use nightly waker_getters APIs

[zjp-CN]

Since rust-lang/rust#96992 has stalled, to prevent potential unsoundness caused by transmuting to &WakerHack, we can use nightly waker_getters APIs by gating it behind nightly feature in embassy-executor without waiting for it to be stablized.

[Dirbaio]

thanks!

[zjp-CN]

Sorry, I realized I could make the cfg code better

pub fn task_from_waker(waker: &Waker) -> TaskRef {
    let (vtable, data) = {
        #[cfg(not(feature = "nightly"))]
        {
            struct WakerHack {
                data: *const (),
                vtable: &'static RawWakerVTable,
            }

            // 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 { core::mem::transmute(waker) };
            (hack.vtable, hack.data)
        }
        #[cfg(feature = "nightly")]
        {
            let raw_waker = waker.as_raw();
            (raw_waker.vtable(), raw_waker.data())
        }
    };

    if vtable != &VTABLE {
        panic!("Found waker not created by the Embassy executor. `embassy_time::Timer` only works with the Embassy executor.")
    }
    // safety: our wakers are always created with `TaskRef::as_ptr`
    unsafe { TaskRef::from_ptr(data as *const TaskHeader) }
}

But it's now merged...

[Dirbaio]

oh, that's true! can you send it in another PR?