M03 Mutability

Mutability

Layer 1: Language Mechanics

Core Question

Why does this data need to change, and who can change it?

Before adding interior mutability, understand:

• Is mutation essential or accidental complexity?
• Who should control mutation?
• Is the mutation pattern safe?

Error → Design Question

Thinking Prompt

Before adding mutability:

1. Is mutation necessary?

   • Maybe transform → return new value
   • Maybe builder → construct immutably

2. Who controls mutation?

   • External caller → &mut T
   • Internal logic → interior mutability
   • Concurrent access → synchronized mutability

3. What's the thread context?

   • Single-thread → Cell/RefCell
   • Multi-thread → Mutex/RwLock/Atomic

Trace Up ↑

When mutability conflicts persist:

E0499/E0502 (borrow conflicts)    ↑ Ask: Is the data structure designed correctly?    ↑ Check: m09-domain (should data be split?)    ↑ Check: m07-concurrency (is async involved?)

Trace Down ↓

From design to implementation:

"Need mutable access from &self"    ↓ T: Copy → Cell<T>    ↓ T: !Copy → RefCell<T>
"Need thread-safe mutation"    ↓ Simple counters → AtomicXxx    ↓ Complex data → Mutex<T> or RwLock<T>
"Need shared mutable state"    ↓ Single-thread: Rc<RefCell<T>>    ↓ Multi-thread: Arc<Mutex<T>>

Borrow Rules

At any time, you can have EITHER:├─ Multiple &T (immutable borrows)└─ OR one &mut T (mutable borrow)
Never both simultaneously.

Quick Reference

Error Code Reference

Interior Mutability Decision

Anti-Patterns

Related Skills