Auto merge of #51442 - tinaun:more-future-impls, r=cramertj

[futures] add a few blanket impls to std

these were defined in the futures crate, but with the core definitions moving to std these would need to move too.

src/liballoc/boxed.rs

@@ -914,6 +914,24 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<PinBox<U>> for PinBox<T> {}
 #[unstable(feature = "pin", issue = "49150")]
 impl<T: ?Sized> Unpin for PinBox<T> {}
 
+#[unstable(feature = "futures_api", issue = "50547")]
+impl<'a, F: ?Sized + Future + Unpin> Future for Box<F> {
+    type Output = F::Output;
+
+    fn poll(mut self: PinMut<Self>, cx: &mut Context) -> Poll<Self::Output> {
+        PinMut::new(&mut **self).poll(cx)
+    }
+}
+
+#[unstable(feature = "futures_api", issue = "50547")]
+impl<'a, F: ?Sized + Future> Future for PinBox<F> {
+    type Output = F::Output;
+
+    fn poll(mut self: PinMut<Self>, cx: &mut Context) -> Poll<Self::Output> {
+        self.as_pin_mut().poll(cx)
+    }
+}
+
 #[unstable(feature = "futures_api", issue = "50547")]
 unsafe impl<F: Future<Output = ()> + Send + 'static> UnsafePoll for PinBox<F> {
     fn into_raw(self) -> *mut () {

src/liballoc/lib.rs

@@ -80,6 +80,7 @@
 #![cfg_attr(test, feature(rand, test))]
 #![feature(allocator_api)]
 #![feature(allow_internal_unstable)]
+#![feature(arbitrary_self_types)]
 #![feature(ascii_ctype)]
 #![feature(box_into_raw_non_null)]
 #![feature(box_patterns)]