rust-chg: have attach_io() simply take reference of AsRawFd object We no longer have to deal with the restriction of the Future type. Before, these file objects couldn't be references and that's the only reason why we had to make stderr an Option<T>. This fixes future type deduction issue of stderr = None, where rustc would complain that T of Option<T> couldn't be deduced. Differential Revision: https://phab.mercurial-scm.org/D8443