M15 Anti Pattern Agent Skill

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:

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

• What would idiomatic code look like?
• References instead of clones
• Iterators instead of index loops
• Pattern matching instead of flags

• Does this fight Rust?
• Fighting borrow checker → restructure
• Excessive unsafe → find safe pattern

Trace Up ↑

To design understanding:

"Why does my code have so many clones?"
    ↑ Ask: Is the ownership model correct?
    ↑ Check: m09-domain (data flow design)
    ↑ Check: m01-ownership (reference patterns)

| Anti-Pattern | Trace To | Question |

|--------------|----------|----------|

| Clone everywhere | m01-ownership | Who should own this data? |

| Unwrap everywhere | m06-error-handling | What's the error strategy? |

| Rc everywhere | m09-domain | Is ownership clear? |

| Fighting lifetimes | m09-domain | Should data structure change? |

Trace Down ↓

To implementation (Layer 1):

"Replace clone with proper ownership"
    ↓ m01-ownership: Reference patterns
    ↓ m02-resource: Smart pointer if needed

"Replace unwrap with proper handling"
    ↓ m06-error-handling: ? operator
    ↓ m06-error-handling: expect with message

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)

Related Skills

| When | See |

|------|-----|

| Ownership patterns | m01-ownership |

| Error handling | m06-error-handling |

| Mental models | m14-mental-model |

| Performance | m10-performance |