Attempting to link to extern C code in this simple rust echo program

main.rs

extern crate rustyline;

use rustyline::error::ReadlineError;
use rustyline::Editor;

#[link(name = "mpc")]
extern {
    fn mpc_new(s: &str) ->  *mut mpc_parser_t;
}
fn main() {
    let mut rl = Editor::<()>::new();
    loop {
        let readline = rl.readline("lispy >> ");
        match readline {
            Ok(line) => {
                if line == "exit" || line == "quit" {
                    break
                }
                rl.add_history_entry(&line);
                println!("{}", line);
            },
            Err(ReadlineError::Interrupted) => {
                println!("CTRL-C");
                break
            },
            Err(ReadlineError::Eof) => {
                println!("CTRL-D");
                break
            },
            Err(err) => {
                println!("Error: {:?}", err);
                break
            }
        }
    }
}

mpc.c and mpc.h are found here:
https://github.com/orangeduck/mpc