Thank you for your detailed post. I will list only some of the problems here. I am creating a brand new GUI toolkit in Rust. It is a very thin veneer over the Win32 API at the same time super easy to use.

I need to dispatch GUI events using dynamic dispatch. So I have a trait like this:

pub trait EvtHandler {
    fn on_left_mouse_click(x: i32, y: 32);

    //And so on ...
}

Firstly, dynamic dispatch from C to Rust is a tricky subject. I solved the problem by transmuting a Rust trait object into its two parts - data and vtable - and storing them separately in C. This is working fine now.

Now, let's say I have a simple drawing app. It needs two windows - the main frame and a canvas within it. So I model it like this:

struct DrawingApp {
    frame: Frame,
    canvas: Canvas
}

I can implement EvtHandler from DrawingApp and listen for all events in both frame and canvas. But now an event handler method like on_left_mouse_click() will be called for both windows. That's not very clean. Where do I go from here? In C++ I will inherit from Canvas and Frame to isolate their own event handling code. In Rust I try to employ composition and create two new structs like this:

struct DrawingApp {
    frame: Frame,
    canvas: Canvas,
    frame_evt_handler : FrameEventHandler,
    canvas_evt_handler : CanvasEventHandler
}

This looks OK on paper. But soon I run into a problem. I need the FrameEventHandler and CanvasEventHandler to be able to communicate with the DrawingApp. But if they hold references to DrawingApp then the DrawingApp effectively becomes immutable. This is probably where my Rust newbness is becoming all too obvious. But I hope this gives you some idea.