In our tool, we will implement two subcommands—add to add a new user, and list to retrieve all available users from the database. Import all the necessary dependencies and add the modules with schema and models:

extern crate clap;
#[macro_use]
extern crate diesel;
extern crate failure;
extern crate serde_derive;

use clap::{
    crate_authors, crate_description, crate_name, crate_version,
    App, AppSettings, Arg, SubCommand,
};
use diesel::prelude::*;
use diesel::r2d2::ConnectionManager;
use failure::Error;

pub mod models;
pub mod schema;

Since we are using the r2d2 crate, we also have to import ConnectionManager to use diesel's abstraction over the traditional database connection.