Cannot move out of value which is behind a shared reference when unwrapping

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Option::unwrap() consumes the option, that is, it accepts the option by value. However, you don’t have a value, you only have a reference to it. That’s what the error is about.

Your code should idiomatically be written like this:

fn my_fn(arg1: &Option<Box<i32>>) -> i32 {
    match arg1 {
        Some(b) => **b,
        None => 0,
    }
}

fn main() {
    let integer = 42;
    my_fn(&Some(Box::new(integer)));
}

(on the Rust playground)

Or you can use Option combinators like Option::as_ref or Option::as_mut paired with Option::map_or, as Shepmaster has suggested:

fn my_fn(arg1: &Option<Box<i32>>) -> i32 {
    arg1.as_ref().map_or(0, |n| **n)
}

This code uses the fact that i32 is automatically copyable. If the type inside the Box weren’t Copy, then you wouldn’t be able to obtain the inner value by value at all – you would only be able to clone it or to return a reference, for example, like here:

fn my_fn2(arg1: &Option<Box<i32>>) -> &i32 {
    arg1.as_ref().map_or(&0, |n| n)
}

Since you only have an immutable reference to the option, you can only return an immutable reference to its contents. Rust is smart enough to promote the literal 0 into a static value to keep in order to be able to return it in case of absence of the input value.

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