Add Clone to graph models, and tests

src/models/graph.rs

@@ -49,6 +49,7 @@ impl IncidenceSlot {
 /// Referenced by [`Vertex`] as a type parameter of the underlying arena. Not intended for direct
 /// external use.
 #[doc(hidden)]
+#[derive(Clone)]
 pub struct VertexIncidenceHeader {
     incidence_count: usize,
     first_incidence: Option<IncidenceSlot>,
@@ -129,12 +130,19 @@ impl IncidenceCursor<Graph> for GraphIncidenceCursor {
 /// planned, performing deletions first allows freed slots to be reused by subsequent additions,
 /// potentially avoiding reallocation.
 ///
+/// # Cloning
+///
+/// [`Vertex`] and [`Edge`] handles obtained from a clone are valid, and refer to the same vertex or
+/// edge in the original, and vice versa. However, once cloned, the two instances no longer affect
+/// each other.
+///
 /// [`add_vertex`]: Self::add_vertex
 /// [`add_edge`]: Self::add_edge
 /// [`degree`]: Self::degree
 /// [`delete_vertex`]: Self::delete_vertex
 /// [`delete_edge`]: Self::delete_edge
 /// [`AppendGraph`]: crate::models::AppendGraph
+#[derive(Clone)]
 pub struct Graph {
     // TODO: Arena index and generation types could be externalized to Graph.
     vertices: Arena<VertexIncidenceHeader, usize, usize>,
@@ -398,6 +406,8 @@ mod trait_tests {
     crate::graph_topology_addition_tests!(Graph);
     crate::graph_topology_deletion_tests!(Graph);
     crate::graph_topology_addition_deletion_tests!(Graph);
+    crate::graph_topology_clone_tests!(Graph);
+    crate::graph_topology_addition_clone_tests!(Graph);
 }
 
 #[cfg(test)]