This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
const nextCache: Cache = nextState.cache;
    pushRootCachePool(root);
    pushCacheProvider(workInProgress, nextCache);
    if (nextCache !== prevState.cache) {
      
      propagateContextChange(workInProgress, CacheContext, renderLanes);
    }
  }
  
  
  const nextChildren = nextState.element;
  if (nextChildren === prevChildren) {
    resetHydrationState();
    return bailoutOnAlreadyFinishedWork(current, workInProgress, renderLanes);
  }
  if (root.hydrate && enterHydrationState(workInProgress)) {
    
    
    
    
    if (supportsHydration) {
      const mutableSourceEagerHydrationData =
        root.mutableSourceEagerHydrationData;
      if (mutableSourceEagerHydrationData != null) {
        for (let i = 0; i < mutableSourceEagerHydrationData.length; i += 2) {
          const mutableSource = ((mutableSourceEagerHydrationData[
            i
          ]: any): MutableSource<any>);
          const version = mutableSourceEagerHydrationData[i + 1];
          setWorkInProgressVersion(mutableSource, version);