Struct core::sync::Exclusive

source ·
#[repr(transparent)]
pub struct Exclusive<T: ?Sized> { /* private fields */ }
🔬This is a nightly-only experimental API. (exclusive_wrapper #98407)
Expand description

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();
    }
});
Run

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();
    })
});
Run

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§

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

Wrap a value in an Exclusive

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

Unwrap the value contained in the Exclusive

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

Get exclusive access to the underlying value.

🔬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.

🔬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.

🔬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§

Formats the value using the given formatter. Read more
Returns the “default value” for a type. Read more
Converts to this type from the input type.
The type of value produced on completion.
Attempt to resolve the future to a final value, registering the current task for wakeup if the value is not yet available. Read more

Auto Trait Implementations§

Blanket Implementations§

Gets the TypeId of self. Read more
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more
Converts to this type from the input type.

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion.