Basic screen to world bevy code. Missing some stuff like the distance calculation.

bevy_screen_to_world.rs

pub fn screen_to_world(
    cursor: Res<Events<CursorMoved>>,
    windows: Res<Windows>,
    mut input_state: ResMut<GlobalInputState>,
    camera_query: Query<(&Camera, &Transform)>,
) {
    // Calculate world space mouse ray.  If there's no new mouse move event, then continue.
    let pos = match input_state.cursor.latest(&cursor) {
        Some(ev) => ev.position,
        None => return,
    };
    let window = windows.get(WindowId::primary()).expect("Couldn't grab primary window.");
    let screen_size = Vec2::from([window.width() as f32, window.height() as f32]);
    let (c, c_t) = camera_query.iter().next().expect("The world didn't have a camera in it!");

    let normal = screen_to_world_dir(&pos, &screen_size, c, c_t);
        
    // This part can easily be done faster.
    let camera_matrix = c_t.compute_matrix();
    let (_, _, t) = camera_matrix.to_scale_rotation_translation();

    let world_coord = t.translation + normal * DISTANCE;    // distance from the camera to put the world coord.
}


// Convert a screen space coordinate to a direction out of the camera.
// Does not include camera positioning in the calculation: to get the screen to world coordinate,
// add camera_transform.translation here.
pub fn screen_to_world_dir(
    coord: &Vec2,
    screen_size: &Vec2,
    camera: &Camera,
    camera_transform: &Transform
) -> Vec3 {
    let proj_mat = camera.projection_matrix;

    // Normalized device coordinates (NDC) describes cursor position from (-1, -1) to (1, 1).
    let cursor_pos_ndc: Vec3 = ((*coord / *screen_size) * 2.0 - Vec2::one()).extend(1.0);

    let camera_matrix = camera_transform.compute_matrix();
    // We can't just use camera_transform.translation because that's actually the LOCAL translation.
    let (_, _, camera_position) = camera_matrix.to_scale_rotation_translation();
    
    let ndc_to_world: Mat4 = camera_matrix * proj_mat.inverse();
    let cursor_position: Vec3 = ndc_to_world.transform_point3(cursor_pos_ndc);
    
    let dir: Vec3 = cursor_position - camera_position;

    return dir.normalize();
}