Conditional compilation fails if enum variants use `from()`

[yawor]

Hi,
I've stumbled upon an error when I've tried to add conditional compilation to my quick_error defined Error enum. Here's an example:

quick_error! {
    #[derive(Debug)]
    pub enum Error {
        Io(err: std::io::Error) {
            cause(err)
            from()
            display("I/O error: {}", err)
        }
        #[cfg(feature="ssh2")]
        Ssh(err: ssh2::Error) {
            cause(err)
            from()
            display("SSH error: {}", err)
        }
        Net(err: std::net::AddrParseError) {
            cause(err)
            from()
            display("Network error: {}", err)
        }
        Zmq(err: zmq::Error) {
            cause(err)
            from()
            display("ZMQ error: {}", err)
        }
        Other(desc: String) {
            description(desc)
            from(desc: &str) -> (String::from(desc))
            from(err: Box<dyn std::error::Error>) -> (String::from(err.description()))
        }
    }
}

If I remove the from() from Ssh variant, then it compiles correctly. When I'm expanding the macro, I can see the #[cfg(feature="ssh2")] everywhere except for the

impl From<ssh2::Error> for Error {
    fn from(err: ssh2::Error) -> Error {
        Error::Ssh(err)
    }
}

[jmjoy]

I meet the same problem.