README.md

@@ -193,9 +193,10 @@ naming a kind that holds it. The fields an embedded struct promotes are columns
 same record; a named struct field, or a pointer to one, fills from its own tags as a
 record of its own, so its references draw apart from its parent's. `fake:"-"` leaves a
 struct field, embedded or named, or a pointer to one, unfilled. Untagged fields keep
-their values, and so does a pointer back to a struct already being filled. The first call for a type compiles its tags and reports
+their values, and so does a pointer back to a struct already being filled; a type
-what they get wrong, with the same error on every later call; a `datatype` in a tag
+whose fields reach more than 1024 structs is refused, naming `fake:"-"` to cut it. The
-names the Go type that already sets it.
+first call for a type compiles its tags and reports what they get wrong, with the same
+error on every later call; a `datatype` in a tag names the Go type that already sets it.
 
 A `*Generator` is safe for concurrent use; a seeded sequence is reproducible only
 when drawn from one goroutine. Changing how a value is composed shifts the seeded

inline.go

@@ -65,7 +65,7 @@ func IsTemplate(arg string) (bool, error) {
 func isTemplate(arg string) (bool, error) {
 	if strings.ContainsRune(arg, '{') || (isJSONStart(strings.TrimSpace(arg)) && json.Valid([]byte(arg))) {
 		if path, lone := loneReference(arg); lone {
-			return false, fmt.Errorf("%s is the path %s written as a template; write %s", arg, path, path)
+			return false, pathAdvice(path, fmt.Sprintf("%s is the path %s written as a template", arg, path))
 		}
 		return true, nil
 	}
@@ -73,7 +73,7 @@ func isTemplate(arg string) (bool, error) {
 		return false, fmt.Errorf("%q holds a %q, which no path may, and it is not valid JSON, so it names no template either", arg, arg[i:i+1])
 	}
 	if path := strings.TrimLeft(arg, "/"); path != arg && path != "" {
-		return false, fmt.Errorf("path %s starts with /, and every path starts at the root already; write %s", arg, path)
+		return false, pathAdvice(path, fmt.Sprintf("path %s starts with /, and every path starts at the root already", arg))
 	}
 	return false, nil
 }
@@ -82,6 +82,14 @@ func isJSONStart(arg string) bool {
 	return strings.HasPrefix(arg, "[") || strings.HasPrefix(arg, `"`)
 }
 
+// pathAdvice refuses a spelling of path, naming path to write, or why no name can spell it.
+func pathAdvice(path, refusal string) error {
+	if err := checkPathNames(path); err != nil {
+		return err
+	}
+	return fmt.Errorf("%s; write %s", refusal, path)
+}
+
 // loneReference is the path a template spells when the value it holds — a format string, a
 // JSON string, or an object holding only a format — is one reference token and nothing else.
 func loneReference(arg string) (string, bool) {
@@ -101,6 +109,9 @@ func loneReference(arg string) (string, bool) {
 	if units[0].kind != 'b' || !isRef(body) || strings.ContainsAny(body, "|(") {
 		return "", false
 	}
+	if strings.HasPrefix(body, "/") {
+		body = "/" + strings.TrimLeft(body, "/")
+	}
 	_, path, err := refShape(body)
 	return path, err == nil
 }

node.go

@@ -396,6 +396,16 @@ func checkName(name string) error {
 	return nil
 }
 
+// checkPathNames rejects a dotted path with a segment no name may be.
+func checkPathNames(path string) error {
+	for _, seg := range strings.Split(path, ".") {
+		if err := checkName(seg); err != nil {
+			return fmt.Errorf("path %w", err)
+		}
+	}
+	return nil
+}
+
 // isOption reports whether a template key configures the node instead of naming a
 // field. These names can never be fields.
 func isOption(name string) bool {

struct.go

@@ -36,9 +36,8 @@ type structResult struct {
 	err   error
 }
 
-// maxStructRecords caps the records one struct type fills, which pointers between struct
+// maxStructs caps the structs compiling one type walks through its fields.
-// types multiply along every path.
+const maxStructs = 1 << 10
-const maxStructRecords = 1 << 10
 
 // structShapeOf compiles a struct type once and remembers the answer. Callers hold the
 // generator's lock.
@@ -50,7 +49,7 @@ func (f *Generator) structShapeOf(t reflect.Type) (*structShape, error) {
 	if label == "" {
 		label = "struct"
 	}
-	sc := &structCompile{root: f.categories, visiting: map[reflect.Type]bool{}, records: maxStructRecords}
+	sc := &structCompile{root: f.categories, visiting: map[reflect.Type]bool{}, structs: maxStructs}
 	shape, err := sc.record(t, label)
 	if err == nil && shape.empty() {
 		err = fmt.Errorf("%s has no fake tags, so nothing to fill", t)
@@ -79,13 +78,13 @@ type nestedStruct struct {
 
 func (s *structShape) empty() bool { return s.record == nil && len(s.nested) == 0 }
 
-// structCompile is what compiling one struct type shares across the records it reaches: the
+// structCompile is what compiling one struct type shares across the structs it reaches: the
 // loaded tree, the types compiling or embedded above, so a pointer back to one is left alone
-// rather than filled without end, and how many more records it may build.
+// rather than filled without end, and how many more structs it may walk.
 type structCompile struct {
 	root     map[string]node
 	visiting map[reflect.Type]bool
-	records  int
+	structs  int
 }
 
 // structFields gathers what one struct type fills: its tagged fields, those its embedded
@@ -100,8 +99,8 @@ type structFields struct {
 }
 
 func (sc *structCompile) record(t reflect.Type, label string) (*structShape, error) {
-	if sc.records--; sc.records < 0 {
+	if err := sc.spend(label); err != nil {
-		return nil, fmt.Errorf(`%s: the struct fields reach more than %d records; leave a pointer unfilled with fake:"-"`, label, maxStructRecords)
+		return nil, err
 	}
 	sc.visiting[t] = true
 	defer delete(sc.visiting, t)
@@ -117,6 +116,13 @@ func (sc *structCompile) record(t reflect.Type, label string) (*structShape, err
 	return c.shape, nil
 }
 
+func (sc *structCompile) spend(label string) error {
+	if sc.structs--; sc.structs >= 0 {
+		return nil
+	}
+	return fmt.Errorf(`%s: the struct fields reach more than %d structs; leave a struct field unfilled with fake:"-"`, label, maxStructs)
+}
+
 // walk gathers the fields of struct type t, which sits at index within c.t.
 func (c *structFields) walk(t reflect.Type, index []int) error {
 	for i := 0; i < t.NumField(); i++ {
@@ -184,6 +190,9 @@ func fieldPath(t reflect.Type, index []int) string {
 }
 
 func (c *structFields) embed(sf reflect.StructField, elem reflect.Type) error {
+	if err := c.spend(c.label + "." + fieldPath(c.t, sf.Index)); err != nil {
+		return err
+	}
 	c.visiting[elem] = true
 	defer delete(c.visiting, elem)
 	tags, nested := len(c.tags), len(c.shape.nested)
@@ -218,10 +227,8 @@ func tagValue(sf reflect.StructField, tag string) (any, error) {
 	case inline:
 		return inputValue(tag)
 	}
-	for _, seg := range strings.Split(tag, ".") {
+	if err := checkPathNames(tag); err != nil {
-		if err := checkName(seg); err != nil {
+		return nil, err
-			return nil, fmt.Errorf("path %w", err)
-		}
 	}
 	return "{/" + tag + "}", nil
 }