nixpkgs-check-by-name reports IO error for Nix versions 2.21+

[philiptaron]

When running nixpkgs-check-by-name with Nix versions greater than 2.20, it reports the following:

I/O error:  Failed to get the definition info for attribute acl: In pkgs/stdenv/linux/default.nix, attribute parent node is not an attribute path node: [email protected]

[infinisil]

Interesting, found the problem: builtins.unsafeGetAttrPos got smarter in Nix 2.21!

# test.nix
let
  attrs = {
    inherit ({ })
      abc def
      hij klm;
  };
in
builtins.mapAttrs (name: _value:
  let
    pos = builtins.unsafeGetAttrPos name attrs;
  in
  [ pos.line pos.column ]
) attrs

$ nix-build '<nixpkgs>' -A nixVersions.nix_2_20 && result/bin/nix-instantiate --eval test.nix --strict
{ abc = [ 3 18 ]; def = [ 3 18 ]; hij = [ 3 18 ]; klm = [ 3 18 ]; }

$ nix-build '<nixpkgs>' -A nixVersions.nix_2_21 && result/bin/nix-instantiate --eval test.nix --strict
{ abc = [ 4 7 ]; def = [ 4 11 ]; hij = [ 5 7 ]; klm = [ 5 11 ]; }

Looks like this isn't part of the test suite in https://github.com/NixOS/nixpkgs-check-by-name/blob/0c73cb7ad5d35f2bd67206bfbf114b580103e946/src/nix_file.rs#L422-L463, would be great to add it there

[philiptaron]

It might have been eldritch horrors in NixOS/nix@1edd6fa