<?php
/* Rewrite Post type URL to be it's ID instead of the slug. */

function custom_link($post_link, $post = 0) {
    if($post->post_type === 'task') { //My post type is "task". Change it to what is required.
        return home_url('task/' . $post->ID . '/'); //change "task" to your post type
    }
    else{
        return $post_link;
    }
}
add_filter('post_type_link', 'custom_link', 1, 3);

function custom_rewrite(){
    add_rewrite_rule('task/([0-9]+)?$', 'index.php?post_type=task&p=$matches[1]', 'top'); // change task to your post type.
}
add_action('init', 'custom_rewrite');