Yesterday I discovered that pretty much all of my C++ submissions in fact did stop working, just as a result of normal language updates! Some of my Rust submissions have died too.
So, I repeat. The arguement that it will break existing submissions is completely bogus. 
Realistically, if anything, it’s in CG’s benefit to enable optimisation flags, it could reduce their server loads substantially!
Do you have any real example of these code that currently work in rust’s debug mode that would not in release mode ?
Aside from panic the behaviour is meant to be the very same. The only actual difference being the execution speed.
Some extra check are disabled in release mode that would panic in debug mode (such as overflow/wrapping).
So the actual argument would be :
EDIT:
I remembered a discussion about that last year :
That thing about debug purposes makes sense though.
Please add some option where you can choose to run your program in release or debug mode. Then set the default to debug. Everybody would be happy.