Turn on Strict Concurrency Checking

In “Build Settings” (for the whole project, not just individual targets), turn on “Strict Concurrency Checking”.

I tried this with the “Complete” setting, and the only errors/warnings we’re getting (as of this writing) are in CommandLineInterfaceTests. Specifically, the call and callWithError functions are mutating the result local variable inside a closure, giving the error:

🛑 Mutation of captured var 'result' in concurrently-executing code

That’s not too hard to fix with just substituting an actor to accumulate the strings that were being assigned to result inside the closure.