Preface

In Swift, ENABLE_NS_ASSERTIONS is ignored, and whether assertions are on or off is dependent of SWIFT_OPTIMIZATION_LEVEL, see here for more information.

assert() is active for -Onone
assertionFailure() is active for -Onone
precondition() is active for -Onone and -O
preconditionFailure() is active for -Onone, -O and -Ounchecked
fatalError() is active for -Onone, -O and -Ounchecked

What I want to achieve

Debug and Beta Builds should have assertions enabled, Release Builds should have assertions disabled.

How I could do it

I could write all my assertions in Swift via the precondition method and compile Release builds with the -Ounchecked flag, beta builds with the -O flag.

What I'm unsure about

The default in iOS for Release builds is -O. Is there anything advised against using -Ounchecked for release builds? Which other unwanted side effects could this cause?

You should definitely not compile -Ounchecked for release in order to use precondition only while testing. Compiling -Ounchecked also disables checks for things like array out of bounds and unwrapping nil, that could lead to some pretty nasty production bugs involving memory corruption.

You can control assertion behaviour independently of compiler optimization settings via the -assert-config argument to swiftc:

$ cat assert.swift 
assert(false, "assertion asserted")
println("made it here...")
$ swiftc -Onone assert.swift; ./assert
assertion failed: assertion asserted: file assert.swift, line 1
Illegal instruction: 4
$ swiftc -O assert.swift; ./assert
made it here...
$ swiftc -O -assert-config Debug assert.swift; ./assert
assertion failed: assertion asserted: file assert.swift, line 1
Illegal instruction: 4
$

It doesn’t look like there’s an Xcode Build Settings toggle for this, but you can add using the “Other Swift Flags” setting.