161 lines
4.7 KiB
Markdown
161 lines
4.7 KiB
Markdown
---
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name: m15-anti-pattern
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description: "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"
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user-invocable: false
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---
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# Anti-Patterns
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> **Layer 2: Design Choices**
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## Core Question
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**Is this pattern hiding a design problem?**
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When reviewing code:
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- Is this solving the symptom or the cause?
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- Is there a more idiomatic approach?
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- Does this fight or flow with Rust?
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---
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## Anti-Pattern → Better Pattern
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| Anti-Pattern | Why Bad | Better |
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|--------------|---------|--------|
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| `.clone()` everywhere | Hides ownership issues | Proper references or ownership |
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| `.unwrap()` in production | Runtime panics | `?`, `expect`, or handling |
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| `Rc` when single owner | Unnecessary overhead | Simple ownership |
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| `unsafe` for convenience | UB risk | Find safe pattern |
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| OOP via `Deref` | Misleading API | Composition, traits |
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| Giant match arms | Unmaintainable | Extract to methods |
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| `String` everywhere | Allocation waste | `&str`, `Cow<str>` |
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| Ignoring `#[must_use]` | Lost errors | Handle or `let _ =` |
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---
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## Thinking Prompt
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When seeing suspicious code:
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1. **Is this symptom or cause?**
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- Clone to avoid borrow? → Ownership design issue
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- Unwrap "because it won't fail"? → Unhandled case
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2. **What would idiomatic code look like?**
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- References instead of clones
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- Iterators instead of index loops
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- Pattern matching instead of flags
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3. **Does this fight Rust?**
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- Fighting borrow checker → restructure
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- Excessive unsafe → find safe pattern
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---
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## Trace Up ↑
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To design understanding:
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```
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"Why does my code have so many clones?"
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↑ Ask: Is the ownership model correct?
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↑ Check: m09-domain (data flow design)
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↑ Check: m01-ownership (reference patterns)
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```
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| Anti-Pattern | Trace To | Question |
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|--------------|----------|----------|
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| Clone everywhere | m01-ownership | Who should own this data? |
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| Unwrap everywhere | m06-error-handling | What's the error strategy? |
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| Rc everywhere | m09-domain | Is ownership clear? |
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| Fighting lifetimes | m09-domain | Should data structure change? |
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---
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## Trace Down ↓
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To implementation (Layer 1):
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```
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"Replace clone with proper ownership"
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↓ m01-ownership: Reference patterns
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↓ m02-resource: Smart pointer if needed
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"Replace unwrap with proper handling"
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↓ m06-error-handling: ? operator
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↓ m06-error-handling: expect with message
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```
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---
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## Top 5 Beginner Mistakes
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| Rank | Mistake | Fix |
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|------|---------|-----|
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| 1 | Clone to escape borrow checker | Use references |
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| 2 | Unwrap in production | Propagate with `?` |
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| 3 | String for everything | Use `&str` |
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| 4 | Index loops | Use iterators |
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| 5 | Fighting lifetimes | Restructure to own data |
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## Code Smell → Refactoring
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| Smell | Indicates | Refactoring |
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|-------|-----------|-------------|
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| Many `.clone()` | Ownership unclear | Clarify data flow |
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| Many `.unwrap()` | Error handling missing | Add proper handling |
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| Many `pub` fields | Encapsulation broken | Private + accessors |
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| Deep nesting | Complex logic | Extract methods |
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| Long functions | Multiple responsibilities | Split |
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| Giant enums | Missing abstraction | Trait + types |
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---
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## Common Error Patterns
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| Error | Anti-Pattern Cause | Fix |
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|-------|-------------------|-----|
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| E0382 use after move | Cloning vs ownership | Proper references |
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| Panic in production | Unwrap everywhere | ?, matching |
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| Slow performance | String for all text | &str, Cow |
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| Borrow checker fights | Wrong structure | Restructure |
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| Memory bloat | Rc/Arc everywhere | Simple ownership |
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---
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## Deprecated → Better
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| Deprecated | Better |
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|------------|--------|
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| Index-based loops | `.iter()`, `.enumerate()` |
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| `collect::<Vec<_>>()` then iterate | Chain iterators |
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| Manual unsafe cell | `Cell`, `RefCell` |
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| `mem::transmute` for casts | `as` or `TryFrom` |
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| Custom linked list | `Vec`, `VecDeque` |
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| `lazy_static!` | `std::sync::OnceLock` |
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---
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## Quick Review Checklist
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- [ ] No `.clone()` without justification
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- [ ] No `.unwrap()` in library code
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- [ ] No `pub` fields with invariants
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- [ ] No index loops when iterator works
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- [ ] No `String` where `&str` suffices
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- [ ] No ignored `#[must_use]` warnings
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- [ ] No `unsafe` without SAFETY comment
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- [ ] No giant functions (>50 lines)
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---
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## Related Skills
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| When | See |
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|------|-----|
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| Ownership patterns | m01-ownership |
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| Error handling | m06-error-handling |
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| Mental models | m14-mental-model |
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| Performance | m10-performance |
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