lexer: c: Highlight #includes as Comment::PreprocFile

[cdown]

When changing https://chrisdown.name over from Pygments to Rouge, one
quite noticeable change was that {% highlight c %} blocks lost the
delineation between #include statements and the actual include, with
them all now just being stylised as a comment.

This patch brings back Comment::PreprocFile support for C-like
languages, as it was with Pygments.

Tested locally by rebuilding chrisdown.name with a gem from this commit,
and seeing that the Comment::PreprocFile highlighting reappeared.

[cdown]

Example change as seen on https://chrisdown.name/2020/01/13/1195725856-and-friends-the-origins-of-mysterious-numbers.html

Pygments:

Rouge HEAD:

Rouge after this PR:

[cdown]

Hey folks, just checking if you have any thoughts on this? Right now I'm running a patched build of Rouge, but especially since you folks just did a release it would be great to get back on mainline if possible :-)

Tagging some folks from PRs I see reviewed recently in the hope that someone has an opinion, sorry to bother. @tancnle @dblessing

[tancnle]

@cdown Thank you for your contribution ❤️ The change looks good overall. I have a look at the current support from Pygments and wonder if we could reuse the vetted pattern. Pygments seems to support the following patterns.

#include <string.h> /* this is a comment */
#include <string.h> // this is a comment

# include <string.h> 
#/* this is a comment */ include <string.h> 
#include  /* this is a comment */  <string.h> 

For this PR, I would suggest at least supporting the top 2 with trailing comments. The change in this PR will display comments as errors so we might need to adjust it slighly.

Implementation wise, I'd suggest

1. Use include mixin in the macro state.

diff --git lib/rouge/lexers/c.rb lib/rouge/lexers/c.rb
index 4df87a4f..ad6624a2 100644
--- lib/rouge/lexers/c.rb
+++ lib/rouge/lexers/c.rb
@@ -66,7 +66,6 @@ module Rouge
         mixin :inline_whitespace

         rule %r/#if\s0/, Comment, :if_0
-        rule %r/#include\s*/, Comment::Preproc, :include
         rule %r/#/, Comment::Preproc, :macro

         rule(//) { pop! }
@@ -171,18 +170,22 @@ module Rouge
       end

       state :macro do
-        # NB: pop! goes back to :bol
-        rule %r/\n/, Comment::Preproc, :pop!
+        mixin :include
         rule %r([^/\n\\]+), Comment::Preproc
         rule %r/\\./m, Comment::Preproc
         mixin :inline_whitespace
         rule %r(/), Comment::Preproc
+        # NB: pop! goes back to :bol
+        rule %r/\n/, Comment::Preproc, :pop!
       end

       state :include do
-        rule %r/\n/, Comment::PreprocFile, :pop!
-        rule %r/<[^\n>]+>/, Comment::PreprocFile
-        rule %r/"[^\n"]+"/, Comment::PreprocFile
+        rule %r/(include)(\s*)(<[^>]+>)([^\n]*)/ do
+          groups Comment::Preproc, Text, Comment::PreprocFile, Comment::Single
+        end
+        rule %r/(include)(\s*)("[^"]+")([^\n]*)/ do
+          groups Comment::Preproc, Text, Comment::PreprocFile, Comment::Single
+        end
       end

       state :if_0 do

2. Add examples with trailing comments in spec/visual/samples/c

#include <string.h> /* this is a comment */
#include <string.h> // this is a comment

I am not a C programmer and would appreciate any feedback 🙏🏼

[cdown]

@tancnle Thank you for the kind review and detailed suggestions! I've made the requested changes, and have also added and visually verified a couple of other valid parsing cases to spec/visual/samples/c.

[tancnle]

Thank you for your prompt response @cdown 👍🏼

[cdown]

You too!