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?

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Error → Design Question

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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

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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?)

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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>>

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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

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Interior Mutability Decision

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Anti-Patterns

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Related Skills