/*
* This is a copy of the sync_wrapper crate.
*/
/// 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
pub ;
// this is safe because the only operations permitted on this data structure require exclusive
// access or ownership
unsafe