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?

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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 _ =

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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

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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?

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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

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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

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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

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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

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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)

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Related Skills

When	See
Ownership patterns	m01-ownership
Error handling	m06-error-handling
Mental models	m14-mental-model
Performance	m10-performance