Agent Guidelines for Rust Code Quality

This document provides guidelines for maintaining high-quality Rust code. These rules MUST be followed by all AI coding agents and contributors.

Your Core Principles

All code you write MUST be fully optimized.

"Fully optimized" includes:

maximizing algorithmic big-O efficiency for memory and runtime
using parallelization and SIMD where appropriate
following proper style conventions for Rust (e.g. maximizing code reuse (DRY))
no extra code beyond what is absolutely necessary to solve the problem the user provides (i.e. no technical debt)

If the code is not fully optimized before handing off to the user, you will be fined $100. You have permission to do another pass of the code if you believe it is not fully optimized.

Preferred Tools

Use cargo for project management, building, and dependency management.
Use indicatif to track long-running operations with progress bars. The message should be contextually sensitive.
Use serde with serde_json for JSON serialization/deserialization.
Use ratatui adnd crossterm for terminal applications/TUIs.
Use axum for creating any web servers or HTTP APIs.
- Keep request handlers async, returning Result<Response, AppError> to centralize error handling.
- Use layered extractors and shared state structs instead of global mutable data.
- Add tower middleware (timeouts, tracing, compression) for observability and resilience.
- Offload CPU-bound work to tokio::task::spawn_blocking or background services to avoid blocking the reactor.
When reporting errors to the console, use tracing::error! or log::error! instead of println!.
If designing applications with a web-based front end interface, e.g. compiling to WASM or using dioxus:
- All deep computation MUST occur within Rust processes (i.e. the WASM binary or the dioxus app Rust process). NEVER use JavaScript for deep computation.
- The front-end MUST use Pico CSS and vanilla JavaScript. NEVER use jQuery or any component-based frameworks such as React.
- The front-end should prioritize speed and common HID guidelines.
- The app should use adaptive light/dark themes by default, with a toggle to switch the themes.
- The typography/theming of the application MUST be modern and unique, similar to that of popular single-page web/mobile. ALWAYS add an appropriate font for headers and body text. You may reference fonts from Google Fonts.
- NEVER use the Pico CSS defaults as-is: a separate CSS/SCSS file is encouraged. The design MUST logically complement the semantics of the application use case.
- ALWAYS rebuild the WASM binary if any underlying Rust code that affects it is touched.
For data processing:
- ALWAYS use polars instead of other data frame libraries for tabular data manipulation.
- If a polars dataframe will be printed, NEVER simultaneously print the number of entries in the dataframe nor the schema as it is redundant.
- NEVER ingest more than 10 rows of a data frame at a time. Only analyze subsets of data to avoid overloading your memory context.
If using Python to implement Rust code using PyO3/maturin:
- Rebuild the Python package with maturin after finishing all Rust code changes.
- ALWAYS use uv for Python package management and to create a .venv if it is not present. NEVER use the base system Python installation.
- Ensure .venv is added to .gitignore.
- Ensure ipykernel and ipywidgets is installed in .venv for Jupyter Notebook compatability. This should not be in package requirements.
- MUST keep functions focused on a single responsibility
- NEVER use mutable objects (lists, dicts) as default argument values
- Limit function parameters to 5 or fewer
- Return early to reduce nesting
- MUST use type hints for all function signatures (parameters and return values)
- NEVER use Any type unless absolutely necessary
- MUST run mypy and resolve all type errors
- Use Optional[T] or T | None for nullable types

Code Style and Formatting

MUST use meaningful, descriptive variable and function names
MUST follow Rust API Guidelines and idiomatic Rust conventions
MUST use 4 spaces for indentation (never tabs)
NEVER use emoji, or unicode that emulates emoji (e.g. ✓, ✗). The only exception is when writing tests and testing the impact of multibyte characters.
Use snake_case for functions/variables/modules, PascalCase for types/traits, SCREAMING_SNAKE_CASE for constants
Limit line length to 100 characters (rustfmt default)
Assume the user is a Python expert, but a Rust novice. Include additional code comments around Rust-specific nuances that a Python developer may not recognize.

Documentation

MUST include doc comments for all public functions, structs, enums, and methods
MUST document function parameters, return values, and errors
Keep comments up-to-date with code changes
Include examples in doc comments for complex functions

Example doc comment:

/// Calculate the total cost of items including tax.
///
/// # Arguments
///
/// * `items` - Slice of item structs with price fields
/// * `tax_rate` - Tax rate as decimal (e.g., 0.08 for 8%)
///
/// # Returns
///
/// Total cost including tax
///
/// # Errors
///
/// Returns `CalculationError::EmptyItems` if items is empty
/// Returns `CalculationError::InvalidTaxRate` if tax_rate is negative
///
/// # Examples
///
/// ```
/// let items = vec![Item { price: 10.0 }, Item { price: 20.0 }];
/// let total = calculate_total(&items, 0.08)?;
/// assert_eq!(total, 32.40);
/// ```
pub fn calculate_total(items: &[Item], tax_rate: f64) -> Result<f64, CalculationError> {

Type System

MUST leverage Rust's type system to prevent bugs at compile time
NEVER use .unwrap() in library code; use .expect() only for invariant violations with a descriptive message
MUST use meaningful custom error types with thiserror
Use newtypes to distinguish semantically different values of the same underlying type
Prefer Option<T> over sentinel values

Error Handling

NEVER use .unwrap() in production code paths
MUST use Result<T, E> for fallible operations
MUST use thiserror for defining error types and anyhow for application-level errors
MUST propagate errors with ? operator where appropriate
Provide meaningful error messages with context using .context() from anyhow

Function Design

MUST keep functions focused on a single responsibility
MUST prefer borrowing (&T, &mut T) over ownership when possible
Limit function parameters to 5 or fewer; use a config struct for more
Return early to reduce nesting
Use iterators and combinators over explicit loops where clearer

Struct and Enum Design

MUST keep types focused on a single responsibility
MUST derive common traits: Debug, Clone, PartialEq where appropriate
Use #[derive(Default)] when a sensible default exists
Prefer composition over inheritance-like patterns
Use builder pattern for complex struct construction
Make fields private by default; provide accessor methods when needed

Testing

MUST write unit tests for all new functions and types
MUST mock external dependencies (APIs, databases, file systems)
MUST use the built-in #[test] attribute and cargo test
Follow the Arrange-Act-Assert pattern
Do not commit commented-out tests
Use #[cfg(test)] modules for test code

Imports and Dependencies

MUST avoid wildcard imports (use module::*) except for preludes, test modules (use super::*), and prelude re-exports
MUST document dependencies in Cargo.toml with version constraints
Use cargo for dependency management
Organize imports: standard library, external crates, local modules
Use rustfmt to automate import formatting

Rust Best Practices

NEVER use unsafe unless absolutely necessary; document safety invariants when used
MUST call .clone() explicitly on non-Copy types; avoid hidden clones in closures and iterators
MUST use pattern matching exhaustively; avoid catch-all _ patterns when possible
MUST use format! macro for string formatting
Use iterators and iterator adapters over manual loops
Use enumerate() instead of manual counter variables
Prefer if let and while let for single-pattern matching

Memory and Performance

MUST avoid unnecessary allocations; prefer &str over String when possible
MUST use Cow<'_, str> when ownership is conditionally needed
Use Vec::with_capacity() when the size is known
Prefer stack allocation over heap when appropriate
Use Arc and Rc judiciously; prefer borrowing

Concurrency

MUST use Send and Sync bounds appropriately
MUST prefer tokio for async runtime in async applications
MUST use rayon for CPU-bound parallelism
Avoid Mutex when RwLock or lock-free alternatives are appropriate
Use channels (mpsc, crossbeam) for message passing

Security

NEVER store secrets, API keys, or passwords in code. Only store them in .env.
- Ensure .env is declared in .gitignore.
MUST use environment variables for sensitive configuration via dotenvy or std::env
NEVER log sensitive information (passwords, tokens, PII)
Use secrecy crate for sensitive data types

Version Control

MUST write clear, descriptive commit messages
NEVER commit commented-out code; delete it
NEVER commit debug println! statements or dbg! macros
NEVER commit credentials or sensitive data

Tools

MUST use rustfmt for code formatting
MUST use clippy for linting and follow its suggestions
MUST ensure code compiles with no warnings (use -D warnings flag in CI, not #![deny(warnings)] in source)
Use cargo for building, testing, and dependency management
Use cargo test for running tests
Use cargo doc for generating documentation
NEVER build with cargo build --features python: this will always fail. Instead, ALWAYS use maturin.

Before Committing

All tests pass (cargo test)
No compiler warnings (cargo build)
Clippy passes (cargo clippy -- -D warnings)
Code is formatted (cargo fmt --check)
If the project creates a Python package and Rust code is touched, rebuild the Python package (source .venv/bin/activate && maturin develop --release --features python)
If the project creates a WASM package and Rust code is touched, rebuild the WASM package (wasm-pack build --target web --out-dir web/pkg)
All public items have doc comments
No commented-out code or debug statements
No hardcoded credentials

Remember: Prioritize clarity and maintainability over cleverness.

Viththagan/CLAUDE.md

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