Revert ".devcontainer: nixfmt-rfc-style -> nixfmt"

[MattSturgeon]

Reverts #430330

As per #425583 and #430330 (comment), I believe this was bumped prematurely.

Unless the devcontainer is using the in-tree copy of nixpkgs, the CI-pinned revision, or some other known-to-be up-to-date revision... If we can guarantee the devcontainer always uses an up-to-date nixpkgs, then this can be closed.

If the devcontainer is using nixpkgs from (e.g.) the NIX_PATH, then we don't know whether pkgs.nixfmt is the rfc-style formatter or the classic version. That's because NIX_PATH may contain an old revision or a stable release.

[jfly]

I know nothing about how devcontainers are used here, but this seems fine to me.

[wolfgangwalther]

I know nothing about .devcontainer either, but nixfmt-rfc-style seems like the safe choice as long as it still exists.

[wolfgangwalther]

It seems like this can be changed whenever the nixfmt in the container at https://github.com/devcontainers/features/pkgs/container/features%2Fnix is the same as nixfmt-rfc-style. Since the last release was 4 months ago, this is unlikely to be the case right now.

[MattSturgeon]

It seems like this can be changed whenever the nixfmt in the container at https://github.com/devcontainers/features/pkgs/container/features%2Fnix is the same as nixfmt-rfc-style.

If so, should I amend the TODO comment to reference that instead of "a stable release"?

[wolfgangwalther]

I'm not 100% sure about my statement. I looked at the source code for that container a bit, and I'm not sure from which nixpkgs revision it pulls packages. Might as well just be the current latest unstable - no idea.

[wolfgangwalther]

We will only find out if somebody actually uses this thing. With nixfmt, the failure-mode will be much more confusing, because it would result in wrong formatting. With nixfmt-rfc-style the failure mode, once this attribute is removed, will be much more obvious - and once somebody hits this, we can fix it.

Thus, I'm merging this to be on the safe side.