refactor: prevent double boxing in boxed

src/future/mod.rs

@@ -308,7 +308,7 @@ pub trait Future {
     fn boxed(self) -> BoxFuture<Self::Item, Self::Error>
         where Self: Sized + Send + 'static
     {
-        ::std::boxed::Box::new(self)
+        Boxed::boxed(self)
     }
 
     /// Map this future's result to a different type, returning a new future of
@@ -1044,3 +1044,19 @@ impl<F> fmt::Debug for ExecuteError<F> {
         }
     }
 }
+
+trait Boxed: Future {
+    fn boxed(self) -> BoxFuture<Self::Item, Self::Error>;
+}
+
+impl<F> Boxed for F where F: Future + Sized + Send + 'static {
+    default fn boxed(self) -> BoxFuture<Self::Item, Self::Error> {
+        ::std::boxed::Box::new(self)
+    }
+}
+
+impl<I, E> Boxed for BoxFuture<I, E> {
+    fn boxed(self) -> BoxFuture<Self::Item, Self::Error> {
+        self
+    }
+}

src/lib.rs

@@ -157,6 +157,7 @@
 #![no_std]
 #![deny(missing_docs, missing_debug_implementations)]
 #![doc(html_root_url = "https://docs.rs/futures/0.1")]
+#![feature(specialization)]
 
 #[macro_use]
 #[cfg(feature = "use_std")]