Add std::cell::clone_ref back, but deprecated.

Author: SimonSapin

src/libcore/cell.rs

@@ -539,6 +539,20 @@ impl<'b, T: ?Sized> Deref for Ref<'b, T> {
     }
 }
 
+/// Copies a `Ref`.
+///
+/// The `RefCell` is already immutably borrowed, so this cannot fail.
+///
+/// A `Clone` implementation would interfere with the widespread
+/// use of `r.borrow().clone()` to clone the contents of a `RefCell`.
+#[deprecated(since = "1.2.0", reason = "moved to a `Ref::clone` associated function")]
+#[unstable(feature = "core",
+           reason = "likely to be moved to a method, pending language changes")]
+#[inline]
+pub fn clone_ref<'b, T:Clone>(orig: &Ref<'b, T>) -> Ref<'b, T> {
+    Ref::clone(orig)
+}
+
 impl<'b, T: ?Sized> Ref<'b, T> {
     /// Copies a `Ref`.
     ///