Constant EMPTY_STRUCT_FIELDS 

pub const EMPTY_STRUCT_FIELDS: &[Field];

The schema of a struct that declares no fields: the one padding member rosidl adds (EMPTY_STRUCT_MEMBER). Codegen emits this for every empty message, so the schema describes the byte the serializer writes and a size bound, a Cyclone descriptor or a schema walk derived from FIELDS agrees with the wire. It has no host storage, so its offset is 0.