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?

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Error → Design Question

Error	Don't Just Say	Ask Instead
E0596	"Add mut"	Should this really be mutable?
E0499	"Split borrows"	Is the data structure right?
E0502	"Separate scopes"	Why do we need both borrows?
RefCell panic	"Use try_borrow"	Is runtime check appropriate?

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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?)

Persistent Error	Trace To	Question
Repeated borrow conflicts	m09-domain	Should data be restructured?
RefCell in async	m07-concurrency	Is Send/Sync needed?
Mutex deadlocks	m07-concurrency	Is the lock design right?

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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.