IMHO Rust isn't a great language for describing UIs because it is too explicit in places that doesn't need to be for coding a UI. Which is why in Slint (another Rust UI framework) we opted for a DSL. The application logic stays in Rust, but the UI is described in a language designed for it.

Your example would look like this:

    export component Hello inherits Rectangle {
        in property <string> text: "World";
        width: 500px; height: 500px;
        background: #505050;
        border-width: 1px; border-color: blue;
        drop-shadow-blur: 15px; drop-shadow-color: #0008;
        VerticalLayout {
            alignment: center; spacing: 12px;
            Text { text: "Hello, \{root.text}!"; font-size: 20px; color: white; horizontal-alignment: center; }
            HorizontalLayout {
                alignment: center; spacing: 8px;
                for c in [#f00, #0f0, #00f, #ff0, #000, #fff]: Rectangle { width: 32px; height: 32px; background: c; }
            }
        }
    }
    

More on why we chose a DSL vs code in Rust: https://slint.dev/blog/domain-specific-language-vs-imperativ...