Add DefaultId helper trait

Author: madsmtm

objc2/src/rc/id_traits.rs

@@ -54,3 +54,24 @@ impl<T: Message> SliceIdMut for [Id<T, Owned>] {
         unsafe { &mut *ptr }
     }
 }
+
+/// Helper trait to implement [`Default`] on types whoose default value is an
+/// [`Id`].
+// TODO: Remove `Sized` bound.
+// TODO: Maybe make this `unsafe` and provide a default implementation?
+pub trait DefaultId: Sized {
+    /// Indicates whether the default value is mutable or immutable.
+    type Ownership: Ownership;
+
+    /// The default [`Id`] for a type.
+    ///
+    /// On most objects the implementation would just be sending a message to
+    /// the `new` selector.
+    fn default_id() -> Id<Self, Self::Ownership>;
+}
+
+impl<T: DefaultId> Default for Id<T, T::Ownership> {
+    fn default() -> Self {
+        T::default_id()
+    }
+}

objc2/src/rc/mod.rs

@@ -35,7 +35,7 @@ mod weak_id;
 
 pub use self::autorelease::{autoreleasepool, AutoreleasePool, AutoreleaseSafe};
 pub use self::id::Id;
-pub use self::id_traits::{SliceId, SliceIdMut};
+pub use self::id_traits::{DefaultId, SliceId, SliceIdMut};
 pub use self::ownership::{Owned, Ownership, Shared};
 pub use self::weak_id::WeakId;