RUST + FLTK: accessing another widget from widget on_push() function

I used FLTK to create a window and two buttons inside, the btn_A has a callback and should change the btn_B label, but I dont see any non-monstrous approach do to this, ples halp? =''[

fn main() {

showMainWindow();

}

pub fn showMainWindow() {

    //WINDOW
    let application=app::App::default();
    let mut win = window::Window::default().with_size(500,300);

    //BTN_A
    let mut btn_A:Listener<_> = button::Button::new(100,100,100,50,"btn_A").into();

    //BTN_B
    let mut btn_B:Listener<_> = button::Button::new(300,100,100,50,"btn_B").into();

    //BTN_A_CALLBACK
    btn_A.handle(|elem,evt| match evt {
     enums::Event::Push => { btn_A(elem); true }
     _ => { false }
     });

    win.end();
    win.show();

    application.run().unwrap();
}


pub fn btn_A(elem:&mut button::Button) {

    elem.deactivate(); //deactivate itself

    //but how do I access btn_B here?
}

In principle all that is needed is to pass a mutable reference to btn_B to your handler function:

pub fn btn_A(elem:&mut button::Button, btn_B: &mut button::Button) {
    ...
}

However there is one slight problem with your code: You named the function the same as the variable that holds your button.

Apart from that in the most recent version of the fltk crate (v.1.2.23, that I used because you did not specify which version you used in your question) there does not seem to be a Listener<_> type.

Here is an example based on the snippet you posted for changing the label of btn_B:

use fltk::{prelude::{WidgetExt, GroupExt, WidgetBase}, window, app, button, enums};

fn main() {
    showMainWindow();
}

pub fn showMainWindow() {
    //WINDOW
    let application = app::App::default();
    let mut win = window::Window::default().with_size(500, 300);

    //BTN_A
    let mut btn_A = button::Button::new(100, 100, 100, 50, "btn_A");

    //BTN_B
    let mut btn_B = button::Button::new(300, 100, 100, 50, "btn_B");

    //BTN_A_CALLBACK
    btn_A.handle(move |elem, evt| match evt {
        enums::Event::Push => {
            btn_A_click(elem, &mut btn_B);
            true
        }
        _ => false,
    });

    win.end();
    win.show();

    application.run().unwrap();
}

pub fn btn_A_click(elem: &mut button::Button, btn_B: &mut button::Button) {
    elem.deactivate(); //deactivate itself

    //but how do I access btn_B here?
    btn_B.set_label("New title.")
}

Also note, that the handle closure now takes ownership of btn_B because of the move keyword.