Rust Patterns

1---2name: rust-patterns3description: Idiomatic Rust patterns, ownership, error handling, traits, concurrency, and best practices for building safe, performant applications.4origin: ECC5---6 7# Rust Development Patterns8 9Idiomatic Rust patterns and best practices for building safe, performant, and maintainable applications.10 11## When to Use12 13- Writing new Rust code14- Reviewing Rust code15- Refactoring existing Rust code16- Designing crate structure and module layout17 18## How It Works19 20This skill enforces idiomatic Rust conventions across six key areas: ownership and borrowing to prevent data races at compile time, `Result`/`?` error propagation with `thiserror` for libraries and `anyhow` for applications, enums and exhaustive pattern matching to make illegal states unrepresentable, traits and generics for zero-cost abstraction, safe concurrency via `Arc<Mutex<T>>`, channels, and async/await, and minimal `pub` surfaces organized by domain.21 22## Core Principles23 24### 1. Ownership and Borrowing25 26Rust's ownership system prevents data races and memory bugs at compile time.27 28```rust29// Good: Pass references when you don't need ownership30fn process(data: &[u8]) -> usize {31    data.len()32}33 34// Good: Take ownership only when you need to store or consume35fn store(data: Vec<u8>) -> Record {36    Record { payload: data }37}38 39// Bad: Cloning unnecessarily to avoid borrow checker40fn process_bad(data: &Vec<u8>) -> usize {41    let cloned = data.clone(); // Wasteful — just borrow42    cloned.len()43}44```45 46### Use `Cow` for Flexible Ownership47 48```rust49use std::borrow::Cow;50 51fn normalize(input: &str) -> Cow<'_, str> {52    if input.contains(' ') {53        Cow::Owned(input.replace(' ', "_"))54    } else {55        Cow::Borrowed(input) // Zero-cost when no mutation needed56    }57}58```59 60## Error Handling61 62### Use `Result` and `?` — Never `unwrap()` in Production63 64```rust65// Good: Propagate errors with context66use anyhow::{Context, Result};67 68fn load_config(path: &str) -> Result<Config> {69    let content = std::fs::read_to_string(path)70        .with_context(|| format!("failed to read config from {path}"))?;71    let config: Config = toml::from_str(&content)72        .with_context(|| format!("failed to parse config from {path}"))?;73    Ok(config)74}75 76// Bad: Panics on error77fn load_config_bad(path: &str) -> Config {78    let content = std::fs::read_to_string(path).unwrap(); // Panics!79    toml::from_str(&content).unwrap()80}81```82 83### Library Errors with `thiserror`, Application Errors with `anyhow`84 85```rust86// Library code: structured, typed errors87use thiserror::Error;88 89#[derive(Debug, Error)]90pub enum StorageError {91    #[error("record not found: {id}")]92    NotFound { id: String },93    #[error("connection failed")]94    Connection(#[from] std::io::Error),95    #[error("invalid data: {0}")]96    InvalidData(String),97}98 99// Application code: flexible error handling100use anyhow::{bail, Result};101 102fn run() -> Result<()> {103    let config = load_config("app.toml")?;104    if config.workers == 0 {105        bail!("worker count must be > 0");106    }107    Ok(())108}109```110 111### `Option` Combinators Over Nested Matching112 113```rust114// Good: Combinator chain115fn find_user_email(users: &[User], id: u64) -> Option<String> {116    users.iter()117        .find(|u| u.id == id)118        .map(|u| u.email.clone())119}120 121// Bad: Deeply nested matching122fn find_user_email_bad(users: &[User], id: u64) -> Option<String> {123    match users.iter().find(|u| u.id == id) {124        Some(user) => match &user.email {125            email => Some(email.clone()),126        },127        None => None,128    }129}130```131 132## Enums and Pattern Matching133 134### Model States as Enums135 136```rust137// Good: Impossible states are unrepresentable138enum ConnectionState {139    Disconnected,140    Connecting { attempt: u32 },141    Connected { session_id: String },142    Failed { reason: String, retries: u32 },143}144 145fn handle(state: &ConnectionState) {146    match state {147        ConnectionState::Disconnected => connect(),148        ConnectionState::Connecting { attempt } if *attempt > 3 => abort(),149        ConnectionState::Connecting { .. } => wait(),150        ConnectionState::Connected { session_id } => use_session(session_id),151        ConnectionState::Failed { retries, .. } if *retries < 5 => retry(),152        ConnectionState::Failed { reason, .. } => log_failure(reason),153    }154}155```156 157### Exhaustive Matching — No Catch-All for Business Logic158 159```rust160// Good: Handle every variant explicitly161match command {162    Command::Start => start_service(),163    Command::Stop => stop_service(),164    Command::Restart => restart_service(),165    // Adding a new variant forces handling here166}167 168// Bad: Wildcard hides new variants169match command {170    Command::Start => start_service(),171    _ => {} // Silently ignores Stop, Restart, and future variants172}173```174 175## Traits and Generics176 177### Accept Generics, Return Concrete Types178 179```rust180// Good: Generic input, concrete output181fn read_all(reader: &mut impl Read) -> std::io::Result<Vec<u8>> {182    let mut buf = Vec::new();183    reader.read_to_end(&mut buf)?;184    Ok(buf)185}186 187// Good: Trait bounds for multiple constraints188fn process<T: Display + Send + 'static>(item: T) -> String {189    format!("processed: {item}")190}191```192 193### Trait Objects for Dynamic Dispatch194 195```rust196// Use when you need heterogeneous collections or plugin systems197trait Handler: Send + Sync {198    fn handle(&self, request: &Request) -> Response;199}200 201struct Router {202    handlers: Vec<Box<dyn Handler>>,203}204 205// Use generics when you need performance (monomorphization)206fn fast_process<H: Handler>(handler: &H, request: &Request) -> Response {207    handler.handle(request)208}209```210 211### Newtype Pattern for Type Safety212 213```rust214// Good: Distinct types prevent mixing up arguments215struct UserId(u64);216struct OrderId(u64);217 218fn get_order(user: UserId, order: OrderId) -> Result<Order> {219    // Can't accidentally swap user and order IDs220    todo!()221}222 223// Bad: Easy to swap arguments224fn get_order_bad(user_id: u64, order_id: u64) -> Result<Order> {225    todo!()226}227```228 229## Structs and Data Modeling230 231### Builder Pattern for Complex Construction232 233```rust234struct ServerConfig {235    host: String,236    port: u16,237    max_connections: usize,238}239 240impl ServerConfig {241    fn builder(host: impl Into<String>, port: u16) -> ServerConfigBuilder {242        ServerConfigBuilder { host: host.into(), port, max_connections: 100 }243    }244}245 246struct ServerConfigBuilder { host: String, port: u16, max_connections: usize }247 248impl ServerConfigBuilder {249    fn max_connections(mut self, n: usize) -> Self { self.max_connections = n; self }250    fn build(self) -> ServerConfig {251        ServerConfig { host: self.host, port: self.port, max_connections: self.max_connections }252    }253}254 255// Usage: ServerConfig::builder("localhost", 8080).max_connections(200).build()256```257 258## Iterators and Closures259 260### Prefer Iterator Chains Over Manual Loops261 262```rust263// Good: Declarative, lazy, composable264let active_emails: Vec<String> = users.iter()265    .filter(|u| u.is_active)266    .map(|u| u.email.clone())267    .collect();268 269// Bad: Imperative accumulation270let mut active_emails = Vec::new();271for user in &users {272    if user.is_active {273        active_emails.push(user.email.clone());274    }275}276```277 278### Use `collect()` with Type Annotation279 280```rust281// Collect into different types282let names: Vec<_> = items.iter().map(|i| &i.name).collect();283let lookup: HashMap<_, _> = items.iter().map(|i| (i.id, i)).collect();284let combined: String = parts.iter().copied().collect();285 286// Collect Results — short-circuits on first error287let parsed: Result<Vec<i32>, _> = strings.iter().map(|s| s.parse()).collect();288```289 290## Concurrency291 292### `Arc<Mutex<T>>` for Shared Mutable State293 294```rust295use std::sync::{Arc, Mutex};296 297let counter = Arc::new(Mutex::new(0));298let handles: Vec<_> = (0..10).map(|_| {299    let counter = Arc::clone(&counter);300    std::thread::spawn(move || {301        let mut num = counter.lock().expect("mutex poisoned");302        *num += 1;303    })304}).collect();305 306for handle in handles {307    handle.join().expect("worker thread panicked");308}309```310 311### Channels for Message Passing312 313```rust314use std::sync::mpsc;315 316let (tx, rx) = mpsc::sync_channel(16); // Bounded channel with backpressure317 318for i in 0..5 {319    let tx = tx.clone();320    std::thread::spawn(move || {321        tx.send(format!("message {i}")).expect("receiver disconnected");322    });323}324drop(tx); // Close sender so rx iterator terminates325 326for msg in rx {327    println!("{msg}");328}329```330 331### Async with Tokio332 333```rust334use tokio::time::Duration;335 336async fn fetch_with_timeout(url: &str) -> Result<String> {337    let response = tokio::time::timeout(338        Duration::from_secs(5),339        reqwest::get(url),340    )341    .await342    .context("request timed out")?343    .context("request failed")?;344 345    response.text().await.context("failed to read body")346}347 348// Spawn concurrent tasks349async fn fetch_all(urls: Vec<String>) -> Vec<Result<String>> {350    let handles: Vec<_> = urls.into_iter()351        .map(|url| tokio::spawn(async move {352            fetch_with_timeout(&url).await353        }))354        .collect();355 356    let mut results = Vec::with_capacity(handles.len());357    for handle in handles {358        results.push(handle.await.unwrap_or_else(|e| panic!("spawned task panicked: {e}")));359    }360    results361}362```363 364## Unsafe Code365 366### When Unsafe Is Acceptable367 368```rust369// Acceptable: FFI boundary with documented invariants (Rust 2024+)370/// # Safety371/// `ptr` must be a valid, aligned pointer to an initialized `Widget`.372unsafe fn widget_from_raw<'a>(ptr: *const Widget) -> &'a Widget {373    // SAFETY: caller guarantees ptr is valid and aligned374    unsafe { &*ptr }375}376 377// Acceptable: Performance-critical path with proof of correctness378// SAFETY: index is always < len due to the loop bound379unsafe { slice.get_unchecked(index) }380```381 382### When Unsafe Is NOT Acceptable383 384```rust385// Bad: Using unsafe to bypass borrow checker386// Bad: Using unsafe for convenience387// Bad: Using unsafe without a Safety comment388// Bad: Transmuting between unrelated types389```390 391## Module System and Crate Structure392 393### Organize by Domain, Not by Type394 395```text396my_app/397├── src/398│   ├── main.rs399│   ├── lib.rs400│   ├── auth/          # Domain module401│   │   ├── mod.rs402│   │   ├── token.rs403│   │   └── middleware.rs404│   ├── orders/        # Domain module405│   │   ├── mod.rs406│   │   ├── model.rs407│   │   └── service.rs408│   └── db/            # Infrastructure409│       ├── mod.rs410│       └── pool.rs411├── tests/             # Integration tests412├── benches/           # Benchmarks413└── Cargo.toml414```415 416### Visibility — Expose Minimally417 418```rust419// Good: pub(crate) for internal sharing420pub(crate) fn validate_input(input: &str) -> bool {421    !input.is_empty()422}423 424// Good: Re-export public API from lib.rs425pub mod auth;426pub use auth::AuthMiddleware;427 428// Bad: Making everything pub429pub fn internal_helper() {} // Should be pub(crate) or private430```431 432## Tooling Integration433 434### Essential Commands435 436```bash437# Build and check438cargo build439cargo check              # Fast type checking without codegen440cargo clippy             # Lints and suggestions441cargo fmt                # Format code442 443# Testing444cargo test445cargo test -- --nocapture    # Show println output446cargo test --lib             # Unit tests only447cargo test --test integration # Integration tests only448 449# Dependencies450cargo audit              # Security audit451cargo tree               # Dependency tree452cargo update             # Update dependencies453 454# Performance455cargo bench              # Run benchmarks456```457 458## Quick Reference: Rust Idioms459 460| Idiom | Description |461|-------|-------------|462| Borrow, don't clone | Pass `&T` instead of cloning unless ownership is needed |463| Make illegal states unrepresentable | Use enums to model valid states only |464| `?` over `unwrap()` | Propagate errors, never panic in library/production code |465| Parse, don't validate | Convert unstructured data to typed structs at the boundary |466| Newtype for type safety | Wrap primitives in newtypes to prevent argument swaps |467| Prefer iterators over loops | Declarative chains are clearer and often faster |468| `#[must_use]` on Results | Ensure callers handle return values |469| `Cow` for flexible ownership | Avoid allocations when borrowing suffices |470| Exhaustive matching | No wildcard `_` for business-critical enums |471| Minimal `pub` surface | Use `pub(crate)` for internal APIs |472 473## Anti-Patterns to Avoid474 475```rust476// Bad: .unwrap() in production code477let value = map.get("key").unwrap();478 479// Bad: .clone() to satisfy borrow checker without understanding why480let data = expensive_data.clone();481process(&original, &data);482 483// Bad: Using String when &str suffices484fn greet(name: String) { /* should be &str */ }485 486// Bad: Box<dyn Error> in libraries (use thiserror instead)487fn parse(input: &str) -> Result<Data, Box<dyn std::error::Error>> { todo!() }488 489// Bad: Ignoring must_use warnings490let _ = validate(input); // Silently discarding a Result491 492// Bad: Blocking in async context493async fn bad_async() {494    std::thread::sleep(Duration::from_secs(1)); // Blocks the executor!495    // Use: tokio::time::sleep(Duration::from_secs(1)).await;496}497```498 499**Remember**: If it compiles, it's probably correct — but only if you avoid `unwrap()`, minimize `unsafe`, and let the type system work for you.