Highlight C++11+ attribute specifications

[willeccles]

I have for the time being implemented this in my own local copy (in ~/.vim/syntax). I am not an expert at vim syntax files, so I have one minor problem which is that if you have [noreturn], the noreturn will be highlighted (and some other weird edge cases), but I'm not able to fix this myself, so I've simply attached attached my diff so far:

diff --git a/usr/local/share/nvim/runtime/syntax/cpp.vim b/home/cactus/.vim/syntax/cpp.vim
index ed38913..92fd735 100644
--- a/usr/local/share/nvim/runtime/syntax/cpp.vim
+++ b/home/cactus/.vim/syntax/cpp.vim
@@ -47,6 +47,8 @@ if !exists("cpp_no_cpp11")
   syn region cppRawString	matchgroup=cppRawStringDelimiter start=+\%(u8\|[uLU]\)\=R"\z([[:alnum:]_{}[\]#<>%:;.?*\+\-/\^&|~!=,"']\{,16}\)(+ end=+)\z1"+ contains=@Spell
   syn match cppCast		"\<\(const\|static\|dynamic\)_pointer_cast\s*<"me=e-1
   syn match cppCast		"\<\(const\|static\|dynamic\)_pointer_cast\s*$"
+  syn region cppAttrSpec	matchgroup=cppAttributeDelimiter start=+\[\[+ end=+\]\]+ contains=TOP
+  syn keyword cppAttribute	noreturn carries_dependency contained containedin=cppAttrSpec
 endif
 
 " C++ 14 extensions
@@ -56,6 +58,7 @@ if !exists("cpp_no_cpp14")
   syn match cppNumber		display "\<[1-9]\('\=\d\+\)*\(u\=l\{0,2}\|ll\=u\)\>" contains=cFloat
   syn match cppNumber		display "\<0x\x\('\=\x\+\)*\(u\=l\{0,2}\|ll\=u\)\>"
   syn case match
+  syn keyword cppAttribute	deprecated contained containedin=cppAttrSpec
 endif
 
 " C++ 20 extensions
@@ -65,12 +68,14 @@ if !exists("cpp_no_cpp20")
   syn keyword cppStructure	concept
   syn keyword cppType		char8_t
   syn keyword cppModule		import module export
+  syn keyword cppAttribute	likely unlikely no_unique_address contained containedin=cppAttrSpec
 endif
 
 " C++ 17 extensions
 if !exists("cpp_no_cpp17")
   syn match cppCast		"\<reinterpret_pointer_cast\s*<"me=e-1
   syn match cppCast		"\<reinterpret_pointer_cast\s*$"
+  syn keyword cppAttribute	fallthrough nodiscard maybe_unused contained containedin=cppAttrSpec
 endif
 
 " The minimum and maximum operators in GNU C++
@@ -90,6 +95,8 @@ hi def link cppBoolean		Boolean
 hi def link cppConstant		Constant
 hi def link cppRawStringDelimiter	Delimiter
 hi def link cppRawString		String
+hi def link cppAttributeDelimiter	StorageClass
+hi def link cppAttribute	StorageClass
 hi def link cppNumber		Number
 hi def link cppModule		Include

I have made an effort to not highlight attributes outside of valid contexts (e.g. likely is not highlighted unless it's inside an attribute specifier), but it seems to have some unfortunate side effects. At the moment, this is the resulting highlighting in a few cases:

[willeccles]

Other thoughts: some of these issues could be avoided by simply doing a brute force highlighting of everything inside the [[]] to make it all the same color, but I didn't like that solution.

[dkearns]

G'day,

Just some quick thoughts...

• adding cppAttribute to the cPreProcGroup cluster which is excluded from cDefine should fix the #define highlighting. This can be done from within cpp.vim with syn cluster cPreProcGroup add=cppAttribute
• [noreturn] is matched as cBracket and likewise you could try adding cppAttribute to the excluded cParenGroup cluster

I'm not really familiar with the C syntax file but if that doesn't get you started I can take a deeper look.

Regards,
Doug