Rust/Cargo Development Skill

You are a Rust development specialist using cargo and related tools. This skill provides comprehensive workflows, best practices, and common patterns for Rust development.

IMPORTANT: Build Strategy

AVOID expensive builds:

• DON'T use cargo build --release or cargo install --path . (very slow)
• DON'T build unless necessary - use cargo check first
• DO use cargo check to verify compilation (fast, no codegen)
• DO use cargo run for iterative development and testing functionality (builds debug + runs in one command)
• DO use debug builds for testing binaries (cargo build without --release)

Decision tree:

1. Just checking if code compiles? → cargo check (fastest)
2. Developing/testing functionality? → cargo run (builds debug + runs - use this for iteration)
3. Need the binary artifact without running? → cargo build (debug)
4. Need optimized performance? → Only then use cargo build --release (slow)

Standard Development Workflow

Command Execution Order

Always follow this sequence when developing or validating Rust code:

cargo test --quiet
cargo check --quiet       # Fast compilation check (PREFER - no binary output)
cargo clippy

IMPORTANT: Use cargo check, NOT cargo build, for validation. Only use cargo build when you actually need the binary artifact.

For iterative development and testing functionality:

cargo run                 # Builds debug and runs (PREFERRED for development iteration)
cargo run -- arg1 arg2    # Pass arguments to test different scenarios
# OR
cargo build --quiet       # Only if you need the binary artifact without running it

Rationale:

1. Tests first: Catch logic errors early
2. Check second: Fast compilation verification without codegen
3. Clippy third: Address code quality and style issues
4. Run/Build last: Use cargo run to iterate on functionality, or cargo build if you need the artifact

Timeout Settings

Rust commands can be long-running, especially for large projects:

# Standard timeout (2 minutes) - sufficient for most operations
cargo test --quiet
cargo check --quiet
cargo build --quiet      # Debug build
# timeout: 120000

# Extended timeout ONLY for release builds (10 minutes) - AVOID if possible
cargo build --release    # WARNING: Very slow, only use when explicitly needed
# timeout: 600000

Best practice: Use the standard 2-minute timeout for check/test/debug builds. Only use extended timeout if you absolutely must do a release build.

Clippy - Linting and Code Quality

Auto-fix Workflow

Always attempt automatic fixes first:

cargo clippy --fix --allow-dirty

Flags:

• --fix: Automatically apply suggested fixes
• --allow-dirty: Allow fixes even with uncommitted changes
• Implies --no-deps and --all-targets

Clippy Strategies

1. Distinguish Warning Types

Separate actionable issues from domain-appropriate patterns:

Actionable bugs:

• Logic errors
• Potential panics
• Memory safety issues
• API misuse

Domain-appropriate suppressions:

• Domain-specific patterns (e.g., cast_possible_truncation in Excel formula evaluators)
• False positives (e.g., unused_assignments for loop invariants)
• Documentation lints for internal tools (e.g., missing_errors_doc)
• Style preferences that don't affect correctness (e.g., needless_pass_by_value)

2. Suppression Hierarchy

Function-level suppression:

#[allow(clippy::cast_possible_truncation)]
fn process_excel_value(val: u64) -> u32 {
    val as u32  // Domain: Excel row numbers are always < u32::MAX
}

Module-level suppression (top of file):

#![allow(clippy::missing_errors_doc)]
#![allow(clippy::cast_possible_truncation)]

// Rest of module code...

Project-level suppression (Cargo.toml):

[lints.clippy]
missing_errors_doc = "allow"
cast_possible_truncation = "allow"
needless_pass_by_value = "allow"

3. Systematic Approach

1. Run cargo clippy to see all warnings
2. Run cargo clippy --fix --allow-dirty to auto-fix
3. Review remaining warnings and categorize:
   • Fix: Legitimate issues
   • Suppress: Domain-specific or false positives
4. Add strategic suppressions at appropriate level
5. Result: Only actionable warnings remain

Common Clippy Lints to Consider

Often suppressed in domain-specific code:

• cast_possible_truncation - When domain guarantees safety
• cast_sign_loss - When values are known positive
• missing_errors_doc - Internal tools/libraries
• missing_panics_doc - When panics are domain-impossible
• needless_pass_by_value - API design choices
• module_name_repetitions - Sometimes necessary for clarity
• too_many_lines - Large but cohesive functions
• unused_assignments - Loop invariants and initialization patterns

Generally should fix:

• redundant_closure - Simplify code
• unnecessary_unwrap - Handle errors properly
• manual_map - Use iterator methods
• match_same_arms - Reduce duplication
• needless_return - Clean up style

Explaining Lints

Get detailed information about any lint:

cargo clippy --explain <LINT_NAME>

Command-line Lint Control

# Warn on specific lint
cargo clippy -- -W clippy::unwrap_used

# Deny specific lint (error)
cargo clippy -- -D clippy::unwrap_used

# Allow specific lint
cargo clippy -- -A clippy::needless_pass_by_value

# Forbid specific lint (cannot be overridden)
cargo clippy -- -F clippy::unwrap_used

Common Cargo Commands

Checking (Prefer This)

cargo check                    # Fast compilation check (PREFER THIS)
cargo check --quiet            # Suppress output
cargo check --all-targets      # Check all targets

Use cargo check instead of cargo build when you just need to verify code compiles.

Building

cargo build                    # Debug build (use for testing binary)
cargo build --quiet            # Suppress output
cargo build --all-targets      # Build all targets (bins, libs, tests, benches)
cargo build --target <TARGET>  # Cross-compile for target

# AVOID unless explicitly needed (very slow):
cargo build --release          # Optimized release build (WARNING: SLOW)

Testing

cargo test                     # Run all tests
cargo test --quiet             # Suppress output except failures
cargo test <NAME>              # Run specific test
cargo test --lib               # Test library only
cargo test --doc               # Test documentation examples
cargo test -- --nocapture      # Show println! output
cargo test -- --test-threads=1 # Run tests serially

Running

cargo run                      # Run default binary (debug build)
cargo run --bin <NAME>         # Run specific binary
cargo run --example <NAME>     # Run example
cargo run -- <ARGS>            # Pass arguments to binary

# AVOID unless you need optimized performance (very slow to build):
cargo run --release            # Run optimized build (WARNING: SLOW)

Documentation

cargo doc                      # Build documentation
cargo doc --open               # Build and open in browser
cargo doc --no-deps            # Only document this crate

Dependency Management

cargo add <CRATE>              # Add dependency
cargo add <CRATE>@<VERSION>    # Add specific version
cargo add --dev <CRATE>        # Add dev dependency
cargo remove <CRATE>           # Remove dependency
cargo update                   # Update dependencies in Cargo.lock
cargo update <CRATE>           # Update specific dependency

Project Management

cargo new <NAME>               # Create new binary project
cargo new --lib <NAME>         # Create new library project
cargo init                     # Initialize in current directory
cargo clean                    # Remove target directory

Publishing and Package Info

cargo search <QUERY>           # Search crates.io
cargo publish                  # Publish to crates.io
cargo package                  # Create distributable package
cargo tree                     # Show dependency tree

Process Management

Cleaning Up Background Processes

Cargo can leave processes running that cause "resource busy" or lock errors:

pkill -f cargo

Use this before running cargo commands if you encounter:

• "resource busy" errors
• Cargo.lock contention
• Build hanging
• File lock errors

Common Lock Issues

Problem: Multiple cargo processes fighting for Cargo.lock

Solution:

pkill -f cargo
cargo clean
cargo build

Configuration Best Practices

Cargo.toml Comments

Place comments on separate lines, not inline:

Correct:

# This dependency is used for CLI parsing
clap = "4.0"

# Linting configuration
[lints.clippy]
# Suppress in domain where values are guaranteed valid
cast_possible_truncation = "allow"

Incorrect:

clap = "4.0"  # This may cause parsing errors
cast_possible_truncation = "allow"  # Don't do this

Lint Configuration Structure

[lints.clippy]
# Code quality - generally enforce
unwrap_used = "warn"
expect_used = "warn"

# Domain-specific allowances
cast_possible_truncation = "allow"
missing_errors_doc = "allow"

# Style preferences
needless_pass_by_value = "allow"
module_name_repetitions = "allow"

Complete Development Workflows

Workflow 1: New Feature Development

# 1. Create feature branch (outside cargo)
git checkout -b feature/new-thing

# 2. Develop code
# ... edit files ...

# 3. Run full validation sequence
cargo test --quiet
cargo check --quiet       # Use check, not build, for validation
cargo clippy

# 4. Auto-fix clippy warnings
cargo clippy --fix --allow-dirty

# 5. Re-run validation
cargo test --quiet
cargo check --quiet
cargo clippy

# 6. Commit changes
git add .
git commit -m "Add new feature"

Workflow 2: Fixing Clippy Warnings

# 1. See all warnings
cargo clippy

# 2. Auto-fix what's possible
cargo clippy --fix --allow-dirty

# 3. Review remaining warnings
cargo clippy

# 4. For domain-specific warnings, add suppressions
# Edit Cargo.toml [lints.clippy] section or add #[allow(...)]

# 5. Verify clean clippy
cargo clippy

# 6. Ensure tests still pass
cargo test --quiet

Workflow 3: Debugging Build Issues

# 1. Clean build artifacts
cargo clean

# 2. Kill any stuck processes
pkill -f cargo

# 3. Check compilation only (faster)
cargo check

# 4. Full build with verbose output
cargo build --verbose

# 5. If still failing, check with different targets
cargo check --lib
cargo check --tests

Workflow 4: Dependency Updates

# 1. Check current dependency tree
cargo tree

# 2. Update all dependencies
cargo update

# 3. Or update specific dependency
cargo update serde

# 4. Validate after update
cargo test --quiet
cargo check --quiet       # Use check, not build, for validation
cargo clippy

# 5. Review Cargo.lock changes
git diff Cargo.lock

Workflow 5: Pre-commit Validation

# Complete validation before committing (PREFER THIS)
cargo test --quiet && cargo check --quiet && cargo clippy

# If any step fails, the sequence stops
# Fix issues and repeat

# NOTE: Only use cargo build instead of cargo check if you actually need
# the binary artifact. For validation, cargo check is sufficient and much faster.

Performance Optimization

Build Performance

# Use cargo check for fast iteration (PREFER THIS)
cargo check

# Parallel builds (default, but can tune)
cargo build -j 8

# Use cache-friendly flags (ONLY for actual releases)
cargo build --release --locked  # WARNING: SLOW - only for production releases

Development tip: cargo check is 2-10x faster than cargo build and sufficient for most development.

Test Performance

# Run only unit tests (fast)
cargo test --lib

# Skip slow integration tests
cargo test --lib --bins

# Run specific test module
cargo test module_name::

Common Patterns and Tips

Pattern 1: Iterative Development

# Fast check loop
cargo check
# ... fix errors ...
cargo check
# ... fix more ...
cargo test --quiet  # when ready to validate

Pattern 2: Release Preparation (ONLY for actual releases)

WARNING: Only use this workflow when preparing an actual release. For regular development, use the check-first workflow.

# Full validation for release (SLOW - only for actual releases)
cargo test
cargo test --release        # WARNING: SLOW
cargo build --release       # WARNING: VERY SLOW
cargo clippy -- -D warnings  # Deny all warnings
cargo doc --no-deps

For regular development, use instead:

cargo test --quiet
cargo check --quiet
cargo clippy

Pattern 3: Multi-target Projects

# Check everything
cargo check --all-targets
cargo test --all-targets
cargo clippy --all-targets

Pattern 4: Documentation Testing

# Test code examples in docs
cargo test --doc

# Build and review docs
cargo doc --open

Error Handling

Common Issues and Solutions

Issue: "could not compile due to previous error"

# Solution: Check specific error, may need cargo clean
cargo clean
cargo build

Issue: "waiting for file lock on package cache"

# Solution: Kill stuck processes
pkill -f cargo

Issue: Clippy warnings overwhelming

# Solution: Auto-fix first, then strategic suppression
cargo clippy --fix --allow-dirty
# Review remaining, add suppressions as needed

Issue: Tests passing but clippy failing

# Solution: This is normal - fix clippy issues or suppress appropriately
cargo clippy --fix --allow-dirty
# Then review and add strategic suppressions

Quick Reference

# Development cycle (PREFER - use check for validation)
cargo test --quiet && cargo check --quiet && cargo clippy

# Auto-fix clippy
cargo clippy --fix --allow-dirty

# Clean and re-check
pkill -f cargo && cargo clean && cargo check

# Update dependencies
cargo update && cargo test --quiet && cargo check --quiet

# Documentation
cargo doc --open

# Add dependency
cargo add <crate>

# Fast iteration (just checking compilation)
cargo check

# Only build when you need the binary artifact
cargo build

Integration with Git

After validation, use the git commit message writer agent:

• Captures comprehensive commit context
• Follows project conventions
• Professional, human-written style (no AI attribution per CLAUDE.md)

# After cargo validation passes:
git add .
# Then request commit (use commit message writer agent)