Macro impl_object_upcast 

macro_rules! impl_object_upcast {
    ($($source:ty => $target:ty),+ $(,)?) => { ... };
}

Implement zero-copy conversions from a derived object reference to one of its base object-reference types.

Safety

Generated bindings must ensure that target is a registered base of source and that every source satisfies the target reference invariants. An incorrect declaration makes the generated safe conversion unsound.