Make the Ownership trait sealed

Author: madsmtm

objc2_id/src/id.rs

@@ -17,15 +17,27 @@ use objc2::Message;
 /// A type used to mark that a struct owns the object(s) it contains,
 /// so it has the sole references to them.
 pub enum Owned {}
+
 /// A type used to mark that the object(s) a struct contains are shared,
 /// so there may be other references to them.
 pub enum Shared {}
 
+mod private {
+    pub trait Sealed {}
+
+    impl Sealed for super::Owned {}
+    impl Sealed for super::Shared {}
+}
+
 /// A type that marks what type of ownership a struct has over the object(s)
 /// it contains; specifically, either [`Owned`] or [`Shared`].
-pub trait Ownership: Any {}
-impl Ownership for Owned {}
-impl Ownership for Shared {}
+///
+/// This trait is sealed and not meant to be implemented outside of the this
+/// crate.
+pub trait Ownership: private::Sealed {}
+
+impl Ownership for super::Owned {}
+impl Ownership for super::Shared {}
 
 /// An pointer for Objective-C reference counted objects.
 ///