core/templates/form/fields.html.twig

{#
   Each field type is rendered by a template fragment, which is determined
   by the name of your form type class (DateTimePickerType -> date_time_picker)
   and the suffix "_widget". This can be controlled by overriding getBlockPrefix()
   in DateTimePickerType.

   See https://symfony.com/doc/current/cookbook/form/create_custom_field_type.html#creating-a-template-for-the-field
#}

{% block date_time_picker_widget %}
    <div class="input-group date" data-toggle="datetimepicker">
        {{ block('datetime_widget') }}
        <span class="input-group-addon">
            <span class="fa fa-calendar" aria-hidden="true"></span>
        </span>
    </div>
{% endblock %}

{% block tags_input_widget %}
    <div class="input-group">
        {{ form_widget(form, {'attr': {'data-toggle': 'tagsinput', 'data-tags': tags|json_encode}}) }}
        <span class="input-group-addon">
            <span class="fa fa-tags" aria-hidden="true"></span>
        </span>
    </div>
{% endblock %}