This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
workInProgress.subtreeFlags = NoFlags;
    workInProgress.deletions = null;
    if (enableProfilerTimer) {
      
      
      
      
      workInProgress.actualDuration = 0;
      workInProgress.actualStartTime = -1;
    }
  }
  
  
  workInProgress.flags = current.flags & StaticMask;
  workInProgress.childLanes = current.childLanes;
  workInProgress.lanes = current.lanes;
  workInProgress.child = current.child;
  workInProgress.memoizedProps = current.memoizedProps;
  workInProgress.memoizedState = current.memoizedState;
  workInProgress.updateQueue = current.updateQueue;
  
  
  const currentDependencies = current.dependencies;
  workInProgress.dependencies =
    currentDependencies === null
      ? null
      : {
          lanes: currentDependencies.lanes,
          firstContext: currentDependencies.firstContext,