---

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

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Error Handling

Use thiserror for Libraries

Copyuse 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

Copyuse anyhow::{Context, Result};

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

Never Panic in Libraries

Copy// ❌ 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

Copypub 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

Copy// ❌ 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

Copy// ❌ 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

Copy// ❌ 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

Copyuse std::borrow::Cow;

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

Prefer Iterators Over Loops

Copy// ❌ 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

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

Structured Concurrency with JoinSet

Copyuse 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

Copyuse tracing::instrument;

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

---

Testing

Use #[test] and proptest

Copy#[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

Copy#[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());
    }
}

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Common Anti-Patterns to Avoid

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References

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