     1
     2
     3
     4
     5
     6
     7
     8
     9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24
    25
    26
    27
    28
    29
    30
    31
    32
    33
    34
    35
    36
    37
    38
    39
    40
    41
    42
    43
    44
    45
    46
    47
    48
    49
    50
    51
    52
    53
    54
    55
    56
    57
    58
    59
    60
    61
    62
    63

    //! Utilities for creating zero-cost wakers that don't do anything.

    use core::ptr::null;
    use core::task::{RawWaker, RawWakerVTable, Waker};

    unsafe fn noop_clone(_data: *const ()) -> RawWaker {
        noop_raw_waker()
    }

    unsafe fn noop(_data: *const ()) {}

    const NOOP_WAKER_VTABLE: RawWakerVTable = RawWakerVTable::new(noop_clone, noop, noop, noop);

    const fn noop_raw_waker() -> RawWaker {
        RawWaker::new(null(), &NOOP_WAKER_VTABLE)
    }

    /// Create a new [`Waker`] which does
    /// nothing when `wake()` is called on it.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::task::noop_waker;
    /// let waker = noop_waker();
    /// waker.wake();
    /// ```
    #[inline]
    pub fn noop_waker() -> Waker {
        // FIXME: Since 1.46.0 we can use transmute in consts, allowing this function to be const.
        unsafe { Waker::from_raw(noop_raw_waker()) }
    }

    /// Get a static reference to a [`Waker`] which
    /// does nothing when `wake()` is called on it.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::task::noop_waker_ref;
    /// let waker = noop_waker_ref();
    /// waker.wake_by_ref();
    /// ```
    #[inline]
    pub fn noop_waker_ref() -> &'static Waker {
        struct SyncRawWaker(RawWaker);
        unsafe impl Sync for SyncRawWaker {}

        static NOOP_WAKER_INSTANCE: SyncRawWaker = SyncRawWaker(noop_raw_waker());

        // SAFETY: `Waker` is #[repr(transparent)] over its `RawWaker`.
        unsafe { &*(&NOOP_WAKER_INSTANCE.0 as *const RawWaker as *const Waker) }
    }

    #[cfg(test)]
    mod tests {
        #[test]
        #[cfg(feature = "std")]
        fn issue_2091_cross_thread_segfault() {
            let waker = std::thread::spawn(super::noop_waker_ref).join().unwrap();
            waker.wake_by_ref();
        }
    }