☔ COM interop on Windows

[grynspan]

Tracking research and work to support COM interfaces in Swift with better ergonomics.

[grynspan]

Tracked as rdar://152309114.

[grynspan]

A few possible ways to improve things:

• Import COM interfaces as Swift classes with foreign refcounting
• Automatically conform imported types to a COMInterface protocol:
  • The protocol would have a static interfaceID: IID { get } property
  • swift_dynamicCast() would know to look for this protocol conformance and call QueryInterface() when appropriate
• Implement ARC atop IUnknown::AddRef() and IUnknown::Release() for COM objects
  • Disallow calling IUnknown::AddRef() and IUnknown::Release() directly from Swift
  • Direct developers to use Unmanaged when they really need to manually refcount
• Teach the importer to map functions of the form HRESULT GetFoo([out] IWhatever**whatever) to func GetFoo() throws -> IWhatever
  • Decide if we want to map GetFoo() to foo() or leave it as-is, while we're in there

[grynspan]

Related:

#83914
swiftlang/swift-installer-scripts#458