Rust Ownership Borrowing

Use this skill to make ownership shape match the program's data flow. Treat borrow-checker errors as design feedback: decide who owns each value, how long access must last, and whether mutation needs exclusivity or a different data shape.

Core Workflow

1. Identify the owner of each value crossing the failing code path.
2. Decide the required access for each function: own, shared borrow, mutable borrow, or shared ownership.
3. Shorten borrows before changing types. Introduce blocks, temporary values, helper functions, or earlier extraction so references end before mutation.
4. Prefer moving values when the caller no longer needs them. Prefer borrowing when the caller retains ownership. Clone only when duplicate ownership is intentional and the cost is acceptable.
5. Replace self-referential or graph-like designs with IDs, indices, arenas, or ownership trees before reaching for Rc<RefCell<_>>.
6. Use Rc for single-threaded shared ownership and Arc for cross-thread shared ownership. Add Cell, RefCell, Mutex, or RwLock only when the mutation model is explicit.
7. Add tests that exercise the ownership-sensitive behavior, not just the compiler error that prompted the change.

Signature Rules

• Take T when the function consumes or stores the value.
• Take &T when the function only reads during the call.
• Take &mut T when the function must mutate and no other access should occur.
• Return owned values unless the returned reference is clearly tied to an input lifetime.
• Avoid accepting &Vec<T>, &String, or &PathBuf when &[T], &str, or &Path expresses the needed capability.
• Avoid adding lifetime parameters to structs unless the struct truly borrows data owned elsewhere. Owned fields are usually simpler.

Common Fix Patterns

Read references/borrow-patterns.md when resolving non-trivial compiler errors or reviewing a borrow-heavy patch.

• Narrow the scope of immutable borrows before taking a mutable borrow.
• Use Option::take, std::mem::take, or std::mem::replace to move a field out while leaving a valid value behind.
• Use collection APIs such as split_at_mut, get_mut, entry, drain, and retain instead of indexing patterns that create overlapping borrows.
• Clone handles like Arc, Rc, and cheap IDs freely when that is the intended ownership handle; avoid cloning payloads to silence the compiler.
• Convert iterator chains to explicit loops only when it makes borrow lifetimes clearer or avoids hidden captures.

Review Checklist

• Ownership follows the domain model; types are not wrapped just to bypass the borrow checker.
• Clone is deliberate and tested for semantic independence where relevant.
• Interior mutability has a clear invariant and a small access surface.
• Lifetimes describe real borrowing relationships, not guesses added until the compiler accepts the code.
• Async or threaded code uses Arc plus the right synchronization primitive; Rc and RefCell stay out of cross-thread paths.