Move AtomicWaker to futures-core

... so `AtomicWaker` could be used inside `futures-channel`.

`AtomicWaker` is still accessible to users only from `futures-util`.

Author: stepancheg

futures-core/Cargo.toml

@@ -1,3 +1,5 @@
+cargo-features = ["rename-dependency"]
+
 [package]
 name = "futures-core-preview"
 edition = "2018"
@@ -17,6 +19,10 @@ name = "futures_core"
 [features]
 default = ["std"]
 std = ["either/use_std"]
+nightly = []
 
 [dependencies]
 either = { version = "1.4", default-features = false, optional = true }
+
+[dev-dependencies]
+futures-preview = { path = "../futures", version = "0.3.0-alpha.9" }

futures-core/src/lib.rs

@@ -1,6 +1,7 @@
 //! Core traits and types for asynchronous operations in Rust.
 
 #![feature(pin, arbitrary_self_types, futures_api)]
+#![cfg_attr(feature = "nightly", feature(cfg_target_has_atomic))]
 
 #![cfg_attr(not(feature = "std"), no_std)]
 

futures-util/src/task/atomic_waker.rs renamed to futures-core/src/task/__internal/atomic_waker.rs

@@ -2,7 +2,7 @@ use core::fmt;
 use core::cell::UnsafeCell;
 use core::sync::atomic::AtomicUsize;
 use core::sync::atomic::Ordering::{Acquire, Release, AcqRel};
-use futures_core::task::{LocalWaker, Waker};
+use crate::task::{LocalWaker, Waker};
 
 /// A synchronization primitive for task wakeup.
 ///
@@ -170,8 +170,9 @@ impl AtomicWaker {
     ///
     /// ```
     /// #![feature(pin, arbitrary_self_types, futures_api)]
-    /// use futures::future::Future;
-    /// use futures::task::{LocalWaker, Poll, AtomicWaker};
+    /// use futures_core::future::Future;
+    /// use futures_core::task::__internal::AtomicWaker;
+    /// use futures_core::task::{LocalWaker, Poll};
     /// use std::sync::atomic::AtomicBool;
     /// use std::sync::atomic::Ordering::SeqCst;
     /// use std::pin::Pin;

futures-core/src/task/__internal/mod.rs

@@ -0,0 +1,10 @@
+#[cfg_attr(
+    feature = "nightly",
+    cfg(all(target_has_atomic = "cas", target_has_atomic = "ptr"))
+)]
+mod atomic_waker;
+#[cfg_attr(
+    feature = "nightly",
+    cfg(all(target_has_atomic = "cas", target_has_atomic = "ptr"))
+)]
+pub use self::atomic_waker::AtomicWaker;

futures-core/src/task/mod.rs

@@ -1,6 +1,8 @@
 //! Task notification.
 
 mod spawn;
+#[doc(hidden)]
+pub mod __internal;
 pub use self::spawn::{Spawn, LocalSpawn, SpawnError};
 
 pub use core::task::{Poll, Waker, LocalWaker, UnsafeWake};

futures-util/src/task/mod.rs

@@ -15,9 +15,4 @@ pub use self::local_waker_ref::{local_waker_ref, local_waker_ref_from_nonlocal,
     feature = "nightly",
     cfg(all(target_has_atomic = "cas", target_has_atomic = "ptr"))
 )]
-mod atomic_waker;
-#[cfg_attr(
-    feature = "nightly",
-    cfg(all(target_has_atomic = "cas", target_has_atomic = "ptr"))
-)]
-pub use self::atomic_waker::AtomicWaker;
+pub use futures_core::task::__internal::AtomicWaker;

futures/Cargo.toml

@@ -36,7 +36,7 @@ futures-test-preview = { path = "../futures-test", version = "0.3.0-alpha.9", de
 tokio = "0.1.11"
 
 [features]
-nightly = ["futures-util-preview/nightly"]
+nightly = ["futures-util-preview/nightly", "futures-core-preview/nightly"]
 std = ["futures-core-preview/std", "futures-executor-preview/std", "futures-io-preview/std", "futures-sink-preview/std", "futures-util-preview/std"]
 default = ["std"]
 compat = ["std", "futures-util-preview/compat"]