Struct std::sync::Exclusive

#[repr(transparent)]
pub struct Exclusive<T> where
    T: ?Sized, { /* private fields */ }

🔬This is a nightly-only experimental API. (exclusive_wrapper #98407)

Exclusive provides only mutable access, also referred to as exclusive access to the underlying value. It provides no immutable, or shared access to the underlying value.

While this may seem not very useful, it allows Exclusive to unconditionally implement Sync. Indeed, the safety requirements of Sync state that for Exclusive to be Sync, it must be sound to share across threads, that is, it must be sound for &Exclusive to cross thread boundaries. By design, a &Exclusive has no API whatsoever, making it useless, thus harmless, thus memory safe.

Certain constructs like Futures can only be used with exclusive access, and are often Send but not Sync, so Exclusive can be used as hint to the rust compiler that something is Sync in practice.

Examples

Using a non-Sync future prevents the wrapping struct from being Sync

use core::cell::Cell;

async fn other() {}
fn assert_sync<T: Sync>(t: T) {}
struct State<F> {
    future: F
}

assert_sync(State {
    future: async {
        let cell = Cell::new(1);
        let cell_ref = &cell;
        other().await;
        let value = cell_ref.get();
    }
});

Exclusive ensures the struct is Sync without stripping the future of its functionality.

#![feature(exclusive_wrapper)]
use core::cell::Cell;
use core::sync::Exclusive;

async fn other() {}
fn assert_sync<T: Sync>(t: T) {}
struct State<F> {
    future: Exclusive<F>
}

assert_sync(State {
    future: Exclusive::new(async {
        let cell = Cell::new(1);
        let cell_ref = &cell;
        other().await;
        let value = cell_ref.get();
    })
});

Parallels with a mutex

In some sense, Exclusive can be thought of as a compile-time version of a mutex, as the borrow-checker guarantees that only one &mut can exist for any value. This is a parallel with the fact that & and &mut references together can be thought of as a compile-time version of a read-write lock.

Implementations

impl<T> Exclusive<T>

pub const fn new(t: T) -> Exclusive<T>

🔬This is a nightly-only experimental API. (exclusive_wrapper #98407)

Wrap a value in an Exclusive

pub const fn into_inner(self) -> T

🔬This is a nightly-only experimental API. (exclusive_wrapper #98407)

Unwrap the value contained in the Exclusive

impl<T> Exclusive<T> where
    T: ?Sized, 

pub const fn get_mut(&mut self) -> &mut T

🔬This is a nightly-only experimental API. (exclusive_wrapper #98407)

Get exclusive access to the underlying value.

pub const fn get_pin_mut(self: Pin<&mut Exclusive<T>>) -> Pin<&mut T>

🔬This is a nightly-only experimental API. (exclusive_wrapper #98407)

Get pinned exclusive access to the underlying value.

Exclusive is considered to structurally pin the underlying value, which means unpinned Exclusives can produce unpinned access to the underlying value, but pinned Exclusives only produce pinned access to the underlying value.

pub const fn from_mut(r: &mut T) -> &mut Exclusive<T>

🔬This is a nightly-only experimental API. (exclusive_wrapper #98407)

Build a mutable references to an Exclusive<T> from a mutable reference to a T. This allows you to skip building an Exclusive with Exclusive::new.

pub const fn from_pin_mut(r: Pin<&mut T>) -> Pin<&mut Exclusive<T>>

🔬This is a nightly-only experimental API. (exclusive_wrapper #98407)

Build a pinned mutable references to an Exclusive<T> from a pinned mutable reference to a T. This allows you to skip building an Exclusive with Exclusive::new.

Trait Implementations

impl<T> Debug for Exclusive<T> where
    T: ?Sized, 

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> Default for Exclusive<T> where
    T: Default + ?Sized, 

fn default() -> Exclusive<T>

Returns the “default value” for a type.

impl<T> From<T> for Exclusive<T>

fn from(t: T) -> Exclusive<T>

Converts to this type from the input type.

impl<T> Future for Exclusive<T> where
    T: Future + ?Sized, 

type Output = <T as Future>::Output

The type of value produced on completion.

fn poll(
    self: Pin<&mut Exclusive<T>>,
    cx: &mut Context<'_>
) -> Poll<<Exclusive<T> as Future>::Output>

Attempt to resolve the future to a final value, registering the current task for wakeup if the value is not yet available.

impl<T> Sync for Exclusive<T> where
    T: ?Sized,