feat(body): remove Sync bound for Body::wrap_stream

A stream wrapped into a Body previously needed to implement `Sync` so
that the Body type implements this autotrait as well (which is needed
due to limitations in async/await). Since a stream only offers one
method that is called with an exclusive reference, this type is
statically proven to be Sync already. In theory it should be fine to add
an `unsafe impl Sync`, but this commit instead adds a SyncWrapper to
enlist the compiler’s help in proving that this is (and remains) correct.

This makes it easier to construct response bodies for client code.

src/common/sync_wrapper.rs

@@ -0,0 +1,115 @@
+/*
+ * This is a copy of the sync_wrapper crate.
+ */
+//! 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")]
+
+/// 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 sync wrapper to prove that your data structure is `Sync` even though it
+/// contains such a `Future`.
+///
+/// # Example
+///
+/// ```ignore
+/// use hyper::common::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 SyncWrapper containing the given value.
+    ///
+    /// # Examples
+    ///
+    /// ```ignore
+    /// use hyper::common::sync_wrapper::SyncWrapper;
+    ///
+    /// let wrapped = SyncWrapper::new(42);
+    /// ```
+    pub 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 wrapper. 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
+    ///
+    /// ```ignore
+    /// use hyper::common::sync_wrapper::SyncWrapper;
+    ///
+    /// let mut wrapped = SyncWrapper::new(42);
+    /// let value = wrapped.get_mut();
+    /// *value = 0;
+    /// assert_eq!(*wrapped.get_mut(), 0);
+    /// ```
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.0
+    }
+
+    /// Consumes this wrapper, returning the underlying data.
+    ///
+    /// This is safe because it requires ownership of the wrapper, aherefore 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
+    ///
+    /// ```ignore
+    /// use hyper::common::sync_wrapper::SyncWrapper;
+    ///
+    /// let mut wrapped = SyncWrapper::new(42);
+    /// assert_eq!(wrapped.into_inner(), 42);
+    /// ```
+    #[allow(dead_code)]
+    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: Send> Sync for SyncWrapper<T> {}