M01 Ownership Agent Skill

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 |