> putting the record data right after the CacheEntry members

I assumed they couldn't do that because they're using it with some kind of generic HashMap<K, V>. In that situation, can "V" be dynamically sized?

A dynamically sized "V" would mean you can't have an array of them, which might preclude some hash map implementations.

> All type parameters have an implicit bound of Sized. The special syntax ?Sized can be used to remove this bound if it’s not appropriate.

, which HashMap does not do, i.e. the keys and values have to have a statically known size.