Add core::future::IntoFuture

This patch adds `core::future::IntoFuture`. However unlike earlier PRs this patch does not integrate it into the `async/.await` lowering. That integration should be done in a follow-up PR.

src/libcore/future/into_future.rs

@@ -0,0 +1,27 @@
+use crate::future::Future;
+
+/// Conversion into a `Future`.
+#[unstable(feature = "into_future", issue = "67644")]
+pub trait IntoFuture {
+    /// The output that the future will produce on completion.
+    #[unstable(feature = "into_future", issue = "67644")]
+    type Output;
+
+    /// Which kind of future are we turning this into?
+    #[unstable(feature = "into_future", issue = "67644")]
+    type Future: Future<Output = Self::Output>;
+
+    /// Creates a future from a value.
+    #[unstable(feature = "into_future", issue = "67644")]
+    fn into_future(self) -> Self::Future;
+}
+
+#[unstable(feature = "into_future", issue = "67644")]
+impl<F: Future> IntoFuture for F {
+    type Output = F::Output;
+    type Future = F;
+
+    fn into_future(self) -> Self::Future {
+        self
+    }
+}

src/libcore/future/mod.rs

@@ -10,12 +10,16 @@ use crate::{
 };
 
 mod future;
+mod into_future;
 mod pending;
 mod ready;
 
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub use self::future::Future;
 
+#[unstable(feature = "into_future", issue = "67644")]
+pub use into_future::IntoFuture;
+
 #[unstable(feature = "future_readiness_fns", issue = "70921")]
 pub use pending::{pending, Pending};
 #[unstable(feature = "future_readiness_fns", issue = "70921")]

src/libstd/future.rs

@@ -2,4 +2,16 @@
 
 #[doc(inline)]
 #[stable(feature = "futures_api", since = "1.36.0")]
-pub use core::future::*;
+pub use core::future::Future;
+
+#[doc(inline)]
+#[unstable(feature = "gen_future", issue = "50547")]
+pub use core::future::{from_generator, get_context, ResumeTy};
+
+#[doc(inline)]
+#[unstable(feature = "future_readiness_fns", issue = "70921")]
+pub use core::future::{pending, ready, Pending, Ready};
+
+#[doc(inline)]
+#[unstable(feature = "into_future", issue = "67644")]
+pub use core::future::IntoFuture;

src/libstd/lib.rs

@@ -266,12 +266,15 @@
 #![feature(external_doc)]
 #![feature(fn_traits)]
 #![feature(format_args_nl)]
+#![feature(future_readiness_fns)]
+#![feature(gen_future)]
 #![feature(generator_trait)]
 #![feature(global_asm)]
 #![feature(hash_raw_entry)]
 #![feature(hashmap_internals)]
 #![feature(int_error_internals)]
 #![feature(int_error_matching)]
+#![feature(into_future)]
 #![feature(integer_atomics)]
 #![feature(lang_items)]
 #![feature(libc)]