Add syntax support for u8 character literals

Author: dkearns

syntax/cpp.vim

@@ -69,6 +69,20 @@ if !exists("cpp_no_cpp17")
   syn match cppCast		"\<reinterpret_pointer_cast\s*$"
   syn match cppFloat		display contained "\<0x\x*\.\x\+p[-+]\=\d\+\([FfLl]\|i[fl]\=\|h\|min\|s\|ms\|us\|ns\|_\i*\)\=\>"
   syn match cppFloat		display contained "\<0x\x\+\.\=p[-+]\=\d\+\([FfLl]\|i[fl]\=\|h\|min\|s\|ms\|us\|ns\|_\i*\)\=\>"
+
+  " TODO: push this up to c.vim if/when supported in C23
+  syn match cppCharacter	"u8'[^\\]'"
+  syn match cppCharacter	"u8'[^']*'" contains=cSpecial
+  if exists("c_gnu")
+    syn match cppSpecialError	  "u8'\\[^'\"?\\abefnrtv]'"
+    syn match cppSpecialCharacter "u8'\\['\"?\\abefnrtv]'"
+  else
+    syn match cppSpecialError	  "u8'\\[^'\"?\\abfnrtv]'"
+    syn match cppSpecialCharacter "u8'\\['\"?\\abfnrtv]'"
+  endif
+  syn match cppSpecialCharacter display "u8'\\\o\{1,3}'"
+  syn match cppSpecialCharacter display "u8'\\x\x\+'"
+
 endif
 
 " C++ 20 extensions
@@ -99,6 +113,9 @@ hi def link cppType		Type
 hi def link cppStorageClass	StorageClass
 hi def link cppStructure		Structure
 hi def link cppBoolean		Boolean
+hi def link cppCharacter	cCharacter
+hi def link cppSpecialCharacter	cSpecialCharacter
+hi def link cppSpecialError	cSpecialError
 hi def link cppConstant		Constant
 hi def link cppRawStringDelimiter	Delimiter
 hi def link cppRawString		String