embassy/embassy-executor/src/executor/raw/waker.rs

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

use super::TaskHeader;

const VTABLE: RawWakerVTable = RawWakerVTable::new(clone, wake, wake, drop);

unsafe fn clone(p: *const ()) -> RawWaker {
    RawWaker::new(p, &VTABLE)
}

unsafe fn wake(p: *const ()) {
    (*(p as *mut TaskHeader)).enqueue()
}

unsafe fn drop(_: *const ()) {
    // nop
}

pub(crate) unsafe fn from_task(p: NonNull<TaskHeader>) -> Waker {
    Waker::from_raw(RawWaker::new(p.as_ptr() as _, &VTABLE))
}

/// Get a task pointer from a waker.
///
/// This can be used as an optimization in wait queues to store task pointers
/// (1 word) instead of full Wakers (2 words). This saves a bit of RAM and helps
/// avoid dynamic dispatch.
///
/// You can use the returned task pointer to wake the task with [`wake_task`](super::wake_task).
///
/// # Panics
///
/// Panics if the waker is not created by the Embassy executor.
pub fn task_from_waker(waker: &Waker) -> NonNull<TaskHeader> {
    // safety: OK because WakerHack has the same layout as Waker.
    // This is not really guaranteed because the structs are `repr(Rust)`, it is
    // indeed the case in the current implementation.
    // TODO use waker_getters when stable. https://github.com/rust-lang/rust/issues/96992
    let hack: &WakerHack = unsafe { mem::transmute(waker) };
    if hack.vtable != &VTABLE {
        panic!("Found waker not created by the Embassy executor. `embassy_executor::time::Timer` only works with the Embassy executor.")
    }

    // safety: we never create a waker with a null data pointer.
    unsafe { NonNull::new_unchecked(hack.data as *mut TaskHeader) }
}

struct WakerHack {
    data: *const (),
    vtable: &'static RawWakerVTable,
}