RenderableInterface was introduced

Classes can now implement \Drupal\Core\Render\RenderableInterface to indicate that instances can return renderable representations (render arrays) of themselves. This universal interface should replace any other methods that return render arrays that represent the object on which the method was called.

Example:

class Foo implements RenderableInterface {

  /**
    * {@inheritdoc}
    */
  public function toRenderable() {
    return [
      '#type' => 'item',
      '#title' => $this->getFakeLabel(),
      '#markup' => $this->getFakeContent(),
    ];
  }

}