drawing: obtain `HeadRef` via new trait for abstract reference objects

src/drawing/band.rs

@@ -5,3 +5,5 @@ pub enum BandFirstSegIndex {
     Straight(LoneLooseSegIndex),
     Bended(SeqLooseSegIndex),
 }
+
+pub struct BandRef {}

src/drawing/head.rs

@@ -1,6 +1,6 @@
 use enum_dispatch::enum_dispatch;
 
-use crate::geometry::shape::MeasureLength;
+use crate::{geometry::shape::MeasureLength, graph::MakeRef};
 
 use super::{
     cane::Cane,
@@ -22,11 +22,8 @@ pub enum Head {
     Cane(CaneHead),
 }
 
-impl Head {
-    pub fn ref_<'a, CW: Copy, R: AccessRules>(
-        &self,
-        drawing: &'a Drawing<CW, R>,
-    ) -> HeadRef<'a, CW, R> {
+impl<'a, CW: Copy, R: AccessRules> MakeRef<'a, HeadRef<'a, CW, R>, Drawing<CW, R>> for Head {
+    fn ref_(&self, drawing: &'a Drawing<CW, R>) -> HeadRef<'a, CW, R> {
         HeadRef::new(*self, drawing)
     }
 }

src/graph.rs

@@ -6,7 +6,9 @@ use std::{
 use enum_dispatch::enum_dispatch;
 use petgraph::stable_graph::NodeIndex;
 
-// Due to apparent limitations of enum_dispatch we're forced to import some types backwards.
+pub trait MakeRef<'a, R: 'a, C> {
+    fn ref_(&self, context: &'a C) -> R;
+}
 
 #[enum_dispatch]
 pub trait GetPetgraphIndex {

src/router/router.rs

@@ -16,7 +16,7 @@ use crate::{
         primitive::{AccessPrimitiveShape, PrimitiveShape},
         shape::{AccessShape, MeasureLength},
     },
-    graph::GetPetgraphIndex,
+    graph::{GetPetgraphIndex, MakeRef},
     layout::Layout,
 };