I see your point, but that also means data marshalling between the higher level language and the rust components. Also, I think GUI is a place where speed plays

I see your point, but that also means data marshalling between the higher level language and the rust components. Also, I think GUI is a place where speed plays a significant role. It is no wonder that most GUI's are written in C/C++. Unless you mean that the lower level GUI code should be rewritten in Rust, so that other languages can provide a higher level interface on top of that. That might indeed sort of work. But the vast amount of work is going to be in Rust. And you still pay some penalty, depending on the mismatch of the higher level language and rust. You will need to have a frictionless integration as between C# and WinForms. The hard work is on that lower level. A simple text input control takes a lot of engineering. You will still end up with something like QT in Rust.