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Tracking Issue for feature(coroutine_clone) #95360

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@canndrew

Description

@canndrew

This is the tracking issue for #[feature(coroutine_clone)].

This feature extends RFC #2132 to allow non-static coroutines to implement Clone/Copy if all their captured variables and all local variables which are held across a yield implement Clone/Copy. For instance, we can now write code like this:

let mut coro0 = || {
    yield 0u32;
    yield 1u32;
};
assert_eq!(CoroutineState::Yielded(0u32), Coroutine::resume(Pin::new(&mut coro0), ()));

// Clone a started coroutine in its current state.
let mut coro1 = coro0.clone();

// Both the original and the clone now yield 1.
assert_eq!(CoroutineState::Yielded(1u32), Coroutine::resume(Pin::new(&mut coro0), ()));
assert_eq!(GeneratorState::Yielded(1u32), Coroutine::resume(Pin::new(&mut coro1), ()));

This also works if the coroutine captures Clone data:

let data = vec!["first", "second"];
let mut coro0 = move || {
    yield data[0];
    yield data[1];
};
assert_eq!(CoroutineState::Yielded("first"), Coroutine:resume(Pin::new(&mut coro0), ()));

let mut coro1 = coro0.clone();
assert_eq!(CoroutineState::Yielded("second"), Coroutine::resume(Pin::new(&mut coro0), ()));
assert_eq!(CoroutineState::Yielded("second"), Coroutine::resume(Pin::new(&mut coro1), ()));

But not if the Coroutine is static or contains non-clonable state.

let coro0 = static || {
    yield 0u32;
    yield 1u32;
};

// ERROR: the trait bound `[static coroutine@...]: Clone` is not satisfied.
let coro1 = coro0.clone();
struct NonClone;

let coro0 = || {
    // NOTE: has type `NonClone` which does not implement `Clone`.
    let non_clone = NonClone;
    yield 0u32;
    drop(non_clone);
}

// ERROR: the trait bound `NonClone: Clone` is not satisified in `[coroutine@...]`
let coro1 = coro0.clone();

Since this feature does not allow static coroutines to be cloned it does not apply to anonymous futures created using async blocks/functions (since these are desugared to static coroutines). Allowing these futures to be clonable is an extension worth considering but is outside the scope of this feature.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

  • Implement the RFC
  • Adjust documentation
  • Stabilization PR

Unresolved Questions

  • Cloning a corotuine that has been polled before is akin to that coroutine returning from its yield multiple times. This is a non-trivial extension of coroutine semantics, with consequences for MIR analyses and correctness reasoning. Let's make sure to properly discuss and explore the consequences of this before stabilizing anything.

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    C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCF-coroutine_clone`#![feature(coroutine_clone)]`S-tracking-impl-incompleteStatus: The implementation is incomplete.T-langRelevant to the language team

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