impl<E> ErrorImpl<E> {
    fn erase(&self) -> &ErrorImpl<()> {
        unsafe { &*(self as *const ErrorImpl<E> as *const ErrorImpl<()>) }
    }
}

impl<E> StdError for ErrorImpl<E> where E: StdError {}

impl<E> StdError for ErrorImpl<E>
where
    E: StdError,
{
    #[cfg(backtrace)]
    fn backtrace(&self) -> Option<&Backtrace> {
        Some(self.erase().backtrace())
    }
    fn source(&self) -> Option<&(dyn StdError + 'static)> {
        self.error.source()
    }
}

        let erased = unsafe { &*(self as *const ErrorImpl<E> as *const ErrorImpl<()>) };
        erased.debug(formatter)

        self.erase().debug(formatter)