Support shared futures on no_std (#2868)

futures-util/Cargo.toml

@@ -12,8 +12,8 @@ Common utilities and extension traits for the futures-rs library.
 
 [features]
 default = ["std", "async-await", "async-await-macro"]
-std = ["alloc", "futures-core/std", "futures-task/std", "slab"]
-alloc = ["futures-core/alloc", "futures-task/alloc"]
+std = ["alloc", "futures-core/std", "futures-task/std", "slab/std"]
+alloc = ["futures-core/alloc", "futures-task/alloc", "slab"]
 async-await = []
 async-await-macro = ["async-await", "futures-macro"]
 compat = ["std", "futures_01"]
@@ -37,12 +37,13 @@ futures-channel = { path = "../futures-channel", version = "=0.4.0-alpha.0", def
 futures-io = { path = "../futures-io", version = "0.3.31", default-features = false, features = ["std"], optional = true }
 futures-sink = { path = "../futures-sink", version = "=0.4.0-alpha.0", default-features = false, optional = true }
 futures-macro = { path = "../futures-macro", version = "=0.4.0-alpha.0", default-features = false, optional = true }
-slab = { version = "0.4.2", optional = true }
+slab = { version = "0.4.2", default-features = false, optional = true }
 memchr = { version = "2.2", optional = true }
 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"
+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(feature = "std")]
+#[cfg(any(feature = "std", all(feature = "alloc", feature = "spin")))]
 mod shared;
-#[cfg(feature = "std")]
+#[cfg(any(feature = "std", all(feature = "alloc", 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(feature = "std")]
+    #[cfg(any(feature = "std", all(feature = "alloc", feature = "spin")))]
     fn shared(self) -> Shared<Self>
     where
         Self: Sized,

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

@@ -1,15 +1,20 @@
 use crate::task::{waker_ref, ArcWake};
+use alloc::sync::{Arc, Weak};
+use core::cell::UnsafeCell;
+use core::fmt;
+use core::hash::Hasher;
+use core::pin::Pin;
+use core::ptr;
+use core::sync::atomic::AtomicUsize;
+use core::sync::atomic::Ordering::{Acquire, SeqCst};
 use futures_core::future::{FusedFuture, Future};
 use futures_core::task::{Context, Poll, Waker};
 use slab::Slab;
-use std::cell::UnsafeCell;
-use std::fmt;
-use std::hash::Hasher;
-use std::pin::Pin;
-use std::ptr;
-use std::sync::atomic::AtomicUsize;
-use std::sync::atomic::Ordering::{Acquire, SeqCst};
-use std::sync::{Arc, Mutex, Weak};
+
+#[cfg(feature = "std")]
+type Mutex<T> = std::sync::Mutex<T>;
+#[cfg(not(feature = "std"))]
+type Mutex<T> = spin::Mutex<T>;
 
 /// Future for the [`shared`](super::FutureExt::shared) method.
 #[must_use = "futures do nothing unless you `.await` or poll them"]
@@ -204,7 +209,10 @@ where
 {
     /// Registers the current task to receive a wakeup when we are awoken.
     fn record_waker(&self, waker_key: &mut usize, cx: &mut Context<'_>) {
+        #[cfg(feature = "std")]
         let mut wakers_guard = self.notifier.wakers.lock().unwrap();
+        #[cfg(not(feature = "std"))]
+        let mut wakers_guard = self.notifier.wakers.lock();
 
         let wakers_mut = wakers_guard.as_mut();
 
@@ -345,7 +353,11 @@ where
         inner.notifier.state.store(COMPLETE, SeqCst);
 
         // Wake all tasks and drop the slab
+        #[cfg(feature = "std")]
         let mut wakers_guard = inner.notifier.wakers.lock().unwrap();
+        #[cfg(not(feature = "std"))]
+        let mut wakers_guard = inner.notifier.wakers.lock();
+
         let mut wakers = wakers_guard.take().unwrap();
         for waker in wakers.drain().flatten() {
             waker.wake();
@@ -375,19 +387,28 @@ where
     fn drop(&mut self) {
         if self.waker_key != NULL_WAKER_KEY {
             if let Some(ref inner) = self.inner {
+                #[cfg(feature = "std")]
                 if let Ok(mut wakers) = inner.notifier.wakers.lock() {
                     if let Some(wakers) = wakers.as_mut() {
                         wakers.remove(self.waker_key);
                     }
                 }
+                #[cfg(not(feature = "std"))]
+                if let Some(wakers) = inner.notifier.wakers.lock().as_mut() {
+                    wakers.remove(self.waker_key);
+                }
             }
         }
     }
 }
 
 impl ArcWake for Notifier {
     fn wake_by_ref(arc_self: &Arc<Self>) {
+        #[cfg(feature = "std")]
         let wakers = &mut *arc_self.wakers.lock().unwrap();
+        #[cfg(not(feature = "std"))]
+        let wakers = &mut *arc_self.wakers.lock();
+
         if let Some(wakers) = wakers.as_mut() {
             for (_key, opt_waker) in wakers {
                 if let Some(waker) = opt_waker.take() {

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(feature = "std")]
+#[cfg(any(feature = "std", all(feature = "alloc", 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