Add negative impls for Sync

src/libcore/cell.rs

@@ -144,7 +144,7 @@
 use clone::Clone;
 use cmp::PartialEq;
 use default::Default;
-use marker::{Copy, Send};
+use marker::{Copy, Send, Sync};
 use ops::{Deref, DerefMut, Drop};
 use option::Option;
 use option::Option::{None, Some};
@@ -660,6 +660,8 @@ pub struct UnsafeCell<T> {
     pub value: T,
 }
 
+impl<T> !Sync for UnsafeCell<T> {}
+
 impl<T> UnsafeCell<T> {
     /// Construct a new instance of `UnsafeCell` which will wrap the specified
     /// value.

src/libcore/marker.rs

@@ -51,6 +51,7 @@ pub unsafe trait Send : MarkerTrait {
 
 impl<T> !Send for *const T { }
 impl<T> !Send for *mut T { }
+impl !Send for Managed { }
 
 /// Types with a constant size known at compile-time.
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -219,6 +220,7 @@ pub unsafe trait Sync : MarkerTrait {
 
 impl<T> !Sync for *const T { }
 impl<T> !Sync for *mut T { }
+impl !Sync for Managed { }
 
 /// A type which is considered "not POD", meaning that it is not
 /// implicitly copyable. This is typically embedded in other types to