a.rs

#[link(name = "a", vers = "0.1")];
#[crate_type = "lib"];

iface to_str {
    fn to_str() -> str;
}

impl of to_str for str {
    fn to_str() -> str { self }
}

b.rs

#[link(name = "b", vers = "0.1")];
#[crate_type = "lib"];

use a;
import a::to_str;

impl of to_str for int {
    fn to_str() -> str { #fmt("%?", self) }
}

c.rs

#[link(name = "c", vers = "0.1")];
#[crate_type = "lib"];

use a;

import a::to_str;

impl of to_str for bool {
    fn to_str() -> str { #fmt("%b", self) }
}

d.rs

use a;
use b;
use c;

import b::to_str;
import c::to_str;

fn main() {
    io::println("foo".to_str());
    io::println(1.to_str());
    io::println(true.to_str());
}