I would translate 'coffee cup' as kaftaso thereby dropping the o of kafo. The translation here probably is correct? However, if you really think about this, you may argue that a cup of coffee can never be empty, as it would cease to be a cup of coffee. On the other hand, it is correct to say that the coffee cup is empty. Food for thoughtĴ
Surely if it's empty it cannot be "my cup of coffee". It would just be "my cup". I suppose in English we might say, "My coffee cup is empty". (Possibly "taso por kafo" in Esperanto?)
I agree that an empty cup is not a cup of anything. If it is a matter of what the cup is intended to hold, then it is a reference to the type of cup, not the contents. So, is coffee here not an adjective? Is it not "kaf-A taso" therefore correct? Or, for that matter, "kaftaso" or 'taskafo? This is Esperanto. You are allowed to make new words.
I don't think so. I spent some time on several sites recently looking up the distinction between 'da' and 'de'.
For 'taso de kafo' I saw translations like 'a cup with coffee in it', 'a cup used for coffee', 'a cup that contains coffee', 'coffee cup' and ' a cup in which coffee is (/is usually) present'.
For 'taso da kafo' I saw translations like 'quantity of coffee contained in a standard sized cup', 'one cup full (as an amount) of coffee', 'cup of coffee', 'a cup sized amount of coffee' and 'cup full of coffee'.
Since the cup in question is empty we must be talking about a 'taso de kafo' not a 'taso da kafo'.