Rust

Purpose

Write Rust that satisfies the borrow checker by design rather than by fighting it. Ownership is a modeling decision made before the code is written, not a compiler obstacle discovered afterward.

When to Use

• Building Rust libraries, services, or CLI tools.
• Resolving lifetime, borrow, or trait-bound errors.
• Designing error types for a library versus an application.
• Writing async Rust with Tokio.
• Auditing or minimizing unsafe blocks.

Capabilities

• Ownership and borrowing design: when to clone, when to borrow, when to use Rc/Arc.
• Error modeling: thiserror for libraries, anyhow for binaries.
• Trait design, generic bounds, and trait objects.
• Async runtimes, Send/Sync bounds, and cancellation semantics.
• Safe encapsulation of unsafe, with documented invariants.

Inputs

• Crate or module source.
• Edition and MSRV.
• Whether the artifact is a library (stable public API) or a binary.

Outputs

• Code that compiles with zero warnings under clippy::pedantic (or a documented allow-list).
• Explicit error enums with #[from] conversions.
• Tests including compile-fail tests for API misuse where warranted.

Workflow

1. Model ownership first — Decide who owns each value and how long it lives before writing the signature.
2. Design the error type — A library's error enum is public API. Enumerate failure modes explicitly.
3. Implement — Start with owned values and clones. Optimize to borrows only where profiling or ergonomics justify it.
4. Constrain generics late — Write it concrete, then generalize if a second caller appears.
5. Gate — cargo clippy -- -D warnings, cargo fmt --check, cargo test, cargo deny check.

Best Practices

• A clone() in non-hot-path code is not a defect. Premature borrow optimization produces unreadable lifetimes.
• Libraries return concrete error enums; binaries use anyhow::Result and add context at each layer.
• Never unwrap() outside tests and main. Use expect("invariant: ...") where the invariant is genuinely proven.
• Prefer impl Trait in argument position for simple bounds; use named generics when the caller must choose the type.
• Every unsafe block gets a // SAFETY: comment stating the invariant that makes it sound.
• Use #[non_exhaustive] on public enums you may extend.

Examples

Library error type:

use thiserror::Error;

#[derive(Debug, Error)]
#[non_exhaustive]
pub enum StoreError {
    #[error("key not found: {0}")]
    NotFound(String),

    #[error("storage backend unavailable")]
    Unavailable(#[source] std::io::Error),

    #[error("value for {key} exceeds {limit} bytes")]
    TooLarge { key: String, limit: usize },
}

pub fn get(key: &str) -> Result<Vec<u8>, StoreError> {
    std::fs::read(key).map_err(|e| match e.kind() {
        std::io::ErrorKind::NotFound => StoreError::NotFound(key.to_owned()),
        _ => StoreError::Unavailable(e),
    })
}

Async with timeout and cancellation:

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

pub async fn fetch_with_deadline(url: &str) -> anyhow::Result<String> {
    let body = timeout(Duration::from_secs(5), reqwest::get(url))
        .await
        .context("request timed out after 5s")?
        .context("request failed")?
        .text()
        .await?;
    Ok(body)
}

Notes

• Borrow-checker errors are usually design feedback. If a lifetime is hard to name, the ownership model is probably wrong.
• Arc<Mutex<T>> is a legitimate answer, not a failure. Contention, not the type, is what costs you.
• Async traits are stable as of Rust 1.75 for non-object-safe use; async-trait is still needed for trait objects.