vim-patch:9.1.0890: %! item not allowed for 'rulerformat' (#31369)

Problem:  %! item not allowed for 'rulerformat'
          (yatinlala)
Solution: also allow to use %! for rulerformat option
          (Yegappan Lakshmanan)

fixes: vim/vim#16091
closes: vim/vim#16118

ac023e8baa

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

src/nvim/options.lua

@@ -8407,6 +8407,7 @@ return {
         All fields except the {item} are optional.  A single percent sign can
         be given as "%%".
 
+        						*stl-%!*
         When the option starts with "%!" then it is used as an expression,
         evaluated and the result is used as the option value.  Example: >vim
         	set statusline=%!MyStatusLine()

src/nvim/optionstr.c

@@ -2191,7 +2191,11 @@ static const char *did_set_statustabline_rulerformat(optset_T *args, bool rulerf
     if (wid && *s == '(' && (errmsg = check_stl_option(p_ruf)) == NULL) {
       ru_wid = wid;
     } else {
-      errmsg = check_stl_option(p_ruf);
+      // Validate the flags in 'rulerformat' only if it doesn't point to
+      // a custom function ("%!" flag).
+      if ((*varp)[1] != '!') {
+        errmsg = check_stl_option(p_ruf);
+      }
     }
   } else if (rulerformat || s[0] != '%' || s[1] != '!') {
     // check 'statusline', 'winbar', 'tabline' or 'statuscolumn'