Get building on thumbv6m targets

As of rust-lang/rust#51953
`cfg(target_has_atomic)` has been updated to separately enable
compare-and-swap (CAS) operations from the different supported sizes.
Because `AtomicWaker` uses these CAS operations we need to also gate its
inclusion on `cfg(target_has_atomic = "cas")`. `thumv6m` is one of the
architectures that supports atomic read-write operations on its pointer
sized integers, but doesn't support CAS operations.

Author: Nemo157

.travis.yml

@@ -27,9 +27,8 @@ matrix:
         - cargo test --manifest-path futures/Cargo.toml --features nightly --test async_await_tests
     - rust: nightly
       script:
-        - rustup component add rust-src
-        - cargo install xargo
-        - xargo build --manifest-path futures/Cargo.toml --target thumbv6m-none-eabi --no-default-features --features nightly
+        - rustup target add thumbv6m-none-eabi
+        - cargo build --manifest-path futures/Cargo.toml --target thumbv6m-none-eabi --no-default-features --features nightly
     - rust: 1.20.0
       script: cargo test --all
     - rust: nightly

futures-core/src/task/mod.rs

@@ -25,9 +25,15 @@ mod data {
 }
 
 
-#[cfg_attr(feature = "nightly", cfg(target_has_atomic = "ptr"))]
+#[cfg_attr(
+    feature = "nightly",
+    cfg(all(target_has_atomic = "cas", target_has_atomic = "ptr"))
+)]
 mod atomic_waker;
-#[cfg_attr(feature = "nightly", cfg(target_has_atomic = "ptr"))]
+#[cfg_attr(
+    feature = "nightly",
+    cfg(all(target_has_atomic = "cas", target_has_atomic = "ptr"))
+)]
 pub use self::atomic_waker::AtomicWaker;
 
 /// A map storing task-local data.

futures/src/lib.rs

@@ -22,8 +22,8 @@
 #![no_std]
 #![doc(html_root_url = "https://docs.rs/futures/0.2.2")]
 
-#![cfg_attr(feature = "nightly", feature(cfg_target_has_atomic))]
 #![cfg_attr(feature = "nightly", feature(use_extern_macros))]
+#![cfg_attr(feature = "nightly", feature(cfg_target_has_atomic))]
 
 extern crate futures_async_runtime;
 extern crate futures_core;
@@ -392,7 +392,10 @@ pub mod task {
         Context, LocalMap, Waker, UnsafeWake,
     };
 
-    #[cfg_attr(feature = "nightly", cfg(target_has_atomic = "ptr"))]
+    #[cfg_attr(
+        feature = "nightly",
+        cfg(all(target_has_atomic = "cas", target_has_atomic = "ptr"))
+    )]
     pub use futures_core::task::AtomicWaker;
 
     #[cfg(feature = "std")]