fix(treesitter): ensure syntaxset augroup exists (#29542)

Problem:
Error when calling vim.treesitter.start() and vim.treesitter.stop() in
init.lua.

Solution:
Ensure syntaxset augroup exists after loading synload.vim.

2 files changed, 24 insertions, 2 deletions

diff --git a/runtime/lua/vim/treesitter/highlighter.lua b/runtime/lua/vim/treesitter/highlighter.lua
index 052b839609..a94c408f4e 100644
--- a/runtime/lua/vim/treesitter/highlighter.lua
+++ b/runtime/lua/vim/treesitter/highlighter.lua
@@ -139,8 +139,11 @@ function TSHighlighter.new(tree, opts)
   -- but use synload.vim rather than syntax.vim to not enable
   -- syntax FileType autocmds. Later on we should integrate with the
   -- `:syntax` and `set syntax=...` machinery properly.
+  -- Still need to ensure that syntaxset augroup exists, so that calling :destroy()
+  -- immediately afterwards will not error.
   if vim.g.syntax_on ~= 1 then
     vim.cmd.runtime({ 'syntax/synload.vim', bang = true })
+    vim.api.nvim_create_augroup('syntaxset', { clear = false })
   end
 
   vim._with({ buf = self.bufnr }, function()
diff --git a/test/functional/treesitter/highlight_spec.lua b/test/functional/treesitter/highlight_spec.lua
index 05c0cdc01e..6eadb3110f 100644
--- a/test/functional/treesitter/highlight_spec.lua
+++ b/test/functional/treesitter/highlight_spec.lua
@@ -10,8 +10,6 @@ local command = n.command
 local api = n.api
 local eq = t.eq
 
-before_each(clear)
-
 local hl_query_c = [[
   (ERROR) @error
 
@@ -120,6 +118,7 @@ describe('treesitter highlighting (C)', function()
   local screen
 
   before_each(function()
+    clear()
     screen = Screen.new(65, 18)
     screen:attach()
     screen:set_default_attr_ids {
@@ -800,6 +799,7 @@ describe('treesitter highlighting (lua)', function()
   local screen
 
   before_each(function()
+    clear()
     screen = Screen.new(65, 18)
     screen:attach()
     screen:set_default_attr_ids {
@@ -838,6 +838,7 @@ describe('treesitter highlighting (help)', function()
   local screen
 
   before_each(function()
+    clear()
     screen = Screen.new(40, 6)
     screen:attach()
     screen:set_default_attr_ids {
@@ -939,6 +940,7 @@ describe('treesitter highlighting (nested injections)', function()
   local screen
 
   before_each(function()
+    clear()
     screen = Screen.new(80, 7)
     screen:attach()
     screen:set_default_attr_ids {
@@ -1006,6 +1008,7 @@ describe('treesitter highlighting (markdown)', function()
   local screen
 
   before_each(function()
+    clear()
     screen = Screen.new(40, 6)
     screen:attach()
     exec_lua([[
@@ -1053,3 +1056,19 @@ printf('Hello World!');
     })
   end)
 end)
+
+it('starting and stopping treesitter highlight in init.lua works #29541', function()
+  t.write_file(
+    'Xinit.lua',
+    [[
+      vim.bo.ft = 'c'
+      vim.treesitter.start()
+      vim.treesitter.stop()
+    ]]
+  )
+  finally(function()
+    os.remove('Xinit.lua')
+  end)
+  clear({ args = { '-u', 'Xinit.lua' } })
+  eq('', api.nvim_get_vvar('errmsg'))
+end)