startup: add init.lua as an alternative user config, fixes #7895

src/nvim/lua/executor.c

@@ -1244,6 +1244,23 @@ void ex_luafile(exarg_T *const eap)
   }
 }
 
+bool load_init_lua(const char *script_path)
+{
+  lua_State *const lstate = nlua_enter();
+
+  if (luaL_loadfile(lstate, script_path)) {
+    nlua_error(lstate, _("E5112: Error while creating lua chunk: %.*s"));
+    return false;
+  }
+
+  if (lua_pcall(lstate, 0, 0, 0)) {
+    nlua_error(lstate, _("E5113: Error while calling lua chunk: %.*s"));
+    return false;
+  }
+
+  return true;
+}
+
 static void nlua_add_treesitter(lua_State *const lstate) FUNC_ATTR_NONNULL_ALL
 {
   tslua_init(lstate);

src/nvim/main.c

@@ -1770,6 +1770,23 @@ static bool do_user_initialization(void)
     do_exrc = p_exrc;
     return do_exrc;
   }
+
+  char_u *init_lua_path = (char_u *)stdpaths_user_conf_subpath("init.lua");
+  if (os_path_exists(init_lua_path)
+      && load_init_lua((const char *)init_lua_path)) {
+    os_setenv("MYVIMRC", (const char *)init_lua_path, 1);
+    char_u *vimrc_path = (char_u *)stdpaths_user_conf_subpath("init.vim");
+
+    if (os_path_exists(vimrc_path)) {
+      EMSG3(_("Conflicting configs: \"%s\" \"%s\""), init_lua_path, vimrc_path);
+    }
+
+    xfree(vimrc_path);
+    xfree(init_lua_path);
+    return false;
+  }
+  xfree(init_lua_path);
+
   char_u *user_vimrc = (char_u *)stdpaths_user_conf_subpath("init.vim");
   if (do_source(user_vimrc, true, DOSO_VIMRC) != FAIL) {
     do_exrc = p_exrc;
@@ -1829,8 +1846,12 @@ static void source_startup_scripts(const mparm_T *const parmp)
         || strequal(parmp->use_vimrc, "NORC")) {
       // Do nothing.
     } else {
-      if (do_source((char_u *)parmp->use_vimrc, false, DOSO_NONE) != OK) {
+      if (path_with_extension(parmp->use_vimrc, "lua")) {
-        EMSG2(_("E282: Cannot read from \"%s\""), parmp->use_vimrc);
+        load_init_lua(parmp->use_vimrc);
+      } else {
+        if (do_source((char_u *)parmp->use_vimrc, false, DOSO_NONE) != OK) {
+          EMSG2(_("E282: Cannot read from \"%s\""), parmp->use_vimrc);
+        }
       }
     }
   } else if (!silent_mode) {

test/functional/core/startup_spec.lua

@@ -432,3 +432,64 @@ describe('clean', function()
   clear('--clean')
   ok(string.match(meths.get_option('runtimepath'), funcs.stdpath('config')) == nil)
 end)
+
+describe('user config init', function()
+  local xhome = 'Xhome'
+  local pathsep = helpers.get_pathsep()
+  local xconfig = xhome .. pathsep .. 'Xconfig'
+  local init_lua_path = table.concat({xconfig, 'nvim', 'init.lua'}, pathsep)
+
+  before_each(function()
+    rmdir(xhome)
+
+    -- TODO, make mkdir_p helper
+    mkdir(xhome)
+    mkdir(xconfig)
+    mkdir(xconfig .. pathsep .. 'nvim')
+
+    write_file(init_lua_path, [[
+      vim.g.lua_rc = 1
+    ]])
+  end)
+
+  after_each(function()
+    rmdir(xhome)
+  end)
+
+  it('loads init.lua from XDG config home by default', function()
+    clear{ args_rm={'-u' }, env={ XDG_CONFIG_HOME=xconfig }}
+
+    eq(1, eval('g:lua_rc'))
+    eq(init_lua_path, eval('$MYVIMRC'))
+  end)
+
+  describe 'with explicitly provided config'(function()
+    local custom_lua_path = table.concat({xhome, 'custom.lua'}, pathsep)
+    before_each(function()
+      write_file(custom_lua_path, [[
+      vim.g.custom_lua_rc = 1
+      ]])
+    end)
+
+    it('loads custom lua config and does not set $MYVIMRC', function()
+      clear{ args={'-u', custom_lua_path }, env={ XDG_CONFIG_HOME=xconfig }}
+      eq(1, eval('g:custom_lua_rc'))
+      eq('', eval('$MYVIMRC'))
+    end)
+  end)
+
+  describe 'VIMRC also exists'(function()
+    before_each(function()
+      write_file(table.concat({xconfig, 'nvim', 'init.vim'}, pathsep), [[
+      let g:vim_rc = 1
+      ]])
+    end)
+
+    it('loads default lua config, but shows an error', function()
+      clear{ args_rm={'-u'}, env={ XDG_CONFIG_HOME=xconfig }}
+      feed('<cr>') -- TODO check this, test execution is blocked without it
+      eq(1, eval('g:lua_rc'))
+      matches('Conflicting configs', meths.exec('messages', true))
+    end)
+  end)
+end)