fix(startup): source init.lua from XDG_CONFIG_DIRS

Problem:

`init.lua` files in `$XDG_CONFIG_DIRS` directories were not being sourced during startup, even though the documentation states they should be searched alongside `init.vim`.

See:

e51f5e17e1/runtime/doc/starting.txt (L495-L496)

Solution:

Modify `do_user_initialization()` to search for `init.lua` in each `$XDG_CONFIG_DIRS` directory before falling back to `init.vim`, matching the behavior for `$XDG_CONFIG_HOME`. Also show `E5422` error if both `init.lua` and `init.vim` exist in the same directory.

Fixes #37405

src/nvim/main.c

@@ -2027,8 +2027,8 @@ static void do_system_initialization(void)
 /// Does one of the following things, stops after whichever succeeds:
 ///
 /// 1. Execution of VIMINIT environment variable.
-/// 2. Sourcing user vimrc file ($XDG_CONFIG_HOME/nvim/init.vim).
-/// 3. Sourcing other vimrc files ($XDG_CONFIG_DIRS[1]/nvim/init.vim, …).
+/// 2. Sourcing user config file ($XDG_CONFIG_HOME/nvim/init.lua or init.vim).
+/// 3. Sourcing other config files ($XDG_CONFIG_DIRS[1]/nvim/init.lua or init.vim, …).
 /// 4. Execution of EXINIT environment variable.
 ///
 /// @return True if it is needed to attempt to source exrc file according to
@@ -2073,6 +2073,11 @@ static bool do_user_initialization(void)
 
   char *const config_dirs = stdpaths_get_xdg_var(kXDGConfigDirs);
   if (config_dirs != NULL) {
+    const char vim_path_tail[] = { 'n', 'v', 'i', 'm', PATHSEP,
+                                   'i', 'n', 'i', 't', '.', 'v', 'i', 'm', NUL };
+    const char lua_path_tail[] = { 'n', 'v', 'i', 'm', PATHSEP,
+                                   'i', 'n', 'i', 't', '.', 'l', 'u', 'a', NUL };
+
     const void *iter = NULL;
     do {
       const char *dir;
@@ -2081,12 +2086,34 @@ static bool do_user_initialization(void)
       if (dir == NULL || dir_len == 0) {
         break;
       }
-      const char path_tail[] = { 'n', 'v', 'i', 'm', PATHSEP,
-                                 'i', 'n', 'i', 't', '.', 'v', 'i', 'm', NUL };
-      char *vimrc = xmalloc(dir_len + sizeof(path_tail) + 1);
+
+      // Build: <xdg_dir>/<appname>/init.lua
+      char *init_lua = xmalloc(dir_len + sizeof(lua_path_tail) + 1);
+      memmove(init_lua, dir, dir_len);
+      init_lua[dir_len] = PATHSEP;
+      memmove(init_lua + dir_len + 1, lua_path_tail, sizeof(lua_path_tail));
+
+      // Build: <xdg_dir>/<appname>/init.vim
+      char *vimrc = xmalloc(dir_len + sizeof(vim_path_tail) + 1);
       memmove(vimrc, dir, dir_len);
       vimrc[dir_len] = PATHSEP;
-      memmove(vimrc + dir_len + 1, path_tail, sizeof(path_tail));
+      memmove(vimrc + dir_len + 1, vim_path_tail, sizeof(vim_path_tail));
+
+      if (os_path_exists(init_lua)
+          && do_source(init_lua, true, DOSO_VIMRC, NULL)) {
+        if (os_path_exists(vimrc)) {
+          semsg(_("E5422: Conflicting configs: \"%s\" \"%s\""), init_lua, vimrc);
+        }
+
+        xfree(vimrc);
+        xfree(init_lua);
+        xfree(config_dirs);
+        do_exrc = p_exrc;
+        return do_exrc;
+      }
+      xfree(init_lua);
+
+      // init.vim
       if (do_source(vimrc, true, DOSO_VIMRC, NULL) != FAIL) {
         do_exrc = p_exrc;
         if (do_exrc) {

test/functional/core/startup_spec.lua

@@ -1421,6 +1421,58 @@ describe('user config init', function()
       )
     end)
   end)
+
+  describe('from XDG_CONFIG_DIRS', function()
+    local xdgdir = 'Xxdgconfigdirs'
+
+    before_each(function()
+      -- Remove init.lua from XDG_CONFIG_HOME so nvim falls back to XDG_CONFIG_DIRS
+      os.remove(init_lua_path)
+      rmdir(xdgdir)
+      mkdir_p(xdgdir .. pathsep .. 'nvim')
+    end)
+
+    after_each(function()
+      rmdir(xdgdir)
+    end)
+
+    it('loads init.lua from XDG_CONFIG_DIRS when no config in XDG_CONFIG_HOME', function()
+      write_file(
+        table.concat({ xdgdir, 'nvim', 'init.lua' }, pathsep),
+        [[vim.g.xdg_config_dirs_lua = 1]]
+      )
+      clear {
+        args_rm = { '-u' },
+        env = { XDG_CONFIG_HOME = xconfig, XDG_DATA_HOME = xdata, XDG_CONFIG_DIRS = xdgdir },
+      }
+      eq(1, eval('g:xdg_config_dirs_lua'))
+      eq(
+        fn.fnamemodify(table.concat({ xdgdir, 'nvim', 'init.lua' }, pathsep), ':p'),
+        eval('$MYVIMRC')
+      )
+    end)
+
+    it('prefers init.lua over init.vim, shows E5422', function()
+      write_file(table.concat({ xdgdir, 'nvim', 'init.lua' }, pathsep), [[vim.g.xdg_lua = 1]])
+      write_file(table.concat({ xdgdir, 'nvim', 'init.vim' }, pathsep), [[let g:xdg_vim = 1]])
+      clear {
+        args_rm = { '-u' },
+        env = { XDG_CONFIG_HOME = xconfig, XDG_DATA_HOME = xdata, XDG_CONFIG_DIRS = xdgdir },
+      }
+      eq(1, eval('g:xdg_lua'))
+      eq(0, eval('get(g:, "xdg_vim", 0)'))
+      t.matches('E5422: Conflicting configs:', eval('v:errmsg'))
+    end)
+
+    it('falls back to init.vim when no init.lua', function()
+      write_file(table.concat({ xdgdir, 'nvim', 'init.vim' }, pathsep), [[let g:xdg_vim = 1]])
+      clear {
+        args_rm = { '-u' },
+        env = { XDG_CONFIG_HOME = xconfig, XDG_DATA_HOME = xdata, XDG_CONFIG_DIRS = xdgdir },
+      }
+      eq(1, eval('g:xdg_vim'))
+    end)
+  end)
 end)
 
 describe('runtime:', function()