In rust and gtk4, why does not gtk::Label satisfy gtk::Widget?

Question

I'm trying to learn rust with its gtk4 bindings and decided to try ListStore as a model for ListBox. In order to bind the model, function which returns Widget is needed, but i need a Label. Why doesn't Label, which inherits Widget (implements IsA<Widget>), satisfy also Widget type?

Error:

error[E0308]: mismatched types
  --> src/main.rs:22:5
   |
21 | fn create_widget<'r>(label: &'r Object) -> Widget {
   |                                            ------ expected `gtk4::Widget` because of return type
22 |     Label::new(Some(label.property_value("name").get().unwrap()))
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `gtk4::Widget`, found struct `gtk4::Label`

Full code:

use gtk::prelude::*;
use gtk::{Application, ApplicationWindow, Paned, Label, Orientation, ListBox, ListBoxRow, Widget};
use gtk::gio::ListStore;
use gtk::glib::Type;
use gtk::glib::Object;


fn main() {
    println!("Hello, world!");// Create a new application
    let app = Application::builder()
        .application_id("org.gtk-rs.example")
        .build();

    // Connect to "activate" signal of `app`
    app.connect_activate(build_ui);

    // Run the application
    app.run();
}

fn create_widget<'r>(label: &'r Object) -> Widget {
    Label::new(Some(label.property_value("name").get().unwrap()))
}

fn build_ui(app: &Application) {
    let listbox_left = ListBox::new();
    let listbox_right = ListBox::new();

    let list_model = ListStore::new(Type::STRING);

    listbox_right.bind_model(Some(&list_model), create_widget);


    let paned = Paned::builder()
        .start_child(&listbox_left)
        .end_child(&listbox_right)
        .orientation(Orientation::Horizontal)
        .build();


    // Create a window
    let window = ApplicationWindow::builder()
        .application(app)
        .title("My GTK App")
        .child(&paned)
        .build();

    for n in 1..11 {
        let n_as_string =(n as u32).to_string();
        let mut my = String::from("Left:");
        my.push_str(&n_as_string);

        let m = Object::with_values(Type::STRING, &[("name", my.to_value())]);

        list_model.append(&m.unwrap());
    }

    // Present window
    window.present();
}

Answer 1

Inheritance is not a language feature in Rust. Instead, the gtk4 crate has Label impl IsA<Widget> (here) which allows you to call Cast::upcast like this:

Label::new(Some(label.property_value("name").get().unwrap())).upcast()

You might need to add

use gtk4::glib::object::Cast;

to the top of your file. Instead you can also make your method return impl IsA<Widget> instead.