Function topo_sort 

pub fn topo_sort<Id, PredsIter>(
    node_ids: impl IntoIterator<Item = Id>,
    preds_fn: impl FnMut(Id) -> PredsIter,
) -> Result<Vec<Id>, Vec<Id>>
where
    Id: Copy + Eq + Hash,
    PredsIter: IntoIterator<Item = Id>,

Topologically sorts a set of nodes. Returns a list where the order of Ids will agree with the order of any path through the graph.

This succeeds if the input is a directed acyclic graph (DAG).

If the input has a cycle, an Err will be returned containing the cycle. Each node in the cycle will be listed exactly once.

https://en.wikipedia.org/wiki/Topological_sorting#Depth-first_search