use nightly waker_getters APIs

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.

embassy-executor/src/lib.rs

@@ -1,4 +1,5 @@
 #![cfg_attr(not(any(feature = "arch-std", feature = "arch-wasm")), no_std)]
+#![cfg_attr(feature = "nightly", feature(waker_getters))]
 #![allow(clippy::new_without_default)]
 #![doc = include_str!("../README.md")]
 #![warn(missing_docs)]

embassy-executor/src/raw/waker.rs

@@ -32,6 +32,7 @@ pub(crate) unsafe fn from_task(p: TaskRef) -> Waker {
 /// # Panics
 ///
 /// Panics if the waker is not created by the Embassy executor.
+#[cfg(not(feature = "nightly"))]
 pub fn task_from_waker(waker: &Waker) -> TaskRef {
     // safety: OK because WakerHack has the same layout as Waker.
     // This is not really guaranteed because the structs are `repr(Rust)`, it is
@@ -46,7 +47,31 @@ pub fn task_from_waker(waker: &Waker) -> TaskRef {
     unsafe { TaskRef::from_ptr(hack.data as *const TaskHeader) }
 }
 
+#[cfg(not(feature = "nightly"))]
 struct WakerHack {
     data: *const (),
     vtable: &'static RawWakerVTable,
 }
+
+/// 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.
+#[cfg(feature = "nightly")]
+pub fn task_from_waker(waker: &Waker) -> TaskRef {
+    let raw_waker = waker.as_raw();
+
+    if raw_waker.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(raw_waker.data() as *const TaskHeader) }
+}