/rust-patterns

name: rust-patterns
description: Idiomatic Rust patterns, ownership, error handling, traits, concurrency, and best practices for building safe, performant applications. Use when writing or reviewing Rust code and ownership, error handling, traits, or concurrency is in question.
metadata:
  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!()
}

// B