/rust-patterns

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<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