Translate non-default directives (breathe extension)

[arwedus]

Is your feature request related to a problem? Please describe.

I cannot combine MyST with other non-default Sphinx extensions, like breathe.

In RST, I would write::

.. doxygenstruct:: MyStruct

(https://breathe.readthedocs.io/en/latest/struct.html)

If I try

```{doxygenstruct} MyStruct
```

in MyST, it returns the following error:

 WARNING: Unknown directive type "doxygenstruct".

Describe the solution you'd like

MyST should just let all directives through and pass handling of "unknown directive" to Sphinx itself.

Describe alternatives you've considered

Additional context

Tested with:

# Sphinx version: 3.5.1
# Python version: 3.6.9 (CPython)
# Docutils version: 0.16 release
# Jinja2 version: 2.11.3

# Loaded extensions:
#   sphinx.ext.mathjax (3.5.1) from /home/username/.local/lib/python3.6/site-packages/sphinx/ext/mathjax.py
#   sphinxcontrib.applehelp (1.0.2) from /home/username/.local/lib/python3.6/site-packages/sphinxcontrib/applehelp/__init__.py
#   sphinxcontrib.devhelp (1.0.2) from /home/username/.local/lib/python3.6/site-packages/sphinxcontrib/devhelp/__init__.py
#   sphinxcontrib.htmlhelp (1.0.3) from /home/username/.local/lib/python3.6/site-packages/sphinxcontrib/htmlhelp/__init__.py
#   sphinxcontrib.serializinghtml (1.1.4) from /home/username/.local/lib/python3.6/site-packages/sphinxcontrib/serializinghtml/__init__.py
#   sphinxcontrib.qthelp (1.0.3) from /home/username/.local/lib/python3.6/site-packages/sphinxcontrib/qthelp/__init__.py
#   alabaster (0.7.12) from /home/username/.local/lib/python3.6/site-packages/alabaster/__init__.py
#   breathe (4.27.0) from /home/username/.local/lib/python3.6/site-packages/breathe/__init__.py
#   sphinx.ext.autodoc.type_comment (3.5.1) from /home/username/.local/lib/python3.6/site-packages/sphinx/ext/autodoc/type_comment.py
#   sphinx.ext.autodoc (3.5.1) from /home/username/.local/lib/python3.6/site-packages/sphinx/ext/autodoc/__init__.py
#   sphinx.ext.doctest (3.5.1) from /home/username/.local/lib/python3.6/site-packages/sphinx/ext/doctest.py
#   sphinx.ext.todo (3.5.1) from /home/username/.local/lib/python3.6/site-packages/sphinx/ext/todo.py
#   sphinx.ext.ifconfig (3.5.1) from /home/username/.local/lib/python3.6/site-packages/sphinx/ext/ifconfig.py
#   sphinx.ext.viewcode (3.5.1) from /home/username/.local/lib/python3.6/site-packages/sphinx/ext/viewcode.py
#   sphinx.ext.extlinks (3.5.1) from /home/username/.local/lib/python3.6/site-packages/sphinx/ext/extlinks.py
#   sphinx_markdown_tables (<module 'sphinx_markdown_tables.__version__' from '/usr/local/lib/python3.6/dist-packages/sphinx_markdown_tables/__version__.py'>) from /usr/local/lib/python3.6/dist-packages/sphinx_markdown_tables/__init__.py
#   sphinxcontrib.confluencebuilder (1.4.0) from /home/username/.local/lib/python3.6/site-packages/sphinxcontrib/confluencebuilder/__init__.py
#   sphinxcontrib.plantuml (unknown version) from /usr/local/lib/python3.6/dist-packages/sphinxcontrib/plantuml.py
#   myst_parser (0.13.5) from /home/username/.local/lib/python3.6/site-packages/myst_parser/__init__.py
#   sphinx_git (unknown version) from /usr/local/lib/python3.6/dist-packages/sphinx_git/__init__.py
#   sphinx_rtd_theme (unknown version) from /usr/local/lib/python3.6/dist-packages/sphinx_rtd_theme/__init__.py

[chrisjsewell]

As with #322, this seems to be something to be an issue specific to https://github.com/michaeljones/breathe and the way it works in registering directives: https://github.com/michaeljones/breathe/blob/4ce8c7a0d79f79f0ae3e7c7fbaa9c4fbb5c84888/breathe/directives.py#L564-L573

There is usually just a simple cache linking directive names to directive classes, but it seems that doxygenstruct is not part of this 🤷

what happens if you do:

```{eval-rst}
.. doxygenstruct:: MyStruct
```

[arwedus]

yes, that works. Actually, that works for me. Breathe directives are so far away from anything that is Markdown, so it's fine to put them in a code fence.

[chrisjsewell]

ah good to hear 👍 it would be nice to get to the bottom of why breathe seems to "break" myst-parser, but indeed for anything like this eval-rst is always the fallback

[jakobandersen]

This should be fixed in the next Breathe release.