pkgs.testers.testEqualContents

The following is the current implementation of this function.

testEqualContents =
    {
      assertion,
      actual,
      expected,
    }:
    runCommand "equal-contents-${lib.strings.toLower assertion}"
      {
        inherit assertion actual expected;
        nativeBuildInputs = [ diffoscopeMinimal ];
      }
      ''
        echo "Checking:"
        printf '%s\n' "$assertion"
        if ! diffoscope --no-progress --text-color=always --exclude-directory-metadata=no -- "$actual" "$expected"
        then
          echo
          echo 'Contents must be equal, but were not!'
          echo
          echo "+: expected,   at $expected"
          echo "-: unexpected, at $actual"
          false
        else
          echo "expected $expected and actual $actual match."
          echo OK
          touch -- "$out"
        fi
      '';