[Backport release-0.9] fix(treesitter): allow foldexpr without highlights (#23673)

fix(treesitter): allow foldexpr without highlights (#23672)

Ref nvim-treesitter/nvim-treesitter#4748

Co-authored-by: Lewis Russell <lewis6991@gmail.com>

runtime/lua/vim/treesitter.lua

@@ -136,16 +136,6 @@ function M.get_parser(bufnr, lang, opts)
   return parsers[bufnr]
 end
 
----@package
----@param bufnr (integer|nil) Buffer number
----@return boolean
-function M._has_parser(bufnr)
-  if bufnr == nil or bufnr == 0 then
-    bufnr = api.nvim_get_current_buf()
-  end
-  return parsers[bufnr] ~= nil
-end
-
 --- Returns a string parser
 ---
 ---@param str string Text to parse

runtime/lua/vim/treesitter/_fold.lua

@@ -279,7 +279,8 @@ function M.foldexpr(lnum)
   lnum = lnum or vim.v.lnum
   local bufnr = api.nvim_get_current_buf()
 
-  if not vim.treesitter._has_parser(bufnr) or not lnum then
+  local parser = vim.F.npcall(vim.treesitter.get_parser, bufnr)
+  if not parser then
     return '0'
   end
 
@@ -287,7 +288,6 @@ function M.foldexpr(lnum)
     foldinfos[bufnr] = FoldInfo.new()
     get_folds_levels(bufnr, foldinfos[bufnr])
 
-    local parser = vim.treesitter.get_parser(bufnr)
     parser:register_cbs({
       on_changedtree = function(tree_changes)
         on_changedtree(bufnr, foldinfos[bufnr], tree_changes)