Swift recently implemented a better concept of sendability, see NS_SWIFT_SENDABLE and NS_SWIFT_UI_ACTOR in Foundation/NSObjCRuntime.h.

Using these attributes, we can automatically mark NSError, NSDate, NSFileHandle and so on Send + Sync. (That NSLock and subclasses are marked sendable seems a bit like a bug? But it shouldn't matter much, since the methods on it is unsafe anyhow, and deallocation on other threads is definitely safe).

More importantly, we can automatically mark subclasses of NSResponder (NSView, NSWindow, ...) as MainThreadOnly.

To finalize the safety of that, it would be nice to run an algorithm like this on every function/method:

if method.return_type.includes_ns_swift_ui_actor() {
    if method.receiver.is_ns_swift_ui_actor() {
        break;
    }
    for argument in method.arguments {
        if argument.is_ns_swift_ui_actor() {
            break;
        }
    }
    method.arguments.push(("mtm", Ty::MainThreadMarker));
}

(Note here the difference between includes_ns_swift_ui_actor and is_ns_swift_ui_actor; includes_ returns true for NSArray<NSView> or Option<NSView>, while is_ doesn't).

So e.g. NSView::new() -> Id<Self> would become NSView::new(_mtm: MainThreadMarker), while NSWindow::view(&self) -> Id<NSView> would stay as-is.

I checked if we could make atomic attributes safe to access from any thread, and it turns out, we cannot, Swift still requires an await around these, so we must as well (can be tested on https://online.swiftplayground.run/):

import AppKit

@MainActor func test(window: NSWindow) {
    if #available(macOS 10.15, *) {
        Task.detached {
            // Call method; should definitely not be possible without `await`.
            await window.setFrame(NSRect.zero, display: false)

            // Access otherwise `atomic` property; turns out that is not possible without `await` either.
            print(await window.title)

            // Assign to property; likewise not possible without `await`.
            //
            // See https://stackoverflow.com/q/69259397 for why we wrap the assignment
            await MainActor.run {
                window.title = "test"
            }
        }
    }
}

If the property is marked nonisolated though, then it is possible.