Function build_join_room_block 

pub fn build_join_room_block(
    conn: &Connection,
    typing: &TypingRegistry,
    room: &Room,
    caller_user_id: i64,
    caller_mxid: &str,
    caller_device_id: &str,
    since_stream: i64,
    upto: i64,
    is_initial_or_full: bool,
    typing_gen_since: u64,
    filter: &SyncFilter,
    now: Instant,
) -> Result<Option<Value>, MatrixError>

One rooms.join.{roomId} block, or None when this is an incremental sync AND nothing changed for this room at all (a room is only listed when something happened — see the module doc’s emptiness rule).