Proposed change:

Implement kani::Arbitrary for PhantomData<T> (blanket impl for all T) and PhantomPinned

Motivation:

I have a struct which represents "bytes that can be deserialized to a T":

struct TypedBytes<'a, T> {
  bytes: &'a [u8],
  _marker: core::marker::PhantomData<T>,
}

I'd like to be able to use #[cfg_attr(kani, derive(kani::Arbitrary))] on this struct, but currently I can't since PhantomData doesn't implement Arbitrary.

Since they are ZSTs, there are no values of either type, so presumably don't interact with symbolic values? I can't see a harm in allowing this, but I'm not familiar with kani, so let me know if I'm not understanding something 😁

Thanks