name: rust-best-practices
description: Rust coding best practices based on Microsoft Pragmatic Rust Guidelines. ALWAYS invoke before writing or modifying Rust code. Covers error handling, API design, performance, and idiomatic patterns.

Rust Best Practices

Based on Microsoft Pragmatic Rust Guidelines and Rust community standards.

Core Principles

1. Leverage the type system — Use types to make invalid states unrepresentable
2. Embrace ownership — Work with the borrow checker, not against it
3. Explicit over implicit — Be clear about fallibility, mutability, and lifetimes
4. Zero-cost abstractions — Use iterators, generics, and traits without runtime cost
5. Fail fast, recover gracefully — Validate early, handle errors explicitly

Error Handling

Use thiserror for Libraries

use thiserror::Error;

#[derive(Error, Debug)]
pub enum MyError {
    #[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),
}

Use anyhow for Applications

use anyhow::{Context, Result};

fn main() -> Result<()> {
    let config = load_config()
        .context("Failed to load configuration")?;
    run_app(config)?;
    Ok(())
}

Never Panic in Libraries

// ❌ BAD
pub fn get_item(index: usize) -> &Item {
    &self.items[index]  // Panics on out-of-bounds
}

// ✅ GOOD
pub fn get_item(&self, index: usize) -> Option<&Item> {
    self.items.get(index)
}

// ✅ GOOD - when you need Result
pub fn get_item(&self, index: usize) -> Result<&Item, Error> {
    self.items.get(index).ok_or(Error::NotFound(index))
}

API Design

Use Builder Pattern for Complex Configs

pub struct Client {
    url: String,
    timeout: Duration,
    retries: u32,
}

impl Client {
    pub fn builder(url: impl Into<String>) -> ClientBuilder {
        ClientBuilder {
            url: url.into(),
            timeout: Duration::from_secs(30),
            retries: 3,
        }
    }
}

pub struct ClientBuilder {
    url: String,
    timeout: Duration,
    retries: u32,
}

impl ClientBuilder {
    pub fn timeout(mut self, timeout: Duration) -> Self {
        self.timeout = timeout;
        self
    }

    pub fn retries(mut self, retries: u32) -> Self {
        self.retries = retries;
        self
    }

    pub fn build(self) -> Client {
        Client {
            url: self.url,
            timeout: self.timeout,
            retries: self.retries,
        }
    }
}

Use Newtype Pattern

// ❌ BAD - primitive obsession
fn create_user(name: String, email: String, age: u32) -> User { ... }

// ✅ GOOD - newtype wrappers
pub struct Username(String);
pub struct Email(String);
pub struct Age(u32);

impl Email {
    pub fn new(email: impl Into<String>) -> Result<Self, ValidationError> {
        let email = email.into();
        if email.contains('@') {
            Ok(Self(email))
        } else {
            Err(ValidationError::InvalidEmail)
        }
    }
}

fn create_user(name: Username, email: Email, age: Age) -> User { ... }

Accept impl Trait for Flexibility

// ❌ BAD - overly specific
pub fn process(items: Vec<String>) { ... }

// ✅ GOOD - accept any iterable
pub fn process(items: impl IntoIterator<Item = impl AsRef<str>>) {
    for item in items {
        println!("{}", item.as_ref());
    }
}

Performance

Avoid Unnecessary Clones

// ❌ BAD
fn process(data: &String) {
    let owned = data.clone();  // Unnecessary allocation
    do_something(owned);
}

// ✅ GOOD
fn process(data: &str) {
    do_something(data);
}

Use Cow for Conditional Ownership

use std::borrow::Cow;

fn normalize(input: &str) -> Cow<'_, str> {
    if input.contains(' ') {
        Cow::Owned(input.replace(' ', "_"))
    } else {
        Cow::Borrowed(input)
    }
}

Prefer Iterators Over Loops

// ❌ BAD
let mut result = Vec::new();
for item in items {
    if item.is_valid() {
        result.push(item.transform());
    }
}

// ✅ GOOD
let result: Vec<_> = items
    .into_iter()
    .filter(|item| item.is_valid())
    .map(|item| item.transform())
    .collect();

Async Patterns

Use tokio Runtime

#[tokio::main]
async fn main() -> anyhow::Result<()> {
    tracing_subscriber::init();
    run().await
}

Structured Concurrency with JoinSet

use tokio::task::JoinSet;

async fn process_all(urls: Vec<String>) -> Vec<Result<Response, Error>> {
    let mut set = JoinSet::new();

    for url in urls {
        set.spawn(async move {
            fetch(&url).await
        });
    }

    let mut results = Vec::new();
    while let Some(res) = set.join_next().await {
        results.push(res.unwrap());
    }
    results
}

Use #[instrument] for Tracing

use tracing::instrument;

#[instrument(skip(password))]
async fn login(username: &str, password: &str) -> Result<Token> {
    tracing::info!("Attempting login");
    // ...
}

Testing

Use #[test] and proptest

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

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

    proptest! {
        #[test]
        fn test_add_commutative(a: i32, b: i32) {
            prop_assert_eq!(add(a, b), add(b, a));
        }
    }
}

Use mockall for Mocking

#[cfg_attr(test, mockall::automock)]
trait Database {
    async fn get(&self, id: u64) -> Result<Record>;
}

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

    #[tokio::test]
    async fn test_with_mock() {
        let mut mock = MockDatabase::new();
        mock.expect_get()
            .returning(|_| Ok(Record::default()));

        let service = Service::new(mock);
        assert!(service.process(1).await.is_ok());
    }
}

Common Anti-Patterns to Avoid

References

• Microsoft Pragmatic Rust Guidelines
• Rust API Guidelines
• The Rust Book