[RFC 0096] Experimental Nix derivations

rfcs/0096-experimental-nix-derivations.md

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+---
+feature: experimental-nix-derivations
+start-date: 2021-06-25
+author: gytis-ivaskevicius
+co-authors:
+shepherd-team: edolstra, blaggacao, maljub01, nrdxp
+shepherd-leader: edolstra
+related-issues:
+---
+
+# Summary
+[summary]: #summary
+
+Add a way for users to use experimental Nix features with improved user-experience
+and better communication to the user that they are using an experimental feature.
+
+# Motivation
+[motivation]: #motivation
+
+Motivation:
+- Revert of (this)[https://github.com/NixOS/nixpkgs/pull/123898].
+- Noticed quite a few peeps who were very supportive of [this](https://github.com/NixOS/nixpkgs/pull/120141) PR (which got reverted).
+- Lack of visibility for the end-users that they are using experimental features.
+
+Expected outcome:
+- Improved UX.
+- Reduced risk of users using experimental features in production environments.
+- Reduced non-bug questions/issues on common support channels.
+
+# Detailed design
+[design]: #detailed-design
+
+`nix` derivation definitions should be updated in such a way so end users would be able to overwrite features with something along the lines of this:
+```nix
+nix.override { experimentalFeatures = ["flakes"]; }
+```
+
+What's so special about this override:
+- Passes configure flag which looks something like this: `--with-experimental-features="nix-command flakes"`
+- Allows building `nix` with selected features enabled by the default
+- When using these overwritten derivations warning should be shown.
+
+
+Warning expected to look something like this:
+```
+WARNING: You are using experimental Nix features which are up to a subject to change. For more information visit https://nixos.org/xyz
+```
+
+Currently, experimental features are undiscoverable and undocumented. It would be nice if there was a way to query which experimental features are available in a Nix release, and to get some documentation about them (like a link to an associated RFC and possibly links to additional resources)
+
+# Examples and Interactions
+[examples-and-interactions]: #examples-and-interactions
+
+If we take Nix flakes as an example:
+The additional line to `nix.conf` is not a deal-breaker for anyone who decides to
+use Nix Flakes. Even if someone were to do so in a production environment is
+likely to get by unnoticed. On another hand, if there is an explicit warning it is
+likely to make multiple peeps question their decisions.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- In case of addition of 'warning' - it may end up being bothersome
+
+
+# Alternatives
+[alternatives]: #alternatives
+
+Not doing this would preserve a lack of visibility. Especially when using projects like [DevOS](https://github.com/divnix/devos) or [flake-utils-plus](https://github.com/gytis-ivaskevicius/flake-utils-plus/)
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+How exactly warning should look like?
+How exactly `experimentalFeatures` feature flag should be implemented? (Via CLI argument? Env variable? Dynamically generated patch/sed?)
+
+# Future work
+[future]: #future-work
+
+Removal/Deprecation of Nix experimental packages on a per case basis.