m15-anti-pattern

# 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 --- ## 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 |