bug: mkOutOfStoreSymlink still links inside nix store for flake and toString input

[yipengsun]

Is there an existing issue for this?

• I have searched the existing issues

Issue description

I'm trying to link my fcitx config out-of-nix-store. I've searched the issues and it looks to me that for flake-based setup, I need to pass a string to mkOutOfStoreSymlink:

# omitted full name for brevity
xdg.configFile."bla".source = mkOutOfStoreSymlink "/home/me/bla";

And it worked like this:

~/.config/bla -> /nix/store/hash-bla -> /home/me/bla

so it's fine. I then want to make this more portable by doing this:

# say my project root is at /home/me
xdg.configFile."bla".source = mkOutOfStoreSymlink (builtins.toString ./bla);

Now it doesn't work. The link terminates at a nix store location.

I've read #589 #2085, and I'm confused on why a literal string works, but toString doesn't.

For my real setup, see https://github.com/yipengsun/nix-config/blob/d3268ce6c3feb6d2cdf56168333582fa476e5bf9/local/profiles/wm/default.nix#L41-L46

Maintainer CC

No response

System information

/nix/store/jaa5l5g36aj1wwlmk9lqhslclmdqms3x-bash-interactive-5.1-p12-dev
copying path '/nix/store/jaa5l5g36aj1wwlmk9lqhslclmdqms3x-bash-interactive-5.1-p12-dev' from 'https://cache.nixos.org'...
 - system: `"x86_64-linux"`
 - host os: `Linux 5.15.16, NixOS, 22.05 (Quokka)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.6.0pre20211217_6e6e998`
 - channels(root): `"nixos-21.11pre292520.540dccb2aea"`
 - nixpkgs: `/nix/store/j5dard0z8nvg3ihsiyv4hkg4105pn1d2-source`

[berbiche]

I then want to make this more portable by doing this:

# say my project root is at /home/me
xdg.configFile."bla".source = mkOutOfStoreSymlink (builtins.toString ./bla);

Now it doesn't work. The link terminates at a nix store location.

This is not possible with Flakes because it uses "pure evaluation"/"restricted evaluation".

Essentially, if builtins.toString ./. returned the absolute path of the a file outside of a nix builder's view, it would be impure and leak building information.

Nix Flakes are pure by nature, which makes it impossible to use the functionality you described.

edit: Flakes are also copied to the Nix store, so the relative directory is always relative to the store.

I hope this helps.

[yipengsun]

I see. that makes sense.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/how-to-refer-to-current-directory-in-shell-nix/9526/9

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/getting-local-path-in-flake/19715/1