Code in guide not working

[ZelphirKaltstahl]

I am following the guide at: https://docs.rs/conrod_core/0.69.0/conrod_core/guide/chapter_3/index.html.

There is a part where it says:

Not quite what we want, yet.

Up until that point, my code is:

#[macro_use] extern crate conrod;
extern crate find_folder;

use glium::Display;
use glium::glutin::{ContextBuilder, WindowBuilder, EventsLoop};
use glium::texture::Texture2d;

use conrod::backend::glium::glium::{self, Surface};
use conrod::backend::glium::Renderer;
use conrod::UiBuilder;
use conrod::{widget, Positionable, Colorable, Widget};


fn main() {
    // boilerplate for creating a window
    const WIDTH: u32 = 400;
    const HEIGHT: u32 = 200;

    // The EventsLoop is given to the Display.
    let mut events_loop = EventsLoop::new();
    let window = WindowBuilder::new().with_title("Hello Conrod").with_dimensions(WIDTH, HEIGHT);
    let context = ContextBuilder::new().with_vsync(true); //.with_multisampling(4);  // multisampling does not work (NoAvailablePixelFormat)
    let display = Display::new(window, context, &events_loop).unwrap();

    // create the UI
    // UiBuilder receives the dimensions in its constuctor.
    let mut ui = UiBuilder::new([WIDTH as f64, HEIGHT as f64]).build();

    // load fonts
    let assets = find_folder::Search::KidsThenParents(3, 5)
        .for_folder("assets")
        .unwrap();
    let font_path = assets.join("fonts/MesloLGM-Regular.ttf");
    ui.fonts.insert_from_file(font_path).unwrap();

    // after building the ui we can add widgets. define a widget named text and
    // keep track of it, so that it later can be given an id.
    widget_ids!(struct Ids { text });
    // generate all ids
    let ids = Ids::new(ui.widget_id_generator());

    // system needs this
    let image_map = conrod::image::Map::<Texture2d>::new();

    // conrod needs renderer
    let mut renderer = Renderer::new(&display).unwrap();

    // the main event loop
    'main: loop {
        // instantiate the widgets
        let ui = &mut ui.set_widgets();
        widget::Text::new("Hello World!")
            .middle_of(ui.window)
            .color(conrod::color::WHITE)
            .font_size(32)
            .set(ids.text, ui);

        // Render the `Ui` and then display it on the screen.
        if let Some(primitives) = ui.draw_if_changed() {
            renderer.fill(&display, primitives, &image_map);
            let mut target = display.draw();
            target.clear_color(0.0, 1.0, 0.0, 1.0);
            renderer.draw(&display, &mut target, &image_map).unwrap();
            target.finish().unwrap();
        }
    }
}

However, Rust complains, that it cannot borrow mutable the ui in the main loop at ui.draw_if_changed():

cannot borrow data in a dereference of `conrod::UiCell<'_>` as mutable (cannot borrow as mutable) [E0596]

trait `DerefMut` is required to modify through a dereference, but it is not implemented for `conrod::UiCell<'_>` [E0596]

My dependencies are:

[dependencies]
conrod = {version="0.55.0", features=["glium", "winit"]}
find_folder = "0.3.0"

My Rust version is:

$ rustc --version
rustc 1.41.1 (f3e1a954d 2020-02-24)

Is the guide outdated, or am I doing something wrong?

[Nachasic]

AFAIK you're supposed to do this in a separate stack layer like this:

// the main event loop
    'main: loop {
        // instantiate the widgets
        {
            let ui = &mut ui.set_widgets();
            widget::Text::new("Hello World!")
                .middle_of(ui.window)
                .color(conrod::color::WHITE)
                .font_size(32)
               .set(ids.text, ui);
        }
        // Render the `Ui` and then display it on the screen.
        if let Some(primitives) = ui.draw_if_changed() {
            renderer.fill(&display, primitives, &image_map);
            let mut target = display.draw();
            target.clear_color(0.0, 1.0, 0.0, 1.0);
            renderer.draw(&display, &mut target, &image_map).unwrap();
            target.finish().unwrap();
        }
    }

[ChiefMilesEdgeworth]

If it were in a separate scope like that, wouldn't it go out of scope and therefore not be usable in the next statement?

[Nachasic]

When you call .set on the widget builder it sets it for the mutable UI reference above.

[ChiefMilesEdgeworth]

Ah. That makes sense. 👍