I think it's pretty normal to have a language dedicated to the UI structure. It should be declarative for easy machine manipulation with a WYSIWYG editor and could be translated to code before compilation.

My reference in this are Delphi .dfm files, which you rarely have to edit manually because they're handled by the IDE form builder. It is still a fantastic IDE to build native UI apps and I don't see a reason why Rust couldn't be used the same way.