Update doc comment and add assertion

src/scheduler/gc_work.rs

@@ -273,11 +273,8 @@ pub(crate) struct ProcessEdgesWorkTracer<E: ProcessEdgesWork> {
 impl<E: ProcessEdgesWork> ObjectTracer for ProcessEdgesWorkTracer<E> {
  /// Forward the `trace_object` call to the underlying `ProcessEdgesWork`,
  /// and flush as soon as the underlying buffer of `process_edges_work` is full.
- ///
- /// This function is inlined because `trace_object` is probably the hottest function in MMTk.
- /// If this function is called in small closures, please profile the program and make sure the
- /// closure is inlined, too.
  fn trace_object(&mut self, object: ObjectReference) -> ObjectReference {
+ debug_assert!(!object.is_null());
  let result = self.process_edges_work.trace_object(object);
  self.flush_if_full();
  result

src/vm/scanning.rs

@@ -26,11 +26,14 @@ impl<ES: Edge, F: FnMut(ES)> EdgeVisitor<ES> for F {
 
 /// Callback trait of scanning functions that directly trace through edges.
 pub trait ObjectTracer {
- /// Call this function for the content of each edge,
- /// and assign the returned value back to the edge.
+ /// Call this function to trace through an object graph edge which points to `object`.
+ /// `object` must point to a valid object, and cannot be `ObjectReference::NULL`.
  ///
- /// Note: This function is performance-critical.
- /// Implementations should consider inlining if necessary.
+ /// The return value is the new object reference for `object` if it is moved, or `object` if
+ /// not moved. If moved, the caller should update the slot that holds the reference to
+ /// `object` so that it points to the new location.
+ ///
+ /// Note: This function is performance-critical, therefore must be implemented efficiently.
  fn trace_object(&mut self, object: ObjectReference) -> ObjectReference;
 }