# Fine as a path...
nix-repl> builtins.filterSource (path: _: ! lib.hasSuffix ".nix" (baseNameOf path)) /nix/store/6b8dw1qsvp9gv8yswcb0za37kh4zagl0-source
"/nix/store/lbz71ain5rbm2l8s45nl6d48rvlgspjc-6b8dw1qsvp9gv8yswcb0za37kh4zagl0-source"

# Or a string...
nix-repl> builtins.filterSource (path: _: ! lib.hasSuffix ".nix" (baseNameOf path)) "/nix/store/6b8dw1qsvp9gv8yswcb0za37kh4zagl0-source"
"/nix/store/lbz71ain5rbm2l8s45nl6d48rvlgspjc-6b8dw1qsvp9gv8yswcb0za37kh4zagl0-source"

# But this fails:
nix-repl> builtins.filterSource (path: _: ! lib.hasSuffix ".nix" (baseNameOf path)) (builtins.fetchGit ./.)
fetching Git repository '/home/kreisys/src/silly-test'warning: no common commits
error: string '/nix/store/6b8dw1qsvp9gv8yswcb0za37kh4zagl0-source' cannot refer to other paths, at (string):1:1