Appending an optional string / array causes the entire expression to be reformatted

[CyberShadow]

Description

I don't really mind either way, but filing this in case someone might find the following useful.

I think this diff could have been less noisy: https://github.com/NixOS/nixpkgs/pull/280482/commits/eac595161a60da7d695d46469e2e849d82da57d6

Small example input

Consider this initial expression:

{
  exampleString = ''
    hello
    beautiful
  '';
  exampleArray = [
    "hello"
    "beautiful"
  ];
}

Now, we want to add some optional things:

{
  exampleString = ''
    hello
    beautiful
  '' + lib.optionalString true ''
    wonderful
    world
  '';
  exampleArray = [
    "hello"
    "beautiful"
  ] ++ lib.optionals true [
    "wonderful"
    "world"
  ];
}

Expected output

No change - I think the above is close to the existing style.

Actual output

nixfmt-rfc-style reformats the entire thing, creating a big diff:

{
  exampleString =
    ''
      hello
      beautiful
    ''
    + lib.optionalString true ''
      wonderful
      world
    '';
  exampleArray =
    [
      "hello"
      "beautiful"
    ]
    ++ lib.optionals true [
      "wonderful"
      "world"
    ];
}

[MattSturgeon]

Is this an issue with the RFC or with the implementation?

It seems this thread is discussing a similar issue too: https://discourse.nixos.org/t/satisfaction-survey-from-the-new-rfc-166-formatting/49758