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:

Is this symptom or cause?

- Clone to avoid borrow? → Ownership design issue - Unwrap "because it won't fail"? → Unhandled case

What would idiomatic code look like?

- References instead of clones - Iterators instead of index loops - Pattern matching instead of flags

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

m15-anti-pattern

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