So I never thought crossSystem was the right idea for a top level argument, but I kept it for backwards compat. What if instead we make runSystem: attrs (née localSystem) and buildSystem: attrs or null (before, rather than after runSystem). Then this PR isn't needed:

let
  runSystem = lib.systems.elaborate
    (builtins.intersectAttrs { platform = null; } config // args.runSystem);

as is done today (with localSystem) is unconditional.

@shlevy @dtzWill @matthewbauer @bgamari @dezgeg @edolstra what do you all think?

That seems massively verbose and non-obvious to figure out.

I guess the real problem is that this new hostPlatform et al stuff were added separately to the existing platform stuff. They should really be merged together with the non-optimal layering violations removed from the existing platform.

BTW don't this localSystem and the like mostly just set hostPlatform in nixpkgs? Why is it called "system" in one place and "platform" in others?

@lopsided98 I'm curious, what's your use case for changing platform? If it's for changing the defconfig for when building the kernel, this should really be moved to the kernel expressions itself one day. I have a WIP branch for that at https://github.com/NixOS/nixpkgs/compare/master...dezgeg:defconfig-in-kernel-pkg?expand=1, hopefully one day I get to debug why it fails at runtime.

@dezgeg Yes, I am changing defconfig, as well as enabling extra kernel options.

That seems massively verbose and non-obvious to figure out.

The PR or my proposal.

I guess the real problem is that this new hostPlatform et al stuff were added separately to the existing platform stuff.

The *System stuff is intentionally separate from the *Platform stuff; the former determines the latter and only the former is ever nullable. I could use the same names though, but I think its important to understand it separate. Also keep in mind while system, platform, and crossSystem are older, I reimplemented everything long ago anyways.

They should really be merged together with

Separately, platform and system, with exactly the same structure, are fields in all of localSystem, crossSystem. buildPlaform, hostPlatform, and targetPlatform.

the non-optimal layering violations removed from the existing platform.

Agreed.

@dezgeg I pinged you in IRC but we should really sync up in real time to go over all this stuff and document it. I am a single point of failure today because I rewrote all that stuff a year ago, and I do not want to be that.

EDIT: eep I accidentally commented on wrong issue, sorry about that! Nothing to see here....

@lopsided98 I recently added 87a68c4. Is that enough for your custom kernel uses to avoid the need for setting nixpkgs.config.platform?

Yes, I have moved all my custom configuration to the kernel derivations. I still feel like the current behavior is not optimal, but I'll close this PR.