warning: methods `f` and `g` are never used
  --> $DIR/issue-6128.rs:6:8
   |
LL | trait Graph<Node, Edge> {
   |       ----- methods in this trait
LL |     fn f(&self, _: Edge);
   |        ^
LL |     fn g(&self, _: Node);
   |        ^
   |
   = note: `#[warn(dead_code)]` on by default

warning: 1 warning emitted