Inaccuracy in 15-1

[djzin]

Because a Box<T> is a pointer, we always know how much space it needs: a
pointer takes up a usize amount of space.

since a box always has the size of a usize, no matter what it's pointing to

These are false statements. For example,

assert_eq!(std::mem::size_of::<Box<str>>(), std::mem::size_of::<usize>() * 2);
assert_eq!(std::mem::size_of::<Box<ToString>>(), std::mem::size_of::<usize>() * 2);

It would be nice to clarify this (maybe in a footnote) whilst keeping it simple for purposes of the example.

[carols10cents]

Ooooops. Yeah, I'll reword this next time we get this chapter back from nostarch so that at least it's not incorrect. Thanks!!! ❤️

[steveklabnik]

So, a box on its own is always a usize. A trait object isn't just a box.