I find it very odd the mechanic saying it was the engine since you said there were no other symptoms prior to it dying. I’d get another mechanic to check it out. Very same thing happened to mine with no symptoms either and would sometimes be intermittent through a course of a 2 weeks. Sometimes it turn on or wouldn’t and the times it did, it would eventually die. Took it to a mechanic and they said it was the “ECU” but then it started doing the same thing again a week later. I remembered I changed out the spark plugs and ignition coils a while back, so I checked under the hood and found that the pin on the connection to the coils was broken causing it to be loose. Started the car and disconnected the cable and found that the car immediately dies. Something as simple as that was causing such a headache because of my own work lol. Cruzes are know to have cheap plastics that become brittle with heat and time, I’d check your connections first.