fix(treesitter): better lang handling of get_parser()

runtime/lua/vim/treesitter.lua

@@ -99,13 +99,28 @@ function M.get_parser(bufnr, lang, opts)
   if bufnr == nil or bufnr == 0 then
     bufnr = a.nvim_get_current_buf()
   end
+
   if lang == nil then
     local ft = vim.bo[bufnr].filetype
+    if ft ~= '' then
       lang = language.get_lang(ft) or ft
       -- TODO(lewis6991): we should error here and not default to ft
       -- if not lang then
       --   error(string.format('filetype %s of buffer %d is not associated with any lang', ft, bufnr))
       -- end
+    else
+      if parsers[bufnr] then
+        return parsers[bufnr]
+      end
+      error(
+        string.format(
+          'There is no parser available for buffer %d and one could not be'
+            .. ' created because lang could not be determined. Either pass lang'
+            .. ' or set the buffer filetype',
+          bufnr
+        )
+      )
+    end
   end
 
   if parsers[bufnr] == nil or parsers[bufnr]:lang() ~= lang then

test/functional/treesitter/parser_spec.lua

@@ -128,7 +128,9 @@ void ui_refresh(void)
   it('does not get parser for empty filetype', function()
     insert(test_text);
 
-    eq(".../language.lua:0: '' is not a valid filetype",
+    eq('.../treesitter.lua:0: There is no parser available for buffer 1 and one'
+         .. ' could not be created because lang could not be determined. Either'
+         .. ' pass lang or set the buffer filetype',
       pcall_err(exec_lua, 'vim.treesitter.get_parser(0)'))
 
     -- Must provide language for buffers with an empty filetype