From 32b9aa5c810712ad97f5b0a793823683ea42cd75 Mon Sep 17 00:00:00 2001
From: M <m@t.se>
Date: Sun, 30 Aug 2026 21:44:35 +0200
Subject: [PATCH] Aim a path token's render edge at what it renders

---
 README.md    |  2 +-
 reference.go | 38 +++++++++++++++++++++++++++++++-------
 render.go    |  4 +---
 3 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/README.md b/README.md
index 749039e..2b60e93 100644
--- a/README.md
+++ b/README.md
@@ -495,7 +495,7 @@ fakes.go        Fakes, New, List, options, seeding
 node.go         the node model and JSON -> node compilation
 render.go       Fake and the recursive renderer (choices, format strings, paths, bound draws)
 template.go     the {token} grammar: scanning, function and path tokens, validation
-reference.go    {..path} binding across the tree, and cycle detection
+reference.go    {..path} binding across the tree, the render graph, and the walks over it
 builtins.go     the {name()} function registry and its implementations
 calc.go         the {calc()} arithmetic evaluator: parser, eval, validation
 data.go         data loading: folders/files -> namespace tree, multi-path merge
diff --git a/reference.go b/reference.go
index eaa7e76..ba5730c 100644
--- a/reference.go
+++ b/reference.go
@@ -59,12 +59,15 @@ func checkBoundLevelsHeld(root map[string]node) error {
 		for _, head := range heads {
 			held := map[node]bool{}
 			cover(t.fields[head], held)
+			// One seen set across the edges: a node that cannot reach the level
+			// cannot reach it by another route either, so it is walked once here.
+			seen := map[node]bool{}
 			for _, e := range renderEdges(t) {
 				if splitArm(e.label).key == head {
 					continue // a path token reading this level, which is the one route allowed
 				}
-				if renders(e.to, held, map[node]bool{}) {
-					return fmt.Errorf("%s: {%s} renders a level that {%s} reads a path into; name the fields you want instead", path, e.label, t.bound[head])
+				if renders(e.to, held, seen) {
+					return fmt.Errorf("%s: {%s} renders %q, which {%s} reads a path into; name the fields you want instead", path, e.label, head, t.bound[head])
 				}
 			}
 		}
@@ -258,11 +261,13 @@ func renderEdges(n node) []renderEdge {
 	case *template:
 		var es []renderEdge
 		for _, name := range append(fieldTokens(n.format), calcOperands(n.format)...) {
-			// A path token renders its head, so the edge is to the head — over-
-			// approximating the sub-field it descends to, which is what keeps the
-			// cycle walk conservative rather than blind.
-			if c, ok := n.fields[splitArm(name).key]; ok {
-				es = append(es, renderEdge{c, name})
+			a := splitArm(name)
+			c, ok := n.fields[a.key]
+			if !ok {
+				continue
+			}
+			for _, leaf := range pathLeaves(c, a.tail) {
+				es = append(es, renderEdge{leaf, name})
 			}
 		}
 		return es
@@ -271,6 +276,25 @@ func renderEdges(n node) []renderEdge {
 	}
 }
 
+// pathLeaves lists what a token's dotted tail renders. A path draws the levels it
+// passes through but renders only what it lands on, so the leaf is the edge — a
+// bare token, whose tail is empty, lands on the field itself. A choice on the way
+// contributes every variant, since any of them may be the one drawn. checkPath has
+// already proved the tail resolves in every variant, so the walk drops nothing.
+func pathLeaves(n node, tail []string) []node {
+	if len(tail) == 0 {
+		return []node{n}
+	}
+	if c, ok := n.(*choice); ok {
+		var out []node
+		for _, it := range c.items {
+			out = append(out, pathLeaves(it, tail)...)
+		}
+		return out
+	}
+	return pathLeaves(child(n, tail[0]), tail[1:])
+}
+
 // checkNoCycles rejects a reference cycle: a node whose rendering can reach itself
 // — directly, mutually, or through a chain — never terminates, so it must fail at
 // New rather than stack-overflow at render. It is a depth-first walk of the render
diff --git a/render.go b/render.go
index 74286d9..cb307e8 100644
--- a/render.go
+++ b/render.go
@@ -190,9 +190,7 @@ func resolve(s *session, arms []arm, t *template, bound *draws) string {
 		bound.variant[a.key] = n
 	}
 	// Hold the draw at every level passed through, so two paths sharing a prefix
-	// share it. The leaf needs no hold: no other token can name it, since
-	// checkNoOverlap rejects a field or calc spelling and checkNotBound a
-	// reference one.
+	// share it.
 	for i, seg := range a.tail {
 		if i < len(a.steps) {
 			held, drew := bound.variant[a.steps[i]]