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
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140

#![allow(cast_ptr_alignment)] // clippy is too strict here

use std::marker::PhantomData;
use std::ops::Deref;
use std::ptr::NonNull;
use std::sync::Arc;
use std::task::{LocalWaker, Waker, Wake, UnsafeWake};

/// A [`LocalWaker`](::std::task::LocalWaker) that is only valid for a given lifetime.
///
/// Note: this type implements [`Deref<Target = LocalWaker>`](::std::ops::Deref),
/// so it can be used to get a `&LocalWaker`.
#[derive(Debug)]
pub struct LocalWakerRef<'a> {
    local_waker: LocalWaker,
    _marker: PhantomData<&'a ()>,
}

impl<'a> LocalWakerRef<'a> {
    /// Create a new [`LocalWakerRef`] from a [`LocalWaker`].
    ///
    /// Note: this function is safe, but it is generally only used
    /// from `unsafe` contexts that need to create a `LocalWaker`
    /// that is guaranteed not to outlive a particular lifetime.
    pub fn new(local_waker: LocalWaker) -> Self {
        LocalWakerRef {
            local_waker,
            _marker: PhantomData,
        }
    }
}

impl<'a> Deref for LocalWakerRef<'a> {
    type Target = LocalWaker;

    fn deref(&self) -> &LocalWaker {
        &self.local_waker
    }
}

// Pointers to this type below are really pointers to `Arc<W>`
struct ReferencedArc<W> {
    _marker: PhantomData<W>,
}

unsafe impl<W: Wake + 'static> UnsafeWake for ReferencedArc<W> {
    #[inline]
    unsafe fn clone_raw(&self) -> Waker {
        let me = self as *const ReferencedArc<W> as *const Arc<W>;
        Arc::clone(&*me).into()
    }

    #[inline]
    unsafe fn drop_raw(&self) {}

    #[inline]
    unsafe fn wake(&self) {
        let me = self as *const ReferencedArc<W> as *const Arc<W>;
        W::wake(&*me)
    }

    #[inline]
    unsafe fn wake_local(&self) {
        let me = self as *const ReferencedArc<W> as *const Arc<W>;
        W::wake_local(&*me)
    }
}

/// Creates a reference to a [`LocalWaker`](::std::task::LocalWaker)
/// from a local [`wake`](::std::task::Wake).
///
/// # Safety
///
/// This function requires that `wake` is "local" (created on the current thread).
/// The resulting [`LocalWaker`](::std::task::LocalWaker) will call
/// [`wake.wake_local()`](::std::task::Wake::wake_local)
/// when awoken, and will call [`wake.wake()`](::std::task::Wake::wake) if
/// awoken after being converted to a [`Waker`](::std::task::Waker).
#[inline]
pub unsafe fn local_waker_ref<W>(wake: &Arc<W>) -> LocalWakerRef<'_>
where
    W: Wake + 'static,
{
    let ptr = wake
        as *const Arc<W>
        as *const ReferencedArc<W>
        as *const dyn UnsafeWake
        as *mut dyn UnsafeWake;
    let local_waker = LocalWaker::new(NonNull::new_unchecked(ptr));
    LocalWakerRef::new(local_waker)
}

// Pointers to this type below are really pointers to `Arc<W>`,
struct NonlocalReferencedArc<W> {
    _marker: PhantomData<W>,
}

unsafe impl<W: Wake + 'static> UnsafeWake for NonlocalReferencedArc<W> {
    #[inline]
    unsafe fn clone_raw(&self) -> Waker {
        let me = self as *const NonlocalReferencedArc<W> as *const Arc<W>;
        Arc::clone(&*me).into()
    }

    #[inline]
    unsafe fn drop_raw(&self) {}

    #[inline]
    unsafe fn wake(&self) {
        let me = self as *const NonlocalReferencedArc<W> as *const Arc<W>;
        W::wake(&*me)
    }

    #[inline]
    unsafe fn wake_local(&self) {
        let me = self as *const NonlocalReferencedArc<W> as *const Arc<W>;
        W::wake(&*me)
    }
}

/// Creates a reference to a [`LocalWaker`](::std::task::LocalWaker)
/// from a non-local [`wake`](::std::task::Wake).
///
/// This function is similar to [`local_waker_ref()`], but does not require
/// that `wake` is local to the current thread. The resulting
/// [`LocalWaker`](::std::task::LocalWaker) will call
/// [`wake.wake()`](::std::task::Wake::wake) when awoken.
#[inline]
pub fn local_waker_ref_from_nonlocal<W>(wake: &Arc<W>) -> LocalWakerRef<'_>
where
    W: Wake + 'static,
{
    let ptr = wake
        as *const Arc<W>
        as *const NonlocalReferencedArc<W>
        as *const dyn UnsafeWake
        as *mut dyn UnsafeWake;
    let local_waker = unsafe { LocalWaker::new(NonNull::new_unchecked(ptr)) };
    LocalWakerRef::new(local_waker)
}