C/C++ incorrect syntax region

[idnsunset]

Hi,
I attempted to add 'c': ['c'] to the g:rst_syntax_code_list to enable C syntax highlighting but run into the issue with c and cpp code blocks as described below.

Given the following rst text:

.. code:: cpp
   using namespace std;

   int main() {}

using

Both 'using' are highlighted as C++ statement. It seems that 'c' and 'cpp' cannot be specified simultaneously in g:rst_syntax_code_list.

[marshallward]

Yes, this is something that I had noticed as well, but looks like I never logged the issue. Thanks for submitting it.

When I tried to sort this out, it seemed to be a problem with one of those syntax files, but I could not make any more progress at the time.

Your example is a bit odd in that there should be a blank line between the directive and the code block. That may be affecting things as well here.

[madjxatw]

I've figured out what exactly happens there.

If you take a look into the C++ syntax file (syntax/cpp.vim), you will find that it loads syntax/c.vim by exectuing runtime! syntax/c.vim. When you put both 'c' and 'cpp' in g:rst_syntax_code_list, the c.vim syntax file will be sourced twice, resulting in all C syntax groups having duplicate patterns.

Taking the example you posted, place the cursor at the opening { and run the following command:

:echo map(synstack(line('.'), col('.')), 'synIDattr(v:val, "name")')

and you will get:

['rstDirectivecpp', 'cBlock', 'cBlock']

This means that the cBlock syntax group is matched twice at {.

Now lets check the definition of cBlock group by running:

:syn list cBlock

and you will get:

--- Syntax items ---
cBlock         xxx start=/{/ end=/}/  contained transparent fold 
                   start=/{/ end=/}/  contained transparent fold

You can see the duplicate patterns defined for cBlock.

When reaching the closing }, the inner cBlock group ends but the remaining texts are still in the outer cBlock group until reaching another }, this is why the second using is highlighted.

I've thought about why 'c' is not added to g:rst_syntax_code_list by default and this might be the reason? However, we don't want to get some C++ keywords such as using highlighted in a C code block, hence we will have to define a separate @rstc syntax cluster containting all C syntax groups, meanwhile avoid source syntax/c.vim twice.

I attempted to fix this by introducing extra code to define the @rstc cluster during processing C++ sutff. See: https://github.com/madjxatw/vim-restructuredtext/blob/a52a1782eb6480554c5641db24fbba2b8309cb3e/syntax/rst.vim#L254-L273
This might not be the best approach, welcome better solution.

[marshallward]

Thank you @madjxatw for solving this mystery, your explanation makes a lot of sense. This looks like an interesting solution to a very difficult problem.

It would be nice if there were a way to catch the more general issue of recursive highlighting, since quite a few syntax languages do this. For example, I think php inherits html!

In the meantime, I'll try to find some time to work through your solution.