Rule proposal: Return before defer

I had code of the form:

let semaphore: DispatchSemaphore(value: 0)
asyncCall(..., completionHandler: { _ in
	if flag { return }
	defer { semaphore.signal() }
	...
})
semaphore.wait()

I added the early return some while after the defer, and it never triggered (it's a rather obscure check in test code). This, of course, livelocks if flag is true -- a small oversight, but it took a while to find.

The problem is that at first sight, the code is fine: defer makes sure that no matter how the function returns, the semaphore is signalled -- well, only those returns below.

Hence, I propose a warning for returns that come before defers, or the other way around.

Start with the Swift example in the issue and determine the intended warning for returns that occur before a defer. No files or tests are named, so the next step is to locate SwiftLint’s rule implementation and testing conventions. Done means the proposal’s problematic pattern is diagnosed consistently and covered by tests.