This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
workInProgress: Fiber,
  firstChild: null | Fiber,
  renderLanes: Lanes,
): void {
  
  
  
  let node = firstChild;
  while (node !== null) {
    if (node.tag === SuspenseComponent) {
      const state: SuspenseState | null = node.memoizedState;
      if (state !== null) {
        scheduleWorkOnFiber(node, renderLanes);
      }
    } else if (node.tag === SuspenseListComponent) {
      
      
      
      
      
      scheduleWorkOnFiber(node, renderLanes);
    } else if (node.child !== null) {
      node.child.return = node;
      node = node.child;
      continue;