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: postgres_benches.rs
    path of file: ./repos/diesel/diesel_bench/benches
    
    the code of the file until where you have to start completion: use super::Bencher;
use rust_postgres::fallible_iterator::FallibleIterator;
use rust_postgres::types::ToSql;
use rust_postgres::{Client, NoTls};
use std::collections::HashMap;

const NO_PARAMS: Vec<&dyn ToSql> = Vec::new();

pub struct User {
    pub id: i32,
    pub name: String,
    pub hair_color: Option<String>,
}

pub struct Post {
    pub id: i32,
    pub user_id: i32,
    pub title: String,
    pub body: Option<String>,
}

pub struct Comment {
    pub id: i32,
    pub post_id: i32,
    pub text: String,
}

fn connection() -> Client {
    dotenvy::dotenv().ok();
    let connection_url = dotenvy::var("POSTGRES_DATABASE_URL")
        .or_else(|_| dotenvy::var("DATABASE_URL"))
        .expect("DATABASE_URL must be set in order to run tests");
    let mut client = Client::connect(&connection_url, NoTls).unwrap();

    client
        .execute("TRUNCATE TABLE comments CASCADE", &[])
        .unwrap();
    client.execute("TRUNCATE TABLE posts CASCADE", &[]).unwrap();
    client.execute("TRUNCATE TABLE users CASCADE", &[]).unwrap();

    client
}

fn insert_users(size: usize, conn: &mut Client, hair_color_init: impl Fn(usize) -> Option<String>) {
    let mut query = String::from("INSERT INTO users (name, hair_color) VALUES");

    let mut params = Vec::with_capacity(2 * size);

    for x in 0..size {
        query += &format!(
            "{} (${}, ${})",
            if x == 0 { "" } else { "," },
            2 * x + 1,
      
    
pub fn loading_associations_sequentially(b: &mut Bencher) {
    let mut client = connection();

    insert_users(100, &mut client, |i| {
        Some(if i % 2 == 0 {
            String::from("black")
        } else {
            String::from("brown")
        })
    });

    let user_ids = client
        .query_raw("SELECT id FROM users", NO_PARAMS)
        .unwrap()
        .map(|row| Ok(row.get("id")))
        .collect::<Vec<i32>>()
        .unwrap();

    let data = user_ids
        .iter()
        .flat_map(|user_id| {
            (0..10).map(move |i| (format!("Post {} by user {}", i, user_id), user_id, None))
        })
        .collect::<Vec<_>>();

    let mut insert_query = String::from("INSERT INTO posts(title, user_id, body) VALUES");

    for x in 0..data.len() {
        insert_query += &format!(
            "{} (${}, ${}, ${})",
            if x == 0 { "" } else { "," },
            3 * x + 1,
            3 * x + 2,
            3 * x + 3
        );
    }

    let data = data
        .iter()
        .flat_map(|(title, user_id, body): &(_, _, Option<String>)| {
            vec![title as &(dyn ToSql + Sync), user_id as _, body as _]
        })
        .collect::<Vec<_>>();

    client.execute(&insert_query as &str, &data).unwrap();

    let all_posts = client
        .query_raw("SELECT id FROM posts", NO_PARAMS)
        .unwrap()
        .map(|row| Ok(row.get("id")))
        .collect::<Vec<i32>>()
        .unwrap();

    let data = all_posts
        .iter()
        .flat_map(|post_id| {
            (0..10).map(move |i| (format!("Comment {} on post {}", i, post_id), post_id))
        })
        .collect::<Vec<_>>();

    let mut insert_query = String::from("INSERT INTO comments(text, post_id) VALUES");

    for x in 0..data.len() {
        insert_query += &format!(
            "{} (${}, ${})",
            if x == 0 { "" } else { "," },
            2 * x + 1,
            2 * x + 2,
        );
    }

    let data = data
        .iter()
        .flat_map(|(title, post_id)| vec![title as _, post_id as _])
        .collect::<Vec<_>>();

    client.execute(&insert_query as &str, &data).unwrap();

    let user_query = client
        .prepare("SELECT id, name, hair_color FROM users")
        .unwrap();

    b.iter(|| {
        let users = client
            .query_raw(&user_query, NO_PARAMS)
            .unwrap()
            .map(|row| {
                Ok(User {
                    id: row.get("id"),
                    name: row.get("name"),
                    hair_color: row.get("hair_color"),
                })
            })
            .collect::<Vec<_>>()
            .unwrap();

        let mut posts_query =
            String::from("SELECT id, title, user_id, body FROM posts WHERE user_id IN(");

        let user_ids = users
            .iter()
            .enumerate()
            .map(|(i, &User { id, .. })| {
                posts_query += &format!("{}${}", if i == 0 { "" } else { "," }, i + 1);
                id
            })
            .collect::<Vec<i32>>();

        posts_query += ")";

        let posts = client
            .query_raw(&posts_query as &str, user_ids)
            .unwrap()
            .map(|row| {
                Ok(Post {
                    id: row.get("id"),
                    user_id: row.get("user_id"),
                    title: row.get("title"),
                    body: row.get("body"),
                })
            })
            .collect::<Vec<_>>()
            .unwrap();

        let mut comments_query =
            String::from("SELECT id, post_id, text FROM comments WHERE post_id IN(");

        let post_ids = posts
            .iter()
            .enumerate()
            .map(|(i, &Post { id, .. })| {
                comments_query += &format!("{}${}", if i == 0 { "" } else { "," }, i + 1);
                id
            })
            .collect::<Vec<i32>>();

        comments_query += ")";

        let comments = client
            .query_raw(&comments_query as &str, post_ids)
            .unwrap()
            .map(|row| {
                Ok(Comment {
                    id: row.get("id"),
                    post_id: row.get("post_id"),
                    text: row.get("text"),
                })
            })
            .collect::<Vec<_>>()
            .unwrap();

        let mut posts = posts
            .into_iter()
            .map(|p| (p.id, (p, Vec::new())))
            .collect::<HashMap<_, _>>();

        let mut users = users
            .into_iter()
            .map(|u| (u.id, (u, Vec::new())))
            .collect::<HashMap<_, _>>();

        for comment in comments {
            posts.get_mut(&comment.post_id).unwrap().1.push(comment);
        }

        for (_, post_with_comments) in posts {
            users
                .get_mut(&post_with_comments.0.user_id)
                .unwrap()
                .1
                .push(post_with_comments);
        }

        users
            .into_iter()
            .map(|(_, users_with_post_and_comment)| users_with_post_and_comment)
            .collect::<Vec<(User, Vec<(Post, Vec<Comment>)>)>>()
    })
}