You're talking about semantics and you're totally right, there's plenty of cases in the implementation where things are not copied when they should be (string s

You're talking about semantics and you're totally right, there's plenty of cases in the implementation where things are not copied when they should be (string slices) or copied when they shouldn't be (boxed double writes into mutable double boxes). As you say though, the semantics are kind of irrelevant when you're talking about actual memory behaviour of allocations. There's actually lots of really interesting consequences of the current implementation, and the various tricks it does around what's observable and what's fast, it's just not what's described here.