California permits write-ins in the June primary for all office. Every election year, there are a few legislative races in which only one person filed to be on the primary ballot. So then anyone who files as a write-in is guaranteed to place second, assuming the write-in candidate is the only declared write-in candidate.
For the 2018 primary, there are three Assembly districts in which Libertarians filed as write-ins, and there is only one candidate on the primary ballot, so the three Libertarians will place second and be on the November ballot. They are Justin Quigley in the 21st district (running against incumbent Adam Gray); Christopher Stare in the 51st district (running against incumbent Wendy Carrillo); and Autumn Browne in the 69th district (running against incumbent Tom Daly). Here is the entire list of declared write-in candidates for all federal and state office in the 2018 primary.