This used to work..

config.rs

use rustc_serialize::Encodable;
use toml;

#[derive(RustcEncodable, Clone)]
pub struct Config {
    pub server: Option<String>,
    pub user: Option<String>,
    pub enterprise: Option<String>,
    pub organization: Option<String>,
    pub project: Option<String>,
    pub git_port: Option<String>,
    pub pipeline: Option<String>
}

impl Config {
    pub fn write_file(&self, path: &Path) -> Result<(), DeliveryError> {
        let write_dir = path.join_many(&[".delivery"]);
        if !write_dir.is_dir() {
            try!(mkdir_recursive(&write_dir, old_io::USER_RWX));
        }
        let write_path = path.join_many(&[".delivery", "cli.toml"]);
        say("white", "Writing configuration to ");
        sayln("yellow", &format!("{}", write_path.display()));
        let mut f = try!(File::create(&write_path));
        let toml_string = toml::encode_str(self);
        sayln("magenta", "New configuration");
        sayln("magenta", "-----------------");
        say("white", &toml_string);
        try!(f.write_all(toml_string.as_bytes()));
        Ok(())
    }
}

// It now fails with:
// src/delivery/config/mod.rs:88:27: 88:43 error: the trait `rustc-serialize::serialize::Encodable` is not implemented for the type `config::Config` [E0277]
// src/delivery/config/mod.rs:88         let toml_string = toml::encode_str(self);