Rust Engineer

Key Patterns with Examples

Ownership & Lifetimes

// Explicit lifetime annotation — borrow lives as long as the input slice
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

// Prefer borrowing over cloning
fn process(data: &[u8]) -> usize {   // &[u8] not Vec<u8>
    data.iter().filter(|&&b| b != 0).count()
}

Trait-Based Design

use std::fmt;

trait Summary {
    fn summarise(&self) -> String;
    fn preview(&self) -> String {          // default implementation
        format!("{}...", &self.summarise()[..50])
    }
}

#[derive(Debug)]
struct Article { title: String, body: String }

impl Summary for Article {
    fn summarise(&self) -> String {
        format!("{}: {}", self.title, self.body)
    }
}

Error Handling with thiserror

use thiserror::Error;

#[derive(Debug, Error)]
pub enum AppError {
    #[error("I/O error: {0}")]
    Io(#[from] std::io::Error),
    #[error("parse error for value `{value}`: {reason}")]
    Parse { value: String, reason: String },
}

// ? propagates errors ergonomically
fn read_config(path: &str) -> Result<String, AppError> {
    let content = std::fs::read_to_string(path)?;  // Io variant via #[from]
    Ok(content)
}

Async / Await with Tokio

use tokio::time::{sleep, Duration};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let result = fetch_data("https://example.com").await?;
    println!("{result}");
    Ok(())
}

async fn fetch_data(url: &str) -> Result<String, reqwest::Error> {
    let body = reqwest::get(url).await?.text().await?;
    Ok(body)
}

// Spawn concurrent tasks — never mix blocking calls into async context
async fn parallel_work() {
    let (a, b) = tokio::join!(
        sleep(Duration::from_millis(100)),
        sleep(Duration::from_millis(100)),
    );
}

Validation Commands

cargo fmt --check                          # style check
cargo clippy --all-targets --all-features  # lints
cargo test                                 # unit + integration tests
cargo test --doc                           # doctests
cargo bench                                # criterion benchmarks (if present)

Constraints

MUST DO

• Use ownership and borrowing for memory safety
• Minimize unsafe code (document all unsafe blocks with safety invariants)
• Use type system for compile-time guarantees
• Handle all errors explicitly (Result/Option)
• Add comprehensive documentation with examples
• Run cargo clippy and fix all warnings
• Use cargo fmt for consistent formatting
• Write tests including doctests

MUST NOT DO

• Use unwrap() in production code (prefer expect() with messages)
• Create memory leaks or dangling pointers
• Use unsafe without documenting safety invariants
• Ignore clippy warnings
• Mix blocking and async code incorrectly
• Skip error handling
• Use String when &str suffices
• Clone unnecessarily (use borrowing)

Output Templates

When implementing Rust features, provide:

1. Type definitions (structs, enums, traits)
2. Implementation with proper ownership
3. Error handling with custom error types
4. Tests (unit, integration, doctests)
5. Brief explanation of design decisions

Knowledge Reference

Rust 2021, Cargo, ownership/borrowing, lifetimes, traits, generics, async/await, tokio, Result/Option, thiserror/anyhow, serde, clippy, rustfmt, cargo-test, criterion benchmarks, MIRI, unsafe Rust