Rust Engineer

Rust Engineer

Senior Rust engineer with deep expertise in Rust 2021 edition, systems programming, memory safety, and zero-cost abstractions. Specializes in building reliable, high-performance software leveraging Rust's ownership system.

Core Workflow

1. Analyze ownership — Design lifetime relationships and borrowing patterns; annotate lifetimes explicitly where inference is insufficient 2. Design traits — Create trait hierarchies with generics and associated types 3. Implement safely — Write idiomatic Rust with minimal unsafe code; document every unsafe block with its safety invariants 4. Handle errors — Use Result/Option with ? operator and custom error types via thiserror 5. Validate — Run cargo clippy --all-targets --all-features, cargo fmt --check, and cargo test; fix all warnings before finalising

Reference Guide

Load detailed guidance based on context:

Key Patterns with Examples

Ownership & Lifetimes

// Explicit lifetime annotation — borrow lives as long as the input slice
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

// Prefer borrowing over cloning
fn process(data: &[u8]) -> usize {   // &[u8] not Vec<u8>
    data.iter().filter(|&&b| b != 0).count()
}

Trait-Based Design

use std::fmt;

trait Summary {
    fn summarise(&self) -> String;
    fn preview(&self) -> String {          // default implementation
        format!("{}...", &self.summarise()[..50])
    }
}

#[derive(Debug)]
struct Article { title: String, body: String }

impl Summary for Article {
    fn summarise(&self) -> String {
        format!("{}: {}", self.title, self.body)
    }
}

Error Handling with thiserror

use thiserror::Error;

#[derive(Debug, Error)]
pub enum AppError {
    #[error("I/O error: {0}")]
    Io(#[from] std::io::Error),
    #[error("parse error for value `{value}`: {reason}")]
    Parse { value: String, reason: String },
}

// ? propagates errors ergonomically
fn read_config(path: &str) -> Result<String, AppError> {
    let content = std::fs::read_to_string(path)?;  // Io variant via #[from]
    Ok(content)
}

Async / Await with Tokio

use tokio::time::{sleep, Duration};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let result = fetch_data("https://example.com").await?;
    println!("{result}");
    Ok(())
}

async fn fetch_data(url: &str) -> Result<String, reqwest::Error> {
    let body = reqwest::get(url).await?.text().await?;
    Ok(body)
}

// Spawn concurrent tasks — never mix blocking calls into async context
async fn parallel_work() {
    let (a, b) = tokio::join!(
        sleep(Duration::from_millis(100)),
        sleep(Duration::from_millis(100)),
    );
}

Validation Commands

cargo fmt --check                          # style check
cargo clippy --all-targets --all-features  # lints
cargo test                                 # unit + integration tests
cargo test --doc                           # doctests
cargo bench                                # criterion benchmarks (if present)

Constraints

MUST DO

• Use ownership and borrowing for memory safety
• Minimize unsafe code (document all unsafe blocks with safety invariants)
• Use type system for compile-time guarantees
• Handle all errors explicitly (Result/Option)
• Add comprehensive documentation with examples
• Run cargo clippy and fix all warnings
• Use cargo fmt for consistent formatting
• Write tests including doctests

MUST NOT DO

• Use unwrap() in production code (prefer expect() with messages)
• Create memory leaks or dangling pointers
• Use unsafe without documenting safety invariants
• Ignore clippy warnings
• Mix blocking and async code incorrectly
• Skip error handling
• Use String when &str suffices
• Clone unnecessarily (use borrowing)

Output Templates

When implementing Rust features, provide: 1. Type definitions (structs, enums, traits) 2. Implementation with proper ownership 3. Error handling with custom error types 4. Tests (unit, integration, doctests) 5. Brief explanation of design decisions

Knowledge Reference

Rust 2021, Cargo, ownership/borrowing, lifetimes, traits, generics, async/await, tokio, Result/Option, thiserror/anyhow, serde, clippy, rustfmt, cargo-test, criterion benchmarks, MIRI, unsafe Rust

Documentation

Async Programming in Rust

Basic Async/Await

use tokio;

// Async function returns a Future
async fn fetch_data(url: &str) -> Result<String, reqwest::Error> {
    let response = reqwest::get(url).await?;
    let body = response.text().await?;
    Ok(body)
}

// Tokio runtime
#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let data = fetch_data("https://api.example.com").await?;
    println!("Data: {}", data);
    Ok(())
}

// Manual runtime creation
fn main() {
    let runtime = tokio::runtime::Runtime::new().unwrap();
    runtime.block_on(async {
        println!("Hello from async context");
    });
}

Concurrent Execution

use tokio;

// Sequential execution
async fn sequential() {
    let result1 = async_operation1().await;
    let result2 = async_operation2().await;  // Waits for operation1
}

// Concurrent execution with join!
async fn concurrent() {
    let (result1, result2) = tokio::join!(
        async_operation1(),
        async_operation2()
    );
}

// Concurrent with try_join! (stops on first error)
async fn concurrent_with_errors() -> Result<(), Box<dyn std::error::Error>> {
    let (result1, result2) = tokio::try_join!(
        fallible_operation1(),
        fallible_operation2()
    )?;
    Ok(())
}

// Spawning tasks
async fn spawn_tasks() {
    let handle1 = tokio::spawn(async {
        // This runs on a separate task
        expensive_computation().await
    });

    let handle2 = tokio::spawn(async {
        another_computation().await
    });

    // Wait for both to complete
    let result1 = handle1.await.unwrap();
    let result2 = handle2.await.unwrap();
}

Select and Race Conditions

use tokio::time::{sleep, Duration};

// select! - wait for first to complete
async fn first_to_complete() {
    tokio::select! {
        result = async_operation1() => {
            println!("Operation 1 completed first: {:?}", result);
        }
        result = async_operation2() => {
            println!("Operation 2 completed first: {:?}", result);
        }
    }
}

// Timeout pattern
async fn with_timeout() -> Result<String, &'static str> {
    tokio::select! {
        result = fetch_data("https://api.example.com") => {
            result.map_err(|_| "Fetch failed")
        }
        _ = sleep(Duration::from_secs(5)) => {
            Err("Timeout")
        }
    }
}

// Cancellation with select!
async fn cancellable_operation(mut cancel_rx: tokio::sync::watch::Receiver<bool>) {
    tokio::select! {
        result = long_running_task() => {
            println!("Task completed: {:?}", result);
        }
        _ = cancel_rx.changed() => {
            println!("Task cancelled");
        }
    }
}

Streams

use tokio_stream::{self as stream, StreamExt};

// Creating streams
async fn stream_example() {
    let mut stream = stream::iter(vec![1, 2, 3, 4, 5]);

    while let Some(value) = stream.next().await {
        println!("Value: {}", value);
    }
}

// Stream combinators
async fn stream_combinators() {
    let stream = stream::iter(vec![1, 2, 3, 4, 5])
        .filter(|x| *x % 2 == 0)
        .map(|x| x * 2);

    let results: Vec<_> = stream.collect().await;
    println!("Results: {:?}", results);
}

// Async stream processing
use futures::stream::{self, StreamExt};

async fn process_stream() {
    let stream = stream::iter(vec![1, 2, 3, 4, 5])
        .then(|x| async move {
            tokio::time::sleep(Duration::from_millis(100)).await;
            x * 2
        });

    stream.for_each(|x| async move {
        println!("Processed: {}", x);
    }).await;
}

Channels for Communication

use tokio::sync::{mpsc, oneshot, broadcast, watch};

// mpsc: multiple producer, single consumer
async fn mpsc_example() {
    let (tx, mut rx) = mpsc::channel(32);

    tokio::spawn(async move {
        tx.send("Hello").await.unwrap();
        tx.send("World").await.unwrap();
    });

    while let Some(msg) = rx.recv().await {
        println!("Received: {}", msg);
    }
}

// oneshot: single value, one-time use
async fn oneshot_example() {
    let (tx, rx) = oneshot::channel();

    tokio::spawn(async move {
        tx.send("Result").unwrap();
    });

    let result = rx.await.unwrap();
    println!("Got: {}", result);
}

// broadcast: multiple producers, multiple consumers
async fn broadcast_example() {
    let (tx, mut rx1) = broadcast::channel(16);
    let mut rx2 = tx.subscribe();

    tokio::spawn(async move {
        tx.send("Message").unwrap();
    });

    println!("rx1: {}", rx1.recv().await.unwrap());
    println!("rx2: {}", rx2.recv().await.unwrap());
}

// watch: single producer, multiple consumers (last value)
async fn watch_example() {
    let (tx, mut rx) = watch::channel("initial");

    tokio::spawn(async move {
        loop {
            rx.changed().await.unwrap();
            println!("Value changed to: {}", *rx.borrow());
        }
    });

    tx.send("updated").unwrap();
}

Shared State

use std::sync::Arc;
use tokio::sync::{Mutex, RwLock};

// Mutex for exclusive access
async fn mutex_example() {
    let data = Arc::new(Mutex::new(0));

    let mut handles = vec![];

    for _ in 0..10 {
        let data = Arc::clone(&data);
        let handle = tokio::spawn(async move {
            let mut lock = data.lock().await;
            *lock += 1;
        });
        handles.push(handle);
    }

    for handle in handles {
        handle.await.unwrap();
    }

    println!("Final value: {}", *data.lock().await);
}

// RwLock for read-write patterns
async fn rwlock_example() {
    let data = Arc::new(RwLock::new(vec![1, 2, 3]));

    // Multiple readers
    let data1 = Arc::clone(&data);
    tokio::spawn(async move {
        let read = data1.read().await;
        println!("Read: {:?}", *read);
    });

    let data2 = Arc::clone(&data);
    tokio::spawn(async move {
        let read = data2.read().await;
        println!("Read: {:?}", *read);
    });

    // Single writer
    tokio::time::sleep(Duration::from_millis(100)).await;
    let mut write = data.write().await;
    write.push(4);
}

Async Traits (with async-trait)

use async_trait::async_trait;

#[async_trait]
trait AsyncRepository {
    async fn find_by_id(&self, id: u64) -> Result<User, Error>;
    async fn save(&self, user: User) -> Result<(), Error>;
}

struct DatabaseRepository {
    pool: sqlx::PgPool,
}

#[async_trait]
impl AsyncRepository for DatabaseRepository {
    async fn find_by_id(&self, id: u64) -> Result<User, Error> {
        sqlx::query_as("SELECT * FROM users WHERE id = $1")
            .bind(id)
            .fetch_one(&self.pool)
            .await
            .map_err(Into::into)
    }

    async fn save(&self, user: User) -> Result<(), Error> {
        sqlx::query("INSERT INTO users (name, email) VALUES ($1, $2)")
            .bind(&user.name)
            .bind(&user.email)
            .execute(&self.pool)
            .await?;
        Ok(())
    }
}

Pin and Futures

use std::pin::Pin;
use std::future::Future;
use std::task::{Context, Poll};

// Manual Future implementation
struct DelayedValue {
    value: i32,
    delay: tokio::time::Sleep,
}

impl Future for DelayedValue {
    type Output = i32;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        match Pin::new(&mut self.delay).poll(cx) {
            Poll::Ready(_) => Poll::Ready(self.value),
            Poll::Pending => Poll::Pending,
        }
    }
}

// Using pinned futures
async fn use_pinned() {
    let future = DelayedValue {
        value: 42,
        delay: tokio::time::sleep(Duration::from_secs(1)),
    };

    let result = future.await;
    println!("Result: {}", result);
}

Background Tasks and Graceful Shutdown

use tokio::signal;

async fn background_task(mut shutdown: tokio::sync::watch::Receiver<bool>) {
    loop {
        tokio::select! {
            _ = tokio::time::sleep(Duration::from_secs(1)) => {
                println!("Background task running...");
            }
            _ = shutdown.changed() => {
                println!("Shutting down background task");
                break;
            }
        }
    }
}

#[tokio::main]
async fn main() {
    let (shutdown_tx, shutdown_rx) = tokio::sync::watch::channel(false);

    let task = tokio::spawn(background_task(shutdown_rx));

    // Wait for ctrl-c
    signal::ctrl_c().await.unwrap();
    println!("Received shutdown signal");

    // Signal shutdown
    shutdown_tx.send(true).unwrap();

    // Wait for task to complete
    task.await.unwrap();
}

Error Handling in Async

use thiserror::Error;

#[derive(Error, Debug)]
enum AsyncError {
    #[error("Network error: {0}")]
    Network(#[from] reqwest::Error),

    #[error("Timeout")]
    Timeout,

    #[error("Task failed")]
    TaskFailed(#[from] tokio::task::JoinError),
}

async fn robust_operation() -> Result<String, AsyncError> {
    let timeout = Duration::from_secs(5);

    let result = tokio::time::timeout(timeout, async {
        reqwest::get("https://api.example.com")
            .await?
            .text()
            .await
    })
    .await
    .map_err(|_| AsyncError::Timeout)??;

    Ok(result)
}

Runtime Configuration

// Custom runtime configuration
fn main() {
    let runtime = tokio::runtime::Builder::new_multi_thread()
        .worker_threads(4)
        .thread_name("my-worker")
        .thread_stack_size(3 * 1024 * 1024)
        .enable_all()
        .build()
        .unwrap();

    runtime.block_on(async {
        println!("Running on custom runtime");
    });
}

// Current-thread runtime (single-threaded)
fn single_threaded() {
    let runtime = tokio::runtime::Builder::new_current_thread()
        .enable_all()
        .build()
        .unwrap();

    runtime.block_on(async {
        println!("Single-threaded async");
    });
}

Best Practices

• Use tokio::spawn for CPU-bound tasks on multi-threaded runtime
• Use spawn_blocking for blocking operations (file I/O, sync code)
• Prefer tokio::sync primitives over std::sync in async code
• Use channels for task communication instead of shared state when possible
• Always handle JoinHandle results (tasks can panic)
• Use select! for cancellation patterns
• Avoid holding locks across .await points
• Use timeout for all external I/O operations
• Implement graceful shutdown with channels
• Use async-trait for trait-based async code
• Prefer try_join! over manual error handling
• Use Arc<Mutex<T>> sparingly (channels often better)
• Test async code with tokio::test macro
• Monitor task spawning to prevent unbounded growth

Error Handling in Rust

Result and Option Basics

// Result: operation that can fail
fn divide(a: f64, b: f64) -> Result<f64, String> {
    if b == 0.0 {
        Err("Division by zero".to_string())
    } else {
        Ok(a / b)
    }
}

// Option: value that might be absent
fn find_user(id: u64) -> Option<User> {
    if id == 1 {
        Some(User { id, name: "Alice".to_string() })
    } else {
        None
    }
}

// Using ? operator for propagation
fn calculate(a: f64, b: f64, c: f64) -> Result<f64, String> {
    let x = divide(a, b)?;  // Returns Err early if division fails
    let y = divide(x, c)?;
    Ok(y)
}

Custom Error Types

use std::fmt;

// Manual error type
#[derive(Debug)]
enum AppError {
    NotFound(String),
    InvalidInput(String),
    DatabaseError(String),
}

impl fmt::Display for AppError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            AppError::NotFound(msg) => write!(f, "Not found: {}", msg),
            AppError::InvalidInput(msg) => write!(f, "Invalid input: {}", msg),
            AppError::DatabaseError(msg) => write!(f, "Database error: {}", msg),
        }
    }
}

impl std::error::Error for AppError {}

// Usage
fn get_user(id: u64) -> Result<User, AppError> {
    if id == 0 {
        return Err(AppError::InvalidInput("ID cannot be zero".to_string()));
    }
    // ... fetch user
    Err(AppError::NotFound(format!("User {} not found", id)))
}

Using thiserror

use thiserror::Error;

#[derive(Error, Debug)]
enum DataError {
    #[error("Data not found: {0}")]
    NotFound(String),

    #[error("Invalid ID: {id}, reason: {reason}")]
    InvalidId { id: u64, reason: String },

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

    #[error("Parse error")]
    Parse(#[from] std::num::ParseIntError),

    #[error("Database error: {0}")]
    Database(#[from] sqlx::Error),
}

// Usage with automatic conversions
fn read_config(path: &str) -> Result<Config, DataError> {
    let content = std::fs::read_to_string(path)?;  // Auto-converts io::Error
    let port: u16 = content.parse()?;  // Auto-converts ParseIntError
    Ok(Config { port })
}

Using anyhow for Applications

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

// Simple error handling for applications
fn process_file(path: &str) -> Result<()> {
    let content = std::fs::read_to_string(path)
        .context(format!("Failed to read file: {}", path))?;

    ensure!(!content.is_empty(), "File is empty");

    if content.len() > 1000 {
        bail!("File too large");
    }

    // Process content...
    Ok(())
}

// Adding context to errors
fn main() -> Result<()> {
    process_file("config.txt")
        .context("Failed to process configuration")?;
    Ok(())
}

Option Combinators

// map: transform Option<T> to Option<U>
let num: Option<i32> = Some(5);
let doubled = num.map(|n| n * 2);  // Some(10)

// and_then: chain operations
let result = Some(5)
    .and_then(|n| if n > 0 { Some(n * 2) } else { None })
    .and_then(|n| Some(n + 1));  // Some(11)

// or: provide alternative
let value = None.or(Some(42));  // Some(42)

// unwrap_or: provide default
let value = None.unwrap_or(42);  // 42

// unwrap_or_else: compute default lazily
let value = None.unwrap_or_else(|| expensive_computation());

// filter: conditional None
let num = Some(5).filter(|&n| n > 10);  // None

// Pattern matching
match find_user(1) {
    Some(user) => println!("Found: {}", user.name),
    None => println!("User not found"),
}

// if let for simple cases
if let Some(user) = find_user(1) {
    println!("Found: {}", user.name);
}

Result Combinators

// map: transform Ok value
let result: Result<i32, String> = Ok(5);
let doubled = result.map(|n| n * 2);  // Ok(10)

// map_err: transform error
let result: Result<i32, &str> = Err("error");
let mapped = result.map_err(|e| e.to_uppercase());  // Err("ERROR")

// and_then: chain fallible operations
fn parse_then_double(s: &str) -> Result<i32, std::num::ParseIntError> {
    s.parse::<i32>()
        .and_then(|n| Ok(n * 2))
}

// or_else: provide alternative computation
let result = Err("error").or_else(|_| Ok(42));  // Ok(42)

// unwrap_or: provide default
let value = Err("error").unwrap_or(42);  // 42

// expect: unwrap with custom panic message
let value = result.expect("Failed to parse number");

// Pattern matching
match divide(10.0, 2.0) {
    Ok(result) => println!("Result: {}", result),
    Err(e) => eprintln!("Error: {}", e),
}

Error Conversion and From Trait

use std::io;
use std::num::ParseIntError;

#[derive(Debug)]
enum MyError {
    Io(io::Error),
    Parse(ParseIntError),
}

impl From<io::Error> for MyError {
    fn from(err: io::Error) -> Self {
        MyError::Io(err)
    }
}

impl From<ParseIntError> for MyError {
    fn from(err: ParseIntError) -> Self {
        MyError::Parse(err)
    }
}

// Now ? operator works with automatic conversion
fn read_and_parse(path: &str) -> Result<i32, MyError> {
    let content = std::fs::read_to_string(path)?;  // io::Error -> MyError
    let number = content.trim().parse()?;  // ParseIntError -> MyError
    Ok(number)
}

Advanced Error Patterns

// Multiple error sources with Box<dyn Error>
use std::error::Error;

fn complex_operation() -> Result<String, Box<dyn Error>> {
    let file = std::fs::read_to_string("data.txt")?;
    let number: i32 = file.trim().parse()?;
    Ok(format!("Number: {}", number))
}

// Error with backtrace (nightly)
#[derive(Debug)]
struct DetailedError {
    message: String,
    backtrace: std::backtrace::Backtrace,
}

impl DetailedError {
    fn new(message: impl Into<String>) -> Self {
        Self {
            message: message.into(),
            backtrace: std::backtrace::Backtrace::capture(),
        }
    }
}

// Recoverable vs unrecoverable errors
fn might_fail(value: i32) -> Result<i32, String> {
    if value < 0 {
        Err("Negative value".to_string())  // Recoverable
    } else if value > 1000 {
        panic!("Value too large!");  // Unrecoverable
    } else {
        Ok(value * 2)
    }
}

Try Blocks (Nightly)

#![feature(try_blocks)]

// Try block for localized error handling
let result: Result<i32, Box<dyn Error>> = try {
    let file = std::fs::read_to_string("config.txt")?;
    let num: i32 = file.trim().parse()?;
    num * 2
};

Error Context Pattern

use thiserror::Error;

#[derive(Error, Debug)]
#[error("{message}")]
struct ContextError {
    message: String,
    #[source]
    source: Option<Box<dyn Error + Send + Sync>>,
}

impl ContextError {
    fn new(message: impl Into<String>) -> Self {
        Self {
            message: message.into(),
            source: None,
        }
    }

    fn with_source(mut self, source: impl Error + Send + Sync + 'static) -> Self {
        self.source = Some(Box::new(source));
        self
    }
}

// Extension trait for adding context
trait Context<T> {
    fn context(self, message: impl Into<String>) -> Result<T, ContextError>;
}

impl<T, E: Error + Send + Sync + 'static> Context<T> for Result<T, E> {
    fn context(self, message: impl Into<String>) -> Result<T, ContextError> {
        self.map_err(|e| ContextError::new(message).with_source(e))
    }
}

Best Practices

• Use Result for recoverable errors, panic! for unrecoverable bugs
• Prefer ? operator over unwrap() in production code
• Use expect() with descriptive messages instead of unwrap()
• Use thiserror for libraries (structured errors)
• Use anyhow for applications (simple error handling)
• Implement std::error::Error trait for custom error types
• Add context to errors as they propagate up the stack
• Use #[from] in thiserror for automatic conversions
• Document error conditions in function documentation
• Use Option::ok_or() to convert Option to Result
• Use Result::ok() to convert Result to Option (discarding error)
• Avoid String as error type (use custom types instead)
• Use ensure! and bail! from anyhow for cleaner checks
• Log errors at boundaries, return them in library code

Ownership, Borrowing, and Lifetimes

Ownership Patterns

// Move semantics (ownership transfer)
fn take_ownership(s: String) {
    println!("{}", s);
} // s dropped here

// Borrowing (immutable reference)
fn borrow(s: &String) {
    println!("{}", s);
} // s NOT dropped, caller still owns

// Mutable borrowing
fn borrow_mut(s: &mut String) {
    s.push_str(" world");
}

// Usage
let s = String::from("hello");
borrow(&s);           // OK, immutable borrow
let mut s2 = s;       // Move, s no longer valid
borrow_mut(&mut s2);  // OK, mutable borrow

Lifetime Annotations

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

// Multiple lifetimes
fn first_word<'a, 'b>(s: &'a str, _other: &'b str) -> &'a str {
    s.split_whitespace().next().unwrap_or("")
}

// Lifetime in structs
struct Excerpt<'a> {
    part: &'a str,
}

impl<'a> Excerpt<'a> {
    fn announce_and_return(&self, announcement: &str) -> &'a str {
        println!("Attention: {}", announcement);
        self.part
    }
}

// Static lifetime (lives for entire program)
const GREETING: &'static str = "Hello, world!";

Smart Pointers

use std::rc::Rc;
use std::cell::RefCell;
use std::sync::{Arc, Mutex};

// Box: heap allocation, single owner
let b = Box::new(5);

// Rc: reference counting (single-threaded)
let rc1 = Rc::new(vec![1, 2, 3]);
let rc2 = Rc::clone(&rc1);  // Increment count
println!("Count: {}", Rc::strong_count(&rc1));  // 2

// Arc: atomic reference counting (thread-safe)
let arc1 = Arc::new(vec![1, 2, 3]);
let arc2 = Arc::clone(&arc1);
std::thread::spawn(move || {
    println!("{:?}", arc2);
});

// RefCell: interior mutability (runtime borrow checking)
let data = RefCell::new(5);
*data.borrow_mut() += 1;  // Mutable borrow at runtime

// Combining Rc + RefCell for shared mutable state
let shared = Rc::new(RefCell::new(vec![1, 2, 3]));
shared.borrow_mut().push(4);

// Combining Arc + Mutex for thread-safe shared state
let counter = Arc::new(Mutex::new(0));
let counter_clone = Arc::clone(&counter);
std::thread::spawn(move || {
    let mut num = counter_clone.lock().unwrap();
    *num += 1;
});

Interior Mutability

use std::cell::{Cell, RefCell};

// Cell: Copy types only
let c = Cell::new(5);
c.set(10);
let val = c.get();

// RefCell: runtime borrow checking
let data = RefCell::new(vec![1, 2, 3]);
data.borrow_mut().push(4);

// Pattern: mock objects with interior mutability
struct MockLogger {
    messages: RefCell<Vec<String>>,
}

impl MockLogger {
    fn new() -> Self {
        Self { messages: RefCell::new(Vec::new()) }
    }

    fn log(&self, msg: &str) {
        self.messages.borrow_mut().push(msg.to_string());
    }

    fn get_messages(&self) -> Vec<String> {
        self.messages.borrow().clone()
    }
}

Pin and Self-Referential Types

use std::pin::Pin;
use std::marker::PhantomPinned;

// Self-referential struct (requires Pin)
struct SelfReferential {
    data: String,
    pointer: *const String,
    _pin: PhantomPinned,
}

impl SelfReferential {
    fn new(data: String) -> Pin<Box<Self>> {
        let mut boxed = Box::pin(Self {
            data,
            pointer: std::ptr::null(),
            _pin: PhantomPinned,
        });

        // Safe: we're not moving the data after this
        let ptr = &boxed.data as *const String;
        unsafe {
            let mut_ref = Pin::as_mut(&mut boxed);
            Pin::get_unchecked_mut(mut_ref).pointer = ptr;
        }

        boxed
    }
}

// Pin in async contexts
async fn pinned_future() {
    // Futures are often self-referential, hence Pin
    let fut = async { 42 };
    let pinned = Box::pin(fut);
    pinned.await;
}

Cow (Clone on Write)

use std::borrow::Cow;

fn process_text(input: &str) -> Cow<str> {
    if input.contains("bad") {
        // Need to modify: allocate new String
        Cow::Owned(input.replace("bad", "good"))
    } else {
        // No modification needed: just borrow
        Cow::Borrowed(input)
    }
}

// Usage
let text1 = "hello world";
let result1 = process_text(text1);  // Borrowed (no allocation)

let text2 = "bad word";
let result2 = process_text(text2);  // Owned (allocated)

Drop Trait and RAII

struct FileGuard {
    name: String,
}

impl FileGuard {
    fn new(name: String) -> Self {
        println!("Opening {}", name);
        Self { name }
    }
}

impl Drop for FileGuard {
    fn drop(&mut self) {
        println!("Closing {}", self.name);
    }
}

// Usage: automatic cleanup
{
    let _file = FileGuard::new("data.txt".to_string());
    // Use file...
} // Drop called automatically here

Common Patterns

// Builder pattern with ownership
struct Config {
    host: String,
    port: u16,
}

impl Config {
    fn builder() -> ConfigBuilder {
        ConfigBuilder::default()
    }
}

struct ConfigBuilder {
    host: Option<String>,
    port: Option<u16>,
}

impl ConfigBuilder {
    fn host(mut self, host: impl Into<String>) -> Self {
        self.host = Some(host.into());
        self
    }

    fn port(mut self, port: u16) -> Self {
        self.port = Some(port);
        self
    }

    fn build(self) -> Result<Config, &'static str> {
        Ok(Config {
            host: self.host.ok_or("host required")?,
            port: self.port.unwrap_or(8080),
        })
    }
}

// Usage
let config = Config::builder()
    .host("localhost")
    .port(3000)
    .build()?;

Best Practices

• Prefer borrowing (&T) over ownership transfer when possible
• Use &str over String for function parameters
• Use &[T] over Vec<T> for function parameters
• Clone only when necessary (profile first)
• Use Cow<'a, T> for conditional cloning
• Document lifetime relationships in complex cases
• Use Arc<Mutex<T>> for shared mutable state across threads
• Use Rc<RefCell<T>> for shared mutable state in single thread
• Implement Drop for RAII patterns
• Use PhantomData to constrain variance when needed

Testing in Rust

Unit Tests

// Tests in same file
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_addition() {
        assert_eq!(2 + 2, 4);
    }

    #[test]
    fn test_subtraction() {
        assert!(10 - 5 == 5);
    }

    #[test]
    #[should_panic(expected = "division by zero")]
    fn test_panic() {
        divide(10, 0);
    }

    #[test]
    fn test_result() -> Result<(), String> {
        let result = divide(10, 2)?;
        assert_eq!(result, 5);
        Ok(())
    }

    #[test]
    #[ignore]
    fn expensive_test() {
        // Run with: cargo test -- --ignored
    }
}

// Assertions
fn assert_examples() {
    assert!(true);
    assert_eq!(2 + 2, 4);
    assert_ne!(2 + 2, 5);

    // Custom messages
    assert!(value > 0, "Value must be positive, got {}", value);
    assert_eq!(result, expected, "Calculation failed");
}

Doctests

/// Adds two numbers together.
///
/// # Examples
///
/// ```
/// use mylib::add;
///
/// let result = add(2, 3);
/// assert_eq!(result, 5);
/// ```
///
/// ```should_panic
/// use mylib::divide;
///
/// divide(10, 0);  // This will panic
/// ```
///
/// ```ignore
/// // This code won't compile but won't fail the test
/// let x = undefined_function();
/// ```
pub fn add(a: i32, b: i32) -> i32 {
    a + b
}

Integration Tests

// tests/integration_test.rs
use mylib;

#[test]
fn test_full_workflow() {
    let config = mylib::Config::new("test.conf");
    let result = mylib::process(&config);
    assert!(result.is_ok());
}

// tests/common/mod.rs - shared test utilities
pub fn setup() -> TestContext {
    TestContext {
        db: create_test_db(),
    }
}

// tests/another_test.rs
mod common;

#[test]
fn test_with_common() {
    let ctx = common::setup();
    // Use ctx...
}

Test Organization

// Nested test modules
#[cfg(test)]
mod tests {
    use super::*;

    mod addition {
        use super::*;

        #[test]
        fn positive_numbers() {
            assert_eq!(add(2, 3), 5);
        }

        #[test]
        fn negative_numbers() {
            assert_eq!(add(-2, -3), -5);
        }
    }

    mod subtraction {
        use super::*;

        #[test]
        fn test_subtract() {
            assert_eq!(subtract(10, 5), 5);
        }
    }
}

Test Fixtures and Setup

struct TestContext {
    temp_dir: std::path::PathBuf,
    db: Database,
}

impl TestContext {
    fn setup() -> Self {
        let temp_dir = std::env::temp_dir().join("test");
        std::fs::create_dir_all(&temp_dir).unwrap();

        Self {
            temp_dir,
            db: Database::connect_test(),
        }
    }
}

impl Drop for TestContext {
    fn drop(&mut self) {
        // Cleanup
        std::fs::remove_dir_all(&self.temp_dir).ok();
        self.db.disconnect();
    }
}

#[test]
fn test_with_fixture() {
    let ctx = TestContext::setup();
    // Test uses ctx...
    // Automatic cleanup via Drop
}

Async Tests

use tokio;

#[tokio::test]
async fn test_async_function() {
    let result = async_operation().await;
    assert_eq!(result, 42);
}

#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
async fn test_with_custom_runtime() {
    let result = concurrent_operation().await;
    assert!(result.is_ok());
}

// Testing async with timeout
#[tokio::test]
async fn test_with_timeout() {
    let timeout = std::time::Duration::from_secs(5);
    let result = tokio::time::timeout(timeout, slow_operation()).await;
    assert!(result.is_ok());
}

Property-Based Testing (proptest)

use proptest::prelude::*;

// Simple property test
proptest! {
    #[test]
    fn test_reversing_twice_is_identity(ref s in ".*") {
        let reversed: String = s.chars().rev().collect();
        let double_reversed: String = reversed.chars().rev().collect();
        assert_eq!(s, &double_reversed);
    }
}

// Custom strategies
proptest! {
    #[test]
    fn test_addition_commutative(a in 0..1000i32, b in 0..1000i32) {
        assert_eq!(a + b, b + a);
    }

    #[test]
    fn test_vector_push_pop(
        ref v in prop::collection::vec(0..100i32, 0..100),
        item in 0..100i32
    ) {
        let mut v = v.clone();
        v.push(item);
        assert_eq!(v.pop(), Some(item));
    }
}

// Complex custom strategies
fn user_strategy() -> impl Strategy<Value = User> {
    (1..1000u64, "[a-z]{3,10}", "[a-z0-9.]+@[a-z]+\\.[a-z]+")
        .prop_map(|(id, name, email)| User { id, name, email })
}

proptest! {
    #[test]
    fn test_user_serialization(user in user_strategy()) {
        let json = serde_json::to_string(&user).unwrap();
        let deserialized: User = serde_json::from_str(&json).unwrap();
        assert_eq!(user, deserialized);
    }
}

Mocking

// Using mockall
use mockall::*;
use mockall::predicate::*;

#[automock]
trait Database {
    fn get_user(&self, id: u64) -> Option<User>;
    fn save_user(&mut self, user: User) -> Result<(), Error>;
}

#[test]
fn test_with_mock() {
    let mut mock = MockDatabase::new();

    mock.expect_get_user()
        .with(eq(1))
        .times(1)
        .returning(|_| Some(User { id: 1, name: "Alice".to_string() }));

    mock.expect_save_user()
        .times(1)
        .returning(|_| Ok(()));

    // Use mock in test
    let user = mock.get_user(1);
    assert!(user.is_some());
}

Benchmarks (Criterion)

// benches/my_benchmark.rs
use criterion::{black_box, criterion_group, criterion_main, Criterion};

fn fibonacci(n: u64) -> u64 {
    match n {
        0 => 1,
        1 => 1,
        n => fibonacci(n - 1) + fibonacci(n - 2),
    }
}

fn criterion_benchmark(c: &mut Criterion) {
    c.bench_function("fib 20", |b| b.iter(|| fibonacci(black_box(20))));
}

criterion_group!(benches, criterion_benchmark);
criterion_main!(benches);

// Cargo.toml:
// [dev-dependencies]
// criterion = "0.5"
//
// [[bench]]
// name = "my_benchmark"
// harness = false

Advanced Benchmarking

use criterion::{BenchmarkId, Criterion, criterion_group, criterion_main};

fn bench_multiple_sizes(c: &mut Criterion) {
    let mut group = c.benchmark_group("sorting");

    for size in [10, 100, 1000, 10000].iter() {
        group.bench_with_input(BenchmarkId::from_parameter(size), size, |b, &size| {
            b.iter_batched(
                || generate_random_vec(size),
                |mut v| v.sort(),
                criterion::BatchSize::SmallInput,
            );
        });
    }

    group.finish();
}

// Comparing implementations
fn bench_comparison(c: &mut Criterion) {
    let mut group = c.benchmark_group("string_search");

    group.bench_function("naive", |b| {
        b.iter(|| naive_search(black_box("haystack"), black_box("needle")))
    });

    group.bench_function("optimized", |b| {
        b.iter(|| optimized_search(black_box("haystack"), black_box("needle")))
    });

    group.finish();
}

criterion_group!(benches, bench_multiple_sizes, bench_comparison);
criterion_main!(benches);

Testing with External Resources

// Testing file I/O
#[test]
fn test_file_operations() {
    use std::io::Write;

    let temp_dir = std::env::temp_dir();
    let file_path = temp_dir.join("test_file.txt");

    // Write
    let mut file = std::fs::File::create(&file_path).unwrap();
    file.write_all(b"test content").unwrap();

    // Read
    let content = std::fs::read_to_string(&file_path).unwrap();
    assert_eq!(content, "test content");

    // Cleanup
    std::fs::remove_file(&file_path).unwrap();
}

// Testing with databases (using sqlx)
#[sqlx::test]
async fn test_database_operations(pool: sqlx::PgPool) -> sqlx::Result<()> {
    sqlx::query("INSERT INTO users (name) VALUES ($1)")
        .bind("Alice")
        .execute(&pool)
        .await?;

    let count: (i64,) = sqlx::query_as("SELECT COUNT(*) FROM users")
        .fetch_one(&pool)
        .await?;

    assert_eq!(count.0, 1);
    Ok(())
}

Snapshot Testing

// Using insta crate
use insta::assert_snapshot;

#[test]
fn test_output_format() {
    let data = generate_complex_output();
    assert_snapshot!(data);
}

#[test]
fn test_json_output() {
    let json = serde_json::to_string_pretty(&get_data()).unwrap();
    assert_snapshot!(json);
}

// Run with: cargo insta test
// Review snapshots: cargo insta review

Code Coverage

// Using tarpaulin
// cargo install cargo-tarpaulin
// cargo tarpaulin --out Html --output-dir coverage

// Using llvm-cov
// cargo install cargo-llvm-cov
// cargo llvm-cov --html

Fuzzing

// Using cargo-fuzz
// cargo install cargo-fuzz
// cargo fuzz init

// fuzz/fuzz_targets/fuzz_target_1.rs
#![no_main]
use libfuzzer_sys::fuzz_target;

fuzz_target!(|data: &[u8]| {
    if let Ok(s) = std::str::from_utf8(data) {
        let _ = mylib::parse_input(s);
    }
});

// Run with: cargo fuzz run fuzz_target_1

Best Practices

• Write tests alongside production code in #[cfg(test)] modules
• Use integration tests in tests/ directory for end-to-end testing
• Include doctests in documentation for examples that must work
• Use descriptive test names that explain what is being tested
• Test edge cases (empty inputs, max values, etc.)
• Use property-based testing for algorithmic code
• Benchmark performance-critical code with criterion
• Run tests in CI with cargo test --all-features
• Use cargo test -- --nocapture to see println! output
• Test error conditions with #[should_panic] or Result
• Mock external dependencies for unit tests
• Use test fixtures for complex setup/teardown
• Run clippy on test code too
• Measure code coverage and aim for high coverage
• Use fuzzing for security-critical parsers
• Test async code with tokio::test
• Use snapshot testing for complex output validation

Traits, Generics, and Type System

Basic Trait Definition

// Simple trait
trait Drawable {
    fn draw(&self);
}

// Trait with default implementation
trait Describable {
    fn describe(&self) -> String {
        String::from("No description available")
    }
}

// Implementing traits
struct Circle {
    radius: f64,
}

impl Drawable for Circle {
    fn draw(&self) {
        println!("Drawing circle with radius {}", self.radius);
    }
}

impl Describable for Circle {
    fn describe(&self) -> String {
        format!("A circle with radius {}", self.radius)
    }
}

Associated Types

// Associated types vs generic parameters
trait Container {
    type Item;

    fn add(&mut self, item: Self::Item);
    fn get(&self, index: usize) -> Option<&Self::Item>;
}

impl Container for Vec<i32> {
    type Item = i32;

    fn add(&mut self, item: i32) {
        self.push(item);
    }

    fn get(&self, index: usize) -> Option<&i32> {
        self.get(index)
    }
}

// Iterator trait (standard library example)
trait MyIterator {
    type Item;

    fn next(&mut self) -> Option<Self::Item>;
}

Generic Traits and Bounds

// Generic trait with multiple bounds
fn print_info<T>(item: &T)
where
    T: std::fmt::Display + std::fmt::Debug,
{
    println!("Display: {}", item);
    println!("Debug: {:?}", item);
}

// Generic struct with trait bounds
struct Pair<T: PartialOrd> {
    first: T,
    second: T,
}

impl<T: PartialOrd> Pair<T> {
    fn new(first: T, second: T) -> Self {
        Self { first, second }
    }

    fn larger(&self) -> &T {
        if self.first > self.second {
            &self.first
        } else {
            &self.second
        }
    }
}

// Blanket implementation
trait MyTrait {
    fn do_something(&self);
}

impl<T: std::fmt::Display> MyTrait for T {
    fn do_something(&self) {
        println!("Value: {}", self);
    }
}

Trait Objects (Dynamic Dispatch)

// Static dispatch (monomorphization)
fn static_dispatch<T: Drawable>(item: &T) {
    item.draw();
}

// Dynamic dispatch (trait objects)
fn dynamic_dispatch(item: &dyn Drawable) {
    item.draw();
}

// Storing trait objects
struct Canvas {
    shapes: Vec<Box<dyn Drawable>>,
}

impl Canvas {
    fn new() -> Self {
        Self { shapes: Vec::new() }
    }

    fn add_shape(&mut self, shape: Box<dyn Drawable>) {
        self.shapes.push(shape);
    }

    fn draw_all(&self) {
        for shape in &self.shapes {
            shape.draw();
        }
    }
}

// Object safety: traits must meet criteria
trait ObjectSafe {
    fn method(&self);  // OK: takes &self
}

trait NotObjectSafe {
    fn generic<T>(&self);  // NOT OK: generic method
    fn by_value(self);     // NOT OK: takes self by value
}

Derive Macros

// Standard derive macros
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
struct User {
    id: u64,
    name: String,
}

// Deriving more traits
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct Point {
    x: i32,
    y: i32,
}

// Custom derive with serde
use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize)]
struct Config {
    host: String,
    port: u16,
}

Advanced Trait Patterns

// Extension trait pattern
trait StringExt {
    fn truncate_to(&self, max_len: usize) -> String;
}

impl StringExt for str {
    fn truncate_to(&self, max_len: usize) -> String {
        if self.len() <= max_len {
            self.to_string()
        } else {
            format!("{}...", &self[..max_len])
        }
    }
}

// Sealed trait pattern (prevent external implementation)
mod sealed {
    pub trait Sealed {}
}

pub trait MySealed: sealed::Sealed {
    fn method(&self);
}

struct MyType;
impl sealed::Sealed for MyType {}
impl MySealed for MyType {
    fn method(&self) {
        println!("Implemented");
    }
}

// Supertraits
trait Printable {
    fn print(&self);
}

trait Loggable: Printable {  // Supertrait: must also impl Printable
    fn log(&self) {
        self.print();  // Can call supertrait methods
    }
}

Associated Constants

trait Config {
    const MAX_SIZE: usize;
    const DEFAULT_TIMEOUT: u64;
}

struct ServerConfig;

impl Config for ServerConfig {
    const MAX_SIZE: usize = 1024;
    const DEFAULT_TIMEOUT: u64 = 30;
}

fn use_config<T: Config>() {
    println!("Max size: {}", T::MAX_SIZE);
}

Generic Associated Types (GATs)

// GATs allow generics in associated types
trait LendingIterator {
    type Item<'a> where Self: 'a;

    fn next<'a>(&'a mut self) -> Option<Self::Item<'a>>;
}

struct WindowsMut<'data, T> {
    data: &'data mut [T],
    index: usize,
}

impl<'data, T> LendingIterator for WindowsMut<'data, T> {
    type Item<'a> = &'a mut [T] where Self: 'a;

    fn next<'a>(&'a mut self) -> Option<Self::Item<'a>> {
        if self.index >= self.data.len() {
            return None;
        }

        let start = self.index;
        self.index += 2;

        Some(&mut self.data[start..start.min(self.data.len())])
    }
}

Marker Traits

use std::marker::{PhantomData, Send, Sync};

// Send: type can be transferred across thread boundaries
// Sync: type can be shared between threads (&T is Send)

// Custom marker trait
trait Trusted {}

struct TrustedData<T> {
    data: T,
    _marker: PhantomData<T>,
}

impl<T: Trusted> TrustedData<T> {
    fn new(data: T) -> Self {
        Self {
            data,
            _marker: PhantomData,
        }
    }
}

Operator Overloading

use std::ops::{Add, Mul};

#[derive(Debug, Clone, Copy)]
struct Vector2D {
    x: f64,
    y: f64,
}

impl Add for Vector2D {
    type Output = Self;

    fn add(self, other: Self) -> Self {
        Self {
            x: self.x + other.x,
            y: self.y + other.y,
        }
    }
}

impl Mul<f64> for Vector2D {
    type Output = Self;

    fn mul(self, scalar: f64) -> Self {
        Self {
            x: self.x * scalar,
            y: self.y * scalar,
        }
    }
}

// Usage
let v1 = Vector2D { x: 1.0, y: 2.0 };
let v2 = Vector2D { x: 3.0, y: 4.0 };
let v3 = v1 + v2;
let v4 = v1 * 2.5;

From/Into Conversion Traits

struct UserId(u64);

impl From<u64> for UserId {
    fn from(id: u64) -> Self {
        UserId(id)
    }
}

// Into is automatically implemented
fn accept_user_id(id: impl Into<UserId>) {
    let user_id = id.into();
    println!("User ID: {}", user_id.0);
}

// TryFrom for fallible conversions
use std::convert::TryFrom;

impl TryFrom<i64> for UserId {
    type Error = &'static str;

    fn try_from(value: i64) -> Result<Self, Self::Error> {
        if value < 0 {
            Err("User ID cannot be negative")
        } else {
            Ok(UserId(value as u64))
        }
    }
}

Const Traits (Nightly)

// Const trait implementations (requires nightly)
#![feature(const_trait_impl)]

#[const_trait]
trait ConstAdd {
    fn add(self, other: Self) -> Self;
}

impl const ConstAdd for i32 {
    fn add(self, other: Self) -> Self {
        self + other
    }
}

const fn compute() -> i32 {
    5.add(10)  // Can use in const context
}

Best Practices

• Prefer associated types when there's one clear type per implementation
• Use generic parameters when multiple types might be used simultaneously
• Keep traits small and focused (single responsibility)
• Use extension traits to add functionality to existing types
• Document trait requirements and invariants
• Use marker traits for compile-time guarantees
• Prefer static dispatch for performance, dynamic dispatch for flexibility
• Use #[derive] when possible instead of manual implementations
• Implement standard traits (Debug, Clone, etc.) for better ecosystem integration
• Use sealed traits to prevent external implementations when needed