Add disambiguation for C++ header files

[pyrmont]

C++ header files can use the .h extension. By default, Rouge highlights these files with the C lexer. This can lead to incorrect highlighting because certain keywords in C++ are not reserved in C. A similar situation can occur with Objective-C files but Rouge provides a disambiguation for Objective-C files to avoid this problem. This commit adds a similar disambiguation for C++.

This fixes #932.

[eliaskosunen]

The list of keywords could be improved. bool is technically a reserved word in C (it's a macro #defined to _Bool in the header <stdbool.h>), so that probably shouldn't be on the list. Other keywords that can appear at the beginning of a line present in C++, but are absent in C include, off the top of my head:

• constexpr
• try, catch and throw
• protected
• noexcept
• static_cast, dynamic_cast and reinterpret_cast
• virtual
• this
• typeid
• thread_local
• decltype
• static_assert
• alignas and alignof
• As of C++20, concept, requires, co_await, co_async, co_yield and consteval

and probably more

Whether these keywords should be used for detection, I don't know.

[pyrmont]

@eliaskosunen Thanks for the quick reply! Would some of those really begin a line in a header file (eg. this, try)? I think the key thing is to identify keywords that may be present in a header file on their own. Based on what I've seen, something like namespace meets this criterion whereas typeid doesn't. Of that list, I'm inclined to want to add protected (since I already added public and private) and virtual. But perhaps that's being too conservative?

[eliaskosunen]

Headers can contain inline function definitions, and with funky formatting, all of these could potentially appear at the beginning of a line:

struct S {
  void foo() {
    try { // `try` as the first word of a line
      bar(
        this); // `this` as the first word of a line
    } catch(...) {
      // ...
    }
  }
};

Should you want to stay conservative, in the least I think you should have, in addition of what you listed, constexpr (appears frequently as the first word of a function signature in post-C++14-world), try, catch and throw.

[pyrmont]

@eliaskosunen Ah, forgot about inline function definitions. Hmmm... I think what I'd like to do at this point is release this fix and see how it goes in everyday use. What do you think?

[eliaskosunen]

This would certainly be better than what we have now.

[pyrmont]

@eliaskosunen Thanks for your help on this. We're on a two-week release cadence at the moment as we work through the backlog so this will go out as part of v3.8.0 on Tuesday 6 August. I'm not affiliated with GitLab at all so am not sure when they'll switch over to using it so you might want to check with them.

[jneen]

@eliaskosunen Back when I worked for Gitlab, I implemented this, which may be of interest to you: https://docs.gitlab.com/ce/user/project/highlighting.html

You can specify the language for any glob of files.