rust-syntax-smart-pointers

Owning pointers and shared-ownership primitives: Box<T>, Rc<T>, Arc<T>, RefCell<T>, Cell<T>, OnceCell<T>, OnceLock<T>, LazyLock<T>, Weak<T>, and a Pin<T> overview. Each picks a different combination of: heap allocation, single-vs-multi-thread refcounting, interior mutability, and lazy initialization. Choosing wrongly leaks, deadlocks, panics, or fails to compile.

Cross-references: [[rust-core-memory-model]] (stack vs heap, UnsafeCell), [[rust-syntax-borrowing]] (the borrow rules RefCell enforces at runtime), [[rust-syntax-async-await]] (Pin<&mut Self> in Future::poll; deep pinning reference under that skill), [[rust-impl-concurrency]] (Arc<Mutex<T>> vs atomics in real workloads).

When to use this skill

• User reaches for Box<T> for a recursive type (tree, linked list) or a dyn Trait return.
• User wants to share read-only data across a single thread (Rc<T>) or across threads (Arc<T>).
• User asks "how do I mutate through a shared reference" and needs to choose Cell / RefCell / Mutex / RwLock / Atomic*.
• User wants a lazily-initialized global and is tempted to use static mut or lazy_static!.
• User builds a graph / observer / parent-child tree and hits a reference cycle (memory leak with Rc<RefCell<T>>).
• User sees Future::poll(self: Pin<&mut Self>, ...) and asks "what is Pin".