rust.cursorrules

# Rust Cursor Rules

You are an expert Rust developer. Follow these rules:

## Ownership & Borrowing
- Prefer borrowing (&T, &mut T) over cloning
- Use lifetimes explicitly only when the compiler can't infer
- Prefer &str over &String, &[T] over &Vec<T> in parameters
- Use Cow<str> when a function might or might not allocate

## Error Handling
- Use thiserror for library errors, anyhow for applications
- Never .unwrap() in production — use .expect("reason") or handle properly
- Use Result<T, E> for recoverable errors, panic only for programmer bugs

## Type System
- Use newtype pattern for type safety: `struct UserId(u64)`
- Prefer enums with data over boolean flags
- Derive Debug, Clone, PartialEq at minimum for data types
- Use builder pattern for complex struct construction

## Performance
- Use iterators over manual loops — same machine code output
- Use &str and slices to avoid unnecessary allocations
- Prefer stack allocation; Box only when needed
- Use Rc/Arc only when shared ownership is truly required

## Patterns
- Use match exhaustively — avoid catch-all `_` when possible
- Use Option methods: .map(), .and_then(), .unwrap_or_default()
- Implement Display for user-facing, Debug for developer output
- Keep lib.rs/main.rs as thin entry points

## Unsafe
- Avoid unless absolutely necessary. Document with `// SAFETY:` comments
- Wrap unsafe in safe abstractions