rust-patterns

SKILL.md

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<Mutex<T>>, 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.

// 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<u8>) -> Record {
    Record { payload: data }
}

// Bad: Cloning unnecessarily to avoid borrow checker
fn process_bad(data: &Vec<u8>) -> usize {
    let cloned = data.clone(); // Wasteful — just borrow
    cloned.len()
}

Use Cow for Flexible Ownership

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

// Good: Propagate errors with context
use anyhow::{Context, Result};

fn load_config(path: &str) -> Result<Config> {
    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

// 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

// Good: Combinator chain
fn find_user_email(users: &[User], id: u64) -> Option<String> {
    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<String> {
    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

// 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

// 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

// Good: Generic input, concrete output
fn read_all(reader: &mut impl Read) -> std::io::Result<Vec<u8>> {
    let mut buf = Vec::new();
    reader.read_to_end(&mut buf)?;
    Ok(buf)
}

// Good: Trait bounds for multiple constraints
fn process<T: Display + Send + 'static>(item: T) -> String {
    format!("processed: {item}")
}

Trait Objects for Dynamic Dispatch

// Use when you need heterogeneous collections or plugin systems
trait Handler: Send + Sync {
    fn handle(&self, request: &Request) -> Response;
}

struct Router {
    handlers: Vec<Box<dyn Handler>>,
}

// Use generics when you need performance (monomorphization)
fn fast_process<H: Handler>(handler: &H, request: &Request) -> Response {
    handler.handle(request)
}

Newtype Pattern for Type Safety

// Good: Distinct types prevent mixing up arguments
struct UserId(u64);
struct OrderId(u64);

fn get_order(user: UserId, order: OrderId) -> Result<Order> {
    // 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<Order> {
    todo!()
}

Structs and Data Modeling

Builder Pattern for Complex Construction

struct ServerConfig {
    host: String,
    port: u16,
    max_connections: usize,
}

impl ServerConfig {
    fn builder(host: impl Into<String>, 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

// Good: Declarative, lazy, composable
let active_emails: Vec<String> = 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

// 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<Vec<i32>, _> = strings.iter().map(|s| s.parse()).collect();

Concurrency

Arc<Mutex<T>> for Shared Mutable State

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

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

use tokio::time::Duration;

async fn fetch_with_timeout(url: &str) -> Result<String> {
    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<String>) -> Vec<Result<String>> {
    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

// Acceptable: FFI boundary with documented invariants (Rust 2024+)
/// # 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

// 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

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

// 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

# 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

// 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<dyn Error> in libraries (use thiserror instead)
fn parse(input: &str) -> Result<Data, Box<dyn std::error::Error>> { 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.