rust-best-practices

name	rust-best-practices
description	Use when writing, modifying, or reviewing Rust code. ALWAYS invoke before Rust edits; covers Microsoft Pragmatic Rust guidance for 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

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());
    }
}

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References

• Microsoft Pragmatic Rust Guidelines
• Rust API Guidelines
• The Rust Book
• references/advanced.md — performance, async, testing, and anti-pattern examples