Add std feature to all crates for forward compatibility

Cargo.toml

@@ -24,8 +24,8 @@ default = ["std"]
 nightly = ["crossbeam-epoch/nightly", "crossbeam-utils/nightly", "crossbeam-queue/nightly"]
 std = [
   "alloc",
-  "crossbeam-channel",
-  "crossbeam-deque",
+  "crossbeam-channel/std",
+  "crossbeam-deque/std",
   "crossbeam-epoch/std",
   "crossbeam-queue/std",
   "crossbeam-utils/std",
@@ -38,11 +38,13 @@ 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]

crossbeam-channel/Cargo.toml

@@ -15,6 +15,10 @@ description = "Multi-producer multi-consumer channels for message passing"
 keywords = ["channel", "mpmc", "select", "golang", "message"]
 categories = ["algorithms", "concurrency", "data-structures"]
 
+[features]
+default = ["std"]
+std = []
+
 [dependencies.crossbeam-utils]
 version = "0.7"
 path = "../crossbeam-utils"

crossbeam-channel/src/lib.rs

@@ -347,6 +347,9 @@
 #![warn(missing_docs)]
 #![warn(missing_debug_implementations)]
 
+#[cfg(not(feature = "std"))]
+compile_error!("`std` feature is currently required to build this crate");
+
 extern crate crossbeam_utils;
 
 mod channel;

crossbeam-deque/Cargo.toml

@@ -15,6 +15,10 @@ description = "Concurrent work-stealing deque"
 keywords = ["chase-lev", "lock-free", "scheduler", "scheduling"]
 categories = ["algorithms", "concurrency", "data-structures"]
 
+[features]
+default = ["std"]
+std = []
+
 [dependencies.crossbeam-epoch]
 version = "0.8"
 path = "../crossbeam-epoch"

crossbeam-deque/src/lib.rs

@@ -89,6 +89,9 @@
 #![warn(missing_docs)]
 #![warn(missing_debug_implementations)]
 
+#[cfg(not(feature = "std"))]
+compile_error!("`std` feature is currently required to build this crate");
+
 extern crate crossbeam_epoch as epoch;
 extern crate crossbeam_utils as utils;
 

crossbeam-epoch/src/lib.rs

@@ -59,6 +59,9 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 #![cfg_attr(feature = "nightly", feature(cfg_target_has_atomic))]
 
+#[cfg(feature = "alloc")]
+compile_error!("`std` or `alloc` feature is currently required to build this crate");
+
 #[macro_use]
 extern crate cfg_if;
 #[cfg(feature = "alloc")]