Auto merge of #51580 - cramertj:async-await, r=eddyb

async/await

This PR implements `async`/`await` syntax for `async fn` in Rust 2015 and `async` closures and `async` blocks in Rust 2018 (tracking issue: https://github.com/rust-lang/rust/issues/50547). Limitations: non-`move` async closures with arguments are currently not supported, nor are `async fn` with multiple different input lifetimes. These limitations are not fundamental and will be removed in the future, however I'd like to go ahead and get this PR merged so we can start experimenting with this in combination with futures 0.3.

Based on https://github.com/rust-lang/rust/pull/51414.
cc @petrochenkov for parsing changes.
r? @eddyb

src/libstd/future.rs

@@ -0,0 +1,116 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Asynchronous values.
+
+use core::cell::Cell;
+use core::marker::Unpin;
+use core::mem::PinMut;
+use core::option::Option;
+use core::ptr::NonNull;
+use core::task::{self, Poll};
+use core::ops::{Drop, Generator, GeneratorState};
+
+#[doc(inline)]
+pub use core::future::*;
+
+/// Wrap a future in a generator.
+///
+/// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
+/// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
+#[unstable(feature = "gen_future", issue = "50547")]
+pub fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> {
+    GenFuture(x)
+}
+
+/// A wrapper around generators used to implement `Future` for `async`/`await` code.
+#[unstable(feature = "gen_future", issue = "50547")]
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
+struct GenFuture<T: Generator<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<Yield = ()>> !Unpin for GenFuture<T> {}
+
+#[unstable(feature = "gen_future", issue = "50547")]
+impl<T: Generator<Yield = ()>> Future for GenFuture<T> {
+    type Output = T::Return;
+    fn poll(self: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
+        set_task_cx(cx, || match unsafe { PinMut::get_mut(self).0.resume() } {
+            GeneratorState::Yielded(()) => Poll::Pending,
+            GeneratorState::Complete(x) => Poll::Ready(x),
+        })
+    }
+}
+
+thread_local! {
+    static TLS_CX: Cell<Option<NonNull<task::Context<'static>>>> = Cell::new(None);
+}
+
+struct SetOnDrop(Option<NonNull<task::Context<'static>>>);
+
+impl Drop for SetOnDrop {
+    fn drop(&mut self) {
+        TLS_CX.with(|tls_cx| {
+            tls_cx.set(self.0.take());
+        });
+    }
+}
+
+#[unstable(feature = "gen_future", issue = "50547")]
+/// Sets the thread-local task context used by async/await futures.
+pub fn set_task_cx<F, R>(cx: &mut task::Context, f: F) -> R
+where
+    F: FnOnce() -> R
+{
+    let old_cx = TLS_CX.with(|tls_cx| {
+        tls_cx.replace(NonNull::new(
+            cx
+                as *mut task::Context
+                as *mut ()
+                as *mut task::Context<'static>
+        ))
+    });
+    let _reset_cx = SetOnDrop(old_cx);
+    f()
+}
+
+#[unstable(feature = "gen_future", issue = "50547")]
+/// Retrieves the thread-local task context used by async/await futures.
+///
+/// This function acquires exclusive access to the task context.
+///
+/// Panics if no task has been set or if the task context has already been
+/// retrived by a surrounding call to get_task_cx.
+pub fn get_task_cx<F, R>(f: F) -> R
+where
+    F: FnOnce(&mut task::Context) -> R
+{
+    let cx_ptr = TLS_CX.with(|tls_cx| {
+        // Clear the entry so that nested `with_get_cx` calls
+        // will fail or set their own value.
+        tls_cx.replace(None)
+    });
+    let _reset_cx = SetOnDrop(cx_ptr);
+
+    let mut cx_ptr = cx_ptr.expect(
+        "TLS task::Context not set. This is a rustc bug. \
+        Please file an issue on https://github.com/rust-lang/rust.");
+    unsafe { f(cx_ptr.as_mut()) }
+}
+
+#[unstable(feature = "gen_future", issue = "50547")]
+/// Polls a future in the current thread-local task context.
+pub fn poll_in_task_cx<F>(f: &mut PinMut<F>) -> Poll<F::Output>
+where
+    F: Future
+{
+    get_task_cx(|cx| f.reborrow().poll(cx))
+}

src/libstd/lib.rs

@@ -263,6 +263,7 @@
 #![feature(fn_traits)]
 #![feature(fnbox)]
 #![feature(futures_api)]
+#![feature(generator_trait)]
 #![feature(hashmap_internals)]
 #![feature(int_error_internals)]
 #![feature(integer_atomics)]
@@ -462,22 +463,6 @@ pub use core::u128;
 #[stable(feature = "core_hint", since = "1.27.0")]
 pub use core::hint;
 
-#[unstable(feature = "futures_api",
-           reason = "futures in libcore are unstable",
-           issue = "50547")]
-pub mod task {
-    //! Types and Traits for working with asynchronous tasks.
-    #[doc(inline)]
-    pub use core::task::*;
-    #[doc(inline)]
-    pub use alloc_crate::task::*;
-}
-
-#[unstable(feature = "futures_api",
-           reason = "futures in libcore are unstable",
-           issue = "50547")]
-pub use core::future;
-
 pub mod f32;
 pub mod f64;
 
@@ -499,6 +484,22 @@ pub mod process;
 pub mod sync;
 pub mod time;
 
+#[unstable(feature = "futures_api",
+           reason = "futures in libcore are unstable",
+           issue = "50547")]
+pub mod task {
+    //! Types and Traits for working with asynchronous tasks.
+    #[doc(inline)]
+    pub use core::task::*;
+    #[doc(inline)]
+    pub use alloc_crate::task::*;
+}
+
+#[unstable(feature = "futures_api",
+           reason = "futures in libcore are unstable",
+           issue = "50547")]
+pub mod future;
+
 // Platform-abstraction modules
 #[macro_use]
 mod sys_common;

src/libstd/macros.rs

@@ -213,6 +213,25 @@ macro_rules! eprintln {
     ($fmt:expr, $($arg:tt)*) => (eprint!(concat!($fmt, "\n"), $($arg)*));
 }
 
+#[macro_export]
+#[unstable(feature = "await_macro", issue = "50547")]
+#[allow_internal_unstable]
+#[allow_internal_unsafe]
+macro_rules! await {
+    ($e:expr) => { {
+        let mut pinned = $e;
+        let mut pinned = unsafe { $crate::mem::PinMut::new_unchecked(&mut pinned) };
+        loop {
+            match $crate::future::poll_in_task_cx(&mut pinned) {
+                // FIXME(cramertj) prior to stabilizing await, we have to ensure that this
+                // can't be used to create a generator on stable via `|| await!()`.
+                $crate::task::Poll::Pending => yield,
+                $crate::task::Poll::Ready(x) => break x,
+            }
+        }
+    } }
+}
+
 /// A macro to select an event from a number of receivers.
 ///
 /// This macro is used to wait for the first event to occur on a number of