I want to get all the variables available in the Symfony form theme file form_div_layout.html.twig, I read the Symfony official documention and searched on the web, but couldn\'
Well, you can get all the available variables in each block by iterating the context:
{% block form_widget_simple %}
{% for key, value in _context %}
- {{ key }}
{% endfor %}
{% spaceless %}
{% set type = type|default('text') %}
{% endspaceless %}
{% endblock form_widget_simple %}
And if you want to use yours, then you'll have to overwrite the classes that actually are rendering those widgets, just take a look at AbtractType::buildView...
As @Gregoire suggested, you can use {{ dump(_context) }}
from version 1.5 (http://twig.sensiolabs.org/doc/functions/dump.html), but be aware that it will print a big amount of info.