Rust Skill

Overview

Rust's ownership system is your ally, not your enemy. This skill guides writing idiomatic Rust that works with the compiler, not against it.

Core principle: If it compiles, it's probably correct. Rust's strictness is a feature. Don't fight the borrow checker—understand what it's telling you.

The Rust Development Process

Phase 1: Design with Ownership

Before writing implementation:

1. Think About Data Ownership

   • Who owns this data?
   • Who needs to read it?
   • Who needs to mutate it?

2. Choose the Right Type

   • Owned: String, Vec<T>, Box<T>
   • Borrowed: &str, &[T], &T
   • Mutable borrow: &mut T

3. Plan Error Handling

   • What can fail?
   • Use Result<T, E> for recoverable errors
   • Use Option<T> for absence of value

Phase 2: Implement Idiomatically

Write Rust, not C-in-Rust:

1. Propagate Errors with ?

   // ✅ Idiomatic error propagation
   fn read_config(path: &str) -> Result<Config, ConfigError> {
       let content = std::fs::read_to_string(path)?;
       let config: Config = toml::from_str(&content)?;
       Ok(config)
   }
   
   // ❌ Manual error handling
   fn read_config(path: &str) -> Result<Config, ConfigError> {
       let content = match std::fs::read_to_string(path) {
           Ok(c) => c,
           Err(e) => return Err(e.into()),
       };
       // ... more verbose code
   }
   

2. Use Iterators, Not Loops

   // ✅ Idiomatic
   let sum: i32 = numbers.iter().filter(|&n| n > 0).sum();
   
   // ❌ Imperative
   let mut sum = 0;
   for n in &numbers {
       if n > 0 {
           sum += n;
       }
   }
   

3. Accept Borrowed, Return Owned

   // ✅ Flexible API
   fn greet(name: &str) -> String {
       format!("Hello, {}!", name)
   }
   
   // ❌ Forces allocation on caller
   fn greet(name: String) -> String {
       format!("Hello, {}!", name)
   }
   

Phase 3: Review for Idioms

Before approving:

1. Check Error Handling

   • No .unwrap() in production code?
   • Errors wrapped with context?
   • Custom error types where appropriate?

2. Check Ownership

   • Minimal cloning?
   • Borrows instead of moves where possible?
   • Lifetimes elided when possible?

3. Check Concurrency

   • Send and Sync bounds understood?
   • No data races possible?
   • Appropriate sync primitives?

Red Flags - STOP and Fix

Error Handling Red Flags

// Unwrap in production code
let value = something.unwrap();  // Will panic!

// Expect without meaningful message
let value = something.expect("failed");  // Unhelpful

// Ignoring Result
let _ = file.write_all(data);  // Silent failure!

// String error types
fn process() -> Result<(), String> {  // Use proper error types

Ownership Red Flags

// Clone to satisfy borrow checker
let data = expensive.clone();  // Usually wrong approach
process(data);

// Unnecessary lifetime annotations
fn first<'a>(s: &'a str) -> &'a str {  // Elision works here
    &s[..1]
}

// Rc/RefCell for single-threaded ownership confusion
let data = Rc::new(RefCell::new(vec![]));  // Smell: rethink design

// Box<dyn Trait> when generics work
fn process(handler: Box<dyn Handler>) {  // Prefer generics

Concurrency Red Flags

// Mutex without Arc in multithreaded context
let mutex = Mutex::new(data);  // Can't share across threads

// Holding lock across await
let guard = mutex.lock().await;
async_operation().await;  // Lock held! Use async-aware mutex

// Thread::spawn without join
thread::spawn(|| work());  // Fire and forget = bug

Common Rationalizations - Don't Accept These

Rust Quality Checklist

Before approving Rust code:

• No unwrap: ? operator or proper error handling
• Errors contextual: Using anyhow or thiserror properly
• Minimal cloning: Borrows preferred where possible
• Idiomatic APIs: Accept &str, return String; accept &[T], return Vec
• Iterators used: Not manual loops for transforms
• Clippy clean: cargo clippy -- -D warnings
• Tests present: Unit and integration tests

Quick Patterns

Error Handling

// ✅ With thiserror
use thiserror::Error;

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

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

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

// ✅ With anyhow for applications
use anyhow::{Context, Result};

fn load_config(path: &str) -> Result<Config> {
    let content = std::fs::read_to_string(path)
        .context("Failed to read config file")?;
    let config: Config = toml::from_str(&content)
        .context("Failed to parse config")?;
    Ok(config)
}

Ownership Patterns

// ✅ Builder pattern
#[derive(Default)]
pub struct RequestBuilder {
    url: Option<String>,
    timeout: Option<Duration>,
}

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

    pub fn build(self) -> Result<Request, BuildError> {
        Ok(Request {
            url: self.url.ok_or(BuildError::MissingUrl)?,
            timeout: self.timeout.unwrap_or(Duration::from_secs(30)),
        })
    }
}

Concurrent Patterns

// ✅ Shared state with Arc<Mutex<T>>
use std::sync::{Arc, Mutex};

let counter = Arc::new(Mutex::new(0));
let handles: Vec<_> = (0..10).map(|_| {
    let counter = Arc::clone(&counter);
    thread::spawn(move || {
        let mut num = counter.lock().unwrap();
        *num += 1;
    })
}).collect();

for handle in handles {
    handle.join().unwrap();
}

Quick Commands

# Format
cargo fmt

# Lint (strict)
cargo clippy -- -D warnings

# Test with output
cargo test -- --nocapture

# Check without building
cargo check

# Security audit
cargo audit

# Documentation
cargo doc --open

# Release build
cargo build --release

References

Detailed patterns and examples in references/:

• ownership-patterns.md - Ownership and borrowing deep dive
• error-handling.md - Error handling patterns
• async-rust.md - Async/await patterns