Paths referenced by plugins.luasnip.from{Lua,Snipmate,Vscode}.*.paths are removed by `nix store gc`

[cezdro]

Discussed in #2162

^{Originally posted by cezdro September 4, 2024}
I configured the snippet path using the relative path like in the example. However, at one point the snippets stopped working and I figured that it happened because Nix thought the directory was only needed at a build time and could now be GC'd (I have nix.gc.automatic enabled). Here is the snippet from the generated init.lua:

require("luasnip.loaders.from_lua").lazy_load({
    ["paths"] = "/nix/store/1wm0qsxvri2pdxd67sxa569inz4pl385-f5jcwka5bpj542475ib1kgfplgnb9x8n-source/modules/home/nvim/plugins/luasnip",
})

require("luasnip.loaders.from_snipmate").lazy_load({
    ["paths"] = "/nix/store/1wm0qsxvri2pdxd67sxa569inz4pl385-f5jcwka5bpj542475ib1kgfplgnb9x8n-source/modules/home/nvim/plugins/luasnip",
})

require("luasnip.loaders.from_vscode").lazy_load({
    ["paths"] = "/nix/store/1wm0qsxvri2pdxd67sxa569inz4pl385-f5jcwka5bpj542475ib1kgfplgnb9x8n-source/modules/home/nvim/plugins/luasnip",
})

I don't know how Nix decides whether the referenced path should be kept or not. I think that if I used mkDerivation the directory wouldn't be treated as a garbage, but I don't think that such thing should be necessary in Nixvim config.

I wrote this in Discussions, because I don't know if this is my fault and if it can be improved in Nixvim.

---

When I created a derivation like this, it wasn't removed on nix store gc:

{ pkgs, ... }: let
  snippets = pkgs.stdenv.mkDerivation {
    name = "snippets";

    src = ./.;

    buildCommand = ''
      mkdir $out
      cp -r $src/*.{json,lua,snippets} $out
    '';
  };
in {
  programs.nixvim = {
    plugins.luasnip = {
      enable = true;

      fromLua = [{paths = "${snippets}";}];
      fromSnipmate = [{paths = "${snippets}";}];
      fromVscode = [{paths = "${snippets}";}];
    };
  };
}

I think Nixvim could do something like this automatically

[MattSturgeon]

Hm. I think the only way we can reliably ensure paths are copied to /nix/store in a persistent way would be to handle them specially in toLuaObject. In theory, every option that could conceivably be set to a path and also end up in init.lua would go through toLuaObject at some point.

We would only be able to do this for paths detected by builtins.isPath though, anything stringified may slip under the radar.

---

Also, I'm wondering if creating a "string with context" causes nix to add some dependency metadata; I know some parts of NixOS use "${path}" to achieve something along these lines.

Does stringifying the path in this way help at all?

EDIT: it seems we already do this 😅
https://github.com/nix-community/nixvim/blob/main/lib%2Fto-lua.nix#L218-L219

[cezdro]

Also, I'm wondering if creating a "string with context" causes nix to add some dependency metadata; I know some parts of NixOS use "${path}" to achieve something along these lines.

Does stringifying the path in this way help at all?

I tried doing this, but it didn't help.