Xcode Instruments, Leaks. Meaning of "thunk for @escaping @callee_guaranted() -> ()"

Question

I often see this line in the Stack Trance when trying to track a memory leak in Xcode Instruments:

thunk for @escaping @callee_guaranteed () -> ()

What does it mean? I can't even translate thunk word, not to mention its technical meaning in this context. The full stack trace looks like this:

0 libsystem_malloc.dylib calloc
1 libobjc.A.dylib weak_resize(weak_table_t*, unsigned long)
2 libobjc.A.dylib weak_register_no_lock
3 libobjc.A.dylib objc_storeWeak
4 SpriteKit -[SKNode(setParent) setParent:]
5 SpriteKit -[SKNode insertChild:atIndex:]
6 SpriteKit -[SKNode addChild:]
7 IOSTest PieceNode.setup() /.../PieceNode.swift:66
8 IOSTest LabeledPieceNode.setup() /.../PieceNode.swift:86
9 IOSTest closure #1 in closure #1 in MaskedRectBoardNodeController.maskedRectBoard(_:didFill:with:alongGravity:) /.../MaskedRectBoardNodeController.swift:48
10 IOSTest thunk for @escaping @callee_guaranteed () -> () /.../<compiler-generated>:0
11 libdispatch.dylib _dispatch_call_block_and_release
12 libdispatch.dylib _dispatch_client_callout
13 libdispatch.dylib _dispatch_main_queue_callback_4CF$VARIANT$mp
14 CoreFoundation __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__
15 CoreFoundation __CFRunLoopRun
16 CoreFoundation CFRunLoopRunSpecific
17 GraphicsServices GSEventRunModal
18 UIKitCore UIApplicationMain
19 IOSTest main /.../PauseMediator.swift:13
20 libdyld.dylib start

Answer 1

A thunk generally is a box around a delayed function call (possibly adding some context, and possibly requiring additional context to complete). In Swift, thunks are generally used to help manage memory or calling conventions around a closure. As a rule, you can ignore the thunk; it's a bit of an implementation detail.

What this is really telling you is that you're leaking an SKNode somewhere, and that SKNode was created in a block dispatched to the main queue (probably using DispatchQueue.main.async). It is highly unlikely that this call stack actually has anything to do with the leak. It's just telling you where the leaked object was created.