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.

Quick Reference

Pattern	Thread-Safe	Runtime Cost	Use When
&mut T	N/A	Zero	Exclusive mutable access
Cell<T>	No	Zero	Copy types, no refs needed
RefCell<T>	No	Runtime check	Non-Copy, need runtime borrow
Mutex<T>	Yes	Lock contention	Thread-safe mutation
RwLock<T>	Yes	Lock contention	Many readers, few writers
Atomic*	Yes	Minimal	Simple types (bool, usize)

Error Code Reference

Error	Cause	Quick Fix
E0596	Borrowing immutable as mutable	Add mut or redesign
E0499	Multiple mutable borrows	Restructure code flow
E0502	&mut while & exists	Separate borrow scopes

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Interior Mutability Decision

Scenario	Choose
T: Copy, single-thread	Cell<T>