How can I reuse a box that I have moved the value out of?

Question

I have some non-copyable type and a function that consumes and (maybe) produces it:

type Foo = Vec;

fn quux(_: Foo) -> Option {
             


        

Answer 1

So, moving out of a Box is a special case... now what?

The std::mem module presents a number of safe functions to move values around, without poking holes (!) into the memory safety of Rust. Of interest here are swap and replace:

pub fn replace(dest: &mut T, src: T) -> T

Which we can use like so:

fn baz(mut abox: Box) -> Option> {
    let foo = std::mem::replace(&mut *abox, Foo::default());

    match quux(foo) {
        Some(new_foo) => {
            *abox = new_foo;
            Some(abox)
        }
        None => None
    }
}

It also helps in the map case, because it does not borrow the Box:

fn baz(mut abox: Box) -> Option> {
    let foo = std::mem::replace(&mut *abox, Foo::default());

    quux(foo).map(|new_foo| { *abox = new_foo; abox })
}