Raise CompilationError for unknown field names in MessageOneofRule.fields

builder.go

@@ -127,10 +127,20 @@
 		return
 	}
 
 	oneofRules := msgRules.GetOneof()
 	for _, rule := range oneofRules {
+		fdescs := make([]protoreflect.FieldDescriptor, 0, len(rule.GetFields()))
+		for _, name := range rule.GetFields() {
+			fdesc := desc.Fields().ByName(protoreflect.Name(name))
+			if fdesc == nil {
+				msgEval.Err = &CompilationError{cause: fmt.Errorf(
+					"field %q not found in message %s", name, desc.FullName())}
+			} else {
+				fdescs = append(fdescs, fdesc)
+			}
+		}
 		oneofEval := &oneofEvaluator{
-			Fields:   rule.GetFields(),
+			Fields:   fdescs,
 			Required: rule.GetRequired(),
 		}
 		msgEval.AppendNested(oneofEval)

message_oneof.go

@@ -27,14 +27,14 @@ import (
 // fields, ensuring that only one is set. If `required` is true, it enforces that one of
 // the fields _must_ be set.
 type oneofEvaluator struct {
-	Fields   []string
+	Fields   []protoreflect.FieldDescriptor
 	Required bool
 }
 
 func (o oneofEvaluator) formatFields() string {
 	quoted := make([]string, len(o.Fields))
-	for idx, val := range o.Fields {
-		quoted[idx] = fmt.Sprintf("'%s'", val)
+	for idx, fdesc := range o.Fields {
+		quoted[idx] = fmt.Sprintf("'%s'", fdesc.Name())
 	}
 	return fmt.Sprintf("[%s]", strings.Join(quoted, ", "))
 }
@@ -50,9 +50,8 @@ func (o oneofEvaluator) EvaluateMessage(msg protoreflect.Message, cfg *validatio
 	err := &ValidationError{}
 	if len(o.Fields) > 0 {
 		count := 0
-		for _, v := range o.Fields {
-			fd := msg.Descriptor().Fields().ByName(protoreflect.Name(v))
-			if fd != nil && msg.Has(fd) {
+		for _, fdesc := range o.Fields {
+			if msg.Has(fdesc) {
 				count++
 			}
 		}