1 files changed, 136 insertions, 0 deletions
diff --git a/library/core/src/ops/generator.rs b/library/core/src/ops/generator.rs
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+++ b/library/core/src/ops/generator.rs
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+use crate::marker::Unpin;
+use crate::pin::Pin;
+
+/// The result of a generator resumption.
+///
+/// This enum is returned from the `Generator::resume` method and indicates the
+/// possible return values of a generator. Currently this corresponds to either
+/// a suspension point (`Yielded`) or a termination point (`Complete`).
+#[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
+#[lang = "generator_state"]
+#[unstable(feature = "generator_trait", issue = "43122")]
+pub enum GeneratorState<Y, R> {
+    /// The generator suspended with a value.
+    ///
+    /// This state indicates that a generator has been suspended, and typically
+    /// corresponds to a `yield` statement. The value provided in this variant
+    /// corresponds to the expression passed to `yield` and allows generators to
+    /// provide a value each time they yield.
+    Yielded(Y),
+
+    /// The generator completed with a return value.
+    ///
+    /// This state indicates that a generator has finished execution with the
+    /// provided value. Once a generator has returned `Complete` it is
+    /// considered a programmer error to call `resume` again.
+    Complete(R),
+}
+
+/// The trait implemented by builtin generator types.
+///
+/// Generators, also commonly referred to as coroutines, are currently an
+/// experimental language feature in Rust. Added in [RFC 2033] generators 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 generators is unstable and will require a
+/// further RFC for stabilization. At this time, though, the syntax is
+/// closure-like:
+///
+/// ```rust
+/// #![feature(generators, generator_trait)]
+///
+/// use std::ops::{Generator, GeneratorState};
+/// use std::pin::Pin;
+///
+/// fn main() {
+///     let mut generator = || {
+///         yield 1;
+///         "foo"
+///     };
+///
+///     match Pin::new(&mut generator).resume(()) {
+///         GeneratorState::Yielded(1) => {}
+///         _ => panic!("unexpected return from resume"),
+///     }
+///     match Pin::new(&mut generator).resume(()) {
+///         GeneratorState::Complete("foo") => {}
+///         _ => panic!("unexpected return from resume"),
+///     }
+/// }
+/// ```
+///
+/// More documentation of generators can be found in the [unstable book].
+///
+/// [RFC 2033]: https://github.com/rust-lang/rfcs/pull/2033
+/// [unstable book]: ../../unstable-book/language-features/generators.html
+#[lang = "generator"]
+#[unstable(feature = "generator_trait", issue = "43122")]
+#[fundamental]
+pub trait Generator<R = ()> {
+    /// The type of value this generator yields.
+    ///
+    /// This associated type corresponds to the `yield` expression and the
+    /// values which are allowed to be returned each time a generator yields.
+    /// For example an iterator-as-a-generator would likely have this type as
+    /// `T`, the type being iterated over.
+    type Yield;
+
+    /// The type of value this generator returns.
+    ///
+    /// This corresponds to the type returned from a generator either with a
+    /// `return` statement or implicitly as the last expression of a generator
+    /// literal. For example futures would use this as `Result<T, E>` as it
+    /// represents a completed future.
+    #[lang = "generator_return"]
+    type Return;
+
+    /// Resumes the execution of this generator.
+    ///
+    /// This function will resume execution of the generator or start execution
+    /// if it hasn't already. This call will return back into the generator's
+    /// last suspension point, resuming execution from the latest `yield`. The
+    /// generator will continue executing until it either yields or returns, at
+    /// which point this function will return.
+    ///
+    /// # Return value
+    ///
+    /// The `GeneratorState` enum returned from this function indicates what
+    /// state the generator is in upon returning. If the `Yielded` variant is
+    /// returned then the generator has reached a suspension point and a value
+    /// has been yielded out. Generators in this state are available for
+    /// resumption at a later point.
+    ///
+    /// If `Complete` is returned then the generator has completely finished
+    /// with the value provided. It is invalid for the generator to be resumed
+    /// again.
+    ///
+    /// # Panics
+    ///
+    /// This function may panic if it is called after the `Complete` variant has
+    /// been returned previously. While generator literals in the language are
+    /// guaranteed to panic on resuming after `Complete`, this is not guaranteed
+    /// for all implementations of the `Generator` trait.
+    fn resume(self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return>;
+}
+
+#[unstable(feature = "generator_trait", issue = "43122")]
+impl<G: ?Sized + Generator<R>, R> Generator<R> for Pin<&mut G> {
+    type Yield = G::Yield;
+    type Return = G::Return;
+
+    fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
+        G::resume((*self).as_mut(), arg)
+    }
+}
+
+#[unstable(feature = "generator_trait", issue = "43122")]
+impl<G: ?Sized + Generator<R> + Unpin, R> Generator<R> for &mut G {
+    type Yield = G::Yield;
+    type Return = G::Return;
+
+    fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
+        G::resume(Pin::new(&mut *self), arg)
+    }
+}