When I call makeDefaultLibrary() on the MTLDevice instance in my Mac playground (which I confirmed is set up correctly), it returns nil. Is this some weird playground setup thing? It doesn't matter whether my shaders (.metal) file is in the sources or resources folder for the playground. I also tried MTLDevice's makeLibrary(source:options:) just to see if it was an issue with the playground finding the Metal shader file, and that didn't help.
This Metal setup code was in a regular Mac app before, and it worked fine in there.
I'm putting this code in a playground because it's my WWDC scholarship submission.