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, a

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. Edit: I submitted a PR to clear up the docs: https://github.com/rust-lang/rust/pull/120110 https://github.com/rust-lang/rust/pull/120110