m03-mutability

Rust mutability design: choosing between &mut , interior mutability, and thread-safe patterns. Layer 1: Language Mechanics Why does this data need to change, and who can change it? Before adding interior mutability, understand: Before adding mutability: Is mutation necessary? Who controls mutation? What's the thread context? When mutability conflicts persist: From design to implementation: - Guides decision-making for E0596, E0499, E0502 errors by asking whether mutation is necessary and who controls it, rather than reflexively adding mut - Covers single-thread patterns (Cell, RefCell) and multi-thread patterns (Mutex, RwLock, Atomic types) with clear selection criteria - Includes borrow rule reference, anti-patterns (RefCell overuse, Mutex in hot loops), and decision tables for interior mutability scenarios - Traces upward to data structure design (m09-domain) and concurrency concerns (m07-concurrency) when borrow conflicts persist - Is mutation essential or accidental complexity? - Who should control mutation? - Is the mutation pattern safe?