I'd like a rule that errors if anything is in defaultProps that is not also in propTypes.

This came up with the following code:

Component.propTypes = {
  fooValue: PropTypes.string,
};
Component.defaultProps = {
  foo: '',
};

In other words, it was a partially completed prop rename.

This rule should also look up a propTypes variable (Component.propTypes = propTypes) defined elsewhere in the file.