what to do with the 'impl <T> Payload<T>'?

gistfile1.rs

extern crate "rustc-serialize" as rustc_serialize;

use rustc_serialize::json;
use rustc_serialize::Encodable;
use rustc_serialize::json::Encoder;
use std::io::IoError;

#[deriving(RustcEncodable)]
struct Data {
    str: &'static str,
    int: i32,
}

#[deriving(RustcEncodable)]
struct Payload<T> {
    timestamp: u32,
    data: T,
}

impl <'a, T: Encodable<Encoder<'a>, IoError>> Payload<T> {
    fn json_encode(&self) -> String {
        json::encode(self)
    }
}

fn main() {
    let data = Data { str: "hello world", int: 42 };
    let payload = Payload { timestamp: 0, data: data };
    let json = payload.json_encode();
    println!("{}", json);
}