In February 2018, @beccadax pitched to add an affordance into the language and/or standard library to opt out of the rethrows checks, but I don't think that went anywhere at the time: Pitch: `rethrows(unchecked)`
Around the same time, I wrote a blog post that illustrates a possible workaround Brent mentioned: A better NSManagedObjectContext​.performAndWait. Check out the "Outsmarting the type checker" section.
I don't know if this workaround still works or if it's even applicable to your problem, but it might be.