Create mutable files with home-manager and Nix

example.nix

{
  home.file."test-file" = {
    text = "Hello world";
    force = true;
    mutable = true;
  };
}

mutability.nix

# This module extends home.file, xdg.configFile and xdg.dataFile with the `mutable` option.
{ config, lib, ... }:
let
  fileOptionAttrPaths =
    [ [ "home" "file" ] [ "xdg" "configFile" ] [ "xdg" "dataFile" ] ];
in {
  options = let

    mergeAttrsList = builtins.foldl' (lib.mergeAttrs) { };

    fileAttrsType = lib.types.attrsOf (lib.types.submodule ({ config, ... }: {
      options.mutable = lib.mkOption {
        type = lib.types.bool;
        default = false;
        description = ''
          Whether to copy the file without the read-only attribute instead of
          symlinking. If you set this to `true`, you must also set `force` to
          `true`. Mutable files are not removed when you remove them from your
          configuration.

          This option is useful for programs that don't have a very good
          support for read-only configurations.
        '';
      };
    }));

  in mergeAttrsList (map (attrPath:
    lib.setAttrByPath attrPath (lib.mkOption { type = fileAttrsType; }))
    fileOptionAttrPaths);

  config = {
    home.activation.mutableFileGeneration = let

      allFiles = (builtins.concatLists (map
        (attrPath: builtins.attrValues (lib.getAttrFromPath attrPath config))
        fileOptionAttrPaths));

      filterMutableFiles = builtins.filter (file:
        (file.mutable or false) && lib.assertMsg file.force
        "if you specify `mutable` to `true` on a file, you must also set `force` to `true`");

      mutableFiles = filterMutableFiles allFiles;

      toCommand = (file:
        let
          source = lib.escapeShellArg file.source;
          target = lib.escapeShellArg file.target;
        in ''
          $VERBOSE_ECHO "${source} -> ${target}"
          $DRY_RUN_CMD cp --remove-destination --no-preserve=mode ${source} ${target}
        '');

      command = ''
        echo "Copying mutable home files for $HOME"
      '' + lib.concatLines (map toCommand mutableFiles);

    in (lib.hm.dag.entryAfter [ "linkGeneration" ] command);
  };
}

vscode.nix

{ config, pkgs, lib, ... }:
let
  # Path logic from:
  # https://github.com/nix-community/home-manager/blob/3876cc613ac3983078964ffb5a0c01d00028139e/modules/programs/vscode.nix
  cfg = config.programs.vscode;

  vscodePname = cfg.package.pname;

  configDir = {
    "vscode" = "Code";
    "vscode-insiders" = "Code - Insiders";
    "vscodium" = "VSCodium";
  }.${vscodePname};

  userDir = if pkgs.stdenv.hostPlatform.isDarwin then
    "Library/Application Support/${configDir}/User"
  else
    "${config.xdg.configHome}/${configDir}/User";

  configFilePath = "${userDir}/settings.json";
  tasksFilePath = "${userDir}/tasks.json";
  keybindingsFilePath = "${userDir}/keybindings.json";

  snippetDir = "${userDir}/snippets";

  pathsToMakeWritable = lib.flatten [
    (lib.optional (cfg.userTasks != { }) tasksFilePath)
    (lib.optional (cfg.userSettings != { }) configFilePath)
    (lib.optional (cfg.keybindings != [ ]) keybindingsFilePath)
    (lib.optional (cfg.globalSnippets != { })
      "${snippetDir}/global.code-snippets")
    (lib.mapAttrsToList (language: _: "${snippetDir}/${language}.json")
      cfg.languageSnippets)
  ];
in {
  home.file = lib.genAttrs pathsToMakeWritable (_: {
    force = true;
    mutable = true;
  });
}

Thank you for your response, very much appreciated.

Alternatively, you can use mkOutOfStoreSymlink to symlink the config to a writable file. Then, however:

This is what I was referring to as symlink.

• you can't use Nix to generate the config contents

That is a drawback, but I can live with it by using some other method to generate it (a script or something like that)

• you have to hardcode the writable file's location, which deduces from reproducibility

That is definitely the main problem, and what I have been doing up until now. I was wondering it I was just doing it wrong and nix provides a mechanism for referencing the paths it owns (like a subfolder within my nix dotfiles folder), but if an advanced user like you tell me that is a literal problem, I trust you.

The cherry on top of this would be to have a mechanism to detect conflicts, or even better, to re-incorporate the modified file into the nix repository