name: rust description: Rust language expertise for writing safe, performant, production-quality Rust code. Use for Rust development, ownership patterns, error handling, async/await, and cargo management. Triggers: rust, cargo, rustc, ownership, borrowing, lifetime, Result, Option, traits, crates.

Rust Language Expertise

Overview

This skill provides guidance for writing safe, efficient, and idiomatic Rust code. It covers Rust's ownership system, error handling patterns, traits and generics, async programming, and cargo ecosystem.

Key Concepts

Ownership, Borrowing, and Lifetimes

// Ownership rules:
// 1. Each value has exactly one owner
// 2. When the owner goes out of scope, the value is dropped
// 3. Ownership can be transferred (moved) or borrowed

// Move semantics
fn take_ownership(s: String) {
    println!("{}", s);
} // s is dropped here

fn main() {
    let s = String::from("hello");
    take_ownership(s);
    // s is no longer valid here
}

// Borrowing (references)
fn borrow(s: &String) {
    println!("{}", s);
}

fn borrow_mut(s: &mut String) {
    s.push_str(" world");
}

// Lifetimes ensure references are valid
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

// Struct with lifetime annotations
struct Parser<'a> {
    input: &'a str,
    position: usize,
}

impl<'a> Parser<'a> {
    fn new(input: &'a str) -> Self {
        Parser { input, position: 0 }
    }

    fn peek(&self) -> Option<char> {
        self.input[self.position..].chars().next()
    }
}

// Common lifetime elision patterns
impl Config {
    // fn get(&self, key: &str) -> Option<&str>
    // is short for:
    // fn get<'a, 'b>(&'a self, key: &'b str) -> Option<&'a str>
    fn get(&self, key: &str) -> Option<&str> {
        self.map.get(key).map(|s| s.as_str())
    }
}

Error Handling

use std::error::Error;
use std::fmt;
use std::io;

// Using thiserror for custom errors
use thiserror::Error;

#[derive(Error, Debug)]
pub enum AppError {
    #[error("IO error: {0}")]
    Io(#[from] io::Error),

    #[error("Parse error at line {line}: {message}")]
    Parse { line: usize, message: String },

    #[error("Not found: {0}")]
    NotFound(String),

    #[error("Validation failed: {0}")]
    Validation(String),
}

// Using anyhow for application code
use anyhow::{Context, Result, bail, ensure};

fn read_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 = serde_json::from_str(&content)
        .context("Failed to parse config JSON")?;

    ensure!(!config.name.is_empty(), "Config name cannot be empty");

    if config.port == 0 {
        bail!("Invalid port number");
    }

    Ok(config)
}

// The ? operator for propagating errors
fn process_file(path: &str) -> Result<Vec<Record>, AppError> {
    let content = std::fs::read_to_string(path)?; // io::Error -> AppError via From
    let records = parse_records(&content)?;
    Ok(records)
}

// Option handling
fn find_user(users: &[User], name: &str) -> Option<&User> {
    users.iter().find(|u| u.name == name)
}

fn get_user_email(users: &[User], name: &str) -> Option<String> {
    users
        .iter()
        .find(|u| u.name == name)
        .and_then(|u| u.email.clone())
}

// Converting between Option and Result
fn require_user(users: &[User], name: &str) -> Result<&User, AppError> {
    users
        .iter()
        .find(|u| u.name == name)
        .ok_or_else(|| AppError::NotFound(format!("User: {}", name)))
}

Traits and Generics

// Defining traits
trait Repository<T> {
    fn get(&self, id: &str) -> Option<&T>;
    fn save(&mut self, item: T) -> Result<(), Box<dyn Error>>;

    // Default implementation
    fn exists(&self, id: &str) -> bool {
        self.get(id).is_some()
    }
}

// Trait bounds
fn process<T: Clone + Debug>(item: &T) {
    let cloned = item.clone();
    println!("{:?}", cloned);
}

// where clauses for complex bounds
fn merge<T, U, V>(a: T, b: U) -> V
where
    T: IntoIterator<Item = V>,
    U: IntoIterator<Item = V>,
    V: Ord + Clone,
{
    let mut result: Vec<V> = a.into_iter().chain(b.into_iter()).collect();
    result.sort();
    result.dedup();
    result.into_iter().next().unwrap()
}

// Associated types
trait Iterator {
    type Item;
    fn next(&mut self) -> Option<Self::Item>;
}

// Implementing traits
struct InMemoryRepo<T> {
    items: HashMap<String, T>,
}

impl<T: Clone> Repository<T> for InMemoryRepo<T> {
    fn get(&self, id: &str) -> Option<&T> {
        self.items.get(id)
    }

    fn save(&mut self, item: T) -> Result<(), Box<dyn Error>> {
        // Implementation
        Ok(())
    }
}

// Blanket implementations
impl<T: Display> ToString for T {
    fn to_string(&self) -> String {
        format!("{}", self)
    }
}

Iterators

// Iterator combinators
fn process_users(users: Vec<User>) -> Vec<String> {
    users
        .into_iter()
        .filter(|u| u.active)
        .map(|u| u.email)
        .filter_map(|email| email)  // Remove None values
        .collect()
}

// Custom iterator
struct Counter {
    current: usize,
    max: usize,
}

impl Iterator for Counter {
    type Item = usize;

    fn next(&mut self) -> Option<Self::Item> {
        if self.current < self.max {
            let val = self.current;
            self.current += 1;
            Some(val)
        } else {
            None
        }
    }
}

// Useful iterator methods
fn examples(numbers: Vec<i32>) {
    // Fold/reduce
    let sum: i32 = numbers.iter().fold(0, |acc, x| acc + x);

    // Any/all
    let has_positive = numbers.iter().any(|&x| x > 0);
    let all_positive = numbers.iter().all(|&x| x > 0);

    // Find
    let first_even = numbers.iter().find(|&&x| x % 2 == 0);

    // Partition
    let (evens, odds): (Vec<_>, Vec<_>) = numbers.iter().partition(|&&x| x % 2 == 0);

    // Enumerate
    for (index, value) in numbers.iter().enumerate() {
        println!("{}: {}", index, value);
    }

    // Zip
    let other = vec![1, 2, 3];
    let pairs: Vec<_> = numbers.iter().zip(other.iter()).collect();
}

Best Practices

Cargo and Project Structure

# Cargo.toml
[package]
name = "myproject"
version = "0.1.0"
edition = "2021"
rust-version = "1.75"

[dependencies]
tokio = { version = "1.35", features = ["full"] }
serde = { version = "1.0", features = ["derive"] }
thiserror = "1.0"
anyhow = "1.0"

[dev-dependencies]
criterion = "0.5"
mockall = "0.12"

[features]
default = []
full = ["feature-a", "feature-b"]
feature-a = []
feature-b = ["dep:optional-dep"]

[[bench]]
name = "my_benchmark"
harness = false

Workspace Structure

myworkspace/
├── Cargo.toml          # Workspace root
├── crates/
│   ├── core/
│   │   ├── Cargo.toml
│   │   └── src/
│   ├── api/
│   │   ├── Cargo.toml
│   │   └── src/
│   └── cli/
│       ├── Cargo.toml
│       └── src/

# Root Cargo.toml
[workspace]
members = ["crates/*"]
resolver = "2"

[workspace.dependencies]
serde = { version = "1.0", features = ["derive"] }
tokio = { version = "1.35", features = ["full"] }

Async/Await

use tokio::sync::{mpsc, Mutex};
use std::sync::Arc;

async fn fetch_data(url: &str) -> Result<String> {
    let response = reqwest::get(url).await?;
    let body = response.text().await?;
    Ok(body)
}

// Concurrent execution
async fn fetch_all(urls: Vec<String>) -> Vec<Result<String>> {
    let futures: Vec<_> = urls.iter().map(|url| fetch_data(url)).collect();
    futures::future::join_all(futures).await
}

// Shared state with Arc<Mutex<T>>
struct AppState {
    counter: Arc<Mutex<u64>>,
}

impl AppState {
    async fn increment(&self) -> u64 {
        let mut counter = self.counter.lock().await;
        *counter += 1;
        *counter
    }
}

// Channel communication
async fn producer_consumer() {
    let (tx, mut rx) = mpsc::channel(32);

    tokio::spawn(async move {
        for i in 0..10 {
            tx.send(i).await.unwrap();
        }
    });

    while let Some(value) = rx.recv().await {
        println!("Received: {}", value);
    }
}

Common Patterns

Builder Pattern

#[derive(Default)]
pub struct RequestBuilder {
    url: Option<String>,
    method: Method,
    headers: HashMap<String, String>,
    body: Option<Vec<u8>>,
    timeout: Duration,
}

impl RequestBuilder {
    pub fn new() -> Self {
        Self {
            method: Method::GET,
            timeout: Duration::from_secs(30),
            ..Default::default()
        }
    }

    pub fn url(mut self, url: impl Into<String>) -> Self {
        self.url = Some(url.into());
        self
    }

    pub fn method(mut self, method: Method) -> Self {
        self.method = method;
        self
    }

    pub fn header(mut self, key: impl Into<String>, value: impl Into<String>) -> Self {
        self.headers.insert(key.into(), value.into());
        self
    }

    pub fn body(mut self, body: impl Into<Vec<u8>>) -> Self {
        self.body = Some(body.into());
        self
    }

    pub fn build(self) -> Result<Request, BuildError> {
        let url = self.url.ok_or(BuildError::MissingUrl)?;
        Ok(Request {
            url,
            method: self.method,
            headers: self.headers,
            body: self.body,
            timeout: self.timeout,
        })
    }
}

Newtype Pattern

// Type safety through newtype
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct UserId(String);

impl UserId {
    pub fn new(id: impl Into<String>) -> Self {
        UserId(id.into())
    }

    pub fn as_str(&self) -> &str {
        &self.0
    }
}

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct OrderId(String);

// Now these are different types - can't mix them up
fn get_user(id: UserId) -> Option<User> { /* ... */ }
fn get_order(id: OrderId) -> Option<Order> { /* ... */ }

Testing

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_basic() {
        let result = add(2, 3);
        assert_eq!(result, 5);
    }

    #[test]
    fn test_with_result() -> Result<(), Box<dyn Error>> {
        let config = parse_config("valid config")?;
        assert_eq!(config.name, "test");
        Ok(())
    }

    #[test]
    #[should_panic(expected = "divide by zero")]
    fn test_panic() {
        divide(1, 0);
    }

    // Async tests with tokio
    #[tokio::test]
    async fn test_async_function() {
        let result = fetch_data("http://example.com").await;
        assert!(result.is_ok());
    }

    // Property-based testing with proptest
    use proptest::prelude::*;

    proptest! {
        #[test]
        fn test_parse_roundtrip(s in "[a-z]+") {
            let parsed = parse(&s)?;
            let serialized = serialize(&parsed);
            prop_assert_eq!(s, serialized);
        }
    }
}

Anti-Patterns

Avoid These Practices

// BAD: Unnecessary clone
fn process(items: &Vec<String>) {
    for item in items.clone() {  // Unnecessary allocation
        println!("{}", item);
    }
}

// GOOD: Iterate by reference
fn process(items: &[String]) {
    for item in items {
        println!("{}", item);
    }
}

// BAD: Using unwrap/expect in library code
fn parse_config(s: &str) -> Config {
    serde_json::from_str(s).unwrap()  // Panics on invalid input
}

// GOOD: Return Result and let caller handle errors
fn parse_config(s: &str) -> Result<Config, serde_json::Error> {
    serde_json::from_str(s)
}

// BAD: Excessive use of Rc<RefCell<T>>
struct Node {
    value: i32,
    children: Vec<Rc<RefCell<Node>>>,
}

// GOOD: Consider arena allocation or indices
struct Arena {
    nodes: Vec<Node>,
}
struct Node {
    value: i32,
    children: Vec<usize>,  // Indices into arena
}

// BAD: String concatenation in loops
fn build_message(parts: &[&str]) -> String {
    let mut result = String::new();
    for part in parts {
        result = result + part + ", ";  // Creates new String each iteration
    }
    result
}

// GOOD: Use push_str or collect
fn build_message(parts: &[&str]) -> String {
    parts.join(", ")
}

// BAD: Boxing errors unnecessarily
fn parse(s: &str) -> Result<Data, Box<dyn Error>> {
    // For libraries, use concrete error types
}

// GOOD: Use concrete error types in libraries
fn parse(s: &str) -> Result<Data, ParseError> {
    // thiserror for library errors, anyhow for applications
}

// BAD: Unsafe without justification
unsafe fn get_unchecked(slice: &[i32], index: usize) -> i32 {
    *slice.get_unchecked(index)
}

// GOOD: Safe by default, unsafe with clear invariants
fn get_unchecked(slice: &[i32], index: usize) -> i32 {
    // SAFETY: Caller must ensure index < slice.len()
    // Only use when bounds checking is a proven bottleneck
    debug_assert!(index < slice.len());
    unsafe { *slice.get_unchecked(index) }
}

// BAD: Ignoring must_use
let _ = fs::remove_file("temp.txt");  // Error silently ignored

// GOOD: Handle the result
fs::remove_file("temp.txt").ok();  // Explicitly ignore
// or
fs::remove_file("temp.txt")?;  // Propagate error