Function edge_cost 

pub fn edge_cost(
    a: Tier,
    b: Tier,
    ty_a: &Ty,
    ty_b: &Ty,
    types: &BTreeMap<Arc<str>, TyDecl>,
) -> Cost

The cost of an edge between two placed nodes.

The two types are what each end produces. The bytes charged are the smaller: see the module docs — the compiler may compute on either side of a boundary and will send whichever is less, so the boundary costs the lesser of the two.