Node-enqueuing tracing.

This commit implements node-enqueuing tracing.  This is mainly for
supporting VMs that does not support edge-enqueuing for some or all
objects.

src/plan/generational/gc_work.rs

@@ -16,6 +16,7 @@ pub struct GenNurseryProcessEdges<VM: VMBinding> {
 
 impl<VM: VMBinding> ProcessEdgesWork for GenNurseryProcessEdges<VM> {
  type VM = VM;
+ type ScanObjectsWorkType = ScanObjects<Self>;
 
  fn new(edges: Vec<Address>, roots: bool, mmtk: &'static MMTK<VM>) -> Self {
  let base = ProcessEdgesBase::new(edges, roots, mmtk);
@@ -39,6 +40,11 @@ impl<VM: VMBinding> ProcessEdgesWork for GenNurseryProcessEdges<VM> {
  debug_assert!(!self.gen.nursery.in_space(new_object));
  unsafe { slot.store(new_object) };
  }
+
+ #[inline(always)]
+ fn create_scan_work(&self, nodes: Vec<ObjectReference>, roots: bool) -> ScanObjects<Self> {
+ ScanObjects::<Self>::new(nodes, false, roots)
+ }
 }
 
 impl<VM: VMBinding> Deref for GenNurseryProcessEdges<VM> {