A useful printing implementation that requires no flags, yet gets removed during compiler optimization for a release build.

Log.swift

import UIKit
import PlaygroundSupport

/// Prints the `items` with an optional separator and terminator.
/// The benefit of this approach is that all calls will be entirely removed during compiler optimization for release builds.
/// Additionally you can provide an optional closure to perform text processing. The processed string gets printed either before or immediately after `message` depending on the returned boolean.
///
/// - Parameters:
///   - items: Zero or more items to print.
///   - separator: An optional string to print between each item; default is a single whitespace.
///   - terminator: Optionally provide a string to append to the end; default is a single new line.
///   - closure: Used to prepare textual output. The closure must return the string to print, and a boolean to determine if the resulting string should be shown above or after the `message` (but before terminator).
public func log(_ items: Any..., separator: String = " ", terminator: String = "\n", unprocessedText closure: (() -> (text: String, showAboveMessage: Bool))? = nil) {
    assert(logPrint(items, separator: separator, terminator: terminator, unprocessedText: closure))
}

// See comment above for log(_:seperator:terminator:closure:)
fileprivate func logPrint(_ items: [Any], separator: String, terminator end: String, unprocessedText closure: (() -> (text: String, showAboveMessage: Bool))?) -> Bool {
    var postfix = ""
    var output = ""
    
    if let processText = closure {
        let processed = processText() as (text: String, showAboveMessage: Bool)
        if processed.showAboveMessage {
            print(processed.text)
        } else {
            postfix = processed.text
        }
    }
    
    var prefix = ""
    for item in items {
        output += prefix + String(describing: item)
        prefix = separator
    }
    print(output + postfix, terminator: end)
    return true
}

log("Processing", separator: ":", terminator: "… ") { () -> (text: String, showAboveMessage: Bool) in
    let today = DispatchTime.now().rawValue
    return (" started \(today) ", false)
}
print("Done!")