Quick tip:
When have a code like this:
{% raw %}{% trans %}prefix.{{ varname }}{% endtrans %}{% endraw %}
And receive this error:
Twig_Error_Syntax: A message inside a trans tag must be a simple text
You can use the following piece of code as workaround.
{% raw %}{{ ("prefix." ~ varname)|trans }}{% endraw %}
PS: maybe this is not the best way to go.