{extra-,}experimental-features in flake nixConfig ignored

[shlevy]

Describe the bug

In a flake with nixConfig.extra-experimental-features = “fetch-closure”, builtins.fetchClosure still is not available. Evaluating the flake with the command line flag works.

nix-env --version output
nix-env (Nix) 2.18.1

[fricklerhandwerk]

Triaged in Nix maintainers meeting:

• @Ericson2314 this may be because configuration is evaluated to late in the process, but needs a repoducer
  • generally don't like the feature, it's a dirty hack and an endless source of bugs
• @roberth: haven't touched the code, but we're probably allowing too much. would be good to define more clearly which settings can even be configured
• @tomberek: will do a bit of probing...
  • confirmed on 2.22
  • the registration seems to be done once, and the evaluation cannot change the setting
  • while the setting change is being caught, it's too late to take effect
  • @Ericson2314 yes, nothing in Nix is designed to do that sort of thing
    • as long as we have global variables we won't be able to do anything about it

[nixos-discourse]

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

https://discourse.nixos.org/t/2024-05-22-nix-team-meeting-minutes-147/45835/1