std::sync

Struct Exclusive

pub struct Exclusive<T>where
    T: ?Sized,
{ /* private fields */ }

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

Expand description

Exclusive provides mutable access, also referred to as exclusive access to the underlying value. However, it only permits immutable, or shared access to the underlying value when that value is Sync.

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<T> for non-Sync T 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)

Gets 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)

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