If you look at the source for into_boxed_slice, it calls shrink_to_fit at the beginning before doing anything else. Hence the documentation is slightly wrong, and no copies occur.
I appreciate the mention of Box<[T]>. I've never (or at least very rarely?) seen people using it, even in rustc, although I'm sure there's somewhere in the compiler that uses it. I've kind of gotten the feeling that Box<[T]> is frowned upon as a needless optimization because the one-word storage difference is so minimal, but I appreciate having the immutable size semantics as a way of ensuring no extra allocations occur by way of the methods on Vec.
Yeah, I think the problem is that the memory model (computer science) article is too short, so the model ended up spitting out information from something seemingly related, i.e., human memory models.
I asked it "what is a memory model in computer science" to see how it handles something slightly obscure. Its answer:
> The multi-store model (also known as Atkinson–Shiffrin memory model) is a memory model in computer science that describes the interactions of threads through memory and their shared use of the data. It allows a compiler to perform many important optimizations.
> More info in the History and significance section of the Memory model (programming) Wikipedia article.
It seems to think that the Atkinson–Shiffrin memory model is somehow related to computer science, which it is not. It's a model of human memory. And that article it references does not once mention the Atkinson–Shiffrin memory model. At least it's easy to verify.
Edit: I submitted a PR to clear up the docs: https://github.com/rust-lang/rust/pull/120110