Rust programming expert

Rust Programming Expert (2024 Edition / v1.85+)

Production Rust patterns: memory safety, async-first, web backends, CLI, and performance.

---

Project Setup

# New project
cargo new my-app --bin
cargo new my-lib --lib

# Essential Cargo.toml
[package]
name = "my-app"
version = "0.1.0"
edition = "2024"

[dependencies]
tokio = { version = "1", features = ["full"] }
axum = "0.8"
serde = { version = "1", features = ["derive"] }
serde_json = "1"
sqlx = { version = "0.8", features = ["postgres", "runtime-tokio", "uuid", "chrono"] }
anyhow = "1"
thiserror = "2"
tracing = "0.1"
tracing-subscriber = { version = "0.3", features = ["env-filter"] }
uuid = { version = "1", features = ["v4", "serde"] }
chrono = { version = "0.4", features = ["serde"] }

[dev-dependencies]
tokio-test = "0.4"

---

Ownership & Borrowing Patterns

Common Ownership Patterns

// Clone when you need owned data in multiple places
fn process(data: Vec<String>) -> Vec<String> {
    let upper: Vec<String> = data.iter().map(|s| s.to_uppercase()).collect();
    upper // data dropped here
}

// Borrow when you only need to read
fn count_long(items: &[String], min_len: usize) -> usize {
    items.iter().filter(|s| s.len() >= min_len).count()
}

// Cow<str> for "borrow or own" flexibility
use std::borrow::Cow;
fn sanitize(input: &str) -> Cow<str> {
    if input.contains('<') {
        Cow::Owned(input.replace('<', "&lt;"))
    } else {
        Cow::Borrowed(input) // zero allocation when no change needed
    }
}

Lifetime Annotations

// Named lifetime: output lives as long as input
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

// Struct holding a reference
struct Parser<'a> {
    input: &'a str,
    pos: usize,
}

impl<'a> Parser<'a> {
    fn new(input: &'a str) -> Self {
        Self { input, pos: 0 }
    }

    fn remaining(&self) -> &'a str {
        &self.input[self.pos..]
    }
}

---

Error Handling

thiserror for Library Errors

use thiserror::Error;

#[derive(Error, Debug)]
pub enum AppError {
    #[error("Not found: {0}")]
    NotFound(String),

    #[error("Unauthorized")]
    Unauthorized,

    #[error("Validation failed: {field} — {message}")]
    Validation { field: String, message: String },

    #[error("Database error")]
    Database(#[from] sqlx::Error),

    #[error("IO error")]
    Io(#[from] std::io::Error),
}

// Result alias
pub type Result<T> = std::result::Result<T, AppError>;

anyhow for Application Code

use anyhow::{Context, Result, bail, ensure};

async fn load_config(path: &str) -> Result<Config> {
    let content = tokio::fs::read_to_string(path)
        .await
        .with_context(|| format!("Failed to read config file: {path}"))?;

    let config: Config = serde_json::from_str(&content)
        .context("Config file is not valid JSON")?;

    ensure!(config.port > 1024, "Port must be > 1024, got {}", config.port);

    Ok(config)
}

Axum Error Response

use axum::{http::StatusCode, response::{IntoResponse, Response}, Json};
use serde_json::json;

impl IntoResponse for AppError {
    fn into_response(self) -> Response {
        let (status, message) = match &self {
            AppError::NotFound(msg) => (StatusCode::NOT_FOUND, msg.clone()),
            AppError::Unauthorized => (StatusCode::UNAUTHORIZED, "Unauthorized".into()),
            AppError::Validation { field, message } => (
                StatusCode::UNPROCESSABLE_ENTITY,
                format!("{field}: {message}"),
            ),
            AppError::Database(_) | AppError::Io(_) => {
                tracing::error!(error = ?self, "Internal error");
                (StatusCode::INTERNAL_SERVER_ERROR, "Internal server error".into())
            }
        };
        (status, Json(json!({ "error": message }))).into_response()
    }
}

---

Async with Tokio

Main Entry Point

#[tokio::main]
async fn main() -> anyhow::Result<()> {
    tracing_subscriber::fmt()
        .with_env_filter(tracing_subscriber::EnvFilter::from_default_env())
        .init();

    let config = load_config("config.json").await?;
    let app = build_app(config).await?;

    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await?;
    tracing::info!("Listening on {}", listener.local_addr()?);
    axum::serve(listener, app).await?;

    Ok(())
}

Concurrent Tasks

use tokio::task::JoinSet;

async fn fetch_all(ids: Vec<String>) -> Vec<Result<Data>> {
    let mut set = JoinSet::new();

    for id in ids {
        set.spawn(async move { fetch_one(&id).await });
    }

    let mut results = Vec::new();
    while let Some(res) = set.join_next().await {
        results.push(res.expect("task panicked"));
    }
    results
}

// Or use join! for fixed set of futures
async fn fetch_dashboard(user_id: &str) -> Result<Dashboard> {
    let (user, posts, stats) = tokio::try_join!(
        fetch_user(user_id),
        fetch_posts(user_id),
        fetch_stats(user_id),
    )?;
    Ok(Dashboard { user, posts, stats })
}

---

Axum Web Backend

App Setup with State

use axum::{Router, extract::State, routing::{get, post}};
use std::sync::Arc;

#[derive(Clone)]
pub struct AppState {
    pub db: sqlx::PgPool,
    pub config: Arc<Config>,
}

pub fn build_router(state: AppState) -> Router {
    Router::new()
        .route("/health", get(health_check))
        .nest("/api/v1", api_routes())
        .with_state(state)
        .layer(
            tower::ServiceBuilder::new()
                .layer(tower_http::trace::TraceLayer::new_for_http())
                .layer(tower_http::cors::CorsLayer::permissive())
        )
}

fn api_routes() -> Router<AppState> {
    Router::new()
        .route("/posts", get(list_posts).post(create_post))
        .route("/posts/:id", get(get_post).delete(delete_post))
}

Handlers

use axum::{
    extract::{Path, Query, State, Json},
    http::StatusCode,
};
use serde::{Deserialize, Serialize};
use uuid::Uuid;

#[derive(Deserialize)]
pub struct ListQuery {
    page: Option<u32>,
    limit: Option<u32>,
}

#[derive(Serialize)]
pub struct PostResponse {
    id: Uuid,
    title: String,
    content: Option<String>,
}

pub async fn list_posts(
    State(state): State<AppState>,
    Query(query): Query<ListQuery>,
) -> Result<Json<Vec<PostResponse>>, AppError> {
    let page = query.page.unwrap_or(1).max(1);
    let limit = query.limit.unwrap_or(10).min(100);
    let offset = (page - 1) * limit;

    let posts = sqlx::query_as!(
        PostResponse,
        "SELECT id, title, content FROM posts WHERE published = true
         ORDER BY created_at DESC LIMIT $1 OFFSET $2",
        limit as i64,
        offset as i64,
    )
    .fetch_all(&state.db)
    .await?;

    Ok(Json(posts))
}

#[derive(Deserialize)]
pub struct CreatePost {
    title: String,
    content: Option<String>,
}

pub async fn create_post(
    State(state): State<AppState>,
    Json(body): Json<CreatePost>,
) -> Result<(StatusCode, Json<PostResponse>), AppError> {
    if body.title.trim().is_empty() {
        return Err(AppError::Validation {
            field: "title".into(),
            message: "must not be empty".into(),
        });
    }

    let post = sqlx::query_as!(
        PostResponse,
        "INSERT INTO posts (title, content) VALUES ($1, $2) RETURNING id, title, content",
        body.title,
        body.content,
    )
    .fetch_one(&state.db)
    .await?;

    Ok((StatusCode::CREATED, Json(post)))
}

---

CLI with Clap

use clap::{Parser, Subcommand};

#[derive(Parser)]
#[command(name = "myapp", about = "My CLI tool", version)]
pub struct Cli {
    #[arg(short, long, default_value = "info")]
    pub log_level: String,

    #[command(subcommand)]
    pub command: Command,
}

#[derive(Subcommand)]
pub enum Command {
    /// Start the server
    Serve {
        #[arg(short, long, default_value = "3000")]
        port: u16,
    },
    /// Run database migrations
    Migrate,
    /// Generate a secret key
    GenKey,
}

#[tokio::main]
async fn main() -> anyhow::Result<()> {
    let cli = Cli::parse();
    match cli.command {
        Command::Serve { port } => serve(port).await?,
        Command::Migrate => migrate().await?,
        Command::GenKey => println!("{}", generate_key()),
    }
    Ok(())
}

---

Performance Tips

// Pre-allocate when size is known
let mut vec = Vec::with_capacity(items.len());

// Use &str instead of String in hot paths
fn is_valid(s: &str) -> bool { /* no allocation */ }

// Avoid unnecessary clones — use references
fn process(items: &[Item]) -> usize { items.len() }

// Use rayon for CPU-bound parallelism
use rayon::prelude::*;
let sum: u64 = big_vec.par_iter().map(|x| expensive(x)).sum();

// Use bytes::Bytes for zero-copy HTTP bodies
use bytes::Bytes;
async fn handler() -> Bytes {
    Bytes::from_static(b"hello")
}

// Profile with cargo-flamegraph
// cargo install flamegraph
// cargo flamegraph --bin my-app

---

Key Rules

1. Prefer Result<T, E> over unwrap() — only unwrap() in tests or truly infallible
2. thiserror for libraries, anyhow for binaries
3. Arc<T> for shared state, Mutex<T> only when mutation needed
4. tokio::spawn for fire-and-forget, JoinSet for tracked tasks
5. sqlx::query_as! macro — compile-time query validation
6. Never block in async context — use tokio::task::spawn_blocking for CPU work
7. tracing not println! — structured logging always
8. Clippy is not optional — cargo clippy -- -D warnings in CI
9. #[derive(Clone)] on state — Axum requires Clone on shared state
10. Edition 2024 in Cargo.toml — use latest language features