Box implementation for Drop does no customization. Seems the compiler is responsible for that

[dadepo]

In Chapter 15, section 3 you have

For example, Box customizes Drop to deallocate the space on the heap that the box points to

source: https://doc.rust-lang.org/book/ch15-03-drop.html

I looked into the implementation of Drop for Box with the intention of seeing how such an implementation would look like and this is what I found:

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
    fn drop(&mut self) {
        // FIXME: Do nothing, drop is currently performed by compiler.
    }
}

So I do not think the quoted snippet from the book is technically correct.