@phausler, did we come up with an answer to this?

It was decided for the erring cases to avoid bridging file reference urls. The are both technically a different type in spirit and they actually behave much more like a reference type than a structure.

The peephole optimization for as casting back to NSURL ought to preserve it's reference nature. Unfortunately we did not get enough time to push through proposals to add any additional types to account for this (since it would mean pretty massive overhauls to the objc side)

Comment by Sanjay Madan (JIRA)

In the meantime, is there a workaround? Given a path URL, how does one get a file reference URL in Swift? I need to watch the contents of a folder, even if the folder is renamed or moved.

Comment by Frédéric Blondiau (JIRA)

I wrote this workaround (in attachement, to avoid text formatting processing... as I also had to watch items renamed or moved.

It seems to work correctly – but I would be pleased to get some confirmation that there are no hidden problems :-)

Workaround.pdf

Comment by Sanjay Madan (JIRA)

Thank you, Fédéric! This seems to work. In the first line I'm curious why you used URL (self as URL) to get the fileResourceIdentifier. Why didn't you just use the corresponding method on NSURL (self) directly? That method returns a dictionary which you can index with .fileResourceIdentifierKey.

Comment by Frédéric Blondiau (JIRA)

Generally, accessing the resourceIdentifiers of the URL thru the properties (instead of accessing the resourceIdentifiers of the NSURL thru the keys) may give more precise information associated with the returned value – here, fileResourceIdentifier is of type (NSCopying & NSSecureCoding & NSObjectProtocol)? instead of Any.
In this case, as we are casting the returned value to Data, I think we can, indeed, avoid bridging to URL using your suggestion. I updated the attachment. Workaround V2.pdf

Note that it is expected behavior that struct URL does not support file reference URLs.

The unfortunate part here is that we don't have a mechanism to keep NSURL.fileReferenceURL from being imported as returning struct URL instead of NSURL. We probably need to add an ObjC thunk in the overlay and mark the API as NS_REFINED_FOR_SWIFT in the NSURL.h header. However, that would be a source breaking change.

Comment by Charles Srstka (JIRA)

Unfortunately, Frédéric's workaround depends on the internal layout of reference URLs, which is undocumented and which could change at any time, particularly given that APFS is just around the corner. I don't recommend it.

A still somewhat hackish, but ultimately much safer option is just to use the Objective-C runtime:

let url = ...

if let refURL = (url as NSURL).perform(#selector(NSURL.fileReferenceURL))?.takeUnretainedValue() as? NSURL {
    print(refURL) // will print something along the lines of 'file:///.file/id=01234546.789012345'
}

This will just invoke the original Objective-C -fileReferenceURL method without involving the reference-mangling Swift bridging mechanism.

Just ran into this. We really should come up with a solution.

There is (or at least used to be) a workaround for incorrectly-nonnull APIs where you could say something like

let foo: TheType? = APIThatReturnsNonnull()

and Swift would skip the nonnull check and instead treat the API as returning a nullable object.

Perhaps we could introduce the same kind of workaround for reference convertible types, where explicitly annotating the return value as the Obj-C type would skip the reference conversion? This would also just be a performance optimization in general, because writing something like

(someNSString.methodThatReturnsString() as NSString).anotherMethod

really has no business bridging the intermediate string into String just to convert it right back to NSString.

I believe we have that already, at least in Swift 4.1. Is it not working?

Oh, I only tested Swift 4.0.3, because this issue is still open.

Testing in a Swift 4.1 snapshot, you're right, someNSURL.fileReferenceURL() as NSURL? is giving me a reference URL back, meaning it's not round-tripping through URL. Glad to know that escape hatch now exists.

Incidentally, someNSURL.fileReferenceURL()! as NSURL doesn't give me a reference URL back. In an ideal world it might be nice to skip the round trip there as well, though I can understand why it doesn't right now.