For some reason, gtk::ListBox doesn't expose a method to get the length of the list. So... we have to hack around.

tl;dr I don't know why, but none of this methods works. Go for binding a model instead.

It will never be an efficient method, but I firstly thought about iterating over every element. This seems possible in Vala:

// ...

 var button = new Button.with_label ("Get count!");
    button.clicked.connect (() => {
        int count = 0;
        listbox.@foreach (() => {

Rust bindings doesn't have a similar `foreach`, so I thought about selecting all the elements, then run `selected_foreach`, then deselecting, like this:

let mut c = 0;
widgets.nucs_listbox.selected_foreach(|_, _| {

But, I don't know why, it doesn't work. The selection fails without any warning.

So, what about getting the index of the last element? It would be drastically more efficient (should be, I don't know how the library get the last element in the first place):

let c = widgets.nucs_listbox.row_at_y(-1);
let c = c.unwrap().get_index();

Does it works? Nope, because get_index method is not available in Rust bindings, even if it appears on the official GTK docs.

So, my suggestion is get the long path and binding the ListBox to a model, no matter how simple it is. It could be overkill, but it's the most solid way to operate on a ListBox.