executor,sync: add support for turbo-wakers.

This is a `core` patch to make wakers 1 word (the task pointer) instead of 2 (task pointer + vtable). It allows having the "waker optimization" we had a while back on `WakerRegistration/AtomicWaker`, but EVERYWHERE, without patching all crates.

Advantages:
- Less memory usage.
- Faster.
- `AtomicWaker` can actually use atomics to load/store the waker, No critical section needed.
- No `dyn` call, which means `cargo-call-stack` can now see through wakes.

Disadvantages:
- You have to patch `core`...
- Breaks all executors and other things that create wakers, unless they opt in to using the new `from_ptr` API.

How to use:

- Run this shell script to patch `core`. https://gist.github.com/Dirbaio/c67da7cf318515181539122c9d32b395
- Enable `build-std`
- Enable `build-std-features = core/turbowakers`
- Enable feature `turbowakers` in `embassy-executor`, `embassy-sync`.
- Make sure you have no other crate creating wakers other than `embassy-executor`. These will panic at runtime.

Note that the patched `core` is equivalent to the unpached one when the `turbowakers` feature is not enabled, so it should be fine to leave it there.

1 files changed, 41 insertions, 0 deletions

diff --git a/embassy-sync/src/waitqueue/atomic_waker.rs b/embassy-sync/src/waitqueue/atomic_waker.rs
new file mode 100644
index 000000000..63fe04a6e
--- /dev/null
+++ b/embassy-sync/src/waitqueue/atomic_waker.rs
@@ -0,0 +1,41 @@
+use core::cell::Cell;
+use core::task::Waker;
+
+use crate::blocking_mutex::raw::CriticalSectionRawMutex;
+use crate::blocking_mutex::Mutex;
+
+/// Utility struct to register and wake a waker.
+pub struct AtomicWaker {
+    waker: Mutex<CriticalSectionRawMutex, Cell<Option<Waker>>>,
+}
+
+impl AtomicWaker {
+    /// Create a new `AtomicWaker`.
+    pub const fn new() -> Self {
+        Self {
+            waker: Mutex::const_new(CriticalSectionRawMutex::new(), Cell::new(None)),
+        }
+    }
+
+    /// Register a waker. Overwrites the previous waker, if any.
+    pub fn register(&self, w: &Waker) {
+        critical_section::with(|cs| {
+            let cell = self.waker.borrow(cs);
+            cell.set(match cell.replace(None) {
+                Some(w2) if (w2.will_wake(w)) => Some(w2),
+                _ => Some(w.clone()),
+            })
+        })
+    }
+
+    /// Wake the registered waker, if any.
+    pub fn wake(&self) {
+        critical_section::with(|cs| {
+            let cell = self.waker.borrow(cs);
+            if let Some(w) = cell.replace(None) {
+                w.wake_by_ref();
+                cell.set(Some(w));
+            }
+        })
+    }
+}