When to use Struct Derives

Derive	When
Debug	Almost always — enables {:?} printing
Clone	When copying the value makes semantic sense
Copy	When type is small, stack-only, and implicit copy is fine (requires Clone)
PartialEq / Eq	When equality comparison makes sense
PartialOrd / Ord	When ordering makes sense
Hash	When used as HashMap/HashSet key (requires Eq)
Default	When a sensible zero/empty value exists
Serialize / Deserialize	When crossing API/file boundaries (via serde)

Practical default: slap #[derive(Debug, Clone)] on almost everything. Add the rest as needed.