This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
_current,
  workInProgress,
  Component,
  renderLanes,
) {
  if (_current !== null) {
    
    
    
    
    _current.alternate = null;
    workInProgress.alternate = null;
    
    workInProgress.flags |= Placement;
  }
  const props = workInProgress.pendingProps;
  let context;
  if (!disableLegacyContext) {
    const unmaskedContext = getUnmaskedContext(
      workInProgress,
      Component,
      false,
    );
    context = getMaskedContext(workInProgress, unmaskedContext);
  }
  prepareToReadContext(workInProgress, renderLanes);
  let value;
  if (__DEV__) {