Trait Coroutine

pub trait Coroutine<R = ()> {
    type Yield;
    type Return;

    // Required method
    fn resume(
        self: Pin<&mut Self>,
        arg: R,
    ) -> CoroutineState<Self::Yield, Self::Return>;
}

🔬This is a nightly-only experimental API. (coroutine_trait #43122)

The trait implemented by builtin coroutine types.

Coroutines are currently an experimental language feature in Rust. Added in RFC 2033 coroutines are currently intended to primarily provide a building block for async/await syntax but will likely extend to also providing an ergonomic definition for iterators and other primitives.

The syntax and semantics for coroutines is unstable and will require a further RFC for stabilization. At this time, though, the syntax is closure-like:

#![feature(coroutines)]
#![feature(coroutine_trait)]
#![feature(stmt_expr_attributes)]

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

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

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

More documentation of coroutines can be found in the unstable book.

Required Associated Types

type Yield

🔬This is a nightly-only experimental API. (coroutine_trait #43122)

The type of value this coroutine yields.

This associated type corresponds to the yield expression and the values which are allowed to be returned each time a coroutine yields. For example an iterator-as-a-coroutine would likely have this type as T, the type being iterated over.

type Return

🔬This is a nightly-only experimental API. (coroutine_trait #43122)

The type of value this coroutine returns.

This corresponds to the type returned from a coroutine either with a return statement or implicitly as the last expression of a coroutine literal. For example futures would use this as Result<T, E> as it represents a completed future.

Required Methods

fn resume( self: Pin<&mut Self>, arg: R, ) -> CoroutineState<Self::Yield, Self::Return>

🔬This is a nightly-only experimental API. (coroutine_trait #43122)

Resumes the execution of this coroutine.

This function will resume execution of the coroutine or start execution if it hasn’t already. This call will return back into the coroutine’s last suspension point, resuming execution from the latest yield. The coroutine will continue executing until it either yields or returns, at which point this function will return.

Return value

The CoroutineState enum returned from this function indicates what state the coroutine is in upon returning. If the Yielded variant is returned then the coroutine has reached a suspension point and a value has been yielded out. Coroutines in this state are available for resumption at a later point.

If Complete is returned then the coroutine has completely finished with the value provided. It is invalid for the coroutine to be resumed again.

Panics

This function may panic if it is called after the Complete variant has been returned previously. While coroutine literals in the language are guaranteed to panic on resuming after Complete, this is not guaranteed for all implementations of the Coroutine trait.

Implementors

impl<G: ?Sized + Coroutine<R> + Unpin, R> Coroutine<R> for &mut G

type Yield = <G as Coroutine<R>>::Yield

type Return = <G as Coroutine<R>>::Return

impl<G: ?Sized + Coroutine<R>, R> Coroutine<R> for Pin<&mut G>

type Yield = <G as Coroutine<R>>::Yield

type Return = <G as Coroutine<R>>::Return

impl<R, G> Coroutine<R> for Exclusive<G>

where G: Coroutine<R> + ?Sized,

type Yield = <G as Coroutine<R>>::Yield

type Return = <G as Coroutine<R>>::Return