Feature gate spinlock in shared future

futures-util/Cargo.toml

@@ -22,6 +22,7 @@ sink = ["futures-sink"]
 io = ["std", "futures-io", "memchr"]
 channel = ["std", "futures-channel"]
 portable-atomic = ["futures-core/portable-atomic"]
+spin = ["dep:spin"]
 
 # Unstable features
 # These features are outside of the normal semver guarantees and require the
@@ -43,9 +44,7 @@ futures_01 = { version = "0.1.25", optional = true, package = "futures" }
 tokio-io = { version = "0.1.9", optional = true }
 pin-utils = "0.1.0"
 pin-project-lite = "0.2.6"
-
-[target.'cfg(target_has_atomic = "8")'.dependencies]
-spin = "0.9.8"
+spin = { version = "0.9.8", optional = true }
 
 [dev-dependencies]
 futures = { path = "../futures", features = ["async-await", "thread-pool"] }

futures-util/src/future/future/mod.rs

@@ -107,9 +107,9 @@ mod remote_handle;
 #[cfg(feature = "std")]
 pub use self::remote_handle::{Remote, RemoteHandle};
 
-#[cfg(all(feature = "alloc", target_has_atomic = "8"))]
+#[cfg(any(feature = "std", feature = "spin"))]
 mod shared;
-#[cfg(all(feature = "alloc", target_has_atomic = "8"))]
+#[cfg(any(feature = "std", feature = "spin"))]
 pub use self::shared::{Shared, WeakShared};
 
 impl<T: ?Sized> FutureExt for T where T: Future {}
@@ -440,7 +440,7 @@ pub trait FutureExt: Future {
     /// into a cloneable future. It enables a future to be polled by multiple
     /// threads.
     ///
-    /// This method is only available when the `std` feature of this
+    /// This method is only available when the `std` or 'spin' feature of this
     /// library is activated, and it is activated by default.
     ///
     /// # Examples
@@ -474,7 +474,7 @@ pub trait FutureExt: Future {
     /// join_handle.join().unwrap();
     /// # });
     /// ```
-    #[cfg(all(feature = "alloc", target_has_atomic = "8"))]
+    #[cfg(any(feature = "std", feature = "spin"))]
     fn shared(self) -> Shared<Self>
     where
         Self: Sized,

futures-util/src/future/mod.rs

@@ -35,7 +35,7 @@ pub use self::future::CatchUnwind;
 #[cfg(feature = "std")]
 pub use self::future::{Remote, RemoteHandle};
 
-#[cfg(all(feature = "alloc", target_has_atomic = "8"))]
+#[cfg(any(feature = "std", feature = "spin"))]
 pub use self::future::{Shared, WeakShared};
 
 mod try_future;

futures/Cargo.toml

@@ -40,6 +40,7 @@ compat = ["std", "futures-util/compat"]
 io-compat = ["compat", "futures-util/io-compat"]
 executor = ["std", "futures-executor/std"]
 thread-pool = ["executor", "futures-executor/thread-pool"]
+spin = ["futures-util/spin"]
 
 # Unstable features
 # These features are outside of the normal semver guarantees and require the