--- name: rust-patterns description: Idiomatic Rust patterns, ownership, error handling, traits, concurrency, and best practices for building safe, performant applications. origin: ECC --- # Rust Development Patterns Idiomatic Rust patterns and best practices for building safe, performant, and maintainable applications. ## When to Use - Writing new Rust code - Reviewing Rust code - Refactoring existing Rust code - Designing crate structure and module layout ## How It Works This 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>`, channels, and async/await, and minimal `pub` surfaces organized by domain. ## Core Principles ### 1. Ownership and Borrowing Rust's ownership system prevents data races and memory bugs at compile time. ```rust // Good: Pass references when you don't need ownership fn process(data: &[u8]) -> usize { data.len() } // Good: Take ownership only when you need to store or consume fn store(data: Vec) -> Record { Record { payload: data } } // Bad: Cloning unnecessarily to avoid borrow checker fn process_bad(data: &Vec) -> usize { let cloned = data.clone(); // Wasteful — just borrow cloned.len() } ``` ### Use `Cow` for Flexible Ownership ```rust use std::borrow::Cow; fn normalize(input: &str) -> Cow<'_, str> { if input.contains(' ') { Cow::Owned(input.replace(' ', "_")) } else { Cow::Borrowed(input) // Zero-cost when no mutation needed } } ``` ## Error Handling ### Use `Result` and `?` — Never `unwrap()` in Production ```rust // Good: Propagate errors with context use anyhow::{Context, Result}; fn load_config(path: &str) -> Result { let content = std::fs::read_to_string(path) .with_context(|| format!("failed to read config from {path}"))?; let config: Config = toml::from_str(&content) .with_context(|| format!("failed to parse config from {path}"))?; Ok(config) } // Bad: Panics on error fn load_config_bad(path: &str) -> Config { let content = std::fs::read_to_string(path).unwrap(); // Panics! toml::from_str(&content).unwrap() } ``` ### Library Errors with `thiserror`, Application Errors with `anyhow` ```rust // Library code: structured, typed errors use thiserror::Error; #[derive(Debug, Error)] pub enum StorageError { #[error("record not found: {id}")] NotFound { id: String }, #[error("connection failed")] Connection(#[from] std::io::Error), #[error("invalid data: {0}")] InvalidData(String), } // Application code: flexible error handling use anyhow::{bail, Result}; fn run() -> Result<()> { let config = load_config("app.toml")?; if config.workers == 0 { bail!("worker count must be > 0"); } Ok(()) } ``` ### `Option` Combinators Over Nested Matching ```rust // Good: Combinator chain fn find_user_email(users: &[User], id: u64) -> Option { users.iter() .find(|u| u.id == id) .map(|u| u.email.clone()) } // Bad: Deeply nested matching fn find_user_email_bad(users: &[User], id: u64) -> Option { match users.iter().find(|u| u.id == id) { Some(user) => match &user.email { email => Some(email.clone()), }, None => None, } } ``` ## Enums and Pattern Matching ### Model States as Enums ```rust // Good: Impossible states are unrepresentable enum ConnectionState { Disconnected, Connecting { attempt: u32 }, Connected { session_id: String }, Failed { reason: String, retries: u32 }, } fn handle(state: &ConnectionState) { match state { ConnectionState::Disconnected => connect(), ConnectionState::Connecting { attempt } if *attempt > 3 => abort(), ConnectionState::Connecting { .. } => wait(), ConnectionState::Connected { session_id } => use_session(session_id), ConnectionState::Failed { retries, .. } if *retries < 5 => retry(), ConnectionState::Failed { reason, .. } => log_failure(reason), } } ``` ### Exhaustive Matching — No Catch-All for Business Logic ```rust // Good: Handle every variant explicitly match command { Command::Start => start_service(), Command::Stop => stop_service(), Command::Restart => restart_service(), // Adding a new variant forces handling here } // Bad: Wildcard hides new variants match command { Command::Start => start_service(), _ => {} // Silently ignores Stop, Restart, and future variants } ``` ## Traits and Generics ### Accept Generics, Return Concrete Types ```rust // Good: Generic input, concrete output fn read_all(reader: &mut impl Read) -> std::io::Result> { let mut buf = Vec::new(); reader.read_to_end(&mut buf)?; Ok(buf) } // Good: Trait bounds for multiple constraints fn process(item: T) -> String { format!("processed: {item}") } ``` ### Trait Objects for Dynamic Dispatch ```rust // Use when you need heterogeneous collections or plugin systems trait Handler: Send + Sync { fn handle(&self, request: &Request) -> Response; } struct Router { handlers: Vec>, } // Use generics when you need performance (monomorphization) fn fast_process(handler: &H, request: &Request) -> Response { handler.handle(request) } ``` ### Newtype Pattern for Type Safety ```rust // Good: Distinct types prevent mixing up arguments struct UserId(u64); struct OrderId(u64); fn get_order(user: UserId, order: OrderId) -> Result { // Can't accidentally swap user and order IDs todo!() } // Bad: Easy to swap arguments fn get_order_bad(user_id: u64, order_id: u64) -> Result { todo!() } ``` ## Structs and Data Modeling ### Builder Pattern for Complex Construction ```rust struct ServerConfig { host: String, port: u16, max_connections: usize, } impl ServerConfig { fn builder(host: impl Into, port: u16) -> ServerConfigBuilder { ServerConfigBuilder { host: host.into(), port, max_connections: 100 } } } struct ServerConfigBuilder { host: String, port: u16, max_connections: usize } impl ServerConfigBuilder { fn max_connections(mut self, n: usize) -> Self { self.max_connections = n; self } fn build(self) -> ServerConfig { ServerConfig { host: self.host, port: self.port, max_connections: self.max_connections } } } // Usage: ServerConfig::builder("localhost", 8080).max_connections(200).build() ``` ## Iterators and Closures ### Prefer Iterator Chains Over Manual Loops ```rust // Good: Declarative, lazy, composable let active_emails: Vec = users.iter() .filter(|u| u.is_active) .map(|u| u.email.clone()) .collect(); // Bad: Imperative accumulation let mut active_emails = Vec::new(); for user in &users { if user.is_active { active_emails.push(user.email.clone()); } } ``` ### Use `collect()` with Type Annotation ```rust // Collect into different types let names: Vec<_> = items.iter().map(|i| &i.name).collect(); let lookup: HashMap<_, _> = items.iter().map(|i| (i.id, i)).collect(); let combined: String = parts.iter().copied().collect(); // Collect Results — short-circuits on first error let parsed: Result, _> = strings.iter().map(|s| s.parse()).collect(); ``` ## Concurrency ### `Arc>` for Shared Mutable State ```rust use std::sync::{Arc, Mutex}; let counter = Arc::new(Mutex::new(0)); let handles: Vec<_> = (0..10).map(|_| { let counter = Arc::clone(&counter); std::thread::spawn(move || { let mut num = counter.lock().expect("mutex poisoned"); *num += 1; }) }).collect(); for handle in handles { handle.join().expect("worker thread panicked"); } ``` ### Channels for Message Passing ```rust use std::sync::mpsc; let (tx, rx) = mpsc::sync_channel(16); // Bounded channel with backpressure for i in 0..5 { let tx = tx.clone(); std::thread::spawn(move || { tx.send(format!("message {i}")).expect("receiver disconnected"); }); } drop(tx); // Close sender so rx iterator terminates for msg in rx { println!("{msg}"); } ``` ### Async with Tokio ```rust use tokio::time::Duration; async fn fetch_with_timeout(url: &str) -> Result { let response = tokio::time::timeout( Duration::from_secs(5), reqwest::get(url), ) .await .context("request timed out")? .context("request failed")?; response.text().await.context("failed to read body") } // Spawn concurrent tasks async fn fetch_all(urls: Vec) -> Vec> { let handles: Vec<_> = urls.into_iter() .map(|url| tokio::spawn(async move { fetch_with_timeout(&url).await })) .collect(); let mut results = Vec::with_capacity(handles.len()); for handle in handles { results.push(handle.await.unwrap_or_else(|e| panic!("spawned task panicked: {e}"))); } results } ``` ## Unsafe Code ### When Unsafe Is Acceptable ```rust // Acceptable: FFI boundary with documented invariants /// # Safety /// `ptr` must be a valid, aligned pointer to an initialized `Widget`. unsafe fn widget_from_raw<'a>(ptr: *const Widget) -> &'a Widget { // SAFETY: caller guarantees ptr is valid and aligned unsafe { &*ptr } } // Acceptable: Performance-critical path with proof of correctness // SAFETY: index is always < len due to the loop bound unsafe { slice.get_unchecked(index) } ``` ### When Unsafe Is NOT Acceptable ```rust // Bad: Using unsafe to bypass borrow checker // Bad: Using unsafe for convenience // Bad: Using unsafe without a Safety comment // Bad: Transmuting between unrelated types ``` ## Module System and Crate Structure ### Organize by Domain, Not by Type ```text my_app/ ├── src/ │ ├── main.rs │ ├── lib.rs │ ├── auth/ # Domain module │ │ ├── mod.rs │ │ ├── token.rs │ │ └── middleware.rs │ ├── orders/ # Domain module │ │ ├── mod.rs │ │ ├── model.rs │ │ └── service.rs │ └── db/ # Infrastructure │ ├── mod.rs │ └── pool.rs ├── tests/ # Integration tests ├── benches/ # Benchmarks └── Cargo.toml ``` ### Visibility — Expose Minimally ```rust // Good: pub(crate) for internal sharing pub(crate) fn validate_input(input: &str) -> bool { !input.is_empty() } // Good: Re-export public API from lib.rs pub mod auth; pub use auth::AuthMiddleware; // Bad: Making everything pub pub fn internal_helper() {} // Should be pub(crate) or private ``` ## Tooling Integration ### Essential Commands ```bash # Build and check cargo build cargo check # Fast type checking without codegen cargo clippy # Lints and suggestions cargo fmt # Format code # Testing cargo test cargo test -- --nocapture # Show println output cargo test --lib # Unit tests only cargo test --test integration # Integration tests only # Dependencies cargo audit # Security audit cargo tree # Dependency tree cargo update # Update dependencies # Performance cargo bench # Run benchmarks ``` ## Quick Reference: Rust Idioms | Idiom | Description | |-------|-------------| | Borrow, don't clone | Pass `&T` instead of cloning unless ownership is needed | | Make illegal states unrepresentable | Use enums to model valid states only | | `?` over `unwrap()` | Propagate errors, never panic in library/production code | | Parse, don't validate | Convert unstructured data to typed structs at the boundary | | Newtype for type safety | Wrap primitives in newtypes to prevent argument swaps | | Prefer iterators over loops | Declarative chains are clearer and often faster | | `#[must_use]` on Results | Ensure callers handle return values | | `Cow` for flexible ownership | Avoid allocations when borrowing suffices | | Exhaustive matching | No wildcard `_` for business-critical enums | | Minimal `pub` surface | Use `pub(crate)` for internal APIs | ## Anti-Patterns to Avoid ```rust // Bad: .unwrap() in production code let value = map.get("key").unwrap(); // Bad: .clone() to satisfy borrow checker without understanding why let data = expensive_data.clone(); process(&original, &data); // Bad: Using String when &str suffices fn greet(name: String) { /* should be &str */ } // Bad: Box in libraries (use thiserror instead) fn parse(input: &str) -> Result> { todo!() } // Bad: Ignoring must_use warnings let _ = validate(input); // Silently discarding a Result // Bad: Blocking in async context async fn bad_async() { std::thread::sleep(Duration::from_secs(1)); // Blocks the executor! // Use: tokio::time::sleep(Duration::from_secs(1)).await; } ``` **Remember**: If it compiles, it's probably correct — but only if you avoid `unwrap()`, minimize `unsafe`, and let the type system work for you.