Agent skill

m01-ownership

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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npx add-skill https://github.com/actionbook/rust-skills/tree/main/skills/m01-ownership

SKILL.md

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:

  1. What is this data's domain role?

    • Entity (unique identity) → owned
    • Value Object (interchangeable) → clone/copy OK
    • Temporary (computation result) → maybe restructure

  2. Is the ownership design intentional?

    • By design → work within constraints
    • Accidental → consider redesign

  3. 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

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