turn a mio::waker into a std::task::waker

waker.rs

use std::{
    sync::Arc,
    task::{RawWaker, RawWakerVTable, Waker},
}
/// turn a mio::waker into a std::task::Waker
fn make_waker(waker: Arc<mio::Waker>) -> Waker {
    static WAKER_VTABLE: &'static RawWakerVTable =
        RawWakerVTable::new(clone_waker, wake, wake_by_ref, drop);

    unsafe fn clone_waker(ptr: *const mio::Waker) -> RawWaker {
        // retrieve the arc, wrapping in ManuallyDrop so refcount is not decreased
        let arc = std::mem::ManuallyDrop::new(Arc::from_raw(ptr));
        // clone the arc. because it is wrapped in ManuallyDrop, the refcount will not be decreased, either
        arc.clone();

        RawWaker::new(ptr, WAKER_VTABLE)
    }

    unsafe fn wake(ptr: *const mio::Waker) {
        Arc::from_raw(ptr).wake().unwrap();
    }

    unsafe fn wake_by_ref(ptr: *const mio::Waker) {
        (ptr as &mio::Waker).wake().unwrap();
    }

    unsafe fn drop(ptr: *const mio::Waker) {
        Arc::from_raw(ptr)
    }

    let ptr = Arc::into_raw(waker);
    let waker = RawWaker::new(ptr, WAKER_VTABLE);

    // safety: valid because we uphold the contract of RawWaker and RawWakerVTable
    unsafe { Waker::from_raw(waker) }
}