I'm not sure what a trash compactor bag is made of, but I imagine it's intended to be disposable.

I recommend a pack liner made of a more durable plastic, as they have a couple of advantages in emergency situations.

1) They can retain air and thus keep you buoyant if you come to grief crossing a river. 2) Assuming you're using a large pack for multi-day trips, you can use a pack liner (and in my country they're designed for this) as emergency shelter if you don't have a bivvy bag or tent.