"Collaterally moved value" error for something that looks fine to me

[sellibitze]

The code

pub type MoveList<M> = Option<Box<MoveNode<M>>>;

pub struct MoveNode<M> {
    m: M,
    n: MoveList<M>
}

pub fn pop_front<M>(ml: MoveList<M>) -> (Option<M>, MoveList<M>) {
    match ml {
        None => (None, None),
        Some(box MoveNode { m, n }) => (Some(m), n)
    }
}

produces the error message “error: use of collaterally moved value” pointing to the second match arm. See http://is.gd/kBotGl -- tested on 2015-01-16 again: same result.

But when the destructuring is split in two parts, it works:

pub fn pop_front<M>(ml: MoveList<M>) -> (Option<M>, MoveList<M>) {
    match ml {
        None => (None, None),
        Some(box ml) => {
            let MoveNode {m, n} = ml;
            (Some(m), n)
        }
    }
}

This looks like a compiler bug to me.

[pythonesque]

You can also make it match properly by calling take() on ml:

pub fn pop_front<M>(mut ml: MoveList<M>) -> (Option<M>, MoveList<M>) {
    match ml.take() {
        None => (None, None),
        Some(box MoveNode {m, n }) => (Some(m), n)
    }
}

[alexcrichton]

Box patterns have since been feature gated, and other usability concerns I believe are covered by #16223