m03-mutability_skill

Overview

This skill helps diagnose and fix Rust mutability and interior-mutability issues. It focuses on compiler errors like E0596, E0499, E0502 and runtime RefCell panics, and guides you to safer design choices. Use it to decide when to add mutability, choose the right mutable container, or restructure data to avoid borrow conflicts.

How this skill works

I inspect error triggers and runtime symptoms and map them to design questions rather than quick fixes. The skill asks whether mutation is necessary, who should control it, and what thread context applies. It then recommends patterns (&mut, Cell, RefCell, Mutex, RwLock, Atomics) and points to restructuring or concurrency checks when borrows repeatedly collide.

When to use it

• When the compiler reports E0596, E0499, or E0502
• When you see RefCell borrow panics at runtime
• When unsure whether to add mutability or use interior mutability
• When choosing thread-safe vs single-thread mutable containers
• When borrow conflicts suggest data should be restructured

Best practices

• Ask if mutation is essential; prefer returning new values or builder patterns when possible
• Decide who owns mutation: external callers (&mut) vs internal interior mutability vs synchronized concurrency
• Match tool to context: Cell/RefCell for single-thread, Mutex/RwLock/Atomic for multi-thread
• Trace persistent borrow errors to domain design — maybe split the data or change ownership
• Avoid RefCell proliferation; prefer clear ownership and handle runtime borrow errors explicitly

Example use cases

• Fix E0596: decide if adding mut is correct or if API should return an owned modified value
• Resolve E0499/E0502: split data into separate fields or change call ordering to avoid overlapping borrows
• Replace RefCell in async code with thread-safe primitives when Send/Sync is required
• Implement a shared counter: use AtomicUsize for simple counts, Mutex for complex state
• Switch single-thread mutable containers to RefCell/Cell to remove unnecessary locking