Get PHPStan working when Craft isn't installed yet

Now only requires `codecept build`

.gitattributes

@@ -16,6 +16,7 @@
 /ecs.php export-ignore
 /gulpfile.js export-ignore
 /lerna.json export-ignore
+/lib/craft/behaviors/CustomFieldBehavior.php export-ignore
 /package-lock.json export-ignore
 /package.json export-ignore
 /packages/ export-ignore

lib/craft/behaviors/CustomFieldBehavior.php

@@ -0,0 +1,33 @@
+<?php
+/**
+ * @link http://craftcms.com/
+ * @copyright Copyright (c) Pixel & Tonic, Inc.
+ * @license http://craftcms.com/license
+ */
+
+namespace craft\behaviors;
+
+use yii\base\Behavior;
+
+/**
+ * This file is never loaded at runtime. It’s only here for PHPStan’n sake.
+ *
+ * @internal
+ */
+class CustomFieldBehavior extends Behavior
+{
+    /**
+     * @var bool Whether the behavior should provide methods based on the field handles.
+     */
+    public bool $hasMethods = false;
+
+    /**
+     * @var bool Whether properties on the class should be settable directly.
+     */
+    public bool $canSetProperties = true;
+
+    /**
+     * @var string[] List of supported field handles.
+     */
+    public static $fieldHandles = [];
+}

phpstan.neon

@@ -14,9 +14,8 @@ parameters:
         - tests/_data/*
         - tests/_output/*
         - tests/_support/_generated/
-    scanDirectories:
-        - tests/_craft/storage/runtime/compiled_classes/
     scanFiles:
+        - lib/craft/behaviors/CustomFieldBehavior.php
         - tests/_support/_generated/AcceptanceTesterActions.php
         - tests/_support/_generated/FunctionalTesterActions.php
         - tests/_support/_generated/GqlTesterActions.php

src/Craft.php

@@ -185,6 +185,11 @@ public static function autoload($className): void
      */
     private static function _autoloadCustomFieldBehavior(): void
     {
+        if (!isset(static::$app)) {
+            // Nothing we can do about it yet
+            return;
+        }
+
         if (!static::$app->getIsInstalled()) {
             // Just load an empty CustomFieldBehavior into memory
             self::_generateCustomFieldBehavior([], null, false, true);