# Rust Anti Patterns

> Use when reviewing code for anti-patterns and when the user asks you for code design advice.

- Skill: `tomevault-io/rust-anti-patterns` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add tomevault-io/rust-anti-patterns`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/rust-anti-patterns/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: tomevault-io (https://skillmd.com/u/tomevault-io)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/tomevault-io/rust-anti-patterns

---


# Anti-Patterns

> **Layer 2: Design Choices**

## Core Question

**Is this pattern hiding a design problem?**

When reviewing code:
- Is this solving the symptom or the cause?
- Is there a more idiomatic approach?
- Does this fight or flow with Rust?

---

## Anti-Pattern → Better Pattern

| Anti-Pattern | Why Bad | Better |
|--------------|---------|--------|
| `.clone()` everywhere | Hides ownership issues | Proper references or ownership |
| `.unwrap()` in production | Runtime panics | `?`, `expect`, or handling |
| `Rc` when single owner | Unnecessary overhead | Simple ownership |
| `unsafe` for convenience | UB risk | Find safe pattern |
| OOP via `Deref` | Misleading API | Composition, traits |
| Giant match arms | Unmaintainable | Extract to methods |
| `String` everywhere | Allocation waste | `&str`, `Cow<str>` |
| Ignoring `#[must_use]` | Lost errors | Handle or `let _ =` |

---

## Thinking Prompt

When seeing suspicious code:

1. **Is this symptom or cause?**
   - Clone to avoid borrow? → Ownership design issue
   - Unwrap "because it won't fail"? → Unhandled case

2. **What would idiomatic code look like?**
   - References instead of clones
   - Iterators instead of index loops
   - Pattern matching instead of flags

3. **Does this fight Rust?**
   - Fighting borrow checker → restructure
   - Excessive unsafe → find safe pattern

---

## Top 5 Beginner Mistakes

| Rank | Mistake | Fix |
|------|---------|-----|
| 1 | Clone to escape borrow checker | Use references |
| 2 | Unwrap in production | Propagate with `?` |
| 3 | String for everything | Use `&str` |
| 4 | Index loops | Use iterators |
| 5 | Fighting lifetimes | Restructure to own data |

## Code Smell → Refactoring

| Smell | Indicates | Refactoring |
|-------|-----------|-------------|
| Many `.clone()` | Ownership unclear | Clarify data flow |
| Many `.unwrap()` | Error handling missing | Add proper handling |
| Many `pub` fields | Encapsulation broken | Private + accessors |
| Deep nesting | Complex logic | Extract methods |
| Long functions | Multiple responsibilities | Split |
| Giant enums | Missing abstraction | Trait + types |

---

## Common Error Patterns

| Error | Anti-Pattern Cause | Fix |
|-------|-------------------|-----|
| E0382 use after move | Cloning vs ownership | Proper references |
| Panic in production | Unwrap everywhere | ?, matching |
| Slow performance | String for all text | &str, Cow |
| Borrow checker fights | Wrong structure | Restructure |
| Memory bloat | Rc/Arc everywhere | Simple ownership |

---

## Deprecated → Better

| Deprecated | Better |
|------------|--------|
| Index-based loops | `.iter()`, `.enumerate()` |
| `collect::<Vec<_>>()` then iterate | Chain iterators |
| Manual unsafe cell | `Cell`, `RefCell` |
| `mem::transmute` for casts | `as` or `TryFrom` |
| Custom linked list | `Vec`, `VecDeque` |
| `lazy_static!` | `std::sync::OnceLock` |

---

## Quick Review Checklist

- [ ] No `.clone()` without justification
- [ ] No `.unwrap()` in library code
- [ ] No `pub` fields with invariants
- [ ] No index loops when iterator works
- [ ] No `String` where `&str` suffices
- [ ] No ignored `#[must_use]` warnings
- [ ] No `unsafe` without SAFETY comment
- [ ] No giant functions (>50 lines)

---
> Source: [christestet/owui-lint](https://github.com/christestet/owui-lint) — distributed by [TomeVault](https://tomevault.io).
<!-- tomevault:4.0:skill_md:2026-06-16 -->