permalink as content

Try to put this in functions.php of your theme. It’s untested in this form, but should work.

/**
 * Change the Permalink on save / update
 */
function wpse_297006_rewrite_slug( $post_id, $post, $update ){
  // check if it's a post
  if (!(get_post_type( $post_id ) === 'post')) return;
  // check if it's not a revision
  if (!(wp_is_post_revision( $post_id ) === false)) return;

  // check if it's not a revision
  $content = get_post_field('post_content', $post_id);
  // set the args
  $args = [
    'ID' => $post_id,
    'post_name' => strtolower( sanitize_title( $content ) ),
  ];

  // unhook this method to prevent infinite loop
  remove_action('wp_insert_post', 'wpse_297006_rewrite_slug');
  // apply $args when post is saved
  wp_update_post( $args );
  // re-hook this method
  add_action('wp_insert_post', 'wpse_297006_rewrite_slug', 10, 3);
};

add_action('wp_insert_post', 'wpse_297006_rewrite_slug', 10, 3);