Make std feature to depend on alloc feature

Cargo.toml

@@ -21,33 +21,48 @@ travis-ci = { repository = "crossbeam-rs/crossbeam" }
 
 [features]
 default = ["std"]
-nightly = ["crossbeam-epoch/nightly", "crossbeam-utils/nightly", "crossbeam-queue/nightly"]
+
+# Enable to use APIs that require `std`.
+# This is enabled by default.
 std = [
-  "crossbeam-channel",
-  "crossbeam-deque",
+  "alloc",
+  "crossbeam-channel/std",
+  "crossbeam-deque/std",
   "crossbeam-epoch/std",
   "crossbeam-queue/std",
   "crossbeam-utils/std",
 ]
-alloc = ["crossbeam-epoch/alloc", "crossbeam-utils/alloc", "crossbeam-queue/alloc"]
+
+# Enable to use APIs that require `alloc`.
+# This is enabled by default and also enabled if the `std` feature is enabled.
+alloc = ["crossbeam-epoch/alloc", "crossbeam-queue/alloc"]
+
+# Enable to use of unstable functionality.
+# This is disabled by default and requires recent nightly compiler.
+# Note that this is outside of the normal semver guarantees and minor versions
+# of crossbeam may make breaking changes to them at any time.
+nightly = ["crossbeam-epoch/nightly", "crossbeam-utils/nightly", "crossbeam-queue/nightly"]
 
 [dependencies]
 cfg-if = "0.1.2"
 
 [dependencies.crossbeam-channel]
 version = "0.4"
 path = "./crossbeam-channel"
+default-features = false
 optional = true
 
 [dependencies.crossbeam-deque]
 version = "0.7.0"
 path = "./crossbeam-deque"
+default-features = false
 optional = true
 
 [dependencies.crossbeam-epoch]
 version = "0.8"
 path = "./crossbeam-epoch"
 default-features = false
+optional = true
 
 [dependencies.crossbeam-queue]
 version = "0.2"

ci/crossbeam-utils.sh

@@ -15,8 +15,5 @@ if [[ "$RUST_VERSION" == "nightly" ]]; then
 
     # Check for no_std environment.
     cargo check --target thumbv7m-none-eabi --no-default-features
-    cargo check --target thumbv7m-none-eabi --no-default-features --features alloc
-    cargo check --target thumbv7m-none-eabi --no-default-features --features alloc,nightly
     cargo check --target thumbv6m-none-eabi --no-default-features --features nightly
-    cargo check --target thumbv6m-none-eabi --no-default-features --features alloc,nightly
 fi

crossbeam-channel/Cargo.toml

@@ -15,9 +15,21 @@ description = "Multi-producer multi-consumer channels for message passing"
 keywords = ["channel", "mpmc", "select", "golang", "message"]
 categories = ["algorithms", "concurrency", "data-structures"]
 
+[features]
+default = ["std"]
+
+# Enable to use APIs that require `std`.
+# This is enabled by default.
+std = ["crossbeam-utils/std"]
+
+[dependencies]
+cfg-if = "0.1.2"
+
 [dependencies.crossbeam-utils]
 version = "0.7"
 path = "../crossbeam-utils"
+default-features = false
+optional = true
 
 [dev-dependencies]
 num_cpus = "1.10.0"

crossbeam-channel/src/lib.rs

@@ -346,33 +346,41 @@
 
 #![warn(missing_docs)]
 #![warn(missing_debug_implementations)]
+#![cfg_attr(not(feature = "std"), no_std)]
 
-extern crate crossbeam_utils;
+#[macro_use]
+extern crate cfg_if;
 
-mod channel;
-mod context;
-mod counter;
-mod err;
-mod flavors;
-mod select;
-mod select_macro;
-mod utils;
-mod waker;
+cfg_if! {
+    if #[cfg(feature = "std")] {
+        extern crate crossbeam_utils;
 
-/// Crate internals used by the `select!` macro.
-#[doc(hidden)]
-pub mod internal {
-    pub use select::SelectHandle;
-    pub use select::{select, select_timeout, try_select};
-}
+        mod channel;
+        mod context;
+        mod counter;
+        mod err;
+        mod flavors;
+        mod select;
+        mod select_macro;
+        mod utils;
+        mod waker;
+
+        /// Crate internals used by the `select!` macro.
+        #[doc(hidden)]
+        pub mod internal {
+            pub use select::SelectHandle;
+            pub use select::{select, select_timeout, try_select};
+        }
 
-pub use channel::{after, never, tick};
-pub use channel::{bounded, unbounded};
-pub use channel::{IntoIter, Iter, TryIter};
-pub use channel::{Receiver, Sender};
+        pub use channel::{after, never, tick};
+        pub use channel::{bounded, unbounded};
+        pub use channel::{IntoIter, Iter, TryIter};
+        pub use channel::{Receiver, Sender};
 
-pub use select::{Select, SelectedOperation};
+        pub use select::{Select, SelectedOperation};
 
-pub use err::{ReadyTimeoutError, SelectTimeoutError, TryReadyError, TrySelectError};
-pub use err::{RecvError, RecvTimeoutError, TryRecvError};
-pub use err::{SendError, SendTimeoutError, TrySendError};
+        pub use err::{ReadyTimeoutError, SelectTimeoutError, TryReadyError, TrySelectError};
+        pub use err::{RecvError, RecvTimeoutError, TryRecvError};
+        pub use err::{SendError, SendTimeoutError, TrySendError};
+    }
+}

crossbeam-deque/Cargo.toml

@@ -15,13 +15,27 @@ description = "Concurrent work-stealing deque"
 keywords = ["chase-lev", "lock-free", "scheduler", "scheduling"]
 categories = ["algorithms", "concurrency", "data-structures"]
 
+[features]
+default = ["std"]
+
+# Enable to use APIs that require `std`.
+# This is enabled by default.
+std = ["crossbeam-epoch/std", "crossbeam-utils/std"]
+
+[dependencies]
+cfg-if = "0.1.2"
+
 [dependencies.crossbeam-epoch]
 version = "0.8"
 path = "../crossbeam-epoch"
+default-features = false
+optional = true
 
 [dependencies.crossbeam-utils]
 version = "0.7"
 path = "../crossbeam-utils"
+default-features = false
+optional = true
 
 [dev-dependencies]
 rand = "0.6"