Mutability Layer 1: Language Mechanics Core Question Why does this data need to change, and who can change it? Before adding interior mutability, understand: - Is mutation essential or accidental complexity? - Who should control mutation? - Is the mutation pattern safe? --- Error → Design Question | Error | Don't Just Say | Ask Instead | |-------|----------------|-------------| | E0596 | "Add mut" | Should this really be mutable? | | E0499 | "Split borrows" | Is the data structure right? | | E0502 | "Separate scopes" | Why do we need both borrows? | | RefCell panic | "Use try borrow" | Is run…