m03-mutability

Fix Rust borrow checker errors by applying interior mutability primitives like Cell or RefCell to safely manage mutable borrows and shared state. This Skill diagnoses E0596 and E0502 errors and guides refactoring data structures to resolve ownership conflicts.

Use Cell for Copy types when you need interior mutability without direct references, and RefCell for types requiring mutable borrows at runtime. This Skill helps select the correct primitive to prevent borrow conflicts and runtime panics based on your single-thread context.

Handle multi-thread mutable access by selecting Mutex, RwLock, or Arc variants to ensure concurrency-safe shared state. This Skill guides choosing the right synchronization primitive to prevent borrow rule violations in concurrent Rust environments.

Rust requires interior mutability when shared objects need mutation without external mutable ownership, enforced by runtime checks or type constraints. This Skill explains the mechanism and helps apply Cell, RefCell, or Mutex to design safer mutable state.

RefCell is not suitable for multi-threaded Rust code because it lacks thread safety; use Mutex or RwLock combined with Arc instead. This Skill helps identify when single-threaded RefCell patterns must transition to concurrency-safe primitives.