Rendering a Liquid template with highlight tags in Jekyll

While developing a blog in Jekyll it may happen that you'll have a need for rendering a custom template. I had this situation when trying to build a code snippet block with Contentful that'd eventually be rendered in a Jekyll blog.

Jekyll's docs suggest that you should parse the template like this:

content = "<pre>{{ contentToDisplay }}</pre>"
template = Liquid::Template.parse(content)

Then you can render the template (passing optionally some variables):

template.render('contentToDisplay' => "x = 1")
"<pre>x = 1</pre>"

It works nicely as long as long as you don't want to use a Jekyll Liquid tag, like the highlight tag that I needed.

When you change the content to:

{% highlight ruby %}
{{ contentToDisplay }}
{% endhighlight %}

then rendering such template will raise a following exception:

undefined method `safe' for nil:NilClass

It took me some time to figure out what the problem was, but eventually it turned out that the Jekyll::Tags::HighlightBlock class expects that there's site in template's context.

So, I ended up having three new problems:

What should be the type of the site variable?
Where do I get the variable from?
How do I pass it to the highlight tag?

After trials & errors I managed to answer all of these questions:

The tag expects site to be a Jekyll::Site
You can get the site object by calling Jekyll.sites.first (I suppose that in 99.9% of cases there's just one site so you shouldn't worry too much)
In order to pass the site context when rendering a template the previous call needs to be converted like this:

@template.render!({ 'contentToDisplay' => "x = 1" }, { registers: { site:  Jekyll.sites.first } })

That's it! Now my template got rendered correctly and the syntax is nicely highlighted:

<figure class="highlight">
  <pre>
    <code class="language-ruby" data-lang="ruby">
      <span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
    </code>
  </pre>
</figure>"