Constant REQUEST_WRITER_GUID_SIZE 

pub const REQUEST_WRITER_GUID_SIZE: usize = 16; // 16usize

Width of RequestId::writer_guid, in bytes.

16, which is upstream’s rmw_request_id_t::writer_guid (int8_t[16]) and rclrs’s RequestId::writer_guid: [u8; 16]. It is NOT PUBLISHER_GID_SIZE (24, RMW_GID_STORAGE_SIZE): upstream keeps the two widths apart, and so does this.