Immovable types prototype where the Move trait is builtin

Author: Zoxc

src/liballoc/boxed.rs

@@ -717,6 +717,7 @@ pub trait FnBox<A> {
     fn call_box(self: Box<Self>, args: A) -> Self::Output;
 }
 
+#[cfg(stage0)]
 #[unstable(feature = "fnbox",
            reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
 impl<A, F> FnBox<A> for F
@@ -729,6 +730,21 @@ impl<A, F> FnBox<A> for F
     }
 }
 
+#[cfg(not(stage0))]
+#[unstable(feature = "fnbox",
+           reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
+impl<A, F> FnBox<A> for F
+    where F: FnOnce<A>,
+          <F as FnOnce<A>>::Output: marker::Move,
+
+{
+    type Output = F::Output;
+
+    fn call_box(self: Box<F>, args: A) -> F::Output {
+        self.call_once(args)
+    }
+}
+
 #[unstable(feature = "fnbox",
            reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {

src/liballoc/lib.rs

@@ -114,6 +114,7 @@
 #![feature(slice_rsplit)]
 #![feature(specialization)]
 #![feature(staged_api)]
+#![cfg_attr(not(stage0), feature(immovable_types))]
 #![feature(str_internals)]
 #![feature(trusted_len)]
 #![feature(unboxed_closures)]