Stored properties in extension hack:

AppDelegate.swift

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        
        var testStr = ""
        var testStr2 = "131231"
        
        testStr.test = 20
        testStr2.test = 40
        debugPrint(testStr.test)
        debugPrint(testStr2.test)
        
        return true
    }
}

extension String {
    
    private struct Structure {
        static var dict = [String: Int]()
    }
    
    var test: Int? {
        get {
            Structure.dict[debugDescription]
        }
        set {
            Structure.dict[debugDescription] = newValue
        }
    }
}