swift 4.1.2 deprecated features and will be removed in a future release

[Sergey Di]

$ cat /etc/lsb-release 
DISTRIB_DESCRIPTION="Ubuntu 16.04.4 LTS"
$ swift --version
Swift version 4.1.2 (swift-4.1.2-RELEASE)
Target: x86_64-unknown-linux-gnu

$ swift build
...
Compile Swift Module 'PerfectNet' (7 sources)
/home/user/antiparsing_v3/.build/checkouts/Perfect-Net.git--9169202650391526425/Sources/NetTCPSSL.swift:779:27: warning: using '!' in this location is deprecated and will be removed in a future release; consider changing this to '?' instead
                typealias ctxCallback = (@convention(c) () -> Swift.Void)!
...