Comma separated list in twig

Don’t know about shortest but this could be clear. Try the following to add comma after all lines in the loop except the last one:

{% for role in user.roles %}
    {{ role.name }}
    {% if not loop.last %},{% endif %}
{% endfor %}

Shorter version as suggested in comments:

{% for role in user.roles %}
    {{ role.name }}
    {{ not loop.last ? ',' }}
{% endfor %}