Smart Folders (Finder -> File -> New Smart Folder). It's not exactly hidden but I never paid too much attention to it. It's essentially a way to create a folder whose content is dynamically updated based on your search conditions. For example, you can create a smart folder that contains all PDF files matching a certain name pattern within a given directory (or the whole disk) [0]. You can get really advanced, there's…
I’ve been sad since they were introduced that they don’t work at the file system level, hence not in CLI applications. Support for them in desktop apps that open directories is also spotty. I would LOVE them if they were reimplemented as a fuse style file system instead of a userland macOS trick.
Like, you're in a smart folder that contains files whose name includes "foo", and you try to create a file whose name does not include "foo", what happens? Do you get an existing filesystem error? Do you get a new "can't create new files in smart folders" error?
Interestingly enough I just discovered that if you drag a file into the Tags section of the Finder window's sidebar, it will remain where it is - but if you create a smart folder whose criteria is "tag:tagname" it won't let you drag anything into it, even though it shows the exact same set of files.
