Agent skill

rust-coding-standards

Master Rust's ownership system, type safety, and zero-cost abstractions for building safe, concurrent, and performant systems. Covers borrowing, lifetimes, traits, error handling, async/await, and testing patterns.

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SKILL.md

Rust Coding Standards

Purpose: Master Rust's ownership system, type safety, and zero-cost abstractions for building safe, concurrent, and performant systems.

Level 1: Quick Reference

Core Principles

  • Ownership: Each value has exactly one owner
  • Borrowing: References allow multiple readers XOR one writer
  • Zero-cost abstractions: High-level code compiles to optimal machine code
  • Explicit over implicit: No hidden allocations or control flow

Ownership & Borrowing Cheat Sheet

rust
// Ownership transfer (move)
let s1 = String::from("hello");
let s2 = s1; // s1 is now invalid

// Immutable borrow (multiple allowed)
let s = String::from("hello");
let r1 = &s;
let r2 = &s;
println!("{} and {}", r1, r2); // OK

// Mutable borrow (exclusive)
let mut s = String::from("hello");
let r1 = &mut s;
r1.push_str(" world");

// Copy types (stack-only)
let x = 5;
let y = x; // x is still valid (i32 implements Copy)

Result & Option

rust
// Option: value may be absent
let name = find_user(42).map(|u| u.name).unwrap_or("Anonymous".into());

// Early return with ?
fn process() -> Result<(), Error> {
    let config = parse_config("app.toml")?;
    Ok(())
}

Essential Checklist

  • cargo clippy -- -D warnings
  • cargo fmt -- --check
  • cargo test
  • cargo doc --no-deps
  • cargo audit
  • Document all unsafe blocks

Memory Safety Rules

  1. No data races: Compiler enforces at compile time
  2. No null pointers: Use Option<T> instead
  3. No dangling pointers: Borrow checker prevents
  4. No use-after-free: Ownership system prevents
  5. No buffer overflows: Bounds checking

Level 2: Implementation Guide

Ownership System

The Three Rules

  1. Each value has a variable that's its owner
  2. There can only be one owner at a time
  3. When the owner goes out of scope, the value is dropped
rust
fn takes_ownership(s: String) {
    println!("{}", s);
} // s dropped here

fn main() {
    let s = String::from("hello");
    takes_ownership(s); // s moved
    // println!("{}", s); // Error: value borrowed after move
}

// Return ownership
fn gives_ownership() -> String {
    String::from("hello")
}

Borrowing & References

rust
// Multiple immutable borrows allowed
fn calculate_length(s: &String) -> usize {
    s.len()
}

// Only one mutable borrow allowed
fn change(s: &mut String) {
    s.push_str(", world");
}

// Non-lexical lifetimes
let mut s = String::from("hello");
let r1 = &s;
let r2 = &s;
println!("{} and {}", r1, r2); // last use of r1 and r2
let r3 = &mut s; // OK: r1 and r2 no longer used

Lifetime Annotations

rust
// Specify reference relationships
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

// Struct with lifetime
struct Excerpt<'a> {
    part: &'a str,
}

// Lifetime elision: compiler infers in common patterns
fn first_word(s: &str) -> &str { /* elided lifetimes */ }

Traits & Generics

rust
pub trait Summary {
    fn summarize(&self) -> String;

    // Default implementation
    fn full_summary(&self) -> String {
        format!("Read more: {}", self.summarize())
    }
}

impl Summary for Article {
    fn summarize(&self) -> String {
        format!("{} by {}", self.headline, self.author)
    }
}

// Trait bounds
pub fn notify<T: Summary + Display>(item: &T) {
    println!("Breaking: {}", item.summarize());
}

// where clause for readability
pub fn process<T, U>(t: &T, u: &U) -> String
where
    T: Display + Clone,
    U: Clone + Debug,
{
    format!("{:?}", u)
}

Common Derived Traits

rust
#[derive(Debug, Clone, PartialEq, Eq, Hash, Default)]
pub struct User {
    pub id: u32,
    pub name: String,
}

Error Handling

rust
use thiserror::Error;

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

    #[error("Database error: {0}")]
    Database(#[from] sqlx::Error),

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

pub type Result<T> = std::result::Result<T, AppError>;

// With anyhow for context
use anyhow::Context;

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

Async/Await

rust
use tokio;

#[tokio::main]
async fn main() -> Result<()> {
    let result = fetch_data().await?;
    process(result).await?;
    Ok(())
}

// Concurrent execution
let futures = urls.iter().map(|url| fetch_url(url));
let results = futures::future::join_all(futures).await;

// Spawn tasks
let handle = tokio::spawn(async move {
    expensive_computation().await
});
let result = handle.await?;

Channels

rust
use tokio::sync::mpsc;

let (tx, mut rx) = mpsc::channel(32);

// Producer
tokio::spawn(async move {
    tx.send(work).await.unwrap();
});

// Consumer
while let Some(result) = rx.recv().await {
    process(result).await;
}

Shared State

rust
use std::sync::Arc;
use tokio::sync::{Mutex, RwLock};

#[derive(Clone)]
struct AppState {
    counter: Arc<Mutex<u64>>,
    cache: Arc<RwLock<HashMap<String, String>>>,
}

async fn increment(state: AppState) {
    let mut counter = state.counter.lock().await;
    *counter += 1;
}

Testing

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

    #[test]
    fn test_basic() {
        assert_eq!(2 + 2, 4);
    }

    #[test]
    #[should_panic(expected = "divide by zero")]
    fn test_panic() {
        divide(10, 0);
    }

    #[test]
    fn test_result() -> Result<()> {
        let result = parse_number("42")?;
        assert_eq!(result, 42);
        Ok(())
    }

    #[tokio::test]
    async fn test_async() {
        let result = fetch_data().await;
        assert!(result.is_ok());
    }
}

Performance Patterns

rust
// Prefer slices over owned strings
fn count_words(text: &str) -> usize {
    text.split_whitespace().count()
}

// Use Cow for conditional cloning
use std::borrow::Cow;

fn process_text(text: &str) -> Cow<str> {
    if text.contains("bad") {
        Cow::Owned(text.replace("bad", "good"))
    } else {
        Cow::Borrowed(text)
    }
}

// Reuse buffers
let mut buffer = String::with_capacity(1024);
for item in items {
    buffer.clear();
    write!(&mut buffer, "{}", item)?;
    send(&buffer)?;
}

Iterators

rust
// Transformation pipeline (zero-cost)
let sum: i32 = vec![1, 2, 3, 4, 5]
    .iter()
    .filter(|&x| x % 2 == 0)
    .map(|x| x * 2)
    .sum();

// Collect into different types
let doubled: Vec<i32> = numbers.iter().map(|x| x * 2).collect();
let set: HashSet<i32> = numbers.into_iter().collect();

Level 3: Deep Dive Resources

For comprehensive coverage of advanced topics, see REFERENCE.md:

Advanced Patterns

  • Newtype Pattern: Type safety via wrapper types
  • Typestate Pattern: Encode state machines in types
  • Builder Pattern: Fluent API construction

Unsafe Code Guidelines

  • Documenting safety invariants
  • Minimizing unsafe scope
  • Testing with Miri

Complete Examples

  • Full async service implementation
  • Property-based testing with proptest
  • Mocking with mockall
  • Integration test patterns

Configuration Files

  • config/rustfmt.toml - Formatter configuration
  • config/clippy.toml - Linter rules
  • templates/ - Project templates

Performance Deep Dive

  • Profiling with flamegraph
  • Compiler optimization flags
  • Memory profiling with valgrind

Learning Path

  1. Start: Rust Book
  2. Practice: Rustlings
  3. Patterns: Rust Design Patterns
  4. Async: Tokio Tutorial

Common Pitfalls

  1. Fighting the borrow checker - design with ownership in mind
  2. Over-using .clone() - profile before optimizing
  3. Ignoring Clippy warnings - they're usually right
  4. Not using ? operator - makes error handling cleaner
  5. Premature unsafe - try safe abstractions first
  6. String vs &str confusion - prefer &str for parameters

Skill Complete: Comprehensive Rust coding standards covering ownership, traits, error handling, concurrency, testing, and performance. See REFERENCE.md for advanced patterns and complete examples.

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