This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
Disconnect from the old current.
     It will get deleted.
    current.alternate = null;
    oldWorkInProgress.alternate = null;
     Connect to the new tree.
    newWorkInProgress.index = oldWorkInProgress.index;
    newWorkInProgress.sibling = oldWorkInProgress.sibling;
    newWorkInProgress.return = oldWorkInProgress.return;
    newWorkInProgress.ref = oldWorkInProgress.ref;
     Replace the child/sibling pointers above it.
    if (oldWorkInProgress === returnFiber.child) {
      returnFiber.child = newWorkInProgress;
    } else {
      let prevSibling = returnFiber.child;
      if (prevSibling === null) {
        throw new Error('Expected parent to have a child.');
      }
      while (prevSibling.sibling !== oldWorkInProgress) {
        prevSibling = prevSibling.sibling;
        if (prevSibling === null) {
          throw new Error('Expected to find the previous sibling.');
        }
      }
      prevSibling.sibling = newWorkInProgress;
    }
    
    
    const deletions = returnFiber.deletions;
    if (deletions === null) {
      returnFiber.deletions = [current];