Rollup merge of #69033 - jonas-schievink:resume-with-context, r=tmandry

Use generator resume arguments in the async/await lowering

This removes the TLS requirement from async/await and enables it in `#![no_std]` crates.

Closes https://github.com/rust-lang/rust/issues/56974

I'm not confident the HIR lowering is completely correct, there seem to be quite a few undocumented invariants in there. The `async-std` and tokio test suites are passing with these changes though.

1 files changed, 78 insertions, 0 deletions

diff --git a/src/libcore/future/mod.rs b/src/libcore/future/mod.rs
index 89ea4713cfd..8dfda7a4a32 100644
--- a/src/libcore/future/mod.rs
+++ b/src/libcore/future/mod.rs
@@ -2,6 +2,84 @@
 
 //! Asynchronous values.
 
+#[cfg(not(bootstrap))]
+use crate::{
+    ops::{Generator, GeneratorState},
+    pin::Pin,
+    ptr::NonNull,
+    task::{Context, Poll},
+};
+
 mod future;
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub use self::future::Future;
+
+/// 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")]
+#[cfg(not(bootstrap))]
+#[derive(Debug, Copy, Clone)]
+pub struct ResumeTy(NonNull<Context<'static>>);
+
+#[unstable(feature = "gen_future", issue = "50547")]
+#[cfg(not(bootstrap))]
+unsafe impl Send for ResumeTy {}
+
+#[unstable(feature = "gen_future", issue = "50547")]
+#[cfg(not(bootstrap))]
+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")]
+#[cfg(not(bootstrap))]
+#[inline]
+pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
+where
+    T: Generator<ResumeTy, Yield = ()>,
+{
+    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")]
+#[cfg(not(bootstrap))]
+#[inline]
+pub unsafe fn poll_with_context<F>(f: Pin<&mut F>, mut cx: ResumeTy) -> Poll<F::Output>
+where
+    F: Future,
+{
+    F::poll(f, cx.0.as_mut())
+}