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+/*
+ * Copyright 2020 Actyx AG
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+//! A mutual exclusion primitive that relies on static type information only
+//!
+//! This library is inspired by [this discussion](https://internals.rust-lang.org/t/what-shall-sync-mean-across-an-await/12020/2).
+#![doc(html_logo_url = "https://developer.actyx.com/img/logo.svg")]
+#![doc(html_favicon_url = "https://developer.actyx.com/img/favicon.ico")]
+#![no_std]
+
+use core::{
+ fmt::{self, Debug, Formatter},
+ pin::Pin,
+ future::Future,
+ task::{Context, Poll},
+};
+
+/// A mutual exclusion primitive that relies on static type information only
+///
+/// In some cases synchronization can be proven statically: whenever you hold an exclusive `&mut`
+/// reference, the Rust type system ensures that no other part of the program can hold another
+/// reference to the data. Therefore it is safe to access it even if the current thread obtained
+/// this reference via a channel. Whenever this is the case, the overhead of allocating and locking
+/// a [`Mutex`] can be avoided by using this static version.
+///
+/// One example where this is often applicable is [`Future`], which requires an exclusive reference
+/// for its [`poll`] method: While a given `Future` implementation may not be safe to access by
+/// multiple threads concurrently, the executor can only run the `Future` on one thread at any
+/// given time, making it [`Sync`] in practice as long as the implementation is `Send`. You can
+/// therefore use the static mutex to prove that your data structure is `Sync` even though it
+/// contains such a `Future`.
+///
+/// # Example
+///
+/// ```
+/// use sync_wrapper::SyncWrapper;
+/// use std::future::Future;
+///
+/// struct MyThing {
+/// future: SyncWrapper<Box<dyn Future<Output = String> + Send>>,
+/// }
+///
+/// impl MyThing {
+/// // all accesses to `self.future` now require an exclusive reference or ownership
+/// }
+///
+/// fn assert_sync<T: Sync>() {}
+///
+/// assert_sync::<MyThing>();
+/// ```
+///
+/// [`Mutex`]: https://doc.rust-lang.org/std/sync/struct.Mutex.html
+/// [`Future`]: https://doc.rust-lang.org/std/future/trait.Future.html
+/// [`poll`]: https://doc.rust-lang.org/std/future/trait.Future.html#method.poll
+/// [`Sync`]: https://doc.rust-lang.org/std/marker/trait.Sync.html
+#[repr(transparent)]
+pub struct SyncWrapper<T>(T);
+
+impl<T> SyncWrapper<T> {
+ /// Creates a new static mutex containing the given value.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use sync_wrapper::SyncWrapper;
+ ///
+ /// let mutex = SyncWrapper::new(42);
+ /// ```
+ pub const fn new(value: T) -> Self {
+ Self(value)
+ }
+
+ /// Acquires a reference to the protected value.
+ ///
+ /// This is safe because it requires an exclusive reference to the mutex. Therefore this method
+ /// neither panics nor does it return an error. This is in contrast to [`Mutex::get_mut`] which
+ /// returns an error if another thread panicked while holding the lock. It is not recommended
+ /// to send an exclusive reference to a potentially damaged value to another thread for further
+ /// processing.
+ ///
+ /// [`Mutex::get_mut`]: https://doc.rust-lang.org/std/sync/struct.Mutex.html#method.get_mut
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use sync_wrapper::SyncWrapper;
+ ///
+ /// let mut mutex = SyncWrapper::new(42);
+ /// let value = mutex.get_mut();
+ /// *value = 0;
+ /// assert_eq!(*mutex.get_mut(), 0);
+ /// ```
+ pub fn get_mut(&mut self) -> &mut T {
+ &mut self.0
+ }
+
+ /// Acquires a pinned reference to the protected value.
+ ///
+ /// See [`Self::get_mut`] for why this method is safe.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use std::future::Future;
+ /// use std::pin::Pin;
+ /// use std::task::{Context, Poll};
+ ///
+ /// use pin_project_lite::pin_project;
+ /// use sync_wrapper::SyncWrapper;
+ ///
+ /// pin_project! {
+ /// struct FutureWrapper<F> {
+ /// #[pin]
+ /// inner: SyncWrapper<F>,
+ /// }
+ /// }
+ ///
+ /// impl<F: Future> Future for FutureWrapper<F> {
+ /// type Output = F::Output;
+ ///
+ /// fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+ /// self.project().inner.get_pin_mut().poll(cx)
+ /// }
+ /// }
+ /// ```
+ pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T> {
+ unsafe { Pin::map_unchecked_mut(self, |this| &mut this.0) }
+ }
+
+ /// Consumes this mutex, returning the underlying data.
+ ///
+ /// This is safe because it requires ownership of the mutex, therefore this method will neither
+ /// panic nor does it return an error. This is in contrast to [`Mutex::into_inner`] which
+ /// returns an error if another thread panicked while holding the lock. It is not recommended
+ /// to send an exclusive reference to a potentially damaged value to another thread for further
+ /// processing.
+ ///
+ /// [`Mutex::into_inner`]: https://doc.rust-lang.org/std/sync/struct.Mutex.html#method.into_inner
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use sync_wrapper::SyncWrapper;
+ ///
+ /// let mut mutex = SyncWrapper::new(42);
+ /// assert_eq!(mutex.into_inner(), 42);
+ /// ```
+ pub fn into_inner(self) -> T {
+ self.0
+ }
+}
+
+// this is safe because the only operations permitted on this data structure require exclusive
+// access or ownership
+unsafe impl<T> Sync for SyncWrapper<T> {}
+
+impl<T> Debug for SyncWrapper<T> {
+ fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+ f.pad("SyncWrapper")
+ }
+}
+
+impl<T: Default> Default for SyncWrapper<T> {
+ fn default() -> Self {
+ Self::new(T::default())
+ }
+}
+
+impl<T> From<T> for SyncWrapper<T> {
+ fn from(value: T) -> Self {
+ Self::new(value)
+ }
+}
+
+/// `Future` which is `Sync`.
+///
+/// # Examples
+///
+/// ```
+/// use sync_wrapper::{SyncWrapper, SyncFuture};
+///
+/// let fut = async { 1 };
+/// let fut = SyncFuture::new(fut);
+/// ```
+pub struct SyncFuture<F> {
+ inner: SyncWrapper<F>
+}
+impl <F: Future> SyncFuture<F> {
+ pub fn new(inner: F) -> Self {
+ Self { inner: SyncWrapper::new(inner) }
+ }
+ pub fn into_inner(self) -> F {
+ self.inner.into_inner()
+ }
+}
+impl <F: Future> Future for SyncFuture<F> {
+ type Output = F::Output;
+ fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+ let inner = unsafe { self.map_unchecked_mut(|x| x.inner.get_mut()) };
+ inner.poll(cx)
+ }
+}
+impl<T> Debug for SyncFuture<T> {
+ fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+ f.pad("SyncFuture")
+ }
+}
+
+/// `Stream` which is `Sync`.
+///
+/// # Examples
+///
+/// ```
+/// use sync_wrapper::SyncStream;
+/// use futures::stream;
+///
+/// let st = stream::iter(vec![1]);
+/// let st = SyncStream::new(st);
+/// ```
+#[cfg(feature = "futures")]
+pub struct SyncStream<S> {
+ inner: SyncWrapper<S>
+}
+#[cfg(feature = "futures")]
+impl <S: futures_core::Stream> SyncStream<S> {
+ pub fn new(inner: S) -> Self {
+ Self { inner: SyncWrapper::new(inner) }
+ }
+ pub fn into_inner(self) -> S {
+ self.inner.into_inner()
+ }
+}
+#[cfg(feature = "futures")]
+impl <S: futures_core::Stream> futures_core::Stream for SyncStream<S> {
+ type Item = S::Item;
+ fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+ let inner = unsafe { self.map_unchecked_mut(|x| x.inner.get_mut()) };
+ inner.poll_next(cx)
+ }
+}
+#[cfg(feature = "futures")]
+impl<T> Debug for SyncStream<T> {
+ fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+ f.pad("SyncStream")
+ }
+}
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