1 files changed, 95 insertions, 0 deletions
diff --git a/library/core/src/future/mod.rs b/library/core/src/future/mod.rs
new file mode 100644
index 00000000000..6d1ad9db744
--- /dev/null
+++ b/library/core/src/future/mod.rs
@@ -0,0 +1,95 @@
+#![stable(feature = "futures_api", since = "1.36.0")]
+
+//! Asynchronous values.
+
+use crate::{
+    ops::{Generator, GeneratorState},
+    pin::Pin,
+    ptr::NonNull,
+    task::{Context, Poll},
+};
+
+mod future;
+mod into_future;
+mod pending;
+mod poll_fn;
+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")]
+pub use ready::{ready, Ready};
+
+#[unstable(feature = "future_poll_fn", issue = "72302")]
+pub use poll_fn::{poll_fn, PollFn};
+
+/// This type is needed because:
+///
+/// a) Generators cannot implement `for<'a, 'b> Generator<&'a mut Context<'b>>`, so we need to pass
+///    a raw pointer (see https://github.com/rust-lang/rust/issues/68923).
+/// b) Raw pointers and `NonNull` aren't `Send` or `Sync`, so that would make every single future
+///    non-Send/Sync as well, and we don't want that.
+///
+/// It also simplifies the HIR lowering of `.await`.
+#[doc(hidden)]
+#[unstable(feature = "gen_future", issue = "50547")]
+#[derive(Debug, Copy, Clone)]
+pub struct ResumeTy(NonNull<Context<'static>>);
+
+#[unstable(feature = "gen_future", issue = "50547")]
+unsafe impl Send for ResumeTy {}
+
+#[unstable(feature = "gen_future", issue = "50547")]
+unsafe impl Sync for ResumeTy {}
+
+/// Wrap a generator in a future.
+///
+/// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
+/// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
+// This is `const` to avoid extra errors after we recover from `const async fn`
+#[doc(hidden)]
+#[unstable(feature = "gen_future", issue = "50547")]
+#[inline]
+pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
+where
+    T: Generator<ResumeTy, Yield = ()>,
+{
+    #[rustc_diagnostic_item = "gen_future"]
+    struct GenFuture<T: Generator<ResumeTy, Yield = ()>>(T);
+
+    // We rely on the fact that async/await futures are immovable in order to create
+    // self-referential borrows in the underlying generator.
+    impl<T: Generator<ResumeTy, Yield = ()>> !Unpin for GenFuture<T> {}
+
+    impl<T: Generator<ResumeTy, Yield = ()>> Future for GenFuture<T> {
+        type Output = T::Return;
+        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+            // Safety: Safe because we're !Unpin + !Drop, and this is just a field projection.
+            let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };
+
+            // Resume the generator, turning the `&mut Context` into a `NonNull` raw pointer. The
+            // `.await` lowering will safely cast that back to a `&mut Context`.
+            match gen.resume(ResumeTy(NonNull::from(cx).cast::<Context<'static>>())) {
+                GeneratorState::Yielded(()) => Poll::Pending,
+                GeneratorState::Complete(x) => Poll::Ready(x),
+            }
+        }
+    }
+
+    GenFuture(gen)
+}
+
+#[doc(hidden)]
+#[unstable(feature = "gen_future", issue = "50547")]
+#[inline]
+pub unsafe fn get_context<'a, 'b>(cx: ResumeTy) -> &'a mut Context<'b> {
+    // SAFETY: the caller must guarantee that `cx.0` is a valid pointer
+    // that fulfills all the requirements for a mutable reference.
+    unsafe { &mut *cx.0.as_ptr().cast() }
+}