Add infrastructure for tracking object insertion

crates/fj-kernel/src/operations/insert.rs

@@ -50,3 +50,30 @@ impl_insert!(
     Surface, surfaces;
     Vertex, vertices;
 );
+
+/// Indicate whether an object has been inserted
+///
+/// Intended to be used as a type parameter bound for structs that need to track
+/// whether their contents have been inserted or not.
+pub trait IsInserted {
+    /// The type of the object for which the insertion status is tracked
+    type T<T>;
+}
+
+/// Indicate that an object has been inserted
+///
+/// See [`IsInserted`].
+pub struct IsInsertedYes;
+
+impl IsInserted for IsInsertedYes {
+    type T<T> = Handle<T>;
+}
+
+/// Indicate that an object has not been inserted
+///
+/// See [`IsInserted`].
+pub struct IsInsertedNo;
+
+impl IsInserted for IsInsertedNo {
+    type T<T> = T;
+}

crates/fj-kernel/src/operations/mod.rs

@@ -10,7 +10,7 @@ pub use self::{
         BuildCycle, BuildFace, BuildHalfEdge, BuildShell, BuildSurface,
         Polygon, Tetrahedron,
     },
-    insert::Insert,
+    insert::{Insert, IsInserted, IsInsertedNo, IsInsertedYes},
     join::JoinCycle,
     update::{UpdateCycle, UpdateFace, UpdateHalfEdge, UpdateShell},
 };