coroutines

The tracking issue for this feature is: #43122

The coroutines feature gate in Rust allows you to define coroutine or coroutine literals. A coroutine is a "resumable function" that syntactically resembles a closure but compiles to much different semantics in the compiler itself. The primary feature of a coroutine is that it can be suspended during execution to be resumed at a later date. Coroutines use the yield keyword to "return", and then the caller can resume a coroutine to resume execution just after the yield keyword.

Coroutines are an extra-unstable feature in the compiler right now. Added in RFC 2033 they're mostly intended right now as a information/constraint gathering phase. The intent is that experimentation can happen on the nightly compiler before actual stabilization. A further RFC will be required to stabilize coroutines and will likely contain at least a few small tweaks to the overall design.

A syntactical example of a coroutine is:

#![feature(coroutines, coroutine_trait)]

use std::ops::{Coroutine, CoroutineState};
use std::pin::Pin;

fn main() {
    let mut coroutine = || {
        yield 1;
        return "foo"
    };

    match Pin::new(&mut coroutine).resume(()) {
        CoroutineState::Yielded(1) => {}
        _ => panic!("unexpected value from resume"),
    }
    match Pin::new(&mut coroutine).resume(()) {
        CoroutineState::Complete("foo") => {}
        _ => panic!("unexpected value from resume"),
    }
}

Coroutines are closure-like literals which can contain a yield statement. The yield statement takes an optional expression of a value to yield out of the coroutine. All coroutine literals implement the Coroutine trait in the std::ops module. The Coroutine trait has one main method, resume, which resumes execution of the coroutine at the previous suspension point.

An example of the control flow of coroutines is that the following example prints all numbers in order:

#![feature(coroutines, coroutine_trait)]

use std::ops::Coroutine;
use std::pin::Pin;

fn main() {
    let mut coroutine = || {
        println!("2");
        yield;
        println!("4");
    };

    println!("1");
    Pin::new(&mut coroutine).resume(());
    println!("3");
    Pin::new(&mut coroutine).resume(());
    println!("5");
}

At this time the main intended use case of coroutines is an implementation primitive for async/await syntax, but coroutines will likely be extended to ergonomic implementations of iterators and other primitives in the future. Feedback on the design and usage is always appreciated!

The Coroutine trait

The Coroutine trait in std::ops currently looks like:

#![allow(unused)]
fn main() {
#![feature(arbitrary_self_types, coroutine_trait)]
use std::ops::CoroutineState;
use std::pin::Pin;

pub trait Coroutine<R = ()> {
    type Yield;
    type Return;
    fn resume(self: Pin<&mut Self>, resume: R) -> CoroutineState<Self::Yield, Self::Return>;
}
}

The Coroutine::Yield type is the type of values that can be yielded with the yield statement. The Coroutine::Return type is the returned type of the coroutine. This is typically the last expression in a coroutine's definition or any value passed to return in a coroutine. The resume function is the entry point for executing the Coroutine itself.

The return value of resume, CoroutineState, looks like:

#![allow(unused)]
fn main() {
pub enum CoroutineState<Y, R> {
    Yielded(Y),
    Complete(R),
}
}

The Yielded variant indicates that the coroutine can later be resumed. This corresponds to a yield point in a coroutine. The Complete variant indicates that the coroutine is complete and cannot be resumed again. Calling resume after a coroutine has returned Complete will likely result in a panic of the program.

Closure-like semantics

The closure-like syntax for coroutines alludes to the fact that they also have closure-like semantics. Namely:

• When created, a coroutine executes no code. A closure literal does not actually execute any of the closure's code on construction, and similarly a coroutine literal does not execute any code inside the coroutine when constructed.

• Coroutines can capture outer variables by reference or by move, and this can be tweaked with the move keyword at the beginning of the closure. Like closures all coroutines will have an implicit environment which is inferred by the compiler. Outer variables can be moved into a coroutine for use as the coroutine progresses.

• Coroutine literals produce a value with a unique type which implements the std::ops::Coroutine trait. This allows actual execution of the coroutine through the Coroutine::resume method as well as also naming it in return types and such.

• Traits like Send and Sync are automatically implemented for a Coroutine depending on the captured variables of the environment. Unlike closures, coroutines also depend on variables live across suspension points. This means that although the ambient environment may be Send or Sync, the coroutine itself may not be due to internal variables live across yield points being not-Send or not-Sync. Note that coroutines do not implement traits like Copy or Clone automatically.

• Whenever a coroutine is dropped it will drop all captured environment variables.

Coroutines as state machines

In the compiler, coroutines are currently compiled as state machines. Each yield expression will correspond to a different state that stores all live variables over that suspension point. Resumption of a coroutine will dispatch on the current state and then execute internally until a yield is reached, at which point all state is saved off in the coroutine and a value is returned.

Let's take a look at an example to see what's going on here:

#![feature(coroutines, coroutine_trait)]

use std::ops::Coroutine;
use std::pin::Pin;

fn main() {
    let ret = "foo";
    let mut coroutine = move || {
        yield 1;
        return ret
    };

    Pin::new(&mut coroutine).resume(());
    Pin::new(&mut coroutine).resume(());
}

This coroutine literal will compile down to something similar to:

#![feature(arbitrary_self_types, coroutines, coroutine_trait)]

use std::ops::{Coroutine, CoroutineState};
use std::pin::Pin;

fn main() {
    let ret = "foo";
    let mut coroutine = {
        enum __Coroutine {
            Start(&'static str),
            Yield1(&'static str),
            Done,
        }

        impl Coroutine for __Coroutine {
            type Yield = i32;
            type Return = &'static str;

            fn resume(mut self: Pin<&mut Self>, resume: ()) -> CoroutineState<i32, &'static str> {
                use std::mem;
                match mem::replace(&mut *self, __Coroutine::Done) {
                    __Coroutine::Start(s) => {
                        *self = __Coroutine::Yield1(s);
                        CoroutineState::Yielded(1)
                    }

                    __Coroutine::Yield1(s) => {
                        *self = __Coroutine::Done;
                        CoroutineState::Complete(s)
                    }

                    __Coroutine::Done => {
                        panic!("coroutine resumed after completion")
                    }
                }
            }
        }

        __Coroutine::Start(ret)
    };

    Pin::new(&mut coroutine).resume(());
    Pin::new(&mut coroutine).resume(());
}

Notably here we can see that the compiler is generating a fresh type, __Coroutine in this case. This type has a number of states (represented here as an enum) corresponding to each of the conceptual states of the coroutine. At the beginning we're closing over our outer variable foo and then that variable is also live over the yield point, so it's stored in both states.

When the coroutine starts it'll immediately yield 1, but it saves off its state just before it does so indicating that it has reached the yield point. Upon resuming again we'll execute the return ret which returns the Complete state.

Here we can also note that the Done state, if resumed, panics immediately as it's invalid to resume a completed coroutine. It's also worth noting that this is just a rough desugaring, not a normative specification for what the compiler does.