{ lib, runCommand, emptyFile, nix-diff }:

assertion: a: b:
let
  drvA = builtins.unsafeDiscardOutputDependency a.drvPath or (throw
    "testEqualDerivation second argument must be a package");
  drvB = builtins.unsafeDiscardOutputDependency
    b.drvPath or (throw "testEqualDerivation third argument must be a package");
  name =
    if a ? name then "testEqualDerivation-${a.name}" else "testEqualDerivation";
in if drvA == drvB then
  emptyFile
else
  runCommand name {
    inherit assertion drvA drvB;
    nativeBuildInputs = [ nix-diff ];
  } ''
    echo "$assertion"
    echo "However, the derivations differ:"
    echo
    echo nix-diff $drvA $drvB
    nix-diff $drvA $drvB
    exit 1
  ''