Why no contracts for release build?

[Dicebot via Digitalmars-d]

Contracts are not supposed to be use for error checking / input 
validation. Quoting docs for enforce: "Also, do not use enforce 
inside of contracts (i.e. inside of in and out blocks and 
invariants), because they will be compiled out when compiling 
with -release. Use assert in contracts. "

Contracts are supposed to verify that there are no fundamental 
flaws in internal logic of the program which will immediately pop 
up in test / debug runs. This may imply very expensive sanity 
checks that are not welcome in release builds for performance 
reasons.

In your case you should simply move `enforce` to the function 
body and remove contract.