Non-Send boxed() (#1563)

Add BoxFutureLocal and boxed_local() function

Author: chmln

futures-core/src/future/mod.rs

@@ -14,6 +14,10 @@ pub use self::future_obj::{FutureObj, LocalFutureObj, UnsafeFutureObj};
 /// statically type your result or need to add some indirection.
 pub type BoxFuture<'a, T> = Pin<alloc::boxed::Box<dyn Future<Output = T> + Send + 'a>>;
 
+#[cfg(feature = "alloc")]
+/// `BoxFuture`, but without the `Send` requirement.
+pub type LocalBoxFuture<'a, T> = Pin<alloc::boxed::Box<dyn Future<Output = T> + 'a>>;
+
 /// A `Future` or `TryFuture` which tracks whether or not the underlying future
 /// should no longer be polled.
 ///

futures-util/src/future/mod.rs

@@ -10,7 +10,7 @@ use futures_core::task::{Context, Poll};
 #[cfg(feature = "alloc")]
 use alloc::boxed::Box;
 #[cfg(feature = "alloc")]
-use futures_core::future::BoxFuture;
+use futures_core::future::{BoxFuture, LocalBoxFuture};
 
 // re-export for `select!`
 #[doc(hidden)]
@@ -499,6 +499,16 @@ pub trait FutureExt: Future {
         Box::pin(self)
     }
 
+    /// Wrap the future in a Box, pinning it.
+    ///
+    /// Similar to `boxed`, but without the `Send` requirement.
+    #[cfg(feature = "alloc")]
+    fn boxed_local<'a>(self) -> LocalBoxFuture<'a, Self::Output>
+        where Self: Sized + 'a
+    {
+        Box::pin(self)
+    }
+
     /// Turns a [`Future<Output = T>`](Future) into a
     /// [`TryFuture<Ok = T, Error = ()`>](futures_core::future::TryFuture).
     fn unit_error(self) -> UnitError<Self>