fix(treesitter): disallow empty filetypes

Fixes #22473

4 files changed, 31 insertions, 3 deletions

diff --git a/runtime/lua/vim/treesitter.lua b/runtime/lua/vim/treesitter.lua
index 09e3814a17..f80da433b1 100644
--- a/runtime/lua/vim/treesitter.lua
+++ b/runtime/lua/vim/treesitter.lua
@@ -47,7 +47,8 @@ function M._create_parser(bufnr, lang, opts)
 
   vim.fn.bufload(bufnr)
 
-  language.add(lang, { filetype = vim.bo[bufnr].filetype })
+  local ft = vim.bo[bufnr].filetype
+  language.add(lang, { filetype = ft ~= '' and ft or nil })
 
   local self = LanguageTree.new(bufnr, lang, opts)
 
diff --git a/runtime/lua/vim/treesitter/language.lua b/runtime/lua/vim/treesitter/language.lua
index 5bcc786e88..5f34d9cd56 100644
--- a/runtime/lua/vim/treesitter/language.lua
+++ b/runtime/lua/vim/treesitter/language.lua
@@ -60,6 +60,16 @@ function M.add(lang, opts)
     filetype = { filetype, { 'string', 'table' }, true },
   })
 
+  if filetype == '' then
+    error(string.format("'%s' is not a valid filetype", filetype))
+  elseif type(filetype) == 'table' then
+    for _, f in ipairs(filetype) do
+      if f == '' then
+        error(string.format("'%s' is not a valid filetype", filetype))
+      end
+    end
+  end
+
   M.register(lang, filetype or lang)
 
   if vim._ts_has_language(lang) then
diff --git a/test/functional/treesitter/language_spec.lua b/test/functional/treesitter/language_spec.lua
index 2cf18242ff..747aea54b7 100644
--- a/test/functional/treesitter/language_spec.lua
+++ b/test/functional/treesitter/language_spec.lua
@@ -33,7 +33,12 @@ describe('treesitter language API', function()
 
   it('shows error for invalid language name', function()
     eq(".../language.lua:0: '/foo/' is not a valid language name",
-      pcall_err(exec_lua, 'vim.treesitter.add("/foo/", nil, false)'))
+      pcall_err(exec_lua, 'vim.treesitter.add("/foo/")'))
+  end)
+
+  it('shows error for invalid filetype', function()
+    eq('.../language.lua:0: \'\' is not a valid filetype',
+      pcall_err(exec_lua, [[vim.treesitter.add('foo', { filetype = '' })]]))
   end)
 
   it('inspects language', function()
diff --git a/test/functional/treesitter/parser_spec.lua b/test/functional/treesitter/parser_spec.lua
index fd9822d1c0..27f2e81ab2 100644
--- a/test/functional/treesitter/parser_spec.lua
+++ b/test/functional/treesitter/parser_spec.lua
@@ -4,6 +4,7 @@ local clear = helpers.clear
 local eq = helpers.eq
 local insert = helpers.insert
 local exec_lua = helpers.exec_lua
+local pcall_err = helpers.pcall_err
 local feed = helpers.feed
 local is_os = helpers.is_os
 local skip = helpers.skip
@@ -124,6 +125,16 @@ void ui_refresh(void)
     }, res)
   end)
 
+  it('does not get parser for empty filetype', function()
+    insert(test_text);
+
+    eq(".../language.lua:0: '' is not a valid filetype",
+      pcall_err(exec_lua, 'vim.treesitter.get_parser(0)'))
+
+    -- Must provide language for buffers with an empty filetype
+    exec_lua("vim.treesitter.get_parser(0, 'c')")
+  end)
+
   it('allows to get a child by field', function()
     insert(test_text);
 
@@ -874,9 +885,10 @@ int x = INT_MAX;
 
   it("can fold via foldexpr", function()
     insert(test_text)
-    exec_lua([[vim.treesitter.get_parser(0, "c")]])
 
     local levels = exec_lua([[
+      vim.opt.filetype = 'c'
+      vim.treesitter.get_parser(0, "c")
       local res = {}
       for i = 1, vim.api.nvim_buf_line_count(0) do
         res[i] = vim.treesitter.foldexpr(i)