core/ops/
coroutine.rs

1use crate::pin::Pin;
2
3/// The result of a coroutine resumption.
4///
5/// This enum is returned from the `Coroutine::resume` method and indicates the
6/// possible return values of a coroutine. Currently this corresponds to either
7/// a suspension point (`Yielded`) or a termination point (`Complete`).
8#[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
9#[lang = "coroutine_state"]
10#[unstable(feature = "coroutine_trait", issue = "43122")]
11pub enum CoroutineState<Y, R> {
12    /// The coroutine suspended with a value.
13    ///
14    /// This state indicates that a coroutine has been suspended, and typically
15    /// corresponds to a `yield` statement. The value provided in this variant
16    /// corresponds to the expression passed to `yield` and allows coroutines to
17    /// provide a value each time they yield.
18    Yielded(Y),
19
20    /// The coroutine completed with a return value.
21    ///
22    /// This state indicates that a coroutine has finished execution with the
23    /// provided value. Once a coroutine has returned `Complete` it is
24    /// considered a programmer error to call `resume` again.
25    Complete(R),
26}
27
28/// The trait implemented by builtin coroutine types.
29///
30/// Coroutines are currently an
31/// experimental language feature in Rust. Added in [RFC 2033] coroutines are
32/// currently intended to primarily provide a building block for async/await
33/// syntax but will likely extend to also providing an ergonomic definition for
34/// iterators and other primitives.
35///
36/// The syntax and semantics for coroutines is unstable and will require a
37/// further RFC for stabilization. At this time, though, the syntax is
38/// closure-like:
39///
40/// ```rust
41/// #![feature(coroutines)]
42/// #![feature(coroutine_trait)]
43/// #![feature(stmt_expr_attributes)]
44///
45/// use std::ops::{Coroutine, CoroutineState};
46/// use std::pin::Pin;
47///
48/// fn main() {
49///     let mut coroutine = #[coroutine] || {
50///         yield 1;
51///         "foo"
52///     };
53///
54///     match Pin::new(&mut coroutine).resume(()) {
55///         CoroutineState::Yielded(1) => {}
56///         _ => panic!("unexpected return from resume"),
57///     }
58///     match Pin::new(&mut coroutine).resume(()) {
59///         CoroutineState::Complete("foo") => {}
60///         _ => panic!("unexpected return from resume"),
61///     }
62/// }
63/// ```
64///
65/// More documentation of coroutines can be found in the [unstable book].
66///
67/// [RFC 2033]: https://github.com/rust-lang/rfcs/pull/2033
68/// [unstable book]: ../../unstable-book/language-features/coroutines.html
69#[lang = "coroutine"]
70#[unstable(feature = "coroutine_trait", issue = "43122")]
71#[fundamental]
72#[must_use = "coroutines are lazy and do nothing unless resumed"]
73pub trait Coroutine<R = ()> {
74    /// The type of value this coroutine yields.
75    ///
76    /// This associated type corresponds to the `yield` expression and the
77    /// values which are allowed to be returned each time a coroutine yields.
78    /// For example an iterator-as-a-coroutine would likely have this type as
79    /// `T`, the type being iterated over.
80    #[lang = "coroutine_yield"]
81    type Yield;
82
83    /// The type of value this coroutine returns.
84    ///
85    /// This corresponds to the type returned from a coroutine either with a
86    /// `return` statement or implicitly as the last expression of a coroutine
87    /// literal. For example futures would use this as `Result<T, E>` as it
88    /// represents a completed future.
89    #[lang = "coroutine_return"]
90    type Return;
91
92    /// Resumes the execution of this coroutine.
93    ///
94    /// This function will resume execution of the coroutine or start execution
95    /// if it hasn't already. This call will return back into the coroutine's
96    /// last suspension point, resuming execution from the latest `yield`. The
97    /// coroutine will continue executing until it either yields or returns, at
98    /// which point this function will return.
99    ///
100    /// # Return value
101    ///
102    /// The `CoroutineState` enum returned from this function indicates what
103    /// state the coroutine is in upon returning. If the `Yielded` variant is
104    /// returned then the coroutine has reached a suspension point and a value
105    /// has been yielded out. Coroutines in this state are available for
106    /// resumption at a later point.
107    ///
108    /// If `Complete` is returned then the coroutine has completely finished
109    /// with the value provided. It is invalid for the coroutine to be resumed
110    /// again.
111    ///
112    /// # Panics
113    ///
114    /// This function may panic if it is called after the `Complete` variant has
115    /// been returned previously. While coroutine literals in the language are
116    /// guaranteed to panic on resuming after `Complete`, this is not guaranteed
117    /// for all implementations of the `Coroutine` trait.
118    #[lang = "coroutine_resume"]
119    fn resume(self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return>;
120}
121
122#[unstable(feature = "coroutine_trait", issue = "43122")]
123impl<G: ?Sized + Coroutine<R>, R> Coroutine<R> for Pin<&mut G> {
124    type Yield = G::Yield;
125    type Return = G::Return;
126
127    fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
128        G::resume((*self).as_mut(), arg)
129    }
130}
131
132#[unstable(feature = "coroutine_trait", issue = "43122")]
133impl<G: ?Sized + Coroutine<R> + Unpin, R> Coroutine<R> for &mut G {
134    type Yield = G::Yield;
135    type Return = G::Return;
136
137    fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
138        G::resume(Pin::new(&mut *self), arg)
139    }
140}