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
| 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? |
Thinking Prompt
Before adding mutability:
-
Is mutation necessary?
- Maybe transform → return new value
- Maybe builder → construct immutably
-
Who controls mutation?
- External caller →
&mut T - Internal logic → interior mutability
- Concurrent access → synchronized mutability
- External caller →
-
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?)
| 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? |
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
| 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 |
Interior Mutability Decision
| Scenario | Choose |
|---|---|
| T: Copy, single-thread | Cell<T> |
| T: !Copy, single-thread | RefCell<T> |
| T: Copy, multi-thread | AtomicXxx |
| T: !Copy, multi-thread | Mutex<T> or RwLock<T> |
| Read-heavy, multi-thread | RwLock<T> |
| Simple flags/counters | AtomicBool, AtomicUsize |
Anti-Patterns
| Anti-Pattern | Why Bad | Better |
|---|---|---|
| RefCell everywhere | Runtime panics | Clear ownership design |
| Mutex for single-thread | Unnecessary overhead | RefCell |
| Ignore RefCell panic | Hard to debug | Handle or restructure |
| Lock inside hot loop | Performance killer | Batch operations |
Related Skills
| When | See |
|---|---|
| Smart pointer choice | m02-resource |
| Thread safety | m07-concurrency |
| Data structure design | m09-domain |
| Anti-patterns | m15-anti-pattern |