Highlight $(...)? correctly

Author: rhysd

syntax/rust.vim

@@ -68,6 +68,7 @@ syn match     rustIdentifier  contains=rustIdentifierPrime "\%([^[:cntrl:][:spac
 syn match     rustFuncName    "\%(r#\)\=\%([^[:cntrl:][:space:][:punct:][:digit:]]\|_\)\%([^[:cntrl:][:punct:][:space:]]\|_\)*" display contained
 
 syn region rustMacroRepeat matchgroup=rustMacroRepeatDelimiters start="$(" end="),\=[*+]" contains=TOP
+syn region rustMacroRepeat matchgroup=rustMacroRepeatDelimiters start="$(" end=")?" contains=TOP
 syn match rustMacroVariable "$\w\+"
 syn match rustRawIdent "\<r#\h\w*" contains=NONE