demo.rs

// Instead of this fuss:

let prefix = Cursor::new("foo".to_owned().into_bytes());
let data = Cursor::new((...).into_bytes()); // Some big Vec/String
let body: Box<Read + Send> = Box::new(prefix.chain(data));

// We can now just write:

let prefix = "foo";
let data = ...; // Some big Vec/String
let body = chain_body(prefix, data);

iron_body.rs

use std::io::{self,Read};
use std::borrow::Borrow;
use iron::response::{Response,WriteBody,ResponseBody};
use iron::modifier::Modifier;

pub struct BodyChain<T: WriteBody + Send, U: WriteBody + Send>(pub T, pub U);

impl <T: WriteBody + Send, U: WriteBody + Send> WriteBody for BodyChain<T, U> {
    fn write_body(&mut self, res: &mut ResponseBody) -> io::Result<()> {
        let &mut BodyChain(ref mut a, ref mut b) = self;
        a.write_body(res).and_then(|_| b.write_body(res))
    }
}

impl <T: WriteBody + Send + 'static, U: WriteBody + Send + 'static> Modifier<Response> for BodyChain<T,U> {
    fn modify(self, res: &mut Response) {
        res.body = Some(Box::new(self));
    }
}