135 lines
4.1 KiB
Markdown
135 lines
4.1 KiB
Markdown
---
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name: m01-ownership
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description: "CRITICAL: Use for ownership/borrow/lifetime issues. Triggers: E0382, E0597, E0506, E0507, E0515, E0716, E0106, value moved, borrowed value does not live long enough, cannot move out of, use of moved value, ownership, borrow, lifetime, 'a, 'static, move, clone, Copy, 所有权, 借用, 生命周期"
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user-invocable: false
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---
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# Ownership & Lifetimes
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> **Layer 1: Language Mechanics**
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## Core Question
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**Who should own this data, and for how long?**
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Before fixing ownership errors, understand the data's role:
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- Is it shared or exclusive?
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- Is it short-lived or long-lived?
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- Is it transformed or just read?
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---
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## Error → Design Question
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| Error | Don't Just Say | Ask Instead |
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|-------|----------------|-------------|
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| E0382 | "Clone it" | Who should own this data? |
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| E0597 | "Extend lifetime" | Is the scope boundary correct? |
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| E0506 | "End borrow first" | Should mutation happen elsewhere? |
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| E0507 | "Clone before move" | Why are we moving from a reference? |
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| E0515 | "Return owned" | Should caller own the data? |
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| E0716 | "Bind to variable" | Why is this temporary? |
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| E0106 | "Add 'a" | What is the actual lifetime relationship? |
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---
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## Thinking Prompt
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Before fixing an ownership error, ask:
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1. **What is this data's domain role?**
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- Entity (unique identity) → owned
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- Value Object (interchangeable) → clone/copy OK
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- Temporary (computation result) → maybe restructure
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2. **Is the ownership design intentional?**
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- By design → work within constraints
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- Accidental → consider redesign
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3. **Fix symptom or redesign?**
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- If Strike 3 (3rd attempt) → escalate to Layer 2
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---
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## Trace Up ↑
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When errors persist, trace to design layer:
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```
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E0382 (moved value)
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↑ Ask: What design choice led to this ownership pattern?
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↑ Check: m09-domain (is this Entity or Value Object?)
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↑ Check: domain-* (what constraints apply?)
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```
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| Persistent Error | Trace To | Question |
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|-----------------|----------|----------|
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| E0382 repeated | m02-resource | Should use Arc/Rc for sharing? |
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| E0597 repeated | m09-domain | Is scope boundary at right place? |
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| E0506/E0507 | m03-mutability | Should use interior mutability? |
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---
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## Trace Down ↓
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From design decisions to implementation:
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```
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"Data needs to be shared immutably"
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↓ Use: Arc<T> (multi-thread) or Rc<T> (single-thread)
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"Data needs exclusive ownership"
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↓ Use: move semantics, take ownership
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"Data is read-only view"
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↓ Use: &T (immutable borrow)
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```
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---
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## Quick Reference
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| Pattern | Ownership | Cost | Use When |
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|---------|-----------|------|----------|
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| Move | Transfer | Zero | Caller doesn't need data |
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| `&T` | Borrow | Zero | Read-only access |
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| `&mut T` | Exclusive borrow | Zero | Need to modify |
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| `clone()` | Duplicate | Alloc + copy | Actually need a copy |
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| `Rc<T>` | Shared (single) | Ref count | Single-thread sharing |
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| `Arc<T>` | Shared (multi) | Atomic ref count | Multi-thread sharing |
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| `Cow<T>` | Clone-on-write | Alloc if mutated | Might modify |
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## Error Code Reference
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| Error | Cause | Quick Fix |
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|-------|-------|-----------|
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| E0382 | Value moved | Clone, reference, or redesign ownership |
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| E0597 | Reference outlives owner | Extend owner scope or restructure |
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| E0506 | Assign while borrowed | End borrow before mutation |
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| E0507 | Move out of borrowed | Clone or use reference |
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| E0515 | Return local reference | Return owned value |
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| E0716 | Temporary dropped | Bind to variable |
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| E0106 | Missing lifetime | Add `'a` annotation |
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---
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## Anti-Patterns
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| Anti-Pattern | Why Bad | Better |
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|--------------|---------|--------|
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| `.clone()` everywhere | Hides design issues | Design ownership properly |
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| Fight borrow checker | Increases complexity | Work with the compiler |
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| `'static` for everything | Restricts flexibility | Use appropriate lifetimes |
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| Leak with `Box::leak` | Memory leak | Proper lifetime design |
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---
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## Related Skills
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| When | See |
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|------|-----|
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| Need smart pointers | m02-resource |
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| Need interior mutability | m03-mutability |
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| Data is domain entity | m09-domain |
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| Learning ownership concepts | m14-mental-model |
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