You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: plugin.rs
    path of file: ./repos/rage/age-core/src
    
    the code of the file until where you have to start completion: //! Common structs and constants for the age plugin system.
//!
//! These are shared between the client implementation in the `age` crate, and the plugin
//! implementations built around the `age-plugin` crate.

use rand::{thread_rng, Rng};
use secrecy::Zeroize;
use std::env;
use std::fmt;
use std::io::{self, BufRead, BufReader, Read, Write};
use std::iter;
use std::path::Path;
use std::process::{ChildStdin, ChildStdout, Command, Stdio};

use crate::{
    format::{grease_the_joint, read, write, Stanza},
    io::{DebugReader, DebugWriter},
};

pub const IDENTITY_V1: &str = "identity-v1";
pub const RECIPIENT_V1: &str = "recipient-v1";

const COMMAND_DONE: &str = "done";
const RESPONSE_OK: &str = "ok";
const RESPONSE_FAIL: &str = "fail";
const RESPONSE_UNSUPPORTED: &str = "unsupported";

/// An error within the plugin protocol.
#[derive(Debug)]
pub enum Error {
    Fail,
    Unsupported,
}

impl fmt::Display for Error {
    fn fmt(&sel
    
    fn mock_plugin() {
        let client_to_plugin = Pipe::new();
        let plugin_to_client = Pipe::new();

        let mut client_conn = Connection {
            input: BufReader::new(PipeReader::new(plugin_to_client.clone())),
            output: PipeWriter::new(client_to_plugin.clone()),
            buffer: String::new(),
            _working_dir: None,
        };
        let mut plugin_conn = Connection {
            input: BufReader::new(PipeReader::new(client_to_plugin)),
            output: PipeWriter::new(plugin_to_client),
            buffer: String::new(),
            _working_dir: None,
        };

        client_conn
            .unidir_send(|mut phase| phase.send("test", &["foo"], b"bar"))
            .unwrap();
        let stanza = plugin_conn
            .unidir_receive::<_, (), (), _, _, _, _>(
                ("test", Ok),
                ("other", |_| Err(())),
                (None, |_| Ok(())),
            )
            .unwrap();
        assert_eq!(
            stanza,
            (
                Ok(vec![Stanza {
                    tag: "test".to_owned(),
                    args: vec!["foo".to_owned()],
                    body: b"bar"[..].to_owned()
                }]),
                Ok(vec![]),
                None
            )
        );
    }