Rust primitive types can be copied by simply copying the bits. Rust structs can also have this property by deriving the Copy trait.

Automatically derive Copy for:
- messages that only have fields with primitive types
- the Rust enum for one-of fields
- messages whose field type are messages that also implement Copy

Generated code for Protobuf enums already derives Copy.

Clippy reports: warning: using `clone` on type `Timestamp` which implements the `Copy` trait