Make async-await feature not depend on std feature

Author: taiki-e

.travis.yml

@@ -126,7 +126,7 @@ matrix:
         - cargo build --manifest-path futures/Cargo.toml
             --target thumbv7m-none-eabi
             --no-default-features
-            --features nightly,alloc
+            --features alloc
 
     - name: cargo check (futures-util)
       rust: nightly
@@ -150,6 +150,7 @@ matrix:
         - cargo check --manifest-path futures-util/Cargo.toml --no-default-features
         - cargo check --manifest-path futures-util/Cargo.toml --no-default-features --features sink
         - cargo check --manifest-path futures-util/Cargo.toml --no-default-features --features alloc,sink
+        - cargo check --manifest-path futures-util/Cargo.toml --no-default-features --features nightly,async-await
 
     - name: cargo doc
       rust: nightly

futures-util/Cargo.toml

@@ -18,7 +18,7 @@ name = "futures_util"
 default = ["std"]
 std = ["alloc", "futures-core-preview/std", "slab"]
 alloc = ["futures-core-preview/alloc"]
-async-await = ["std"]
+async-await = []
 compat = ["std", "futures_01"]
 io-compat = ["io", "compat", "tokio-io"]
 bench = []

futures-util/src/lib.rs

@@ -30,14 +30,17 @@ extern crate futures_core;
 pub use futures_core::ready;
 pub use pin_utils::pin_mut;
 
+#[cfg(feature = "std")]
 #[cfg(feature = "async-await")]
 #[macro_use]
 #[doc(hidden)]
 pub mod async_await;
+#[cfg(feature = "std")]
 #[cfg(feature = "async-await")]
 #[doc(hidden)]
 pub use self::async_await::*;
 
+#[cfg(feature = "std")]
 #[cfg(feature = "select-macro")]
 #[doc(hidden)]
 pub mod rand_reexport { // used by select!

futures/src/lib.rs

@@ -67,6 +67,7 @@ compile_error!("The `cfg-target-has-atomic` feature requires the `nightly` featu
 // Macro reexports
 pub use futures_core::ready; // Readiness propagation
 pub use futures_util::pin_mut;
+#[cfg(feature = "std")]
 #[cfg(feature = "async-await")]
 pub use futures_util::{pending, poll}; // Async-await
 
@@ -515,10 +516,12 @@ pub mod never {
 
 // proc-macro re-export --------------------------------------
 
+#[cfg(feature = "std")]
 #[cfg(feature = "async-await")]
 #[doc(hidden)]
 pub use futures_util::rand_reexport;
 
+#[cfg(feature = "std")]
 #[cfg(feature = "async-await")]
 #[doc(hidden)]
 pub mod inner_macro {
@@ -527,6 +530,7 @@ pub mod inner_macro {
     pub use futures_util::select;
 }
 
+#[cfg(feature = "std")]
 #[cfg(feature = "async-await")]
 futures_util::document_join_macro! {
     #[macro_export]
@@ -550,6 +554,7 @@ futures_util::document_join_macro! {
     }
 }
 
+#[cfg(feature = "std")]
 #[cfg(feature = "async-await")]
 futures_util::document_select_macro! {
     #[macro_export]