After upgrading from Linux Mint 13 to 17 (and Eclipse from Juno to Luna) I noticed that auto-complete (also called content assist) which is normally invoked using Ctrl+Space is not working anymore. After searching I found it is not actually caused by Eclipse, but instead the IBus (Keyboard) preferences eat Ctrl+Space so Eclipse never receives it! This is a very poor choice of global shortcut and it has been reported as a bug here. You can confirm Eclipse is not receiving it by going into Window->Preferences->General->Keys and attempting to enter Ctrl+Space for a key.
Until it is fixed, you can work-around it by right-clicking the keyboard icon in the system tray (next to the clock), choose ‘Preferences’ and select a different shortcut for ‘Next Input Method’.