8 files changed, 303 insertions, 23 deletions

diff --git a/embassy-sync/src/channel.rs b/embassy-sync/src/channel.rs
index ff7129303..48f4dafd6 100644
--- a/embassy-sync/src/channel.rs
+++ b/embassy-sync/src/channel.rs
@@ -263,6 +263,12 @@ impl<'ch, T> Future for DynamicReceiveFuture<'ch, T> {
     }
 }
 
+impl<'ch, M: RawMutex, T, const N: usize> From<ReceiveFuture<'ch, M, T, N>> for DynamicReceiveFuture<'ch, T> {
+    fn from(value: ReceiveFuture<'ch, M, T, N>) -> Self {
+        Self { channel: value.channel }
+    }
+}
+
 /// Future returned by [`Channel::send`] and  [`Sender::send`].
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct SendFuture<'ch, M, T, const N: usize>
@@ -321,6 +327,15 @@ impl<'ch, T> Future for DynamicSendFuture<'ch, T> {
 
 impl<'ch, T> Unpin for DynamicSendFuture<'ch, T> {}
 
+impl<'ch, M: RawMutex, T, const N: usize> From<SendFuture<'ch, M, T, N>> for DynamicSendFuture<'ch, T> {
+    fn from(value: SendFuture<'ch, M, T, N>) -> Self {
+        Self {
+            channel: value.channel,
+            message: value.message,
+        }
+    }
+}
+
 pub(crate) trait DynamicChannel<T> {
     fn try_send_with_context(&self, message: T, cx: Option<&mut Context<'_>>) -> Result<(), TrySendError<T>>;
 
@@ -507,6 +522,16 @@ where
         Receiver { channel: self }
     }
 
+    /// Get a sender for this channel using dynamic dispatch.
+    pub fn dyn_sender(&self) -> DynamicSender<'_, T> {
+        DynamicSender { channel: self }
+    }
+
+    /// Get a receiver for this channel using dynamic dispatch.
+    pub fn dyn_receiver(&self) -> DynamicReceiver<'_, T> {
+        DynamicReceiver { channel: self }
+    }
+
     /// Send a value, waiting until there is capacity.
     ///
     /// Sending completes when the value has been pushed to the channel's queue.
@@ -648,7 +673,7 @@ mod tests {
     }
 
     #[test]
-    fn dynamic_dispatch() {
+    fn dynamic_dispatch_into() {
         let c = Channel::<NoopRawMutex, u32, 3>::new();
         let s: DynamicSender<'_, u32> = c.sender().into();
         let r: DynamicReceiver<'_, u32> = c.receiver().into();
@@ -657,6 +682,16 @@ mod tests {
         assert_eq!(r.try_receive().unwrap(), 1);
     }
 
+    #[test]
+    fn dynamic_dispatch_constructor() {
+        let c = Channel::<NoopRawMutex, u32, 3>::new();
+        let s = c.dyn_sender();
+        let r = c.dyn_receiver();
+
+        assert!(s.try_send(1).is_ok());
+        assert_eq!(r.try_receive().unwrap(), 1);
+    }
+
     #[futures_test::test]
     async fn receiver_receives_given_try_send_async() {
         let executor = ThreadPool::new().unwrap();
diff --git a/embassy-sync/src/fmt.rs b/embassy-sync/src/fmt.rs
index 78e583c1c..2ac42c557 100644
--- a/embassy-sync/src/fmt.rs
+++ b/embassy-sync/src/fmt.rs
@@ -1,5 +1,5 @@
 #![macro_use]
-#![allow(unused_macros)]
+#![allow(unused)]
 
 use core::fmt::{Debug, Display, LowerHex};
 
@@ -229,7 +229,6 @@ impl<T, E> Try for Result<T, E> {
     }
 }
 
-#[allow(unused)]
 pub(crate) struct Bytes<'a>(pub &'a [u8]);
 
 impl<'a> Debug for Bytes<'a> {
diff --git a/embassy-sync/src/lib.rs b/embassy-sync/src/lib.rs
index b0ccfde57..61b173e80 100644
--- a/embassy-sync/src/lib.rs
+++ b/embassy-sync/src/lib.rs
@@ -1,6 +1,4 @@
 #![cfg_attr(not(any(feature = "std", feature = "wasm")), no_std)]
-#![cfg_attr(nightly, feature(async_fn_in_trait, impl_trait_projections))]
-#![cfg_attr(nightly, allow(stable_features, unknown_lints))]
 #![allow(async_fn_in_trait)]
 #![allow(clippy::new_without_default)]
 #![doc = include_str!("../README.md")]
@@ -15,6 +13,7 @@ mod ring_buffer;
 pub mod blocking_mutex;
 pub mod channel;
 pub mod mutex;
+pub mod once_lock;
 pub mod pipe;
 pub mod priority_channel;
 pub mod pubsub;
diff --git a/embassy-sync/src/once_lock.rs b/embassy-sync/src/once_lock.rs
new file mode 100644
index 000000000..31cc99711
--- /dev/null
+++ b/embassy-sync/src/once_lock.rs
@@ -0,0 +1,236 @@
+//! Syncronization primitive for initializing a value once, allowing others to await a reference to the value.
+
+use core::cell::Cell;
+use core::future::poll_fn;
+use core::mem::MaybeUninit;
+use core::sync::atomic::{AtomicBool, Ordering};
+use core::task::Poll;
+
+/// The `OnceLock` is a synchronization primitive that allows for
+/// initializing a value once, and allowing others to `.await` a
+/// reference to the value. This is useful for lazy initialization of
+/// a static value.
+///
+/// **Note**: this implementation uses a busy loop to poll the value,
+/// which is not as efficient as registering a dedicated `Waker`.
+/// However, the if the usecase for is to initialize a static variable
+/// relatively early in the program life cycle, it should be fine.
+///
+/// # Example
+/// ```
+/// use futures_executor::block_on;
+/// use embassy_sync::once_lock::OnceLock;
+///
+/// // Define a static value that will be lazily initialized
+/// static VALUE: OnceLock<u32> = OnceLock::new();
+///
+/// let f = async {
+///
+/// // Initialize the value
+/// let reference = VALUE.get_or_init(|| 20);
+/// assert_eq!(reference, &20);
+///
+/// // Wait for the value to be initialized
+/// // and get a static reference it
+/// assert_eq!(VALUE.get().await, &20);
+///
+/// };
+/// block_on(f)
+/// ```
+pub struct OnceLock<T> {
+    init: AtomicBool,
+    data: Cell<MaybeUninit<T>>,
+}
+
+unsafe impl<T> Sync for OnceLock<T> {}
+
+impl<T> OnceLock<T> {
+    /// Create a new uninitialized `OnceLock`.
+    pub const fn new() -> Self {
+        Self {
+            init: AtomicBool::new(false),
+            data: Cell::new(MaybeUninit::zeroed()),
+        }
+    }
+
+    /// Get a reference to the underlying value, waiting for it to be set.
+    /// If the value is already set, this will return immediately.
+    pub async fn get(&self) -> &T {
+        poll_fn(|cx| match self.try_get() {
+            Some(data) => Poll::Ready(data),
+            None => {
+                cx.waker().wake_by_ref();
+                Poll::Pending
+            }
+        })
+        .await
+    }
+
+    /// Try to get a reference to the underlying value if it exists.
+    pub fn try_get(&self) -> Option<&T> {
+        if self.init.load(Ordering::Relaxed) {
+            Some(unsafe { self.get_ref_unchecked() })
+        } else {
+            None
+        }
+    }
+
+    /// Set the underlying value. If the value is already set, this will return an error with the given value.
+    pub fn init(&self, value: T) -> Result<(), T> {
+        // Critical section is required to ensure that the value is
+        // not simultaniously initialized elsewhere at the same time.
+        critical_section::with(|_| {
+            // If the value is not set, set it and return Ok.
+            if !self.init.load(Ordering::Relaxed) {
+                self.data.set(MaybeUninit::new(value));
+                self.init.store(true, Ordering::Relaxed);
+                Ok(())
+
+            // Otherwise return an error with the given value.
+            } else {
+                Err(value)
+            }
+        })
+    }
+
+    /// Get a reference to the underlying value, initializing it if it does not exist.
+    pub fn get_or_init<F>(&self, f: F) -> &T
+    where
+        F: FnOnce() -> T,
+    {
+        // Critical section is required to ensure that the value is
+        // not simultaniously initialized elsewhere at the same time.
+        critical_section::with(|_| {
+            // If the value is not set, set it.
+            if !self.init.load(Ordering::Relaxed) {
+                self.data.set(MaybeUninit::new(f()));
+                self.init.store(true, Ordering::Relaxed);
+            }
+        });
+
+        // Return a reference to the value.
+        unsafe { self.get_ref_unchecked() }
+    }
+
+    /// Consume the `OnceLock`, returning the underlying value if it was initialized.
+    pub fn into_inner(self) -> Option<T> {
+        if self.init.load(Ordering::Relaxed) {
+            Some(unsafe { self.data.into_inner().assume_init() })
+        } else {
+            None
+        }
+    }
+
+    /// Take the underlying value if it was initialized, uninitializing the `OnceLock` in the process.
+    pub fn take(&mut self) -> Option<T> {
+        // If the value is set, uninitialize the lock and return the value.
+        critical_section::with(|_| {
+            if self.init.load(Ordering::Relaxed) {
+                let val = unsafe { self.data.replace(MaybeUninit::zeroed()).assume_init() };
+                self.init.store(false, Ordering::Relaxed);
+                Some(val)
+
+            // Otherwise return None.
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Check if the value has been set.
+    pub fn is_set(&self) -> bool {
+        self.init.load(Ordering::Relaxed)
+    }
+
+    /// Get a reference to the underlying value.
+    /// # Safety
+    /// Must only be used if a value has been set.
+    unsafe fn get_ref_unchecked(&self) -> &T {
+        (*self.data.as_ptr()).assume_init_ref()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn once_lock() {
+        let lock = OnceLock::new();
+        assert_eq!(lock.try_get(), None);
+        assert_eq!(lock.is_set(), false);
+
+        let v = 42;
+        assert_eq!(lock.init(v), Ok(()));
+        assert_eq!(lock.is_set(), true);
+        assert_eq!(lock.try_get(), Some(&v));
+        assert_eq!(lock.try_get(), Some(&v));
+
+        let v = 43;
+        assert_eq!(lock.init(v), Err(v));
+        assert_eq!(lock.is_set(), true);
+        assert_eq!(lock.try_get(), Some(&42));
+    }
+
+    #[test]
+    fn once_lock_get_or_init() {
+        let lock = OnceLock::new();
+        assert_eq!(lock.try_get(), None);
+        assert_eq!(lock.is_set(), false);
+
+        let v = lock.get_or_init(|| 42);
+        assert_eq!(v, &42);
+        assert_eq!(lock.is_set(), true);
+        assert_eq!(lock.try_get(), Some(&42));
+
+        let v = lock.get_or_init(|| 43);
+        assert_eq!(v, &42);
+        assert_eq!(lock.is_set(), true);
+        assert_eq!(lock.try_get(), Some(&42));
+    }
+
+    #[test]
+    fn once_lock_static() {
+        static LOCK: OnceLock<i32> = OnceLock::new();
+
+        let v: &'static i32 = LOCK.get_or_init(|| 42);
+        assert_eq!(v, &42);
+
+        let v: &'static i32 = LOCK.get_or_init(|| 43);
+        assert_eq!(v, &42);
+    }
+
+    #[futures_test::test]
+    async fn once_lock_async() {
+        static LOCK: OnceLock<i32> = OnceLock::new();
+
+        assert!(LOCK.init(42).is_ok());
+
+        let v: &'static i32 = LOCK.get().await;
+        assert_eq!(v, &42);
+    }
+
+    #[test]
+    fn once_lock_into_inner() {
+        let lock: OnceLock<i32> = OnceLock::new();
+
+        let v = lock.get_or_init(|| 42);
+        assert_eq!(v, &42);
+
+        assert_eq!(lock.into_inner(), Some(42));
+    }
+
+    #[test]
+    fn once_lock_take_init() {
+        let mut lock: OnceLock<i32> = OnceLock::new();
+
+        assert_eq!(lock.get_or_init(|| 42), &42);
+        assert_eq!(lock.is_set(), true);
+
+        assert_eq!(lock.take(), Some(42));
+        assert_eq!(lock.is_set(), false);
+
+        assert_eq!(lock.get_or_init(|| 43), &43);
+        assert_eq!(lock.is_set(), true);
+    }
+}
diff --git a/embassy-sync/src/priority_channel.rs b/embassy-sync/src/priority_channel.rs
index bd75c0135..e77678c24 100644
--- a/embassy-sync/src/priority_channel.rs
+++ b/embassy-sync/src/priority_channel.rs
@@ -325,7 +325,7 @@ where
 ///
 /// Sent data may be reordered based on their priorty within the channel.
 /// For example, in a [`Max`](heapless::binary_heap::Max) [`PriorityChannel`]
-/// containing `u32`'s, data sent in the following order `[1, 2, 3]` will be recieved as `[3, 2, 1]`.
+/// containing `u32`'s, data sent in the following order `[1, 2, 3]` will be received as `[3, 2, 1]`.
 pub struct PriorityChannel<M, T, K, const N: usize>
 where
     T: Ord,
diff --git a/embassy-sync/src/ring_buffer.rs b/embassy-sync/src/ring_buffer.rs
index d95ffa7c9..81e60c42b 100644
--- a/embassy-sync/src/ring_buffer.rs
+++ b/embassy-sync/src/ring_buffer.rs
@@ -3,7 +3,7 @@ use core::ops::Range;
 pub struct RingBuffer<const N: usize> {
     start: usize,
     end: usize,
-    empty: bool,
+    full: bool,
 }
 
 impl<const N: usize> RingBuffer<N> {
@@ -11,13 +11,13 @@ impl<const N: usize> RingBuffer<N> {
         Self {
             start: 0,
             end: 0,
-            empty: true,
+            full: false,
         }
     }
 
     pub fn push_buf(&mut self) -> Range<usize> {
-        if self.start == self.end && !self.empty {
-            trace!("  ringbuf: push_buf empty");
+        if self.is_full() {
+            trace!("  ringbuf: push_buf full");
             return 0..0;
         }
 
@@ -38,11 +38,11 @@ impl<const N: usize> RingBuffer<N> {
         }
 
         self.end = self.wrap(self.end + n);
-        self.empty = false;
+        self.full = self.start == self.end;
     }
 
     pub fn pop_buf(&mut self) -> Range<usize> {
-        if self.empty {
+        if self.is_empty() {
             trace!("  ringbuf: pop_buf empty");
             return 0..0;
         }
@@ -64,20 +64,20 @@ impl<const N: usize> RingBuffer<N> {
         }
 
         self.start = self.wrap(self.start + n);
-        self.empty = self.start == self.end;
+        self.full = false;
     }
 
     pub fn is_full(&self) -> bool {
-        self.start == self.end && !self.empty
+        self.full
     }
 
     pub fn is_empty(&self) -> bool {
-        self.empty
+        self.start == self.end && !self.full
     }
 
     #[allow(unused)]
     pub fn len(&self) -> usize {
-        if self.empty {
+        if self.is_empty() {
             0
         } else if self.start < self.end {
             self.end - self.start
@@ -89,7 +89,7 @@ impl<const N: usize> RingBuffer<N> {
     pub fn clear(&mut self) {
         self.start = 0;
         self.end = 0;
-        self.empty = true;
+        self.full = false;
     }
 
     fn wrap(&self, n: usize) -> usize {
diff --git a/embassy-sync/src/signal.rs b/embassy-sync/src/signal.rs
index bea67d8be..520f1a896 100644
--- a/embassy-sync/src/signal.rs
+++ b/embassy-sync/src/signal.rs
@@ -111,7 +111,21 @@ where
         poll_fn(move |cx| self.poll_wait(cx))
     }
 
-    /// non-blocking method to check whether this signal has been signaled.
+    /// non-blocking method to try and take the signal value.
+    pub fn try_take(&self) -> Option<T> {
+        self.state.lock(|cell| {
+            let state = cell.replace(State::None);
+            match state {
+                State::Signaled(res) => Some(res),
+                state => {
+                    cell.set(state);
+                    None
+                }
+            }
+        })
+    }
+
+    /// non-blocking method to check whether this signal has been signaled. This does not clear the signal.  
     pub fn signaled(&self) -> bool {
         self.state.lock(|cell| {
             let state = cell.replace(State::None);
diff --git a/embassy-sync/src/zerocopy_channel.rs b/embassy-sync/src/zerocopy_channel.rs
index f704cbd5d..cfce9a571 100644
--- a/embassy-sync/src/zerocopy_channel.rs
+++ b/embassy-sync/src/zerocopy_channel.rs
@@ -1,10 +1,7 @@
 //! A zero-copy queue for sending values between asynchronous tasks.
 //!
-//! It can be used concurrently by multiple producers (senders) and multiple
-//! consumers (receivers), i.e. it is an  "MPMC channel".
-//!
-//! Receivers are competing for messages. So a message that is received by
-//! one receiver is not received by any other.
+//! It can be used concurrently by a producer (sender) and a
+//! consumer (receiver), i.e. it is an  "SPSC channel".
 //!
 //! This queue takes a Mutex type so that various
 //! targets can be attained. For example, a ThreadModeMutex can be used