Imported from majiayu000/claude-skill-registry (skills/development/modern-rust-expert-jonathanbelolo-composable-rust/SKILL.md). Install upstream with npx skills add majiayu000/claude-skill-registry --skill modern-rust-expert-jonathanbelolo-composable-rust. Copyright stays with the author.

Modern Rust Expert

Expert knowledge for writing cutting-edge, idiomatic Rust code with Rust edition 2024, strict clippy compliance, and functional-but-pragmatic philosophy.

When to Use This Skill

Automatically apply this knowledge when:

• Writing any Rust code in this project
• Fixing clippy warnings
• Structuring new modules or crates
• Reviewing or refactoring Rust code

Rust Edition 2024 Specifics

Version Requirements

• Edition: 2024
• MSRV: 1.85.0 (minimum for edition 2024)
• Always check rustc --version and update with rustup update stable if needed

Edition 2024 & Rust 1.85+ Features Available

1. Async Functions in Traits (Stable!)

Use async fn directly in trait definitions without async-trait crate:

// Modern Rust - async fn in traits (no macro needed!)
trait Database: Send + Sync {
    async fn save(&self, data: &[u8]) -> Result<(), Error>;
    async fn load(&self, id: &str) -> Result<Vec<u8>, Error>;
}

// Implementation
impl Database for PostgresDb {
    async fn save(&self, data: &[u8]) -> Result<(), Error> {
        // Direct async implementation
        sqlx::query("INSERT INTO ...").execute(&self.pool).await?;
        Ok(())
    }
}

Use this for: All async trait methods in Environment traits (Database, HttpClient, etc.)

2. Return Position Impl Trait in Traits (RPITIT)

Return impl Trait from trait methods:

trait EventPublisher {
    // Can return impl Future instead of Box<dyn Future>
    fn publish(&self, event: Event) -> impl Future<Output = Result<()>> + Send;
}

Benefit: No heap allocation, better performance than Box<dyn Future>.

3. Let-Else Statements

Early return with pattern matching:

// Modern pattern for error handling
fn process_order(action: OrderAction) -> Result<Order, Error> {
    let OrderAction::PlaceOrder { customer_id, items } = action else {
        return Err(Error::InvalidAction);
    };

    // customer_id and items are in scope here
    Ok(Order { customer_id, items })
}

// Compare to old style:
fn process_order_old(action: OrderAction) -> Result<Order, Error> {
    match action {
        OrderAction::PlaceOrder { customer_id, items } => {
            Ok(Order { customer_id, items })
        }
        _ => Err(Error::InvalidAction),
    }
}

Use for: Extracting enum variants with early return.

4. Enhanced Const Generics

Use const generics in more contexts:

// Const generic for effect buffer size
struct EffectBuffer<Action, const N: usize> {
    effects: [Option<Effect<Action>>; N],
}

impl<Action, const N: usize> EffectBuffer<Action, N> {
    const fn new() -> Self {
        Self {
            effects: [const { None }; N],
        }
    }
}

Use for: Stack-allocated buffers with configurable size.

5. Inline Const Expressions

Use const { } blocks in const contexts:

const DEFAULT_CAPACITY: usize = 16;

struct Cache<T> {
    // Inline const expression
    data: [Option<T>; const { DEFAULT_CAPACITY * 2 }],
}

6. C-String Literals

Create CStr at compile time:

// Modern: c"string" literal
let path = c"/tmp/data";  // Type: &'static CStr

// Old way required:
use std::ffi::CString;
let path = CString::new("/tmp/data").unwrap();

Use for: FFI code and system calls.

7. If/While Let Chains

Combine multiple patterns with &&:

// Modern: let chains
fn handle_event(state: &State, action: Action) {
    if let OrderAction::PlaceOrder { items, .. } = action
        && !items.is_empty()
        && state.can_place_order()
    {
        // All conditions met
    }
}

// Old way required nested ifs or match

8. Improved Pattern Matching

Rest patterns in slices:

match events.as_slice() {
    [first, .., last] => {
        // first and last are available
    }
    _ => {}
}

Patterns in let statements:

let [first, second, ..] = &events[..] else {
    return Err(Error::NotEnoughEvents);
};

9. Expanded Const Fn Capabilities

More operations allowed in const fn:

const fn calculate_capacity(base: usize) -> usize {
    // These are all allowed in const fn now:
    if base < 16 { 16 } else { base }
}

const fn create_default<T>() -> Option<T> {
    None  // Can return generic types
}

// Use in const contexts
const CAPACITY: usize = calculate_capacity(10);

10. Precise Capturing in RPIT

Control which lifetimes are captured:

trait Store {
    // Only capture 'a, not all lifetimes
    fn get_state<'a>(&'a self) -> impl Future<Output = State> + use<'a>;
}

Features NOT Available (Require Nightly)

To avoid confusion, these are NOT in stable yet:

• ❌ gen blocks for generators
• ❌ Type alias impl trait (TAIT)
• ❌ Specialization
• ❌ Generic const expressions (full support)
• ❌ #[derive] on enum with generics in some cases

Recommended Patterns for This Project

1. Use async fn in traits - No more async-trait dependency
2. Prefer let-else - Cleaner error handling
3. Use RPITIT - Return impl Future instead of Box<dyn Future>
4. Const generics for buffers - Stack allocation where possible
5. Let chains - Combine multiple conditions elegantly

Clippy Configuration & Compliance

Workspace Lint Configuration

Always configure lints at workspace level in root Cargo.toml:

[workspace.lints.rust]
unsafe_code = "forbid"
missing_docs = "warn"

[workspace.lints.clippy]
# Pedantic lints - USE LOWER PRIORITY so specific lints can override
pedantic = { level = "warn", priority = -1 }

# Deny common issues
unwrap_used = "deny"
expect_used = "deny"
panic = "deny"
todo = "deny"
unimplemented = "deny"

# Performance
missing_const_for_fn = "warn"

# Cognitive complexity
cognitive_complexity = "warn"

# Documentation
missing_errors_doc = "warn"
missing_panics_doc = "warn"

Critical: Lint groups like pedantic MUST have priority = -1 to avoid conflicts with specific lints.

Individual Crate Configuration

In each crate's Cargo.toml, inherit workspace lints:

[lints]
workspace = true

Common Clippy Issues & Solutions

1. Mixed Attributes Style

Problem: Having both outer (///) and inner (//!) doc comments on the same item.

Wrong:

/// Module for actions
///
/// This contains action types.
pub mod action {
    //! Action implementations
}

Correct:

/// Module for actions
///
/// This contains action types.
///
/// Action implementations.
pub mod action {}

Rule: Pick one style per item. For modules, use outer docs (///) and move inner content to the outer doc comment.

2. Documentation Backticks

Rule: ALL type names, function names, trait names in documentation MUST be in backticks.

Wrong:

/// - Database: The database trait
/// - HttpClient: HTTP client

Correct:

/// - `Database`: The database trait
/// - `HttpClient`: HTTP client

3. Missing # Panics Documentation

If a function can panic, document it:

/// Create a test clock
///
/// # Panics
///
/// Panics if the hardcoded timestamp fails to parse
/// (should never happen in practice).
#[allow(clippy::expect_used)]  // Justified in test utilities
pub fn test_clock() -> Clock {
    // ...
}

4. Non-Debug Types

Some types (like Future) don't implement Debug. Manually implement it:

pub enum Effect<Action> {
    Future(Pin<Box<dyn Future<Output = Option<Action>> + Send>>),
}

// Manual Debug implementation
impl<Action> std::fmt::Debug for Effect<Action>
where
    Action: std::fmt::Debug,
{
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Effect::Future(_) => write!(f, "Effect::Future(<future>)"),
            // ... other arms
        }
    }
}

5. Missing const fn

If clippy suggests a function can be const, make it const:

// Clippy will suggest this can be const
pub const fn merge(effects: Vec<Effect<Action>>) -> Effect<Action> {
    Effect::Parallel(effects)
}

6. Wildcard Imports

Avoid wildcard imports in library code:

Wrong:

use super::*;

Correct:

use super::{Arc, Effect, Reducer, RwLock};

Exception: Wildcard imports are acceptable in tests and examples.

7. Match Arm Consistency

Consolidate identical match arms:

Wrong:

match effect {
    Effect::None => {},
    Effect::Parallel(_) => {},
    Effect::Sequential(_) => {},
}

Correct:

match effect {
    Effect::None | Effect::Parallel(_) | Effect::Sequential(_) => {
        // Placeholder implementation
    },
}

8. Doc List Indentation

Module docs must be indented or separated with blank lines:

Wrong:

pub mod mocks {
    //! Mock implementations
}

Correct (moved to outer):

/// Mock implementations
pub mod mocks {}

Rustfmt Configuration

Stable Features Only

Only use rustfmt features available in stable Rust. Many formatting options require nightly.

Safe rustfmt.toml:

edition = "2024"
max_width = 100
hard_tabs = false
tab_spaces = 4
newline_style = "Unix"
fn_params_layout = "Tall"
match_block_trailing_comma = true
chain_width = 60
use_try_shorthand = true
use_field_init_shorthand = true
force_explicit_abi = true

Avoid (require nightly):

• imports_granularity
• group_imports
• wrap_comments
• format_code_in_doc_comments
• normalize_comments
• reorder_impl_items
• brace_style
• match_arm_blocks

Functional-but-Pragmatic Philosophy

Core Principles

1. Prefer immutability, but allow &mut self when performance matters
2. Prefer pure functions, but recognize async/await patterns
3. Prefer composition, but allow practical escape hatches
4. Favor readability over theoretical purity

Practical Applications

Effect-as-Value Pattern

Describe side effects as values, don't execute them immediately:

// Good: Return effect description
fn reduce(&self, state: &mut State, action: Action, env: &Env) -> Vec<Effect> {
    vec![Effect::Database(SaveOrder), Effect::PublishEvent(event)]
}

// Bad: Execute side effects directly
fn reduce(&self, state: &mut State, action: Action, env: &Env) {
    env.database.save(state);  // ❌ Side effect in reducer!
}

Mutable State in Reducers

It's OK to mutate state in place in reducers for performance:

trait Reducer {
    fn reduce(
        &self,
        state: &mut Self::State,  // ✅ Mutable for performance
        action: Self::Action,
        env: &Self::Environment,
    ) -> Vec<Effect<Self::Action>>;
}

This is pragmatic: tests are still fast and deterministic, and we avoid unnecessary cloning.

Static Dispatch Over Dynamic

Prefer generic types (static dispatch) over trait objects (dynamic dispatch):

// Prefer this (static dispatch, zero-cost)
struct Store<S, A, E, R>
where
    R: Reducer<State = S, Action = A, Environment = E>
{
    reducer: R,
}

// Over this (dynamic dispatch, runtime cost)
struct Store<S, A, E> {
    reducer: Box<dyn Reducer<State = S, Action = A>>,
}

Exception: Use dynamic dispatch when you need runtime polymorphism (plugins, hot-swapping implementations).

Error Handling

Strict Rules:

• NEVER use unwrap() or expect() in library code (deny by clippy)
• NEVER use panic!() in library code (deny by clippy)
• NEVER leave todo!() or unimplemented!() in production code (deny by clippy)

Exceptions (must be explicitly allowed):

// Test utilities can use expect with justification
#[allow(clippy::expect_used)]
pub fn test_clock() -> Clock {
    DateTime::parse_from_rfc3339("2025-01-01T00:00:00Z")
        .expect("hardcoded timestamp should always parse")
        .with_timezone(&Utc)
}

Always document why you're allowing it.

Code Organization Patterns

Module Documentation

Pattern 1: Small modules - Use outer docs only:

/// Action types and utilities
///
/// Actions represent all possible state transitions.
pub mod action {}

Pattern 2: Large modules - No inner docs if you have comprehensive outer docs:

/// Store runtime for coordinating reducer execution
///
/// Detailed explanation here.
///
/// Store runtime implementation details.
pub mod store {
    // No //! needed, everything is in outer docs
}

Re-exports

Use pub use for convenience, but document what you're re-exporting:

// Re-export commonly used types from dependencies
pub use chrono::{DateTime, Utc};
pub use serde::{Deserialize, Serialize};

// Re-export from submodules for convenience
pub use mocks::{FixedClock, test_clock};

Workspace Structure

For multi-crate projects:

project/
├── Cargo.toml          # Workspace root with [workspace.dependencies]
├── core/
│   ├── Cargo.toml      # Uses workspace.dependencies
│   └── src/lib.rs
├── runtime/
│   ├── Cargo.toml      # Depends on core
│   └── src/lib.rs
└── testing/
    ├── Cargo.toml      # Depends on core + runtime
    └── src/lib.rs

Rules:

• Share dependencies via [workspace.dependencies]
• Use dependency.workspace = true in crate manifests
• Shared package metadata via [workspace.package]
• Workspace lints via [workspace.lints]

Common Gotchas & Solutions

1. Workspace Dependencies Cannot Be Optional

Wrong:

[workspace.dependencies]
sqlx = { version = "0.8", optional = true }  # ❌ Error!

Correct:

[workspace.dependencies]
sqlx = { version = "0.8" }  # Define without optional

# In individual crate Cargo.toml:
[dependencies]
sqlx = { workspace = true, optional = true }  # ✅ Make it optional here

2. Edition 2024 Requires Newer MSRV

If you get:

error: rust-version 1.83.0 is older than first version (1.85.0) required by edition 2024

Solution: Update MSRV:

[workspace.package]
rust-version = "1.85.0"  # Minimum for edition 2024

3. Future Doesn't Implement Debug

Pin<Box> doesn't implement Debug. See "Non-Debug Types" section above for manual implementation.

4. Async Functions in Placeholders

Placeholders in async functions trigger unused_async:

#[allow(clippy::unused_async)]  // Justified: placeholder implementation
async fn execute_effect(&self, effect: Effect<A>) {
    // Will be implemented in Phase 1
}

Testing Patterns

Test Organization

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_something() {
        // Unit test
    }

    #[tokio::test]
    async fn test_async_something() {
        // Async test
    }
}

Property-Based Testing

Use proptest for property tests:

use proptest::prelude::*;

proptest! {
    #[test]
    fn property_holds(input: Vec<u8>) {
        prop_assert!(check_property(&input));
    }
}

Performance Considerations

Zero-Cost Abstractions

With static dispatch, generics compile to optimal code:

// This generic code:
fn reduce<D: Database>(state: &mut State, db: &D) {
    db.save(state);
}

// Monomorphizes to:
fn reduce_with_postgres(state: &mut State, db: &PostgresDatabase) {
    db.save(state);  // Direct call, no vtable lookup
}

Allocation Minimization

In hot paths, consider SmallVec for small collections:

use smallvec::SmallVec;

// Stack-allocated for ≤ 4 items, heap for more
fn reduce(&self, ...) -> SmallVec<[Effect; 4]> {
    let mut effects = SmallVec::new();
    effects.push(Effect::Save);
    effects
}

Documentation Standards

Crate-Level Documentation

Start lib.rs with comprehensive module docs:

//! # Crate Name
//!
//! Brief description.
//!
//! ## Overview
//!
//! Detailed explanation.
//!
//! ## Example
//!
//! ```ignore
//! // Example code here
//! ```

Function Documentation

/// Brief one-line description
///
/// Longer detailed explanation if needed.
///
/// # Arguments
///
/// - `state`: Description
/// - `action`: Description
///
/// # Returns
///
/// What the function returns.
///
/// # Errors
///
/// When this function returns an error (if applicable).
///
/// # Panics
///
/// When this function panics (if applicable).
///
/// # Example
///
/// ```
/// // Example usage
/// ```
pub fn my_function(state: &State, action: Action) -> Result<(), Error> {
    // ...
}

Type Documentation

/// Brief description of the type
///
/// # Type Parameters
///
/// - `S`: State type
/// - `A`: Action type
///
/// # Example
///
/// ```
/// // Example usage
/// ```
pub struct MyType<S, A> {
    // ...
}

Quick Reference Checklist

Before committing Rust code:

• Run cargo fmt --all
• Run cargo clippy --all-targets --all-features -- -D warnings
• Run cargo test --all-features
• Run cargo doc --no-deps --all-features
• All type names in docs have backticks
• No unwrap/panic/todo in library code
• Functions that panic have # Panics section
• No wildcard imports (use super::*)
• Manual Debug impl for non-Debug types
• Const fn where applicable
• Static dispatch preferred over dynamic

Common Commands

# Format code
cargo fmt --all

# Check without building
cargo check --all-features

# Build
cargo build --all-features

# Test
cargo test --all-features

# Lint
cargo clippy --all-targets --all-features -- -D warnings

# Documentation
cargo doc --no-deps --all-features --open

# Documentation with warnings as errors
RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --all-features

Project-Specific Guidelines

This Project's Philosophy

1. Functional Core, Imperative Shell: Pure business logic, effects as values
2. Explicit over Implicit: All side effects are visible
3. Type Safety: Make invalid states unrepresentable
4. Testability: Business logic tests run at memory speed
5. Performance: Zero-cost abstractions via static dispatch
6. Pragmatism: Functional patterns, but practical when needed

Architecture Patterns

• Action: Unified type for commands and events
• Reducer: Pure function (State, Action, Env) → (State, Effects)
• Effect: Values describing side effects (not execution)
• Environment: Trait-based dependency injection
• Store: Runtime that coordinates everything

When in Doubt

• Check the architecture spec: specs/architecture.md
• Follow patterns from existing code
• Prefer explicitness over cleverness
• If clippy complains, there's usually a good reason
• Ask for clarification rather than guessing

Remember: This skill is automatically applied to all Rust code in this project. Follow these guidelines to write idiomatic, performant, and maintainable Rust code.