"Heaven Sent" is interesting in that it is the exact opposite of a time loop! >!In that, time doesn't loop, and it's the Doctor himself that's reset between each 'loop'!<

This tripped me up on first watch, because I thought it was a time loop episode, but realised that that would create some plot holes >!If it was a time loop episode, then everything would reset, and thus the whole point of bashing the diamond wouldn't make sense!<

But it's explicitly not a time loop, and therefore certain observations change between each loop >!(gradually over time, the stars change postions)!< which is how he realises each time he's looping.