Fix at compile how readField reads an arm, deleting its two panics and its held lookup per read

format.go

@@ -315,8 +315,18 @@ type arm struct {
 	steps       []pathStep
 	leaves      []node // every node the path may land on, one per variant it passes
 	cover       node   // what holding the path pins: the first choice it passes, else its leaf
+	kind        armKind
 }
 
+// armKind is how expand reads an arm, fixed at compile.
+type armKind uint8
+
+const (
+	freshRead   armKind = iota // drawn afresh at every read
+	heldRead                   // drawn once per expansion, kept in its hold
+	refPathRead                // drawn once per draw group, kept in its memo
+)
+
 func (a arm) isRefPath() bool {
 	return isRef(a.head) && len(a.tail) > 0
 }

reference.go

@@ -112,6 +112,12 @@ func compileArm(t *template, a *arm) {
 	}
 	w := compilePath(head, a.tail)
 	a.steps, a.leaves, a.cover = w.steps, w.leaves, w.cover
+	switch _, held := t.compiled.held[a.head]; {
+	case a.isRefPath():
+		a.kind = refPathRead
+	case held:
+		a.kind = heldRead
+	}
 }
 
 // resolveLink binds every reference t reads, refusing one to t's own category, and keys

render.go

@@ -158,21 +158,15 @@ func expand(s *generatorState, t *template, sc renderScope) string {
 // read one row, either read twice gives one value, and a shown operand is the operand
 // computed. Every other name is drawn afresh, so {word} {word} still draws twice.
 func readField(s *generatorState, t *template, hold *drawMemo, sc renderScope, a arm) readValue {
-	if a.isRefPath() {
+	if a.kind == refPathRead {
 		return readReference(s, t, sc, a)
 	}
 	if sc.draws.trace != nil {
 		traceRead(sc.draws.trace, t, sc, a)
 	}
-	if _, expansionHolds := t.compiled.held[a.head]; !expansionHolds {
+	if a.kind == freshRead {
-		if len(a.tail) > 0 {
-			panic(internalError("%q reads a path into %q, which the expansion does not hold", a.spelling, a.head))
-		}
 		return readValue{text: render(s, t.head(a.head), sc)}
 	}
-	if hold == nil {
-		panic(internalError("%q reads a name the expansion holds, with no hold to keep it in", a.spelling))
-	}
 	return readMemo(s, t, hold, nil, sc, a)
 }