fix(toc): ensure format defined for labels.vim

refer: #2883

autoload/vimtex/parser/toc/labels.vim

@@ -11,17 +11,18 @@ endfunction
 " }}}1
 
 let s:matcher = {
-      \ 'label_dict' : {},
-      \ 'prefilter_cmds' : ['label'],
-      \ 'priority' : 1,
-      \ 're' : g:vimtex#re#not_comment . '\\label\{\zs.{-}\ze\}',
+      \ 'label_dict': {},
+      \ 'prefilter_cmds': ['label'],
+      \ 'priority': 1,
+      \ 're': g:vimtex#re#not_comment .. '\\label\{\zs.{-}\ze\}',
+      \ 'format': '%s%s',
       \}
 function! s:matcher.init() abort dict " {{{1
   let l:labels = vimtex#parser#auxiliary#labels()
 
   let self.label_dict = {}
   for l:x in l:labels
-    let self.label_dict[l:x.word] = ' (' . l:x.menu . ')'
+    let self.label_dict[l:x.word] = ' (' .. l:x.menu .. ')'
   endfor
 
   let l:wininfo = getwininfo(win_getid())[0]
@@ -32,7 +33,7 @@ function! s:matcher.init() abort dict " {{{1
   let l:width -= 10
   let l:w1 = l:width/2
   let l:w2 = l:width - l:w1
-  let self.format = '%-' . l:w1 . 's%' . l:w2 . 's'
+  let self.format = '%-' .. l:w1 .. 's%' .. l:w2 .. 's'
 endfunction
 
 " }}}1
@@ -41,13 +42,13 @@ function! s:matcher.get_entry(context) abort dict " {{{1
   let l:label = get(self.label_dict, l:key, '')
 
   return {
-        \ 'title'  : printf(self.format, l:key, l:label),
-        \ 'number' : '',
-        \ 'file'   : a:context.file,
-        \ 'line'   : a:context.lnum,
-        \ 'level'  : a:context.max_level - a:context.level.current,
-        \ 'rank'   : a:context.lnum_total,
-        \ 'type'   : 'label',
+        \ 'title':  printf(self.format, l:key, l:label),
+        \ 'number': '',
+        \ 'file':   a:context.file,
+        \ 'line':   a:context.lnum,
+        \ 'level':  a:context.max_level - a:context.level.current,
+        \ 'rank':   a:context.lnum_total,
+        \ 'type':   'label',
         \ }
 endfunction
 " }}}1