Constraints on associated trait types

问题:

Here's a (somewhat contrived) example to illustrate what I would like to do

pub trait Node: Eq + Hash {
    type Edge: Edge;
    fn get_in_edges(&self)  -> Vec<&Self::Edge>;
    fn get_out_edges(&self) -> Vec<&Self::Edge>;
}

pub trait Edge {
    type Node: Node;
    fn get_src(&self) -> &Self::Node;
    fn get_dst(&self) -> &Self::Node;
}

pub trait Graph {
    type Node: Node;
    type Edge: Edge;
    fn get_nodes(&self) -> Vec<Self::Node>;
}

pub fn dfs<G: Graph>(root: &G::Node) {
    let mut stack = VecDeque::new();
    let mut visited = HashSet::new();

    stack.push_front(root);
    while let Some(n) = stack.pop_front() {
        if visited.contains(n) {
            continue
        }
        visited.insert(n);
        for e in n.get_out_edges() {
            stack.push_front(e.get_dst());
        }
    }
}

Is there a way to express in the Graph trait that Graph::Node must be the same type as Graph::Edge::Node and that Graph::Edge must be the same type as Graph::Node::Edge?

I remember reading something about a feature (not implemented at the time) that would allow richer constraints for this sort of thing, but I don't remember its name and cannot find it.

回答1:

In Graph's definition, you can constrain each associated type's associated type (!) to be equal to the corresponding associated type in Graph.

pub trait Graph {
    type Node: Node<Edge = Self::Edge>;
    type Edge: Edge<Node = Self::Node>;
    fn get_nodes(&self) -> Vec<Self::Node>;
}