GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/TracingStatus.h

Index: tools/clang/blink_gc_plugin/TracingStatus.h
diff --git a/tools/clang/blink_gc_plugin/TracingStatus.h b/tools/clang/blink_gc_plugin/TracingStatus.h
index 9eb1080de85d13845a910530b559d739883ebe6b..46e96547a9d4fa79c27ec4f11b77749a6c6d2931 100644
--- a/tools/clang/blink_gc_plugin/TracingStatus.h
+++ b/tools/clang/blink_gc_plugin/TracingStatus.h
@@ -5,12 +5,33 @@
 #ifndef TOOLS_BLINK_GC_PLUGIN_TRACING_STATUS_H_
 #define TOOLS_BLINK_GC_PLUGIN_TRACING_STATUS_H_
 
-// TracingStatus is a three-point value ordered by unneeded < unknown < needed.
+// TracingStatus is a four-point value ordered by
+//       illegal < unneeded < unknown < needed
+//
+// It is used to categorize tracing of fields:
+//
+//  * illegal  field is invalid/illegal to trace.
+//  * unneeded field has type with no traceable fields of its own;
+//             it may have an empty trace() method. Not harmful
+//             to trace, but not needed.
+//  * unknown  initial TracingStatus value.
+//  * needed   field is a heap reference or an object containing
+//             traceable fields.
+//
+// Tracing status |illegal| is considered an error; treating |unneeded| also
+// as an error would detect and report unnecessary tracing of objects that
+// probably don't need to be on the Blink GC heap. However, template use
+// and instantiation can leave us with classes that do have empty trace
+// methods and no traceable fields -- reporting these as errors/warnings
+// wouldn't work. Hence, only consider |illegal| as an error TracingStatus
+// state.
 class TracingStatus {
  public:
+  static TracingStatus Illegal() { return kIllegal; }
   static TracingStatus Unneeded() { return kUnneeded; }
   static TracingStatus Unknown() { return kUnknown; }
   static TracingStatus Needed() { return kNeeded; }
+  bool IsIllegal() const { return status_ == kIllegal; }
   bool IsUnneeded() const { return status_ == kUnneeded; }
   bool IsUnknown() const { return status_ == kUnknown; }
   bool IsNeeded() const { return status_ == kNeeded; }
@@ -21,7 +42,7 @@ class TracingStatus {
     return status_ == other.status_;
   }
  private:
-  enum Status { kUnneeded, kUnknown, kNeeded };
+  enum Status { kIllegal, kUnneeded, kUnknown, kNeeded };
   TracingStatus(Status status) : status_(status) {}
   Status status_;
 };