some things that support for can be coded into the garbage collector:

build a list of all objects that reference this one.

how much storage is accessible from this object.

how much storage would be freed if this object were released?
  run a mark phase suppressing the marking of this object, then mark
  this object, accumulating the storage requirements of unmarked
  objects reachable from the target object.

real time display of memory stats:
  memory in use as of end of last gc
  additional allocations since last gc
  fragmentation
  max heap size