//! 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();

    }

}