Ownership & Lifetimes
Layer 1: Language Mechanics
Core Question
Who should own this data, and for how long?
Before fixing ownership errors, understand the data's role:
- Is it shared or exclusive?
- Is it short-lived or long-lived?
- Is it transformed or just read?
Error → Design Question
| Error | Don't Just Say | Ask Instead | |-------|----------------|-------------| | E0382 | "Clone it" | Who should own this data? | | E0597 | "Extend lifetime" | Is the scope boundary correct? | | E0506 | "End borrow first" | Should mutation happen elsewhere? | | E0507 | "Clone before move" | Why are we moving from a reference? | | E0515 | "Return owned" | Should caller own the data? | | E0716 | "Bind to variable" | Why is this temporary? | | E0106 | "Add 'a" | What is the actual lifetime relationship? |
Thinking Prompt
Before fixing an ownership error, ask:
-
What is this data's domain role?
- Entity (unique identity) → owned
- Value Object (interchangeable) → clone/copy OK
- Temporary (computation result) → maybe restructure
-
Is the ownership design intentional?
- By design → work within constraints
- Accidental → consider redesign
-
Fix symptom or redesign?
- If Strike 3 (3rd attempt) → escalate to Layer 2
Trace Up ↑
When errors persist, trace to design layer:
E0382 (moved value)
↑ Ask: What design choice led to this ownership pattern?
↑ Check: m09-domain (is this Entity or Value Object?)
↑ Check: domain-* (what constraints apply?)
| Persistent Error | Trace To | Question | |-----------------|----------|----------| | E0382 repeated | m02-resource | Should use Arc/Rc for sharing? | | E0597 repeated | m09-domain | Is scope boundary at right place? | | E0506/E0507 | m03-mutability | Should use interior mutability? |
Trace Down ↓
From design decisions to implementation:
"Data needs to be shared immutably"
↓ Use: Arc<T> (multi-thread) or Rc<T> (single-thread)
"Data needs exclusive ownership"
↓ Use: move semantics, take ownership
"Data is read-only view"
↓ Use: &T (immutable borrow)
Quick Reference
| Pattern | Ownership | Cost | Use When |
|---------|-----------|------|----------|
| Move | Transfer | Zero | Caller doesn't need data |
| &T | Borrow | Zero | Read-only access |
| &mut T | Exclusive borrow | Zero | Need to modify |
| clone() | Duplicate | Alloc + copy | Actually need a copy |
| Rc<T> | Shared (single) | Ref count | Single-thread sharing |
| Arc<T> | Shared (multi) | Atomic ref count | Multi-thread sharing |
| Cow<T> | Clone-on-write | Alloc if mutated | Might modify |
Error Code Reference
| Error | Cause | Quick Fix |
|-------|-------|-----------|
| E0382 | Value moved | Clone, reference, or redesign ownership |
| E0597 | Reference outlives owner | Extend owner scope or restructure |
| E0506 | Assign while borrowed | End borrow before mutation |
| E0507 | Move out of borrowed | Clone or use reference |
| E0515 | Return local reference | Return owned value |
| E0716 | Temporary dropped | Bind to variable |
| E0106 | Missing lifetime | Add 'a annotation |
Anti-Patterns
| Anti-Pattern | Why Bad | Better |
|--------------|---------|--------|
| .clone() everywhere | Hides design issues | Design ownership properly |
| Fight borrow checker | Increases complexity | Work with the compiler |
| 'static for everything | Restricts flexibility | Use appropriate lifetimes |
| Leak with Box::leak | Memory leak | Proper lifetime design |
Related Skills
| When | See | |------|-----| | Need smart pointers | m02-resource | | Need interior mutability | m03-mutability | | Data is domain entity | m09-domain | | Learning ownership concepts | m14-mental-model |