Add support for filter section

[Guillaume Gomez]

Would you be open to add support for filter section?

[René Kijewski]

So, this filter first collects all produced text into a buffer, e.g. String, then the collected data is passed to the filter? How does this work with escaped data?

{% filter upper %}
{{ "<script>" }}
{% endfilter %}

→

<SCRIPT>

because "<script>" gets escaped eagerly? Or

<SCRIPT>

because the escaping is only done after the whole section was processed?

Could you tell me how you want to use a filter section?

[Guillaume Gomez]

I suppose that if we are in "html mode", we want it to be escaped indeed. So generated code would actually look like:

let _x = escape_html(format!("<script>"));

I need it for the tera to askama migration in docs.rs. Examples:

• https://github.com/rust-lang/docs.rs/blob/4b3a59af7b7ae4f234537035912acb0adf721b37/templates/crate/build_details.html#L33
• https://github.com/rust-lang/docs.rs/blob/4b3a59af7b7ae4f234537035912acb0adf721b37/templates/core/about/metadata.html#L17

[Dirkjan Ochtman]

I feel like this has come up before, but I can't find any prior issues discussing it.

What alternatives have you come up with?

I don't like it that much, on account of the idea that I don't feel like the code generated it for it is conceptually obvious. But, I can't think of a strong reason to reject it, either... I notice that your use cases are relying on filters we haven't implemented yet (dedent and highlight), do you have have an implementation for those already?

[Guillaume Gomez]

Yes we already have them. Conceptually, I see it as very close as what we do for the rest in general: we generate a format! (in a sub-block would be best I think) and call filters on it and "return" the result of all this that we put either in a variable that will be used later on.

[Dirkjan Ochtman]

Okay, I'm open to reviewing changes for this. I assume we'll collect, filter, then escape as @Kijewski suggested.