Is there any benefit to riders on the front of a paceline? You can save a lot on air resistance by drafting a rider closely, i.e. riding close to their rear wheel.

What about the rider in front? Is there a benefit, a loss, or is there no difference?

Edit - is there any way or position ensuring front cannot feel any energy loss while you enjoy resting behind or speedup ?
Yes, it's small but real.  Aerodynamic drag in cycling pelotons: New insights by CFD simulation and wind tunnel testing Journal of Wind Engineering and Industrial Aerodynamics
Volume 179, August 2018 is an interesting paper on the subject, with both theory and experiment.  The introduction is a nice summary of previous work, including simpler cases, but many of those papers are inacessible without a university login, unlike the one I've linked.

Figure 9 and Section 3.8 are most of interest, especially Fig. 9b which indicates that a reduction in drag of 4% on the lead rider is possible.  Figure 22a shows a simulation leading to an even greater reduction in drag for the front rider.

You can see from the figures in the paper that behind a single rider there's a low-pressure region. You can regard this as sucking the rider backwards. By partially filling this low pressure region with another rider, its effect is reduced.