Support oneshot in no_std environment

.travis.yml

@@ -94,6 +94,7 @@ matrix:
         - cargo build --manifest-path futures/Cargo.toml --no-default-features --features alloc
         - cargo build --manifest-path futures-core/Cargo.toml --no-default-features --features alloc
         - cargo build --manifest-path futures-sink/Cargo.toml --no-default-features --features alloc
+        - cargo build --manifest-path futures-channel/Cargo.toml --no-default-features --features alloc
         - cargo build --manifest-path futures-util/Cargo.toml --no-default-features --features alloc
 
     - name: cargo build --target=thumbv6m-none-eabi

futures-channel/Cargo.toml

@@ -15,9 +15,12 @@ Channels for asynchronous communication using futures-rs.
 name = "futures_channel"
 
 [features]
-std = ["futures-core-preview/std"]
-sink = ["futures-sink-preview"]
 default = ["std"]
+std = ["alloc", "futures-core-preview/std"]
+alloc = ["futures-core-preview/alloc"]
+sink = ["futures-sink-preview"]
+nightly = ["futures-core-preview/nightly"]
+cfg-target-has-atomic = ["futures-core-preview/cfg-target-has-atomic"]
 
 [dependencies]
 futures-core-preview = { path = "../futures-core", version = "=0.3.0-alpha.17", default-features = false }

futures-channel/src/lib.rs

@@ -3,9 +3,11 @@
 //! This crate provides channels that can be used to communicate between
 //! asynchronous tasks.
 //!
-//! All items of this library are only available when the `std` feature of this
+//! All items of this library are only available when the `std` or `alloc` feature of this
 //! library is activated, and it is activated by default.
 
+#![cfg_attr(feature = "cfg-target-has-atomic", feature(cfg_target_has_atomic))]
+
 #![cfg_attr(not(feature = "std"), no_std)]
 
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms, unreachable_pub)]
@@ -17,9 +19,27 @@
 
 #![doc(html_root_url = "https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alpha.17/futures_channel")]
 
-#[cfg(feature = "std")]
-mod lock;
-#[cfg(feature = "std")]
-pub mod mpsc;
-#[cfg(feature = "std")]
-pub mod oneshot;
+#[cfg(all(feature = "cfg-target-has-atomic", not(feature = "nightly")))]
+compile_error!("The `cfg-target-has-atomic` feature requires the `nightly` feature as an explicit opt-in to unstable features");
+
+macro_rules! cfg_target_has_atomic {
+    ($($item:item)*) => {$(
+        #[cfg_attr(
+            feature = "cfg-target-has-atomic",
+            cfg(all(target_has_atomic = "cas", target_has_atomic = "ptr"))
+        )]
+        $item
+    )*};
+}
+
+cfg_target_has_atomic! {
+    #[cfg(feature = "alloc")]
+    extern crate alloc;
+
+    #[cfg(feature = "alloc")]
+    mod lock;
+    #[cfg(feature = "std")]
+    pub mod mpsc;
+    #[cfg(feature = "alloc")]
+    pub mod oneshot;
+}

futures-channel/src/oneshot.rs

@@ -1,13 +1,12 @@
 //! A channel for sending a single message between asynchronous tasks.
 
+use alloc::sync::Arc;
+use core::fmt;
+use core::pin::Pin;
+use core::sync::atomic::AtomicBool;
+use core::sync::atomic::Ordering::SeqCst;
 use futures_core::future::Future;
 use futures_core::task::{Context, Poll, Waker};
-use std::pin::Pin;
-use std::sync::Arc;
-use std::sync::atomic::AtomicBool;
-use std::sync::atomic::Ordering::SeqCst;
-use std::error::Error;
-use std::fmt;
 
 use crate::lock::Lock;
 
@@ -387,7 +386,8 @@ impl fmt::Display for Canceled {
     }
 }
 
-impl Error for Canceled {}
+#[cfg(feature = "std")]
+impl std::error::Error for Canceled {}
 
 impl<T> Receiver<T> {
     /// Gracefully close this receiver, preventing any subsequent attempts to

futures/Cargo.toml

@@ -36,14 +36,14 @@ tokio = "0.1.11"
 assert_matches = "1.3.0"
 
 [features]
-nightly = ["futures-core-preview/nightly", "futures-util-preview/nightly"]
+default = ["std"]
 std = ["alloc", "futures-core-preview/std", "futures-executor-preview/std", "futures-io-preview/std", "futures-sink-preview/std", "futures-util-preview/std", "futures-util-preview/io", "futures-util-preview/channel"]
+alloc = ["futures-core-preview/alloc", "futures-sink-preview/alloc", "futures-channel-preview/alloc", "futures-util-preview/alloc"]
+nightly = ["futures-core-preview/nightly", "futures-channel-preview/nightly", "futures-util-preview/nightly"]
 async-await = ["futures-util-preview/async-await", "futures-util-preview/select-macro"]
-default = ["std"]
 compat = ["std", "futures-util-preview/compat"]
 io-compat = ["compat", "futures-util-preview/io-compat"]
-cfg-target-has-atomic = ["futures-core-preview/cfg-target-has-atomic", "futures-util-preview/cfg-target-has-atomic"]
-alloc = ["futures-core-preview/alloc", "futures-sink-preview/alloc", "futures-util-preview/alloc"]
+cfg-target-has-atomic = ["futures-core-preview/cfg-target-has-atomic", "futures-channel-preview/cfg-target-has-atomic", "futures-util-preview/cfg-target-has-atomic"]
 
 [package.metadata.docs.rs]
 all-features = true

futures/src/lib.rs

@@ -73,7 +73,11 @@ pub use futures_util::{
     join, try_join, pending, poll,
 };
 
-#[cfg(feature = "std")]
+#[cfg_attr(
+    feature = "cfg-target-has-atomic",
+    cfg(all(target_has_atomic = "cas", target_has_atomic = "ptr"))
+)]
+#[cfg(feature = "alloc")]
 pub mod channel {
     //! Cross-task communication.
     //!
@@ -86,10 +90,13 @@ pub mod channel {
     //!   channel for sending values between tasks, analogous to the
     //!   similarly-named structure in the standard library.
     //!
-    //! This module is only available when the `std` feature of this
+    //! This module is only available when the `std` or `alloc` feature of this
     //! library is activated, and it is activated by default.
 
-    pub use futures_channel::{oneshot, mpsc};
+    pub use futures_channel::oneshot;
+
+    #[cfg(feature = "std")]
+    pub use futures_channel::mpsc;
 }
 
 #[cfg(feature = "compat")]