Callback enters unreachable code when used as alias inside a separate component

[abalogstromer]

Bug Description

I was trying to workaround this issue (#2268). I thought to wrap the callback in another component to by-pass the duplication issue of the callback. When I wanted to build the code, it failed with the following error:

[build] thread 'main' panicked at internal/compiler/llr/optim_passes/count_property_use.rs:53:22:
[build] internal error: entered unreachable code

The reason for trying this is that I expose an adapter to be used in C++. There I want to invoke the callback from the adapter and then on slint side I want to bind the callback in different components wherever I need to react to it.

Reproducible Code (if applicable)

export global ExternalAdapter {
    callback action();
}


component WrapperCall {
    callback action_call <=> ExternalAdapter.action;
}

export component ExampleComponent {
    WrapperCall {
        action_call => {
            debug("Action called");
        }
    }
}

Environment Details

• Slint Version: 1.8.0
• Platform/OS: Linux
• Programming Language: C++
• Backend/Renderer: Skia

Product Impact

No response

[ogoffart]

Thanks for the bug report.

I can reproduce it in the master branch

[ogoffart]

In retrospect, it was a mistake to allow aliasing callback to global callback.
This was done before there was ways to access global from native code.

We can't support setting callback alias to global as the property access within the callback would be on the wrong scope anyway.

Eg:

export component ExampleComponent {
    in-out property <bool> some-condition: true;
    in-out property <string> some-property;
    // What if some-condition becomes false? We don't have the machinery to remove callback now.
    if some-condition: WrapperCall {
        action_call => {
            // Also the some-property access can't be performed properly when called from global
            debug("Action called", some-property);
        }
    }
}