Systemd/shell-completion/zsh/_sd_unit_files

#autoload

_sd_unit_files() {
    local files expl
    files=( '*:files:->files' )

    _description files expl 'unit file'
    _files "$expl[@]" -g '*.(automount|busname|device|mount|path|service|snapshot|socket|swap|target|timer)'
}