In the U.S. constitution under the 13th amendment it states "Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction." by this logic slavery as punishment of a crime is permitted by the U.S. constitution.
This bolsters the economy because you have efficient workers that basically create something for nothing. This will help bring back jobs from overseas and increase global competitiveness.
Our government spends 74 billion dollars a year on prisons housing around 25% of the worlds inmates. By instituting this law we will be able to close many higher secutiryt prisons which not only makes our system more streamlined but cheaper to run.
I believe giving murders and rapist a sentence of military conscription or hard labor isn't a great idea at all. Many criminals have mental problems and are very unpredictable. Giving such person a gun would be a bad idea. Also, working with a group of criminals as slaves can cause many problems like uprisings and fights. Instead, prisons should focus on rehab so one day, they can turn their lives around.
We don't want murderers and rapists in the military. They had no respect for people's rights at home they won't abroad Geneva Convention or not. It would mean atrocities and inevitably scandals. It is on the whole a very bad idea. It's allowed by the 13th amendment yes but it's a bad idea.
The main issue I have with slavery is that it's so deplorable, especially in this day and age. One can have a whole argument on whether or not a person convicted of rape or murder should have basic human rights, but in my opinion, they should.
Then you have issues of trying to control people who often have mental issues, which can lead to more innocent people being harmed. Another problem area is that of finding people willing to make use of such people. Mainly due to moral reasons, but also due to safety concerns for themselves and their equipment. Then there's the problem of motivating said slaves. Being inhumane about it is wrong on many levels, but also expecting them to be efficient or diligent in any job other than the most basic kinds is also improbable.
So in summary, I wouldn't see this working in practice