@adeelkhan: What would happen if any of these that use interpolate_html were mistakenly also wrapped with {% filter force_escape %}? Will the linter find that and complain? Is it a problem?

For now linter would not complain, but i think this can be added. I dont think it is correct to use both and any developer would use them simultaneously, but i think django template would not complain too. The question is we can use a tag inside a filter in django templates. I have to check that.

The idea is linter inspects between blocktrans/endblocktrans content to see if there is any html/interpolated variable inside it or not. Based on it decides
1) If foudn regular string, move up and treat it as regular blocktrans tag and find filter expression for such block.
2) If found any html content, flag it as violation, we would not allow this.
3) If found any interpolation variable eg {var}, then look for connected interpolate_html expression. It connects them using output variable used like in below example variable is tmsgx

So there are three cases:

  {% blocktrans trimmed asvar tmsgx %}
      {{span_start}} hour{% plural %}{{span_end}} hours
  {% interpolate_html tmsgx span_start='<span>'|safe span_end='</span>'|safe %}
  {% endblocktrans %}
{% endfilter %}

Linter would complain as it finds html tags inside blocktrans/endblocktrans, so it doesnt checks if its under filter/endfilter block. We would not allow html usage inside blocktrans/endblocktrans.

  {% blocktrans trimmed asvar tmsgx %}
    {{span_start}} hour{% plural %}{{span_end}} hours
  {% endblocktrans %}
  {% interpolate_html tmsgx span_start='<span>'|safe span_end='</span>'|safe %}
{% endfilter %}

Now as there is no html/interpolate variable inside blocktrans/endblocktrans, linter would expect it as a regular block and move up to find {% filter force_escape %} expression.

  {% blocktrans trimmed asvar tmsgx %}
      {span_start} hour{% plural %}{span_end} hours
  {% endblocktrans %}
  {% interpolate_html tmsgx span_start='<span>'|safe span_end='</span>'|safe %}
{% endfilter %}

Linter finding interpolation variable like {span_start} would try to find related interpolate_html and further checks it has the correct arguments and any html used is passed through safe filter.

There is no check on the position of interpolate_tag. I guess a blocktrans/endblocktrans could be in one position and its not necessary for its interpolate_html tag to be aligned with it. It can be anywhere in file.