Merge pull request #2305 from hannobraun/validation

Update documentation of `AdjacentHalfEdgesNotConnected` validation check

crates/fj-core/src/validation/checks/face_winding.rs

@@ -1,9 +1,10 @@
 use fj_math::Winding;
 
 use crate::{
+    geometry::Geometry,
     storage::Handle,
     topology::{Cycle, Face},
-    validation::ValidationCheck,
+    validation::{ValidationCheck, ValidationConfig},
 };
 
 /// Interior [`Cycle`] of [`Face`] has invalid winding
@@ -38,8 +39,8 @@ pub struct InteriorCycleHasInvalidWinding {
 impl ValidationCheck<Face> for InteriorCycleHasInvalidWinding {
     fn check(
         object: &Face,
-        geometry: &crate::geometry::Geometry,
-        _: &crate::validation::ValidationConfig,
+        geometry: &Geometry,
+        _: &ValidationConfig,
     ) -> impl Iterator<Item = Self> {
         object.region().interiors().iter().filter_map(|interior| {
             let exterior = object.region().exterior();

crates/fj-core/src/validation/checks/half_edge_connection.rs

@@ -7,7 +7,7 @@ use crate::{
     validation::{validation_check::ValidationCheck, ValidationConfig},
 };
 
-/// Adjacent [`HalfEdge`]s in [`Cycle`] are not connected
+/// # Adjacent [`HalfEdge`]s in [`Cycle`] are not connected
 ///
 /// Each [`HalfEdge`] only references its start vertex. The end vertex is always
 /// assumed to be the start vertex of the next [`HalfEdge`] in the cycle. This
@@ -16,6 +16,25 @@ use crate::{
 ///
 /// However, the *position* of that shared vertex is redundantly defined in both
 /// [`HalfEdge`]s. This check verifies that both positions are the same.
+///
+/// ## Implementation Note
+///
+/// Having the vertex positions redundantly defined is not desirable, but
+/// currently we lack the facilities to project a single definition (whether
+/// local to a curve, local to a surface, or global in 3D space) into other
+/// local contexts, where they are required for approximation/triangulation.
+///
+/// As of this writing, there is no issue for creating these facilities and
+/// consolidating these redundant definitions, but the following issue tracks a
+/// prerequisite of that:
+///
+/// <https://github.com/hannobraun/fornjot/issues/2118>
+///
+/// If there was a single definition for each vertex position, we wouldn't need
+/// this validation check in its current form, but we would still need another
+/// one that fills a similar gap. Namely, we would still need to check whether a
+/// half-edge's start and end vertices are actually located on that half-edge's
+/// curve.
 #[derive(Clone, Debug, thiserror::Error)]
 #[error(
     "Adjacent `HalfEdge`s in `Cycle` are not connected\n\