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name: rust-best-practices description: Rust development best practices, patterns, and conventions. Use when writing Rust code, reviewing .rs files, discussing ownership, lifetimes, borrowing, or Cargo configuration. Triggers on mentions of Rust, Cargo, ownership, borrowing, lifetimes, traits, async Rust, tokio.

Rust Best Practices

Ownership and Borrowing

Prefer Borrowing Over Ownership

Copy// Bad - takes ownership unnecessarily
fn print_name(name: String) {
    println!("{}", name);
}

// Good - borrows immutably
fn print_name(name: &str) {
    println!("{}", name);
}

Use References Appropriately

Copy// Immutable borrow for reading
fn calculate_length(s: &String) -> usize {
    s.len()
}

// Mutable borrow for modification
fn push_char(s: &mut String, c: char) {
    s.push(c);
}

Error Handling

Use Result and Option

Copyfn find_user(id: u32) -> Option<User> {
    users.get(&id).cloned()
}

fn parse_config(path: &str) -> Result<Config, ConfigError> {
    let content = fs::read_to_string(path)?;
    toml::from_str(&content).map_err(ConfigError::Parse)
}

Custom Error Types

Copyuse thiserror::Error;

#[derive(Error, Debug)]
pub enum AppError {
    #[error("database error: {0}")]
    Database(#[from] sqlx::Error),

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

    #[error("validation error: {field}")]
    Validation { field: String },
}

Propagate with ?

Copyfn process_file(path: &str) -> Result<Data, Error> {
    let content = fs::read_to_string(path)?;
    let parsed = serde_json::from_str(&content)?;
    let validated = validate(parsed)?;
    Ok(validated)
}

Structs and Enums

Builder Pattern

Copy#[derive(Default)]
pub struct ServerBuilder {
    port: Option<u16>,
    host: Option<String>,
}

impl ServerBuilder {
    pub fn port(mut self, port: u16) -> Self {
        self.port = Some(port);
        self
    }

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

    pub fn build(self) -> Result<Server, BuildError> {
        Ok(Server {
            port: self.port.unwrap_or(8080),
            host: self.host.unwrap_or_else(|| "localhost".into()),
        })
    }
}

Newtype Pattern

Copypub struct UserId(u64);
pub struct Email(String);

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

Enums for State

Copyenum ConnectionState {
    Disconnected,
    Connecting { attempt: u32 },
    Connected { session_id: String },
    Error { message: String },
}

Traits

Implement Standard Traits

Copy#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct User {
    pub id: u64,
    pub name: String,
}

Trait Objects vs Generics

Copy// Static dispatch (monomorphization)
fn process<T: Handler>(handler: T) { ... }

// Dynamic dispatch (trait object)
fn process(handler: &dyn Handler) { ... }
fn process(handler: Box<dyn Handler>) { ... }

Extension Traits

Copypub trait StringExt {
    fn truncate_ellipsis(&self, max_len: usize) -> String;
}

impl StringExt for str {
    fn truncate_ellipsis(&self, max_len: usize) -> String {
        if self.len() <= max_len {
            self.to_string()
        } else {
            format!("{}...", &self[..max_len - 3])
        }
    }
}

Lifetimes

Elision Rules

Copy// Lifetimes elided - single input reference
fn first_word(s: &str) -> &str { ... }

// Explicit when multiple inputs
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str { ... }

Struct Lifetimes

Copystruct Parser<'a> {
    input: &'a str,
    position: usize,
}

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

Async Rust

Tokio Patterns

Copy#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let result = fetch_data().await?;
    Ok(())
}

async fn fetch_data() -> Result<Data, Error> {
    let client = reqwest::Client::new();
    let response = client.get(URL).send().await?;
    response.json().await.map_err(Into::into)
}

Concurrent Operations

Copyuse futures::future::join_all;

async fn fetch_all(urls: Vec<String>) -> Vec<Result<Response, Error>> {
    let futures: Vec<_> = urls.into_iter()
        .map(|url| fetch_one(url))
        .collect();

    join_all(futures).await
}

Testing

Unit Tests

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

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

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

Integration Tests

Copy// tests/integration_test.rs
use mylib::process;

#[test]
fn test_full_workflow() {
    let result = process("input");
    assert!(result.is_ok());
}

Performance

Avoid Unnecessary Allocations

Copy// Bad - allocates new String
fn process(s: &str) -> String {
    s.to_uppercase()
}

// Good when possible - use Cow
use std::borrow::Cow;
fn process(s: &str) -> Cow<str> {
    if s.chars().any(|c| c.is_lowercase()) {
        Cow::Owned(s.to_uppercase())
    } else {
        Cow::Borrowed(s)
    }
}

Use Iterators

Copy// Good - lazy, zero-cost
let sum: i32 = numbers.iter()
    .filter(|n| **n > 0)
    .map(|n| n * 2)
    .sum();

Anti-Patterns to Avoid

• .unwrap() in production code
• Excessive .clone() to "fix" borrow checker
• unsafe without clear justification
• Ignoring compiler warnings
• Not using clippy
• Global mutable state (lazy_static with Mutex)