pub fn dfs_next_buffered_filter<N, VM, G>(
    dfs: &mut DfsPostOrder<N, VM>,
    graph: G,
    mut buffered_filter: impl BufferedEdgeFilter<G>,
) -> Option<N>
where
    N: Copy + PartialEq,
    VM: VisitMap<N>,
    G: IntoEdges<NodeId = N>,
{
    // Adapted from DfsPostOrder::next in petgraph 0.5.0.
    while let Some(&nx) = dfs.stack.last() {
        if dfs.discovered.visit(nx) {
            // First time visiting `nx`: Push neighbors, don't pop `nx`
            let neighbors = graph.edges(nx).flat_map(|edge| {
                buffered_filter
                    .filter(edge)
                    .into_iter()
                    .map(|edge| edge.target())
            });
            for succ in neighbors {
                if !dfs.discovered.is_visited(&succ) {
                    dfs.stack.push(succ);
                }
            }
        } else {
            dfs.stack.pop();
            if dfs.finished.visit(nx) {
                // Second time: All reachable nodes must have been finished
                return Some(nx);
            }
        }
    }
    None
}
`DfsPostOrder::next`, adapted for a buffered filter that's also FnMut.
guppy/src/petgraph_support/dfs.rs