Can Box move its contents when the whole Box is moved?

Question

If I Box::new a value, take a pointer to it (the borrow checker won't allow to take a reference, since I'm about to move the box), and then move the Box, can a move of the value (e.g. a reallocation) happen?

I thought that Box just stores the values address, so that moving the Box would only move the address. Is there therefore a reason why borrow checker prohibits moving it when its contents are immutably borrowed?

Playground

Answer 1

No, moving a Box will not move the value in the heap.

Box makes a guarantee:

a Box<T> is guaranteed to be represented as a single pointer

And Rust makes the guarantee that moves are always bitwise copies (if they copy at all).

Is there therefore a reason why borrow checker prohibits moving it when its contents are immutably borrowed?

This is covered by:

• Is there any way to allow moving a container that has a borrowed element but not dropping it?
• Why can't I store a value and a reference to that value in the same struct?