When downcasting in Rust, you (attempt to) downcast to the erased type. In that snippet, you would only be able to successfully downcast to a TabBar.
You can switch between dyn Trait types like so. But I'm not sure this actually accomplishes anything you're trying to do.
As for why Rc::downcast is called that way, it's due to its smart-pointer nature:
The inherent methods of
Rcare all associated functions, which means that you have to call them as e.g.,Rc::get_mut(&mut value)instead ofvalue.get_mut(). This avoids conflicts with methods of the inner typeT.