thanks, but I'm confused about the "for example hash keys were already copy-on-write" comment.

By principle hash-keys are (and must be) immutable.

Immutable data can be shared w/o allocating new memory, while mutable objects need to be copied to separate space.

COW is a performance trick to "lazily" delay that copy to the time when the mutation (the write) happens.

So there must be a misunderstanding, since hash-keys are always read-only, there is never a write.