message: Silence unused method warning for edge_bound
diff: @@ -735,6 +735,7 @@ impl<N, E, Ty, Ix> StableGraph<N, E, Ty, Ix>
  self.g.raw_edges()
  }
  + #[cfg(feature = "serde-1")]
  fn edge_bound(&self) -> usize {
  self.edge_references()
  .next_back()
  