fn with_connector<C: NetworkConnector>(connector: &mut C) -> HttpResult<Request> {
    let stream = try!(connector.connect(host, port, scheme));
    let stream = BufferedWriter::new(box stream as Box<NetworkStream + Send>);
    //                               ^~~ parameter `C` may not live long enough, consider an explicit lifetime 'static
}

pub trait NetworkConnector {
    type Stream: NetworkStream + Send;

    pub fn connect(...) -> IoResult<Self::Stream>;
}

fn with_connector<'a, C: NetworkConnector<Stream = T>, T: NetworkStream+'a>(connector: &C) -> Result<Box<NetworkStream + 'a>, ()> {
    let stream = try!(connector.connect());
    Ok(box stream as Box<NetworkStream + 'a>)
}

trait NetworkStream {}

pub trait NetworkConnector {
    type Stream: NetworkStream;

    fn connect(&self) -> Result<Self::Stream, ()>;
}