lib.fileset.fileFilter: Restrict second argument to paths

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](https://github.com/NixOS/nixpkgs/pull/257356).
  - 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

1 files changed, 15 insertions, 5 deletions

diff --git a/lib/fileset/default.nix b/lib/fileset/default.nix
index d90c770633d83..81be1af9d8a1a 100644
--- a/lib/fileset/default.nix
+++ b/lib/fileset/default.nix
@@ -366,7 +366,7 @@ in {
           type :: String,
           ...
         } -> Bool)
-        -> FileSet
+        -> Path
         -> FileSet
 
     Example:
@@ -397,14 +397,24 @@ in {
       Other attributes may be added in the future.
     */
     predicate:
-    # The file set to filter based on the predicate function
-    fileset:
+    # The path whose files to filter
+    path:
     if ! isFunction predicate then
       throw ''
         lib.fileset.fileFilter: First argument is of type ${typeOf predicate}, but it should be a function instead.''
+    else if ! isPath path then
+      if path._type or "" == "fileset" then
+        throw ''
+          lib.fileset.fileFilter: Second argument is a file set, but it should be a path instead.
+              If you need to filter files in a file set, use `intersection fileset (fileFilter pred ./.)` instead.''
+      else
+        throw ''
+          lib.fileset.fileFilter: Second argument is of type ${typeOf path}, but it should be a path instead.''
+    else if ! pathExists path then
+      throw ''
+        lib.fileset.fileFilter: Second argument (${toString path}) is a path that does not exist.''
     else
-      _fileFilter predicate
-        (_coerce "lib.fileset.fileFilter: Second argument" fileset);
+      _fileFilter predicate path;
 
   /*
     The file set containing all files that are in both of two given file sets.