When should I use references (&T) versus smart pointers like Box or Rc?

This really boils down to ownership, and more importantly: how do you want this to be cleaned up?

For a function that reads something, acts on it and doesn't do any sort of cleanup: Use a reference.

If you're transferring ownership, the default move semantic or Box if it's on the heap is the right answer.

If you don't really know who owns an object, and want it to go away whenever it is no longer in use by anyone: use Rc or Arc.

And if you're in async, learn to love Arc!