futures_task/
noop_waker.rs

1//! Utilities for creating zero-cost wakers that don't do anything.
2
3use core::ptr::null;
4use core::task::{RawWaker, RawWakerVTable, Waker};
5
6#[inline(always)]
7unsafe fn noop_clone(_data: *const ()) -> RawWaker {
8    noop_raw_waker()
9}
10
11unsafe fn noop(_data: *const ()) {}
12
13const NOOP_WAKER_VTABLE: RawWakerVTable = RawWakerVTable::new(noop_clone, noop, noop, noop);
14
15const fn noop_raw_waker() -> RawWaker {
16    RawWaker::new(null(), &NOOP_WAKER_VTABLE)
17}
18
19/// Create a new [`Waker`] which does
20/// nothing when `wake()` is called on it.
21///
22/// # Examples
23///
24/// ```
25/// use futures::task::noop_waker;
26/// let waker = noop_waker();
27/// waker.wake();
28/// ```
29#[inline]
30pub fn noop_waker() -> Waker {
31    // FIXME: Since 1.46.0 we can use transmute in consts, allowing this function to be const.
32    unsafe { Waker::from_raw(noop_raw_waker()) }
33}
34
35/// Get a static reference to a [`Waker`] which
36/// does nothing when `wake()` is called on it.
37///
38/// # Examples
39///
40/// ```
41/// use futures::task::noop_waker_ref;
42/// let waker = noop_waker_ref();
43/// waker.wake_by_ref();
44/// ```
45#[inline]
46pub fn noop_waker_ref() -> &'static Waker {
47    struct SyncRawWaker(RawWaker);
48    unsafe impl Sync for SyncRawWaker {}
49
50    static NOOP_WAKER_INSTANCE: SyncRawWaker = SyncRawWaker(noop_raw_waker());
51
52    // SAFETY: `Waker` is #[repr(transparent)] over its `RawWaker`.
53    unsafe { &*(&NOOP_WAKER_INSTANCE.0 as *const RawWaker as *const Waker) }
54}
55
56#[cfg(test)]
57mod tests {
58    #[test]
59    #[cfg(feature = "std")]
60    fn issue_2091_cross_thread_segfault() {
61        let waker = std::thread::spawn(super::noop_waker_ref).join().unwrap();
62        waker.wake_by_ref();
63    }
64}
futures_task/noop_waker.rs

futures_task/
noop_waker.rs
