1---
2name: m15-anti-pattern
3description: "Use when reviewing code for anti-patterns. Keywords: anti-pattern, common mistake, pitfall, code smell, bad practice, code review, is this an anti-pattern, better way to do this, common mistake to avoid, why is this bad, idiomatic way, beginner mistake, fighting borrow checker, clone everywhere, unwrap in production, should I refactor, 反模式, 常见错误, 代码异味, 最佳实践, 地道写法"
4user-invocable: false
5---
6 
7# Anti-Patterns
8 
9> **Layer 2: Design Choices**
10 
11## Core Question
12 
13**Is this pattern hiding a design problem?**
14 
15When reviewing code:
16- Is this solving the symptom or the cause?
17- Is there a more idiomatic approach?
18- Does this fight or flow with Rust?
19 
20---
21 
22## Anti-Pattern → Better Pattern
23 
24| Anti-Pattern | Why Bad | Better |
25|--------------|---------|--------|
26| `.clone()` everywhere | Hides ownership issues | Proper references or ownership |
27| `.unwrap()` in production | Runtime panics | `?`, `expect`, or handling |
28| `Rc` when single owner | Unnecessary overhead | Simple ownership |
29| `unsafe` for convenience | UB risk | Find safe pattern |
30| OOP via `Deref` | Misleading API | Composition, traits |
31| Giant match arms | Unmaintainable | Extract to methods |
32| `String` everywhere | Allocation waste | `&str`, `Cow<str>` |
33| Ignoring `#[must_use]` | Lost errors | Handle or `let _ =` |
34 
35---
36 
37## Thinking Prompt
38 
39When seeing suspicious code:
40 
411. **Is this symptom or cause?**
42   - Clone to avoid borrow? → Ownership design issue
43   - Unwrap "because it won't fail"? → Unhandled case
44 
452. **What would idiomatic code look like?**
46   - References instead of clones
47   - Iterators instead of index loops
48   - Pattern matching instead of flags
49 
503. **Does this fight Rust?**
51   - Fighting borrow checker → restructure
52   - Excessive unsafe → find safe pattern
53 
54---
55 
56## Trace Up ↑
57 
58To design understanding:
59 
60```
61"Why does my code have so many clones?"
62    ↑ Ask: Is the ownership model correct?
63    ↑ Check: m09-domain (data flow design)
64    ↑ Check: m01-ownership (reference patterns)
65```
66 
67| Anti-Pattern | Trace To | Question |
68|--------------|----------|----------|
69| Clone everywhere | m01-ownership | Who should own this data? |
70| Unwrap everywhere | m06-error-handling | What's the error strategy? |
71| Rc everywhere | m09-domain | Is ownership clear? |
72| Fighting lifetimes | m09-domain | Should data structure change? |
73 
74---
75 
76## Trace Down ↓
77 
78To implementation (Layer 1):
79 
80```
81"Replace clone with proper ownership"
82    ↓ m01-ownership: Reference patterns
83    ↓ m02-resource: Smart pointer if needed
84 
85"Replace unwrap with proper handling"
86    ↓ m06-error-handling: ? operator
87    ↓ m06-error-handling: expect with message
88```
89 
90---
91 
92## Top 5 Beginner Mistakes
93 
94| Rank | Mistake | Fix |
95|------|---------|-----|
96| 1 | Clone to escape borrow checker | Use references |
97| 2 | Unwrap in production | Propagate with `?` |
98| 3 | String for everything | Use `&str` |
99| 4 | Index loops | Use iterators |
100| 5 | Fighting lifetimes | Restructure to own data |
101 
102## Code Smell → Refactoring
103 
104| Smell | Indicates | Refactoring |
105|-------|-----------|-------------|
106| Many `.clone()` | Ownership unclear | Clarify data flow |
107| Many `.unwrap()` | Error handling missing | Add proper handling |
108| Many `pub` fields | Encapsulation broken | Private + accessors |
109| Deep nesting | Complex logic | Extract methods |
110| Long functions | Multiple responsibilities | Split |
111| Giant enums | Missing abstraction | Trait + types |
112 
113---
114 
115## Common Error Patterns
116 
117| Error | Anti-Pattern Cause | Fix |
118|-------|-------------------|-----|
119| E0382 use after move | Cloning vs ownership | Proper references |
120| Panic in production | Unwrap everywhere | ?, matching |
121| Slow performance | String for all text | &str, Cow |
122| Borrow checker fights | Wrong structure | Restructure |
123| Memory bloat | Rc/Arc everywhere | Simple ownership |
124 
125---
126 
127## Deprecated → Better
128 
129| Deprecated | Better |
130|------------|--------|
131| Index-based loops | `.iter()`, `.enumerate()` |
132| `collect::<Vec<_>>()` then iterate | Chain iterators |
133| Manual unsafe cell | `Cell`, `RefCell` |
134| `mem::transmute` for casts | `as` or `TryFrom` |
135| Custom linked list | `Vec`, `VecDeque` |
136| `lazy_static!` | `std::sync::OnceLock` |
137 
138---
139 
140## Quick Review Checklist
141 
142- [ ] No `.clone()` without justification
143- [ ] No `.unwrap()` in library code
144- [ ] No `pub` fields with invariants
145- [ ] No index loops when iterator works
146- [ ] No `String` where `&str` suffices
147- [ ] No ignored `#[must_use]` warnings
148- [ ] No `unsafe` without SAFETY comment
149- [ ] No giant functions (>50 lines)
150 
151---
152 
153## Related Skills
154 
155| When | See |
156|------|-----|
157| Ownership patterns | m01-ownership |
158| Error handling | m06-error-handling |
159| Mental models | m14-mental-model |
160| Performance | m10-performance |
161