After reading through spec a little. Abstract values are opaque and their members can't be accessed. The addedExternalValue struct allows working in both opaque and non-opaque modes, so I just documented this distinction.

It will default to non-opaque mode if type implementsAsValue. Otherwise, an opaque value can always be constructed.

Also, the drop logic I used is something I already tested in another crate with Miri, so itshould work.

Oh, and CEL is designed for use with protobuf, so what#81 is doing might be a better fit. I'll probably fork off with these changes into a different crate if I get really invested in this because my use case calls for a lot of things that go against or aren't aligned with the spec.