pub unsafe trait DowncastTarget: ClassType + 'static { }Expand description
Classes that can be safely downcasted to.
DowncastTarget is an unsafe marker trait that can be implemented on
types that also implement ClassType.
Ideally, every type that implements ClassType would also be a valid
downcast target, however this would be unsound when used with generics,
because we can only trivially decide whether the “base container” is an
instance of some class type, but anything related to the generic arguments
is unknown.
This trait is implemented automatically by the extern_class! and
define_class! macros.
§Safety
The type must not have any generic arguments other than AnyObject.
§Examples
Implementing DowncastTarget for NSString:
// SAFETY: NSString does not have any generic parameters.
unsafe impl DowncastTarget for NSString {}However, implementing it for NSArray can only be done when the object
type is AnyObject.
// SAFETY: NSArray does not have any generic parameters set (the generic
// defaults to `AnyObject`).
unsafe impl DowncastTarget for NSArray {}
// This would not be valid, since downcasting can only trivially determine
// whether the base class (in this case `NSArray`) matches the receiver
// class type.
// unsafe impl<T: Message> DowncastTarget for NSArray<T> {}Dyn Compatibility§
This trait is not dyn compatible.
In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.