You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: all_piston_window.rs
    path of file: ./repos/conrod/backends/conrod_piston/examples
    
    the code of the file until where you have to start completion: //! An example demonstrating all widgets in a long, vertically scrollable window.

extern crate conrod_core;
extern crate conrod_example_shared;
extern crate conrod_piston;
extern crate find_folder;
extern crate piston_window;

use self::piston_window::texture::UpdateTexture;
use self::piston_window::OpenGL;
use self::piston_window::{Flip, G2d, G2dTexture, Texture, TextureSettings};
use self::piston_window::{PistonWindow, UpdateEvent, Window, WindowSettings};

pub fn main() {
    const WIDTH: u32 = conrod_example_shared::WIN_W;
    const HEIGHT: u32 = conrod_example_shared::WIN_H;

    // Construct the window.
    let mut window: PistonWindow =
        WindowSettings::new("All Widgets - Piston Backend", [WIDTH, HEIGHT])
            .graphics_api(OpenGL::V3_2) // If not working, try `OpenGL::V2_1`.
            .samples(4)
            .exit_on_esc(true)
            .vsync(true)
            .build()
            .unwrap();

    // construct our `Ui`.
    let mut ui = conrod_core::UiBuilder::new([WIDTH as f64, HEIGHT as f64])
        .theme(conrod_example_shared::theme())
        .build();

    // Add a `Font` to the `Ui`'s `font::Map` from file.
    let assets = find_folder::Search::KidsThenParents(3, 5)
        .for_folder("assets")
        .unwrap();
    let font_path = assets.join("fonts/NotoSans/NotoSans-Regular.ttf");
    ui.fonts.insert_from_file(font_path).unwrap();

    // Create texture context to perform operations on textures.
    let mut texture_context = window.create_texture_context();

    // Create a texture to use for efficiently caching text on the GPU.
    let mut text_vertex_data = Vec::new();
    let (mut glyph_cache, mut text_texture_cache) = {
        const SCALE_TOLERANCE: f32 = 0.1;
        const POSITION_TOLERANCE: f32 = 0.1;
        let cache = conrod_core::text::GlyphCache::builder()
            .dimensions(WIDTH, HEIGHT)
            .scale_tolerance(SCALE_TOLERANCE)
            .position_tolerance(POSITION_TOLERANCE)
            .build();
        let buffer_len = WIDTH as usize * HEIGHT as usize;
        let init = vec![128; buffer_len];
        let settings = TextureSettings::new();
        let texture =
            G2dTexture::from_memory_alpha(&mut texture_context, &init, WIDTH, HEIGHT, &settings)
                .unwrap();
        (cache, texture)
    };

    // Instantiate the generated list of widget identifiers.
    let ids = conrod_example_shared::Ids::new(ui.widget_id_generator());

    // Load the rust logo from file to a piston_window texture.
    let rust_logo: G2dTexture = {
        let assets = find_folder::Search::ParentsThenKids(5, 3)
            .for_folder("assets")
            .unwrap();
        let path = assets.join("images/rust.png");
        let settings = TextureSettings::new();
        Texture::from_path(&mut texture_context, &path, Flip::None, &settings).unwrap()
    };

    // Create our `conrod_core::image::Map` which describes each of our widget->image mappings.
    let mut image_map = conrod_core::image::Map::new();
    let rust_logo = image_map.insert(rust_logo);

    // A demonstration of some state that we'd like to control with the App.
    let mut app = conrod_example_shared::DemoApp::new(rust_logo);

    // Poll events from the window.
    while let Some(event) = window.next() {
        // Convert the src event to a conrod event.
        let size = window.size();
        let (win_w, win_h) = (
            size.width as conrod_core::Scalar,
            size.height as conrod_core::Scalar,
        );
        if let Some(e) = conrod_piston::event::convert(event.clone(), win_w, win_h) {
            ui.handle_event(e);
        }

        event.update(|_| {
            let mut ui = ui.set_widgets();
            conrod_example_shared::gui(&mut ui, &ids, &mut app);
        });

        window.draw_2d(&event, |context, graphics, device| {
            if let Some(primitives) = ui.draw_if_changed() {
                // A function used for caching glyphs to the texture cache.
                let cache_queued_glyphs = |_graphics: &mut G2d,
                                           cache: &mut G2dTexture,
                                           rect: conrod_core::text::rt::Rect<u32>,
                                           data: &[u8]| {
                    let offset = [rect.min.x, rect.min.y];
                    let size = [rect.width
    
pub fn main() {
    const WIDTH: u32 = conrod_example_shared::WIN_W;
    const HEIGHT: u32 = conrod_example_shared::WIN_H;

    // Construct the window.
    let mut window: PistonWindow =
        WindowSettings::new("All Widgets - Piston Backend", [WIDTH, HEIGHT])
            .graphics_api(OpenGL::V3_2) // If not working, try `OpenGL::V2_1`.
            .samples(4)
            .exit_on_esc(true)
            .vsync(true)
            .build()
            .unwrap();

    // construct our `Ui`.
    let mut ui = conrod_core::UiBuilder::new([WIDTH as f64, HEIGHT as f64])
        .theme(conrod_example_shared::theme())
        .build();

    // Add a `Font` to the `Ui`'s `font::Map` from file.
    let assets = find_folder::Search::KidsThenParents(3, 5)
        .for_folder("assets")
        .unwrap();
    let font_path = assets.join("fonts/NotoSans/NotoSans-Regular.ttf");
    ui.fonts.insert_from_file(font_path).unwrap();

    // Create texture context to perform operations on textures.
    let mut texture_context = window.create_texture_context();

    // Create a texture to use for efficiently caching text on the GPU.
    let mut text_vertex_data = Vec::new();
    let (mut glyph_cache, mut text_texture_cache) = {
        const SCALE_TOLERANCE: f32 = 0.1;
        const POSITION_TOLERANCE: f32 = 0.1;
        let cache = conrod_core::text::GlyphCache::builder()
            .dimensions(WIDTH, HEIGHT)
            .scale_tolerance(SCALE_TOLERANCE)
            .position_tolerance(POSITION_TOLERANCE)
            .build();
        let buffer_len = WIDTH as usize * HEIGHT as usize;
        let init = vec![128; buffer_len];
        let settings = TextureSettings::new();
        let texture =
            G2dTexture::from_memory_alpha(&mut texture_context, &init, WIDTH, HEIGHT, &settings)
                .unwrap();
        (cache, texture)
    };

    // Instantiate the generated list of widget identifiers.
    let ids = conrod_example_shared::Ids::new(ui.widget_id_generator());

    // Load the rust logo from file to a piston_window texture.
    let rust_logo: G2dTexture = {
        let assets = find_folder::Search::ParentsThenKids(5, 3)
            .for_folder("assets")
            .unwrap();
        let path = assets.join("images/rust.png");
        let settings = TextureSettings::new();
        Texture::from_path(&mut texture_context, &path, Flip::None, &settings).unwrap()
    };

    // Create our `conrod_core::image::Map` which describes each of our widget->image mappings.
    let mut image_map = conrod_core::image::Map::new();
    let rust_logo = image_map.insert(rust_logo);

    // A demonstration of some state that we'd like to control with the App.
    let mut app = conrod_example_shared::DemoApp::new(rust_logo);

    // Poll events from the window.
    while let Some(event) = window.next() {
        // Convert the src event to a conrod event.
        let size = window.size();
        let (win_w, win_h) = (
            size.width as conrod_core::Scalar,
            size.height as conrod_core::Scalar,
        );
        if let Some(e) = conrod_piston::event::convert(event.clone(), win_w, win_h) {
            ui.handle_event(e);
        }

        event.update(|_| {
            let mut ui = ui.set_widgets();
            conrod_example_shared::gui(&mut ui, &ids, &mut app);
        });

        window.draw_2d(&event, |context, graphics, device| {
            if let Some(primitives) = ui.draw_if_changed() {
                // A function used for caching glyphs to the texture cache.
                let cache_queued_glyphs = |_graphics: &mut G2d,
                                           cache: &mut G2dTexture,
                                           rect: conrod_core::text::rt::Rect<u32>,
                                           data: &[u8]| {
                    let offset = [rect.min.x, rect.min.y];
                    let size = [rect.width(), rect.height()];
                    let format = piston_window::texture::Format::Rgba8;
                    text_vertex_data.clear();
                    text_vertex_data.extend(data.iter().flat_map(|&b| vec![255, 255, 255, b]));
                    UpdateTexture::update(
                        cache,
                        &mut texture_context,
                        format,
                        &text_vertex_data[..],
                        offset,
                        size,
                    )
                    .expect("failed to update texture")
                };

                // Specify how to get the drawable texture from the image. In this case, the image
                // *is* the texture.
                fn texture_from_image<T>(img: &T) -> &T {
                    img
                }

                // Draw the conrod `render::Primitives`.
                conrod_piston::draw::primitives(
                    primitives,
                    context,
                    graphics,
                    &mut text_texture_cache,
                    &mut glyph_cache,
                    &image_map,
                    cache_queued_glyphs,
                    texture_from_image,
                );

                texture_context.encoder.flush(device);
            }
        });
    }
}