can't implement trait (e.g., Clone) for recursive datatype that uses Option

[tjhance]

Suppose you want to implement Clone for a recursive datatype:

struct Node {
    child: Option<Box<Node>>,
}

impl Clone for Node {
    fn clone(&self) -> Self {
        Node {
            child: self.child.clone()
        }
    }
}

This currently gives a "cyclic self-reference" error.

It should theoretically be possible to prove termination here. And it should be optional, anyway (since it's exec code).

[tjhance]

the presence of Option is the part that trips it up. If you inline the Option-handling instead of relying on Option::clone it works fine:

struct Node {
    child: Option<Box<Node>>,
}

impl Clone for Node {
    fn clone(&self) -> Self {
        Node {
            child: match &self.child {
                Some(node) => Some(Box::new((&**node).clone())),
                None => None,
            }
        }
    }
}