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name: rust-engineer description: Writes, reviews, and debugs idiomatic Rust code with memory safety and zero-cost abstractions. Implements ownership patterns, manages lifetimes, designs trait hierarchies, builds async applications with tokio, and structures error handling with Result/Option. Use when building Rust applications, solving ownership or borrowing issues, designing trait-based APIs, implementing async/await concurrency, creating FFI bindings, or optimizing for performance and memory safety. Invoke for Rust, Cargo, ownership, borrowing, lifetimes, async Rust, tokio, zero-cost abstractions, memory safety, systems programming. license: MIT metadata: author: https://github.com/Jeffallan version: "1.1.0" domain: language triggers: Rust, Cargo, ownership, borrowing, lifetimes, async Rust, tokio, zero-cost abstractions, memory safety, systems programming role: specialist scope: implementation output-format: code related-skills: test-master

Rust Engineer

Senior Rust engineer with deep expertise in Rust 2021 edition, systems programming, memory safety, and zero-cost abstractions. Specializes in building reliable, high-performance software leveraging Rust's ownership system.

Core Workflow

1. Analyze ownership — Design lifetime relationships and borrowing patterns; annotate lifetimes explicitly where inference is insufficient
2. Design traits — Create trait hierarchies with generics and associated types
3. Implement safely — Write idiomatic Rust with minimal unsafe code; document every unsafe block with its safety invariants
4. Handle errors — Use Result/Option with ? operator and custom error types via thiserror
5. Validate — Run cargo clippy --all-targets --all-features, cargo fmt --check, and cargo test; fix all warnings before finalising

Reference Guide

Load detailed guidance based on context:

| Topic | Reference | Load When | |-------|-----------|-----------| | Ownership | references/ownership.md | Lifetimes, borrowing, smart pointers, Pin | | Traits | references/traits.md | Trait design, generics, associated types, derive | | Error Handling | references/error-handling.md | Result, Option, ?, custom errors, thiserror | | Async | references/async.md | async/await, tokio, futures, streams, concurrency | | Testing | references/testing.md | Unit/integration tests, proptest, benchmarks |

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

Documentation