[oslog] [stdlib-private] Add a prototype of the new os_log swift APIs

that accept string interpolations.

The prototypes are added to stdlib/private directory and will be
used only in tests and in experimental code.

stdlib/private/OSLog/OSLog.swift

@@ -0,0 +1,69 @@
+//===----------------- OSLog.swift ----------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===-----------------------------------------------------------------------===//
+
+// This file contains the new swift APIs for OS log that accept string
+// interpolations. This is a prototype meant for experimentation and testing.
+// Do not use it outside of tests.
+
+@_exported import os
+
+@available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *)
+public struct Logger {
+  internal let logObject: OSLog
+
+  /// Create a custom OS log object.
+  public init(subsystem: String, category: String) {
+    logObject = OSLog(subsystem: subsystem, category: category)
+  }
+
+  /// Return the default OS log object.
+  public init() {
+    logObject = OSLog.default
+  }
+
+  /// Log a string interpolation at a given level. The level is `default` if
+  /// it is not specified.
+  public func log(level: OSLogType = .default, _ message: OSLogMessage) {
+    osLog(logObject, level, message)
+  }
+
+  // TODO: define overloads for logging at specific levels: debug, info, notice,
+  // error, fault based on the Swift forum "logging-levels" discussion.
+}
+
+/// Given an instance of the custom string interpolation type: `OSLogMessage`,
+/// extract the format string, serialize the arguments to a byte buffer,
+/// and pass them to the OS logging system.
+@available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *)
+internal func osLog(
+  _ logObject: OSLog,
+  _ logLevel: OSLogType,
+  _ message: OSLogMessage
+) {
+  guard logObject.isEnabled(type: logLevel) else { return }
+
+  let bufferSize = message.bufferSize
+  let bufferMemory = UnsafeMutablePointer<UInt8>.allocate(capacity: bufferSize)
+  var builder = OSLogByteBufferBuilder(bufferMemory)
+
+  message.serializeArguments(into: &builder)
+
+  message.formatString.withCString { cFormatString in
+    ___os_log_impl(UnsafeMutableRawPointer(mutating: #dsohandle),
+                   logObject,
+                   logLevel,
+                   cFormatString,
+                   bufferMemory,
+                   UInt32(bufferSize))
+  }
+  bufferMemory.deallocate()
+}