Note
This PR was dependent on #267381

Description of changes

Currently fileFilter has the type

(Attrs -> Bool) -> Fileset -> Fileset

The predicate function currently supports properties name and type of the file. However, it might be really useful to allow filtering files by their path. But for the sake of reproducibility, we shouln't allow the absolute path. So instead it should be a subpath instead. But then the question is, what should it be relative to?

So I propose to change the type signature of fileFilter to

(Attrs -> Bool) -> Path -> Fileset

instead, because it allows us to use the Path as the base for the subpath, allowing a future interface like this:

fileFilter (file: elem "target" file.components) ./.

While this change is backwards-incompatible, I think it's okay because:

• The fileFilter function is not yet in a stable NixOS release, it was only merged about a month ago.
  • All public uses of the function on GitHub only pass a path
• Any fileFilter pred fileset can also be expressed as intersection fileset (fileFilter pred path) without loss of functionality.
  • This is furthermore pointed out in the new error message when a file set is passed
• Once fully stable, we can still relax the restriction, but it's harder to add new restrictions.

Ping @roberth @fricklerhandwerk

This work is sponsored by Antithesis ✨

Things done

• Added tests
• Updated docs
• Updated design document