Agent skill

m15-anti-pattern

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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npx add-skill https://github.com/actionbook/rust-skills/tree/main/skills/m15-anti-pattern

SKILL.md

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

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