Whoa wait, the system frameworks actually include os_log APIs for Swift? I thought they didn't have that, like they don't have os_activity. That's interesting.

What is the answer fixing these warnings?

@kdawgwilk try this:
let str = buf.baseAddress!.withMemoryRebound(to: Int8.self, capacity: 8, { $0 })

Update for Xcode 9.4 / Swift 4.1:

import Foundation
import os.activity

// Bridging Obj-C variabled defined as c-macroses. See `activity.h` header.
private let OS_ACTIVITY_NONE = unsafeBitCast(dlsym(UnsafeMutableRawPointer(bitPattern: -2), "_os_activity_none"), to: OS_os_activity.self)
private let OS_ACTIVITY_CURRENT = unsafeBitCast(dlsym(UnsafeMutableRawPointer(bitPattern: -2), "_os_activity_current"), to: OS_os_activity.self)

public struct Activity {

   private let activity: OS_os_activity!

   public init(_ description: StaticString, dso: UnsafeRawPointer? = #dsohandle, options: Options = []) {
      activity = description.withUTF8Buffer {
         if let dso = UnsafeMutableRawPointer(mutating: dso), let address = $0.baseAddress {
            let str = UnsafeRawPointer(address).assumingMemoryBound(to: Int8.self)
            return _os_activity_create(dso, str, OS_ACTIVITY_CURRENT, os_activity_flag_t(rawValue: options.rawValue))
         } else {
            return nil
         }
      }
   }

   public init(_ description: StaticString, dso: UnsafeRawPointer? = #dsohandle, parent: Activity, options: Options = []) {
      let parentActivity: OS_os_activity = parent.activity != nil ? parent.activity : OS_ACTIVITY_CURRENT
      activity = description.withUTF8Buffer {
         if let dso = UnsafeMutableRawPointer(mutating: dso), let address = $0.baseAddress {
            let str = UnsafeRawPointer(address).assumingMemoryBound(to: Int8.self)
            return _os_activity_create(dso, str, parentActivity, os_activity_flag_t(rawValue: options.rawValue))
         } else {
            return nil
         }
      }
   }

   private init(_ activity: OS_os_activity) {
      self.activity = activity
   }

}

extension Activity {

   public static var none: Activity {
      return Activity(OS_ACTIVITY_NONE)
   }

   public static var current: Activity {
      return Activity(OS_ACTIVITY_CURRENT)
   }

   public static func initiate(_ description: StaticString, dso: UnsafeRawPointer? = #dsohandle, options: Options = [],
                               execute body: @convention(block) () -> ()) {
      description.withUTF8Buffer {
         if let dso = UnsafeMutableRawPointer(mutating: dso), let address = $0.baseAddress {
            let str = UnsafeRawPointer(address).assumingMemoryBound(to: Int8.self)
            _os_activity_initiate(dso, str, os_activity_flag_t(rawValue: options.rawValue), body)
         }
      }
   }

   public func apply(execute body: @convention(block) () -> ()) {
      if activity != nil {
         os_activity_apply(activity, body)
      }
   }

   public func enter() -> Scope {
      var scope = Scope()
      if activity != nil {
         os_activity_scope_enter(activity, &scope.state)
      }
      return scope
   }

   /**
    * Label an activity that is auto-generated by AppKit/UIKit with a name that is
    * useful for debugging macro-level user actions.  The API should be called
    * early within the scope of the IBAction and before any sub-activities are
    * created.
    * This API can only be called once and only on the activity created by AppKit/UIKit.
    */
   public static func labelUserAction(_ description: StaticString, dso: UnsafeRawPointer? = #dsohandle) {
      description.withUTF8Buffer {
         if let dso = UnsafeMutableRawPointer(mutating: dso), let address = $0.baseAddress {
            let str = UnsafeRawPointer(address).assumingMemoryBound(to: Int8.self)
            _os_activity_label_useraction(dso, str)
         }
      }
   }

}

extension Activity {

   public struct Options: OptionSet {

      public let rawValue: UInt32

      public init(rawValue: UInt32) {
         self.rawValue = rawValue
      }

      public static let `default` = Options(rawValue: OS_ACTIVITY_FLAG_DEFAULT.rawValue)
      public static let detached = Options(rawValue: OS_ACTIVITY_FLAG_DETACHED.rawValue)
      public static let ifNonePresent = Options(rawValue: OS_ACTIVITY_FLAG_IF_NONE_PRESENT.rawValue)
   }

   public struct Scope {

      fileprivate var state = os_activity_scope_state_s()

      public mutating func leave() {
         os_activity_scope_leave(&state)
      }
   }
}

@vgorloff What is OS_os_activity?

@zwaldowski (all), Do you occasionally get blocked by the error 'String' is not convertible to 'StaticString'?

EDIT: This seems to be the Swift compiler behavior, when a Return type is not specified when wrapping a function expecting a return. Makes sense.

Fix:

func applyMainExclusionPath() -> CGRect {
        // would be invoked from within Activity
        
        return Activity("apply main exclusion path").active { () -> CGRect in

I haven't been able to try this out yet, but it looks interesting.
Compared to the native API, the process of entering and leaving scopes is a bit verbose, and when someone forgets to leave it, I imagine it may cause problems.

To avoid the deferred leave boilerplate, would it be possible to define the scope as a class instead and leave the scope on deinit?

public class Scope {

    fileprivate var state = os_activity_scope_state_s()

    deinit {
        os_activity_scope_leave(&state)
    }
}

To avoid the deferred leave boilerplate, would it be possible to define the scope as a class instead and leave the scope on deinit?

public class Scope {

    fileprivate var state = os_activity_scope_state_s()

    deinit {
        os_activity_scope_leave(&state)
    }
}

This approach doesn't work on Swift 5.0. Unfortunately, we still have to dance around the defer { scope.leave() }.

Re: class Scope, Swift ARC deinitialization isn't defined to happen at the end of scope like in C++. It's free to be deallocated (and thus end scope) as soon as you last use it.

@vgorloff Works great even in Swift 5.2/Xcode 11, thanks!

@LinMaris Its just a protocol, same as os_activity_t. The piece works perfectly!

public protocol OS_os_activity : NSObjectProtocol {
}

public typealias os_activity_t = OS_os_activity