flakes: adopt repl-flake behavior as default

[Kranzes]

Motivation

I think the behavior of repl-flake makes sense to be the default when flakes are enabled. Robert and I's idea was to avoid actually deprecating the repl-flake experimental feature with a warning to avoid annoyance and because it's frankly not the most important thing we want to deal with, so this should be pretty straightforward.

I already discussed with @fricklerhandwerk about making this change at NixCon and I finally implemented it with @roberth at NixCamp.

Context

Priorities

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[roberth]

to avoid annoyance

and potential conflict when the warning needs to be resolved in a file that's used with both nix versions (before and after this). E.g. a shared NixOS module where some users use nixos 23.05 (nix 2.13, before this PR) and other use nixos-unstable (will have this PR).

[tomberek]

The breaking change should be more prominent or clear in the notes?

Primary breaking change
  is for the common usage of `nix repl '<nixpkgs>'` which can be recovered with
  `nix repl --file '<nixpkgs>'` or `nix repl --expr 'import <nixpkgs>{}'`.

https://github.com/NixOS/nix/blob/b19bd4f348aae8346563b63c806da2c82ca90ab3/doc/manual/src/release-notes/rl-2.10.md?plain=1#L4C55-L6C75

[roberth]

The breaking change

Formally not a breaking change, but it would be nice to address the flakes audience. I think you can use Tom's quote to tell flakes users what to do in the release notes.

[Ericson2314]

I agree we should be making this more stable, but I a have a few doubts about this. IMO the name "repl flakes" is misleading: the point was that nix repl uses the same CLI syntax as all the other commands, and it just so happens that that CLI syntax (installables) today features flakes prominently.

Whether or not flakes are in used / the flakes experimental feature is enabled, I think it is is important that we make the nix repl CLI consistent. I think that means I would rather get rid of the the ability to disable ReplFlakes entirely than make it part of Flakes.

[roberth]

@Ericson2314

[regardless] the flakes experimental feature is enabled, I think it is is important that we make the nix repl CLI consistent.

Do you mean that the nix repl ./file behavior should first be removed in order to achieve consistency with the rest of the CLI?

By "promoting" this change from a niche flag to a flag with a significant user base (...), we can effectively do a partial roll-out of this change that doesn't affect non-flakes users yet. This PR could then be a step towards an unconditional removal of nix repl ./file.

[Ericson2314]

@roberth I think we would then want to make it so either flakes or repl-flake is enabled.

To make it consistent we'd need to eventually merge repl-flake with nix-command --- it makes perfect sense for flakes to be disabled but to still have a consistent CLI. Unless we think that contra RFC 136 Flakes will be stabilized before the CLI (and I don't) it doesn't really make sense to make the only way of getting the new syntax enabling Flakes.

[Kranzes]

This PR was merged over an iPad on wheels.