XDG: Windows: resolve $LOCALAPPDATA, $TEMP (#5278)

After #4964 environment variables in the XDG "fallback" table are no
longer expanded.

Fallback to correctly expanded $LOCALAPPDATA, $TEMP. If that fails
(unlikely), fallback to hard-coded paths (e.g. ~/AppData/Local).

Closes #5255

src/nvim/os/stdpaths.c

@@ -16,20 +16,29 @@ static const char *xdg_env_vars[] = {
   [kXDGDataDirs] = "XDG_DATA_DIRS",
 };
 
+#ifdef WIN32
+static const char *const xdg_defaults_env_vars[] = {
+  [kXDGConfigHome] = "LOCALAPPDATA",
+  [kXDGDataHome] = "LOCALAPPDATA",
+  [kXDGCacheHome] = "TEMP",
+  [kXDGRuntimeDir] = NULL,
+  [kXDGConfigDirs] = NULL,
+  [kXDGDataDirs] = NULL,
+};
+#endif
+
 /// Defaults for XDGVarType values
 ///
 /// Used in case environment variables contain nothing. Need to be expanded.
 static const char *const xdg_defaults[] = {
 #ifdef WIN32
-  // Windows
-  [kXDGConfigHome] = "$LOCALAPPDATA",
-  [kXDGDataHome]   = "$LOCALAPPDATA",
-  [kXDGCacheHome]  = "$TEMP",
+  [kXDGConfigHome] = "~\\AppData\\Local",
+  [kXDGDataHome] = "~\\AppData\\Local",
+  [kXDGCacheHome] = "~\\AppData\\Local\\Temp",
   [kXDGRuntimeDir] = NULL,
   [kXDGConfigDirs] = NULL,
   [kXDGDataDirs] = NULL,
 #else
-  // Linux, BSD, CYGWIN, Apple
   [kXDGConfigHome] = "~/.config",
   [kXDGDataHome] = "~/.local/share",
   [kXDGCacheHome] = "~/.cache",
@@ -50,7 +59,14 @@ char *stdpaths_get_xdg_var(const XDGVarType idx)
   const char *const env = xdg_env_vars[idx];
   const char *const fallback = xdg_defaults[idx];
 
-  const char *const env_val = os_getenv(env);
+  const char *env_val = os_getenv(env);
+
+#ifdef WIN32
+  if (env_val == NULL) {
+    env_val = os_getenv(xdg_defaults_env_vars[idx]);
+  }
+#endif
+
   char *ret = NULL;
   if (env_val != NULL) {
     ret = xstrdup(env_val);

test/functional/options/defaults_spec.lua

@@ -9,8 +9,6 @@ local eval = helpers.eval
 local eq = helpers.eq
 local neq = helpers.neq
 
-if helpers.pending_win32(pending) then return end
-
 local function init_session(...)
   local args = { helpers.nvim_prog, '-i', 'NONE', '--embed',
     '--cmd', 'set shortmess+=I background=light noswapfile noautoindent',
@@ -24,6 +22,8 @@ end
 
 describe('startup defaults', function()
   describe(':filetype', function()
+    if helpers.pending_win32(pending) then return end
+
     local function expect_filetype(expected)
       local screen = Screen.new(48, 4)
       screen:attach()
@@ -99,8 +99,37 @@ describe('startup defaults', function()
 end)
 
 describe('XDG-based defaults', function()
-  -- Need to be in separate describe() block to not run clear() twice.
+  -- Need separate describe() blocks to not run clear() twice.
   -- Do not put before_each() here for the same reasons.
+
+  describe('with empty/broken environment', function()
+    it('sets correct defaults', function()
+      clear({env={
+        XDG_CONFIG_HOME=nil,
+        XDG_DATA_HOME=nil,
+        XDG_CACHE_HOME=nil,
+        XDG_RUNTIME_DIR=nil,
+        XDG_CONFIG_DIRS=nil,
+        XDG_DATA_DIRS=nil,
+        LOCALAPPDATA=nil,
+        HOMEPATH=nil,
+        HOMEDRIVE=nil,
+        HOME=nil,
+        TEMP=nil,
+        VIMRUNTIME=nil,
+        USER=nil,
+      }})
+
+      eq('.', meths.get_option('backupdir'))
+      eq('.', meths.get_option('viewdir'))
+      eq('.', meths.get_option('directory'))
+      eq('.', meths.get_option('undodir'))
+    end)
+  end)
+
+  -- TODO(jkeyes): tests below fail on win32 because of path separator.
+  if helpers.pending_win32(pending) then return end
+
   describe('with too long XDG variables', function()
     before_each(function()
       clear({env={