rust-anti-patterns

name	rust-anti-patterns
description	Identifies and fixes common Rust anti-patterns. Use during code review, when seeing excessive clones/unwraps/index loops, or when code fights the borrow checker instead of working with it.

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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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

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Anti-Pattern Reference

Anti-Pattern	Why Bad	Better
.clone() everywhere	Hides ownership issues	Proper references or ownership
.unwrap() in production	Runtime panics	?, expect, or matching
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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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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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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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
once_cell::Lazy	std::sync::LazyLock

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Related Skills

• ownership — fixing ownership anti-patterns
• error-handling — replacing unwrap chains
• performance — fixing allocation anti-patterns
• type-driven-design — fixing primitive obsession