igmarin/agent-profiles/rust-ownership-borrowing

Resolve skill names through ../../skill-map.json; use the source pack to disambiguate. Load only the workflow and resources needed for the authorized task.

Ownership and borrowing

Apply the execution contract before this procedure.

Goal

No allocation that a borrow would cover. Shared mutability only when the type system requires it.

Inputs / outputs

• In: a function or type that owns or shares data
• Out: API that takes the least ownership it needs

Reads / writes

• Read: the function and its callers
• Write: none unless a playbook opened the task

Approval

None to drop clones. State lifecycle and concurrency cost before adding Arc, Rc, Mutex, RwLock, or RefCell; stop for approval if that choice changes the accepted design or scope.

RULES — no exceptions

1. Prefer &T / &mut T over .clone()
2. Accept &[T] not &Vec<T>; &str not &String
3. Cow<'_, T> only when you sometimes own
4. Arc<T> across threads; Rc<T> single-thread only
5. RefCell / Mutex / RwLock last; document why
6. Copy for tiny, obviously copyable types; otherwise explicit Clone
7. Move large values; Box if the move itself is the cost
8. Elide lifetimes until the compiler asks

Example

// ❌
fn count_words(text: &String) -> usize {
    text.clone().split_whitespace().count()
}
 
// ✅
fn count_words(text: &str) -> usize {
    text.split_whitespace().count()
}

Clone is justified when storing, sending 'static to a thread, or the type is Copy.

Steps

1. Grep the touched fn for .clone(
2. If the clone is not stored or sent, borrow
3. Widen params to slices/str

Validation

cargo test on the crate (or cargo test <test_name> -- --exact if a test is already red). Diff should drop clones or justify each remainder in one comment.

Pitfalls

Integration

Predecessor: rust-essentials. Successor: type-driven-design. Do not hold a lock across .await.