@nuclearace thanks for the suggestion.
As of swift 5.1, string interpolation is broken in the dynamicallyCall(withArguments:) and dynamicallyCall(withKeywordArguments:) methods, so it's hard to be sure. But until the swift compiler has beefier reflection (if ever) manually writing out labels for debug and logged data, especially in testing and playground code, seems inevitable. See DefaultStringInterpolation cannot be used as an argument for DynamicCallable
I did manage to get interpolation working by using a class-based implementation of StringInterpolationProtocol on a custom type, but had to misuse ExpressibleByArrayLiteral in lieu of a ExpressibleBy<T> (this works for a fake ExpressibleByTuple too 
) I'll post the code back on the original thread.
Here are a couple of callsite examples:
log("\(0...1, label: "closed")", "\(2..<3, label: "halfOpen")") // using ExpressibleByStringInterpolation
log(partialUpTo: [...4], partialFrom: [5...]) // using ExpressibleByArrayLiteral
I definitely prefer typing the second form. (the array notation notation is only needed if you want string interpolation support before swift 6.