Code Quality Guide

Load this skill when writing, reviewing, or refactoring Rust code in the SWS project.

When to load: editing any file under src/, adding a new module, changing error handling, touching async code, or reviewing a PR that modifies Rust source.

Core Engineering Principles

• Correctness above all. Build production-grade software, not prototypes. Prioritize correctness, reliability, and maintainability over expedient shortcuts.

• Every change requires a test. Every code change must be accompanied by a test that fails before the change and passes after it. No behavioral change is complete without objective verification.

• Enforce invariants explicitly. Critical assumptions and invariants must be asserted, not silently ignored. Fail fast on invalid states rather than masking defects with defensive conditionals that obscure root causes.

• Own regressions end-to-end. Any test failures introduced by your change are your responsibility to investigate and resolve. Do not defer by comparing against another branch or attempting to prove the failure is pre-existing. Diagnose the failure, identify the root cause, and either fix it or provide conclusive evidence that it is unrelated.

• Evidence over assumptions. Every debugging hypothesis must be validated with reproducible evidence. Never speculate, infer causality without proof, or implement fixes based on unverified assumptions. Root-cause analysis must be grounded in observable facts.

• Optimize only after measurement. Performance work must be driven by profiling, benchmarks, or measurable evidence. Do not trade correctness or maintainability for speculative micro-optimizations.

API Design

• Design APIs that make invalid states impossible or difficult to represent.
• Encode invariants in the type system whenever practical instead of relying on runtime validation.
• Public APIs should have clear ownership semantics and minimal surprises.
• Favor composability over specialization.

State & Concurrency

• Minimize mutable state. Keep state transitions explicit and deterministic.
• Prefer ownership and message passing over shared mutable state. Synchronize shared state explicitly.
• Avoid holding locks across .await. Acquire locks for the shortest possible duration.
• Keep async critical sections small. Never block asynchronous executors (see rust-backend skill: no block_on in async context).
• Ensure task cancellation leaves the system in a valid state.

Maintainability

• Prefer straightforward code over clever code.
• Eliminate duplication through sound abstractions, not indirection.
• Keep functions focused on a single responsibility. Keep modules cohesive.
• Refactor when complexity increases instead of layering additional special cases.
• Code should be understandable without external explanation.

Explicitness & Predictability

• Make control flow, ownership, and lifetimes obvious.
• Prefer explicit conversions over implicit behavior.
• Identical inputs must produce identical outputs unless randomness or external state is explicitly part of the contract.
• Avoid hidden side effects and surprising defaults.

Resilience

• Validate external input immediately. Never silently ignore malformed input.
• Fail fast on impossible states. Recover gracefully from expected operational failures.
• Preserve service availability whenever recovery is possible.

Code Review Checklist

Every contribution should leave the codebase:

• More correct.
• More explicit.
• More maintainable.
• Better tested.
• At least as performant.
• Easier to reason about than before.

Cross-References

The following concerns are governed by dedicated skills — defer to them for specifics:

Concern	Skill
Error handling (Result<T>, anyhow::Context, StatusCode), unsafe policy, async runtime, file system, patterns to avoid	rust-backend/SKILL.md
Test organization, fixtures, handler/static-file test patterns	testing/SKILL.md
Profiling, allocations, hot-path optimization, benchmarking	performance/SKILL.md
Path traversal, TLS, security headers, CORS, input validation	security/SKILL.md