---

name: rust-patterns description: | Idiomatic Rust patterns for ownership, errors, traits, and configuration. Use when:

• Writing or reviewing Rust code
• Designing error handling with Result and thiserror
• Implementing traits and composition patterns
• Working with ownership and borrowing
• Configuring Cargo features Keywords: Rust, ownership, borrowing, Result, thiserror, anyhow, trait, lifetimes, Cargo features, unsafe

---

Rust Patterns

Ownership-first, zero-cost abstractions, no hidden complexity.

Error Handling

Always use Result<T, E>. Never panic for expected failures:

Copy// Use thiserror for library error types
#[derive(Debug, thiserror::Error)]
pub enum UserError {
    #[error("user not found: {0}")]
    NotFound(String),
    #[error("invalid email format")]
    InvalidEmail,
    #[error("database error: {0}")]
    Database(#[from] sqlx::Error),
}

// Use anyhow for applications (context chaining)
fn fetch_user(id: &str) -> anyhow::Result<User> {
    let user = db.get(id)
        .context("fetching user from database")?;
    Ok(user)
}

Propagate with ?, add context at boundaries.

Ownership Patterns

Borrowing > Cloning:

Copy// Good: Borrow for read-only
fn process(items: &[Item]) -> usize { ... }

// Good: Take ownership when storing/transforming
fn consume(items: Vec<Item>) -> Output { ... }

// Avoid: Excessive cloning
fn bad(items: &Vec<Item>) {
    let copy = items.clone(); // Usually unnecessary
}

Fight the borrow checker → redesign, don't circumvent.

Trait Design

Small, focused traits (1-3 methods):

Copytrait Readable {
    type Item;
    fn read(&self) -> Self::Item;
}

trait Writable {
    type Item;
    fn write(&mut self, item: Self::Item);
}

// Compose through bounds
fn copy<R, W>(src: &R, dst: &mut W)
where
    R: Readable<Item = Vec<u8>>,
    W: Writable<Item = Vec<u8>>,
{ ... }

Consumer-side interfaces. Static dispatch by default.

Configuration

Cargo features for compile-time options:

Copy[features]
default = ["json"]
json = ["serde_json"]
database = ["sqlx"]
full = ["json", "database"]

Copy#[cfg(feature = "json")]
pub mod json_support { ... }

Unsafe

Minimize. Document with // SAFETY: comments:

Copy// SAFETY: We verified ptr is non-null and properly aligned
// in the caller's bounds check above
unsafe { *ptr }

Abstract behind safe interfaces.

Anti-Patterns

• unwrap() / expect() for recoverable errors
• Result<T, String> (use typed errors)
• Excessive Rc<RefCell<T>> (redesign ownership)
• Monolithic traits (10+ methods)
• Reflection instead of generics
• Fighting borrow checker with unsafe