https://discuss.asciidoctor.org/Embed-asciidoc-into-Haml-tp683p691.html
Jason nailed it. Haml is migrating filters for markup languages like Textile, AsciiDoc and Markdown to a separate gem, ambiguously named haml-contrib.
I'm a bit frustrated with the Haml project. I first submitted a pull request for an AsciiDoc filter at the beginning of the year. That pull request was rejected with a request to resubmit the change to the haml-contrib project. I appealed for them to reconsider since adding an extra dependency introduces unnecessary complexity. I got no response to my post [1]. After I while, I accepted it was a hopeless argument and sent a pull request to haml-contrib as requested [2]. That pull request has been sitting there for over 3 months. There's no indication from the developers when its going to be merged.
If we don't see any life from the Haml project, I think we should consider putting the filter into Awestruct so that it can be activated with a simple 'require' statement in _ext/pipeline.rb.
If you are open to alternatives, this is one of several reasons why I strongly recommend Slim over Haml. Slim accepted the AsciiDoc filter almost immediately and you can start using it immediately. Slim is also much more concise than Haml. He're an example:
[source,slim]
----
.span6
asciidoc:
I'm writin' me some *AsciiDoc*!
----
NOTE: The colon goes at the end of the filter name in Slim.
You can also render inline AsciiDoc in either Haml or Slim as so:
[source,slim]
----
.span6
p Here's a paragraph with #{Asciidoctor.render '*strong*', :doctype => :inline} text.
----
Hope that helps!
-Dan