Add minimal future APIs to support async fn.

Author: Without Boats

src/libcore/future.rs

@@ -0,0 +1,6 @@
+//! Futures and supporting APIs.
+#![unstable(feature = "async_await", issue = "50547")]
+
+/// The Future trait.
+#[unstable(feature = "async_await", issue = "50547")]
+pub trait Future { }

src/libcore/lib.rs

@@ -191,10 +191,11 @@ pub mod ascii;
 pub mod sync;
 pub mod cell;
 pub mod char;
-pub mod panic;
-pub mod panicking;
+pub mod future;
 pub mod iter;
 pub mod option;
+pub mod panic;
+pub mod panicking;
 pub mod raw;
 pub mod result;
 

src/libcore/raw.rs

@@ -97,3 +97,30 @@ pub struct TraitObject {
     pub data: *mut (),
     pub vtable: *mut (),
 }
+
+/// An adapter from a generator to a future.
+///
+/// This is a permanently unstable API used by the compiler. You should not
+/// depend on it.
+#[allow(missing_debug_implementations, dead_code)]
+#[unstable(feature = "gen_future", issue = "50547")]
+pub struct GenFuture<G, R> {
+    gen: G,
+    marker: ::marker::PhantomData<R>,
+}
+
+/// Construct a future from a generator (used in lowering async functions).
+///
+/// This is a permanently unstable API used by the compiler. You should not
+/// depend on it.
+#[unstable(feature = "gen_future", issue = "50547")]
+pub fn gen_future<G>(gen: G) -> GenFuture<G, G::Return> where
+    G: ::ops::Generator<Yield = ()>
+{
+    GenFuture { gen, marker: ::marker::PhantomData }
+}
+
+#[unstable(feature = "gen_future", issue = "50547")]
+impl<G> ::future::Future for GenFuture<G, G::Return> where
+    G: ::ops::Generator<Yield = ()>
+{ }

src/libstd/lib.rs

@@ -242,6 +242,7 @@
 #![feature(array_error_internals)]
 #![feature(ascii_ctype)]
 #![feature(asm)]
+#![feature(async_await)]
 #![feature(attr_literals)]
 #![feature(box_syntax)]
 #![feature(cfg_target_has_atomic)]
@@ -390,6 +391,8 @@ pub use core::cmp;
 pub use core::convert;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::default;
+#[unstable(feature = "async_await", issue = "50547")]
+pub use core::future;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::hash;
 #[stable(feature = "rust1", since = "1.0.0")]