m01-ownership

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 (multi-thread) or Rc (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 Shared (single) Ref count Single-thread sharing Arc Shared (multi) Atomic ref count Multi-thread sharing Cow 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