> Pins are very often `Unpin`, but this isn't relevant to their function.

This idea is what I've seen get a lot of people lost, but it's pretty natural. By analogy, a DANGER HIGH VOLTAGE sign doesn't itself need to be dangerous or high voltage in order to be useful. If your DANGER HIGH VOLTAGE sign (the sign) is dangerous or high voltage, you'd probably want a second DANGER HIGH VOLTAGE sign pointing out this fact about the first sign. The same for Pin: if you're wanting the pinned pointer (the pointer itself) to not be Unpin, your pinned pointer would need to be pinned by some second pinned pointer.