In any case we'll need assignment, mem::replace, mem::swap, or something like that. Because given a &mut reference to an object there is no way to move this object (or any of it's fields) out without replacing it's memory area with something valid, as long as Rust forbids references to uninitialized memory.
As for dummy values for replacement, you can always make them yourself for any type by using some wrapper type. For example, I often use Option for this purpose, where Some(T) is the value of type T, and None acts as dummy. This is what I mean:
struct Tree(Option>);
enum Node {
Leaf(T),
Children(Vec>),
}
impl Tree where T: PartialEq {
fn remove(&mut self, value: &T) {
match self.0.take() {
Some(Node::Leaf(ref leaf_value)) if leaf_value == value =>
(),
node @ Some(Node::Leaf(..)) =>
*self = Tree(node),
Some(Node::Children(node_children)) => {
let children: Vec<_> =
node_children
.into_iter()
.filter_map(|mut tree| { tree.remove(value); tree.0 })
.map(|node| Tree(Some(node)))
.collect();
if !children.is_empty() {
*self = Tree(Some(Node::Children(children)));
}
},
None =>
panic!("something went wrong"),
}
}
}
playground link
