Chapter 2: Ownership & Borrowing Mastery

This chapter delves deeper into Rust's ownership model and borrowing system through design patterns that showcase advanced memory management techniques. Building on the foundation patterns from Chapter 1, these patterns will teach you to leverage Rust's unique features for efficient, safe, and elegant code. After mastering these patterns, you'll have a deep understanding of how to work with Rust's ownership system rather than fighting against it.

Patterns Covered

Strategy Pattern

Simple trait usage, dynamic dispatch vs static dispatch

Core Rust concepts introduced:

• Trait Objects and Dynamic Dispatch (dyn Trait)
• Static vs Dynamic Dispatch Trade-offs
• Object Safety Rules
• Vtables and Runtime Polymorphism
• Box Smart Pointers for Heap Allocation
• Performance Implications of Dispatch Types

Adapter Pattern

Trait implementations for foreign types, orphan rule

New Rust concepts introduced:

• The Orphan Rule and Coherence
• Newtype Wrapper for External Types
• Foreign Function Interface (FFI) Considerations
• Trait Implementation Restrictions
• Upstream/Downstream Crate Relationships
• Blanket Implementations and Conflicts

Interior Mutability Pattern

When and why to use Cell/RefCell/Mutex

New Rust concepts introduced:

• Interior Mutability vs Inherited Mutability
• Cell<T> for Copy Types
• RefCell<T> for Runtime Borrow Checking
• Mutex<T> for Thread-Safe Interior Mutability
• Runtime vs Compile-Time Borrow Checking
• Shared Ownership with Rc<RefCell<T>>
• Memory Safety Trade-offs

Cow (Clone on Write)

Efficient memory usage, understanding when cloning is needed

New Rust concepts introduced:

• Cow<'a, T> Enum and Borrowed/Owned Variants
• Lazy Cloning and Performance Optimization
• Lifetime Elision in Return Types
• ToOwned Trait and Custom Implementations
• Zero-Copy String Processing
• API Design for Flexible Input Types
• Memory Efficiency Patterns

Command Pattern

Closures, function pointers, and ownership of captured variables

New Rust concepts introduced:

• Closure Traits: Fn, FnMut, and FnOnce
• Function Pointers (fn) vs Closures
• Capture Modes: By Reference, By Value, By Move
• move Keyword for Closure Ownership
• Boxed Closures for Dynamic Storage
• Higher-Order Functions and Combinators
• Callback Patterns and Event Systems

Learning Approach

Each pattern in this chapter builds upon the ownership concepts from Chapter 1 while introducing more sophisticated borrowing and memory management techniques. You'll learn:

1. When to use different dispatch mechanisms and their performance implications
2. How to work with external types while respecting Rust's coherence rules
3. When compile-time borrowing isn't enough and how to safely use runtime alternatives
4. How to avoid unnecessary cloning while maintaining API flexibility
5. How to capture and store computation using Rust's powerful closure system

Pattern Structure

Each pattern chapter follows this enhanced structure:

1. Problem Statement - Real-world scenarios where the pattern applies
2. Traditional Implementation - How other languages approach the problem
3. Rust Implementation - Leveraging ownership and borrowing for better solutions
4. Performance Analysis - Memory and runtime characteristics
5. Best Practices - When to use each approach and common pitfalls
6. Advanced Techniques - Pushing the pattern to its limits
7. Core Rust Concepts Introduced - New ownership and borrowing features

By the end of this chapter, you'll understand Rust's ownership model at a deep level and be able to choose the right ownership patterns for complex scenarios. You'll also be prepared for the concurrent and parallel patterns in later chapters.