How to use Bootstrap input groups (http://getbootstrap.com/components/#input-groups) in a Symfony form theme

form.html.twig

{% form_theme form 'theme.html.twig' %}

{# the theme above will take care of rendering the input group #}
{{ form(form) }}

theme.html.twig

{# form_name will be the name of your form type #}
{# field_name is the name of the field #}
{# See http://symfony.com/doc/current/cookbook/form/form_customization.html#how-to-customize-an-individual-field #}
{% block _form_name_field_name_widget -%}
<div class="input-group">
    {{- block('form_widget_simple') -}}
    <span class="input-group-btn">
        <button type="button" class="btn btn-default">
            Do Stuff
        </button>
    </span>
</div>
{%- endblock %}