Add CastNone trait

Closes gtk-rs#842

glib/src/object.rs

@@ -288,6 +288,55 @@ pub trait Cast: ObjectType {
 
 impl<T: ObjectType> Cast for T {}
 
+// rustdoc-stripper-ignore-next
+/// Convenience trait mirroring `Cast`, implemented on `Option<Object>` types.
+pub trait CastNone: Sized {
+    type Inner;
+    fn downcast<T: ObjectType>(self) -> Result<T, Self>
+    where
+        Self::Inner: CanDowncast<T>;
+    fn downcast_ref<T: ObjectType>(&self) -> Option<&T>
+    where
+        Self::Inner: CanDowncast<T>;
+    fn upcast<T: ObjectType>(self) -> Option<T>
+    where
+        Self::Inner: IsA<T>;
+    fn upcast_ref<T: ObjectType>(&self) -> Option<&T>
+    where
+        Self::Inner: IsA<T>;
+}
+impl<I: ObjectType + Sized> CastNone for Option<I> {
+    type Inner = I;
+
+    fn downcast<T: ObjectType>(self) -> Result<T, Self>
+    where
+        Self::Inner: CanDowncast<T>,
+    {
+        self.ok_or(None)
+            .and_then(|i| i.downcast().map_err(|e| Some(e)))
+    }
+
+    fn downcast_ref<T: ObjectType>(&self) -> Option<&T>
+    where
+        Self::Inner: CanDowncast<T>,
+    {
+        self.as_ref().and_then(|i| i.downcast_ref())
+    }
+    fn upcast<T: ObjectType>(self) -> Option<T>
+    where
+        Self::Inner: IsA<T>,
+    {
+        self.map(|i| i.upcast())
+    }
+
+    fn upcast_ref<T: ObjectType>(&self) -> Option<&T>
+    where
+        Self::Inner: IsA<T>,
+    {
+        self.as_ref().map(|i| i.upcast_ref())
+    }
+}
+
 // rustdoc-stripper-ignore-next
 /// Marker trait for the statically known possibility of downcasting from `Self` to `T`.
 pub trait CanDowncast<T> {}