zanna@peach:~/apt-1.9.4ubuntu0.1$ grep -r 'DoDistUpgrade'
cmdline/apt-get.cc: {"dist-upgrade", &DoDistUpgrade, _("Distribution upgrade, see apt-get(8)")},
cmdline/apt-get.cc: {"full-upgrade", &DoDistUpgrade, nullptr},
cmdline/apt.cc: {"full-upgrade", &DoDistUpgrade, _("upgrade the system by removing/installing/upgrading packages")},
cmdline/apt.cc: {"dist-upgrade", &DoDistUpgrade, nullptr},
apt-private/private-upgrade.h:APT_PUBLIC bool DoDistUpgrade(CommandLine &CmdL);
apt-private/private-upgrade.cc:// DoDistUpgrade - Automatic smart upgrader …