core/sync/
exclusive.rs

1//! Defines [`Exclusive`].
2
3use core::fmt;
4use core::future::Future;
5use core::marker::Tuple;
6use core::ops::{Coroutine, CoroutineState};
7use core::pin::Pin;
8use core::task::{Context, Poll};
9
10/// `Exclusive` provides only _mutable_ access, also referred to as _exclusive_
11/// access to the underlying value. It provides no _immutable_, or _shared_
12/// access to the underlying value.
13///
14/// While this may seem not very useful, it allows `Exclusive` to _unconditionally_
15/// implement [`Sync`]. Indeed, the safety requirements of `Sync` state that for `Exclusive`
16/// to be `Sync`, it must be sound to _share_ across threads, that is, it must be sound
17/// for `&Exclusive` to cross thread boundaries. By design, a `&Exclusive` has no API
18/// whatsoever, making it useless, thus harmless, thus memory safe.
19///
20/// Certain constructs like [`Future`]s can only be used with _exclusive_ access,
21/// and are often `Send` but not `Sync`, so `Exclusive` can be used as hint to the
22/// Rust compiler that something is `Sync` in practice.
23///
24/// ## Examples
25/// Using a non-`Sync` future prevents the wrapping struct from being `Sync`
26/// ```compile_fail
27/// use core::cell::Cell;
28///
29/// async fn other() {}
30/// fn assert_sync<T: Sync>(t: T) {}
31/// struct State<F> {
32///     future: F
33/// }
34///
35/// assert_sync(State {
36///     future: async {
37///         let cell = Cell::new(1);
38///         let cell_ref = &cell;
39///         other().await;
40///         let value = cell_ref.get();
41///     }
42/// });
43/// ```
44///
45/// `Exclusive` ensures the struct is `Sync` without stripping the future of its
46/// functionality.
47/// ```
48/// #![feature(exclusive_wrapper)]
49/// use core::cell::Cell;
50/// use core::sync::Exclusive;
51///
52/// async fn other() {}
53/// fn assert_sync<T: Sync>(t: T) {}
54/// struct State<F> {
55///     future: Exclusive<F>
56/// }
57///
58/// assert_sync(State {
59///     future: Exclusive::new(async {
60///         let cell = Cell::new(1);
61///         let cell_ref = &cell;
62///         other().await;
63///         let value = cell_ref.get();
64///     })
65/// });
66/// ```
67///
68/// ## Parallels with a mutex
69/// In some sense, `Exclusive` can be thought of as a _compile-time_ version of
70/// a mutex, as the borrow-checker guarantees that only one `&mut` can exist
71/// for any value. This is a parallel with the fact that
72/// `&` and `&mut` references together can be thought of as a _compile-time_
73/// version of a read-write lock.
74#[unstable(feature = "exclusive_wrapper", issue = "98407")]
75#[doc(alias = "SyncWrapper")]
76#[doc(alias = "SyncCell")]
77#[doc(alias = "Unique")]
78// `Exclusive` can't have `PartialOrd`, `Clone`, etc. impls as they would
79// use `&` access to the inner value, violating the `Sync` impl's safety
80// requirements.
81#[derive(Default)]
82#[repr(transparent)]
83pub struct Exclusive<T: ?Sized> {
84    inner: T,
85}
86
87// See `Exclusive`'s docs for justification.
88#[unstable(feature = "exclusive_wrapper", issue = "98407")]
89unsafe impl<T: ?Sized> Sync for Exclusive<T> {}
90
91#[unstable(feature = "exclusive_wrapper", issue = "98407")]
92impl<T: ?Sized> fmt::Debug for Exclusive<T> {
93    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
94        f.debug_struct("Exclusive").finish_non_exhaustive()
95    }
96}
97
98impl<T: Sized> Exclusive<T> {
99    /// Wrap a value in an `Exclusive`
100    #[unstable(feature = "exclusive_wrapper", issue = "98407")]
101    #[must_use]
102    #[inline]
103    pub const fn new(t: T) -> Self {
104        Self { inner: t }
105    }
106
107    /// Unwrap the value contained in the `Exclusive`
108    #[unstable(feature = "exclusive_wrapper", issue = "98407")]
109    #[rustc_const_unstable(feature = "exclusive_wrapper", issue = "98407")]
110    #[must_use]
111    #[inline]
112    pub const fn into_inner(self) -> T {
113        self.inner
114    }
115}
116
117impl<T: ?Sized> Exclusive<T> {
118    /// Gets exclusive access to the underlying value.
119    #[unstable(feature = "exclusive_wrapper", issue = "98407")]
120    #[must_use]
121    #[inline]
122    pub const fn get_mut(&mut self) -> &mut T {
123        &mut self.inner
124    }
125
126    /// Gets pinned exclusive access to the underlying value.
127    ///
128    /// `Exclusive` is considered to _structurally pin_ the underlying
129    /// value, which means _unpinned_ `Exclusive`s can produce _unpinned_
130    /// access to the underlying value, but _pinned_ `Exclusive`s only
131    /// produce _pinned_ access to the underlying value.
132    #[unstable(feature = "exclusive_wrapper", issue = "98407")]
133    #[must_use]
134    #[inline]
135    pub const fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T> {
136        // SAFETY: `Exclusive` can only produce `&mut T` if itself is unpinned
137        // `Pin::map_unchecked_mut` is not const, so we do this conversion manually
138        unsafe { Pin::new_unchecked(&mut self.get_unchecked_mut().inner) }
139    }
140
141    /// Build a _mutable_ reference to an `Exclusive<T>` from
142    /// a _mutable_ reference to a `T`. This allows you to skip
143    /// building an `Exclusive` with [`Exclusive::new`].
144    #[unstable(feature = "exclusive_wrapper", issue = "98407")]
145    #[must_use]
146    #[inline]
147    pub const fn from_mut(r: &'_ mut T) -> &'_ mut Exclusive<T> {
148        // SAFETY: repr is ≥ C, so refs have the same layout; and `Exclusive` properties are `&mut`-agnostic
149        unsafe { &mut *(r as *mut T as *mut Exclusive<T>) }
150    }
151
152    /// Build a _pinned mutable_ reference to an `Exclusive<T>` from
153    /// a _pinned mutable_ reference to a `T`. This allows you to skip
154    /// building an `Exclusive` with [`Exclusive::new`].
155    #[unstable(feature = "exclusive_wrapper", issue = "98407")]
156    #[must_use]
157    #[inline]
158    pub const fn from_pin_mut(r: Pin<&'_ mut T>) -> Pin<&'_ mut Exclusive<T>> {
159        // SAFETY: `Exclusive` can only produce `&mut T` if itself is unpinned
160        // `Pin::map_unchecked_mut` is not const, so we do this conversion manually
161        unsafe { Pin::new_unchecked(Self::from_mut(r.get_unchecked_mut())) }
162    }
163}
164
165#[unstable(feature = "exclusive_wrapper", issue = "98407")]
166impl<T> From<T> for Exclusive<T> {
167    #[inline]
168    fn from(t: T) -> Self {
169        Self::new(t)
170    }
171}
172
173#[unstable(feature = "exclusive_wrapper", issue = "98407")]
174impl<F, Args> FnOnce<Args> for Exclusive<F>
175where
176    F: FnOnce<Args>,
177    Args: Tuple,
178{
179    type Output = F::Output;
180
181    extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
182        self.into_inner().call_once(args)
183    }
184}
185
186#[unstable(feature = "exclusive_wrapper", issue = "98407")]
187impl<F, Args> FnMut<Args> for Exclusive<F>
188where
189    F: FnMut<Args>,
190    Args: Tuple,
191{
192    extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
193        self.get_mut().call_mut(args)
194    }
195}
196
197#[unstable(feature = "exclusive_wrapper", issue = "98407")]
198impl<T> Future for Exclusive<T>
199where
200    T: Future + ?Sized,
201{
202    type Output = T::Output;
203
204    #[inline]
205    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
206        self.get_pin_mut().poll(cx)
207    }
208}
209
210#[unstable(feature = "coroutine_trait", issue = "43122")] // also #98407
211impl<R, G> Coroutine<R> for Exclusive<G>
212where
213    G: Coroutine<R> + ?Sized,
214{
215    type Yield = G::Yield;
216    type Return = G::Return;
217
218    #[inline]
219    fn resume(self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
220        G::resume(self.get_pin_mut(), arg)
221    }
222}