc_const_stable(feature = "const_waker", since = "1.82.0")]
pub const fn build(self) -> Context<'a> {
let ContextBuilder { waker, local_waker, ext, _marker, _marker2 } = self;
Context { waker, local_waker, ext: AssertUnwindSafe(ext), _marker, _marker2 }
}
}
/// A `Waker` is a handle for waking up a task by notifying its executor that it
/// is ready to be run.
///
/// This handle encapsulates a [`RawWaker`] instance, which defines the
/// executor-specific wakeup behavior.
///
/// The typical life of a `Waker` is that it is constructed by an executor, wrapped in a
/// [`Context`], then passed to [`Future::poll()`]. Then, if the future chooses to return
/// [`Poll::Pending`], it must also store the waker somehow and call [`Waker::wake()`] when
/// the future should be polled again.
///
/// Implements [`Clone`], [`Send`], and [`Sync`]; therefore, a waker may be invoked
/// from any thread, including ones not in any way managed by the executor. For example,
/// this might be done to wake a future when a blocking function call completes on another
/// thread.
///
/// Note that it is preferable to use `waker.clone_from(&new_waker)` instead
/// of `*waker = new_waker.clone()`, as the former will avoid cloning the waker
/// unnecessarily if the two wakers [wake the same task](Self::will_wake).
///
/// Constructing a `Waker` from a [`RawWaker`] is unsafe.
/// Implementing the [`Wake`] trait is a safe alternative that requires memory allocation.
///
/// [`Future::poll()`]: core::future::Future::poll
/// [`Poll::Pending`]: core::task::Poll::Pending
/// [`Wake`]: ../../alloc/task/trait.Wake.html
#[repr(transparent)]
#[stable(feature = "futures_api", since = "1.36.0")]
pub struct Waker {
waker: RawWaker,
}
#[stable(feature = "futures_api", since = "1.36.0")]
impl Unpin for Waker {}
#[stable(feature = "futures_api", since = "1.36.0")]
unsafe impl Send for Waker {}
#[stable(feature = "futures_api", since = "1.36.0")]
unsafe impl Sync for Waker {}
impl Waker {
/// Wakes up the task associated with this `Waker`.
///
/// As long as the executor keeps running and the task is not finished, it is
/// guaranteed that each invocation of [`wake()`](Self::wake) (or
/// [`wake_by_ref()`](Self::wake_by_ref)) will be followed by at least one
/// [`poll()`] of the task to which this `Waker` belongs. This makes
/// it possible to temporarily yield to other tasks while running potentially
/// unbounded processing loops.
///
/// Note that the above implies that multiple wake-ups may be coalesced into a
/// single [`poll()`] invocation by the runtime.
///
/// Also note that yielding to competing tasks is not guaranteed: it is the
/// executor’s choice which task to run and the executor may choose to run the
/// current task again.
///
/// [`poll()`]: crate::future::Future::poll
#[inline]
#[stable(feature = "futures_api", since = "1.36.0")]
pub fn wake(self) {
// The actual wakeup call is delegated through a virtual function call
// to the implementation which is defined by the executor.
// Don't call `drop` -- the waker will be consumed by `wake`.
let this = ManuallyDrop::new(self);
// SAFETY: This is safe because `Waker::from_raw` is the only way
// to initialize `wake` and `data` requiring the user to acknowledge
// that the contract of `RawWaker` is upheld.
unsafe { (this.waker.vtable.wake)(this.waker.data) };
}
/// Wakes up the task associated with this `Waker` without consuming the `Waker`.
///
/// This is similar to [`wake()`](Self::wake), but may be slightly less efficient in
/// the case where an owned `Waker` is available. This method should be preferred to
/// calling `waker.clone().wake()`.
#[inline]
#[stable(feature = "futures_api", since = "1.36.0")]
pub fn wake_by_ref(&self) {
// The actual wakeup call is delegated through a virtual function call
// to the implementation which is defined by the executor.
// SAFETY: see `wake`
unsafe { (self.waker.vtable.wake_by_ref)(self.waker.data) }
}
/// Returns `true` if this `Waker` and another `Waker` would awake 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 — for example,
/// this type's [`clone_from`](Self::clone_from) implementation uses it to
/// avoid cloning the waker when they would wake the same task anyway.
#[inline]
#[must_use]
#[stable(feature = "futures_api", since = "1.36.0")]
pub fn will_wake(&self, other: &Waker) -> bool {
// We optimize this by comparing vtable addresses instead of vtable contents.
// This is permitted since the function is documented as best-effort.
let RawWaker { data: a_data, vtable: a_vtable } = self.waker;
let RawWaker { data: b_data, vtable: b_vtable } = other.waker;
a_data == b_data && ptr::eq(a_vtable, b_vtable)
}
/// Creates a new `Waker` from [`RawWaker`].
///
/// # Safety
///
/// The behavior of the returned `Waker` is undefined if the contract defined
/// in [`RawWaker`]'s and [`RawWakerVTable`]'s documentation is not upheld.
///
/// (Authors wishing to avoid unsafe code may implement the [`Wake`] trait instead, at the
/// cost of a required heap allocation.)
///
/// [`Wake`]: ../../alloc/task/trait.Wake.html
#[inline]
#[must_use]
#[stable(feature = "futures_api", since = "1.36.0")]
#[rustc_const_stable(feature = "const_waker", since = "1.82.0")]
pub const unsafe fn from_raw(waker: RawWaker) -> Waker {
Waker { waker }
}
/// Returns a reference to a `Waker` that does nothing when used.
///
// Note! Much of the documentation for this method is duplicated
// in the docs for `LocalWaker::noop`.
// If you edit it, consider editing the other copy too.
//
/// This is mostly useful for writing tests that need a [`Context`] to poll
/// some futures, but are not expecting those futures to wake the waker or
/// do not need to do anything specific if it happens.
///
/// More generally, using `Waker::noop()` to poll a future
/// means discarding the notification of when the future should be polled again.
/// So it should only be used when such a notification will not be needed to make progress.
///
/// If an owned `Waker` is needed, `clone()` this one.
///
/// # Examples
///
/// ```
/// #![feature(noop_waker)]
///
/// use std::future::Future;
/// use std::task;
///
/// let mut cx = task::Context::from_waker(task::Waker::noop());
///
/// let mut future = Box::pin(async { 10 });
/// assert_eq!(future.as_mut().poll(&mut cx), task::Poll::Ready(10));
/// ```
#[inline]
#[must_use]
#[unstable(feature = "noop_waker", issue = "98286")]
pub const fn noop() -> &'static Waker {
const WAKER: &Waker = &Waker { waker: RawWaker::NOOP };
WAKER
}
/// Gets a reference to the underlying [`RawWaker`].
#[inline]
#[must_use]
#[unstable(feature = "waker_getters", issue = "96992")]
pub fn as_raw(&self) -> &RawWaker {
&self.waker
}
}
#[stable(feature = "futures_api", since = "1.36.0")]
impl Clone for Waker {
#[inline]
fn clone(&self) -> Self {
Waker {
// SAFETY: This is safe because `Waker::from_raw` is the only way
// to initialize `clone` and `data` requiring the user to acknowledge
// that the contract of [`RawWaker`] is upheld.
waker: unsafe { (self.waker.vtable.clone)(self.waker.data) },
}
}
/// Assigns a clone of `source` to `self`, unless [`self.will_wake(source)`][Waker::will_wake] anyway.
///
/// This method is preferred over simply assigning `source.clone()` to `self`,
/// as it avoids cloning the waker if `self` is already the same waker.
///
/// # Examples
///
/// ```
/// use std::future::Future;
/// use std::pin::Pin;
/// use std::sync::{Arc, Mutex};
/// use std::task::{Context, Poll, Waker};
///
/// struct Waiter {
/// shared: Arc<Mutex<Shared>>,
/// }
///
/// struct Shared {
/// waker: Waker,
/// // ...
/// }
///
/// impl Future for Waiter {
/// type Output = ();
/// fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
/// let mut shared = self.shared.lock().unwrap();
///
/// // update the waker
/// shared.waker.clone_from(cx.waker());
///
/// // readiness logic ...
/// # Poll::Ready(())
/// }
/// }
///
/// ```
#[inline]
fn clone_from(&mut self, source: &Self) {
if !self.will_wake(source) {
*self = source.clone();
}
}
}
#[stable(feature = "futures_api", since = "1.36.0")]
impl Drop for Waker {
#[inline]
fn drop(&mut self) {
// SAFETY: This is safe because `Waker::from_raw` is the only way
// to initialize `drop` and `data` requiring the user to acknowledge
// that the contract of `RawWaker` is upheld.
unsafe { (self.waker.vtable.drop)(self.waker.data) }
}
}
#[stable(feature = "futures_api", since = "1.36.0")]
impl fmt::Debug for Waker {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let vtable_ptr = self.waker.vtable as *const RawWakerVTable;
f.debug_struct("Waker")
.field("data", &self.waker.data)
.field("vtable", &vtable_ptr)
.finish()
}
}
/// A `LocalWaker` is analogous to a [`Waker`], but it does not implement [`Send`] or [`Sync`].
///
/// This handle encapsulates a [`RawWaker`] instance, which defines the
/// executor-specific wakeup behavior.
///
/// Local wakers can be requested from a `Context` with the [`local_waker`] method.
///
/// The typical life of a `LocalWaker` is that it is constructed by an executor, wrapped in a
/// [`Context`] using [`ContextBuilder`], then passed to [`Future::poll()`]. Then, if the future chooses to return
/// [`Poll::Pending`], it must also store the waker somehow and call [`LocalWaker::wake()`] when
/// the future should be polled again.
///
/// Implements [`Clone`], but neither [`Send`] nor [`Sync`]; therefore, a local waker may
/// not be moved to other threads. In general, when deciding to use wakers or local wakers,
/// local wakers are preferable unless the waker needs to be sent across threads. This is because
/// wakers can incur in additional cost related to memory synchronization.
///
/// Note that it is preferable to use `local_waker.clone_from(&new_waker)` instead
/// of `*local_waker = new_waker.clone()`, as the former will avoid cloning the waker
/// unnecessarily if the two wakers [wake the same task](Self::will_wake).
///
/// # Examples
/// Usage of a local waker to implement a future analogous to `std::thread::yield_now()`.
/// ```
/// #![feature(local_waker)]
/// use std::future::{Future, poll_fn};
/// use std::task::Poll;
///
/// // a future that returns pending once.
/// fn yield_now() -> impl Future<Output=()> + Unpin {
/// let mut yielded = false;
/// poll_fn(move |cx| {
/// if !yielded {
/// yielded = true;
/// cx.local_waker().wake_by_ref();
/// return Poll::Pending;
/// }
/// return Poll::Ready(())
/// })
/// }
///
/// # async fn __() {
/// yield_now().await;
/// # }
/// ```
///
/// [`Future::poll()`]: core::future::Future::poll
/// [`Poll::Pending`]: core::task::Poll::Pending
/// [`local_waker`]: core::task::Context::local_waker
#[unstable(feature = "local_waker", issue = "118959")]
#[repr(transparent)]
pub struct LocalWaker {
waker: RawWaker,
}
#[unstable(feature = "local_waker", issue = "118959")]
impl Unpin for LocalWaker {}
impl LocalWaker {
/// Wakes up the task associated with this `LocalWaker`.
///
/// As long as the executor keeps running and the task is not finished, it is
/// guaranteed that each invocation of [`wake()`](Self::wake) (or
/// [`wake_by_ref()`](Self::wake_by_ref)) will be followed by at least one
/// [`poll()`] of the task to which this `LocalWaker` belongs. This makes
/// it possible to temporarily yield to other tasks while running potentially
/// unbounded processing loops.
///
/// Note that the above implies that multiple wake-ups may be coalesced into a
/// single [`poll()`] invocation by the runtime.
///
/// Also note that yielding to competing tasks is not guaranteed: it is the
/// executor’s choice which task to run and the executor may choose to run the
/// current task again.
///
/// [`poll()`]: crate::future::Future::poll
#[inline]
#[unstable(feature = "local_waker", issue = "118959")]
pub fn wake(self) {
// The actual wakeup call is delegated through a virtual function call
// to the implementation which is defined by the executor.
// Don't call `drop` -- the waker will be consumed by `wake`.
let this = ManuallyDrop::new(self);
// SAFETY: This is safe because `Waker::from_raw` is the only way
// to initialize `wake` and `data` requiring the user to acknowledge
// that the contract of `RawWaker` is upheld.
unsafe { (this.waker.vtable.wake)(this.waker.data) };
}
/// Wakes up the task associated with this `LocalWaker` without consuming the `LocalWaker`.
///
/// This is similar to [`wake()`](Self::wake), but may be slightly less efficient in
/// the case where an owned `Waker` is available. This method should be preferred to
/// calling `waker.clone().wake()`.
#[inline]
#[unstable(feature = "local_waker", issue = "118959")]
pub fn wake_by_ref(&self) {
// The actual wakeup call is delegated through a virtual function call
// to the implementation which is defined by the executor.
// SAFETY: see `wake`
unsafe { (self.waker.vtable.wake_by_ref)(self.waker.data) }
}
/// Returns `true` if this `LocalWaker` and another `LocalWaker` would awake 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 — for example,
/// this type's [`clone_from`](Self::clone_from) implementation uses it to
/// avoid cloning the waker when they would wake the same task anyway.
#[inline]
#[must_use]
#[unstable(feature = "local_waker", issue = "118959")]
pub fn will_wake(&self, other: &LocalWaker) -> bool {
// We optimize this by comparing vtable addresses instead of vtable contents.
// This is permitted since the function is documented as best-effort.
let RawWaker { data: a_data, vtable: a_vtable } = self.waker;
let RawWaker { data: b_data, vtable: b_vtable } = other.waker;
a_data == b_data && ptr::eq(a_vtable, b_vtable)
}
/// Creates a new `LocalWaker` from [`RawWaker`].
///
/// The behavior of the returned `LocalWaker` is undefined if the contract defined
/// in [`RawWaker`]'s and [`RawWakerVTable`]'s documentation is not upheld.
/// Therefore this method is unsafe.
#[inline]
#[must_use]
#[unstable(feature = "local_waker", issue = "118959")]
#[rustc_const_unstable(feature = "local_waker", issue = "118959")]
pub const unsafe fn from_raw(waker: RawWaker) -> LocalWaker {
Self { waker }
}
/// Returns a reference to a `LocalWaker` that does nothing when used.
///
// Note! Much of the documentation for this method is duplicated
// in the docs for `Waker::noop`.
// If you edit it, consider editing the other copy too.
//
/// This is mostly useful for writing tests that need a [`Context`] to poll
/// some futures, but are not expecting those futures to wake the waker or
/// do not need to do anything specific if it happens.
///
/// More generally, using `LocalWaker::noop()` to poll a future
/// means discarding the notification of when the future should be polled again,
/// So it should only be used when such a notification will not be needed to make progress.
///
/// If an owned `LocalWaker` is needed, `clone()` this one.
///
/// # Examples
///
/// ```
/// #![feature(local_waker)]
/// #![feature(noop_waker)]
///
/// use std::future::Future;
/// use std::task::{ContextBuilder, LocalWaker, Waker, Poll};
///
/// let mut cx = ContextBuilder::from_waker(Waker::noop())
/// .local_waker(LocalWaker::noop())
/// .build();
///
/// let mut future = Box::pin(async { 10 });
/// assert_eq!(future.as_mut().poll(&mut cx), Poll::Ready(10));
/// ```
#[inline]
#[must_use]
#[unstable(feature = "noop_waker", issue = "98286")]
pub const fn noop() -> &'static LocalWaker {
const WAKER: &LocalWaker = &LocalWaker { waker: RawWaker::NOOP };
WAKER
}
/// Gets a reference to the underlying [`RawWaker`].
#[inline]
#[must_use]
#[unstable(feature = "waker_getters", issue = "96992")]
pub fn as_raw(&self) -> &RawWaker {
&self.waker
}
}
#[unstable(feature = "local_waker", issue = "118959")]
impl Clone for LocalWaker {
#[inline]
fn clone(&self) -> Self {
LocalWaker {
// SAFETY: This is safe because `Waker::from_raw` is the only way
// to initialize `clone` and `data` requiring the user to acknowledge
// that the contract of [`RawWaker`] is upheld.
waker: unsafe { (self.waker.vtable.clone)(self.waker.data) },
}
}
#[inline]
fn clone_from(&mut self, source: &Self) {
if !self.will_wake(source) {
*self = source.clone();
}
}
}
#[unstable(feature = "local_waker", issue = "118959")]
impl AsRef<LocalWaker> for Waker {
fn as_ref(&self) -> &LocalWaker {
// SAFETY: LocalWaker is just Waker without thread safety
unsafe { transmute(self) }
}
}
#[unstable(feature = "local_waker", issue = "118959")]
impl Drop for LocalWaker {
#[inline]
fn drop(&mut self) {
// SAFETY: This is safe because `LocalWaker::from_raw` is the only way
// to initialize `drop` and `data` requiring the user to acknowledge
// that the contract of `RawWaker` is upheld.
unsafe { (self.waker.vtable.drop)(self.waker.data) }
}
}
#[unstable(feature = "local_waker", issue = "118959")]
impl fmt::Debug for LocalWaker {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let vtable_ptr = self.waker.vtable as *const RawWakerVTable;
f.debug_struct("LocalWaker")
.field("data", &self.waker.data)
.field("vtable", &vtable_ptr)
.finish()
}
}
#[unstable(feature = "local_waker", issue = "118959")]
impl !Send for LocalWaker {}
#[unstable(feature = "local_waker", issue = "118959")]
impl !Sync for LocalWaker {}