one probably wouldn't even need to run a lot of services at the same time - just start and stop different processes serially

Actually this is not true since fds are reused. one has to hold on to the fd (e.g. with a process running inside the VM).

$ nix build -f . swiftPackages.swift-format
$ result/bin/swift-format --version
6.2.3

{
  lib,
  fetchFromGitHub,
  mkSwiftPackage,
  swift-argument-parser,
  swift-markdown,
  swift-syntax,
  stdenv,
  swift_release,
}:

mkSwiftPackage (finalAttrs: {
  pname = "swift-format";
  version = swift_release;

  src = fetchFromGitHub {
    owner = "swiftlang";
    repo = "swift-format";
    tag = "swift-${finalAttrs.version}-RELEASE";
    hash = "sha256-DhTtGU2B9BIyN6PyLBlg7kIs3cVyRl6Pa9G0txzta0g=";
  };

  postPatch =
    # Fix the deployment target or compiling code using XCTest fails.
    lib.optionalString stdenv.hostPlatform.isDarwin ''
      substituteInPlace Package.swift \
        --replace-fail '.macOS("13.0")' ".macOS(\"$MACOSX_DEPLOYMENT_TARGET\")"
    '';

  buildInputs = [
    swift-argument-parser
    swift-markdown
    swift-syntax
  ];
})

❯ nix-shell -I nixpkgs=channel:nixpkgs-unstable -p "python3.withPackages (ps: [ ps.numpy ])"
this derivation will be built:
  /nix/store/fsnqmdg1k6cvsvkb24h9bjcx1i3w80ci-python3-3.13.9-env.drv
building '/nix/store/fsnqmdg1k6cvsvkb24h9bjcx1i3w80ci-python3-3.13.9-env.drv'...
created 219 symlinks in user environment

[nix-shell:~]$ python
Python 3.13.9 (main, Oct 14 2025, 13:52:31) [Clang 21.1.2 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    import numpy
ModuleNotFoundError: No module named 'numpy'
>>>