parse method for custom roles (implement MockInliner.parse)

[matthew-brett]

I really miss the parse method of the inliner object available within roles : https://github.com/executablebooks/MyST-Parser/blob/dcbd5ef/myst_parser/mocking.py#L46

It's very nice to be able to use Python / Sphinx code to extend Markdown with roles, but, without the parse method, it means I have to do a lot of hand-building with docutils / Sphinx nodes, in order do a fairly simple macro-like insertion:

https://github.com/matthew-brett/cfd2020/blob/2d15c9f/cfdcode/ucb_page.py#L91

Here, what I really wanted to do was something like:

nodes = inliner.parse("""\
This page has content from the [%s](%s) notebook of an older version of the 
UC Berkeley data science course. See {ref}`ucb-license`.""" % (text, url)

but I don't think there's any simple way to get the parsing machinery, in order do that. In particular, making the pending reference node at the end is extremely fiddly, so I gave up and made a hardish-coded URL.

If that was available it would have made my life a lot easier, and I think it would also make it possible to make a nice helper for simple custom roles, along the lines of the code I was working on here.

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[chrisjsewell]

As you can see its on the TODO list, I just hadn't found a role that required it yet 😄 I'll bump it up the priority list 👍

[matthew-brett]

Thanks.

On reflection - I wonder whether it would be worth having a parse and a parse_md method - so the role can be used with ReST as well as Markdown? Likewise for the directives?