cross-compilation: native derivations and stdenv.cross

[vcunat]

The problem is closely related to #10874. We've developed a habit to test stdenv.cross to find if we're cross-compiling or not. However, that does not work for native build inputs, i.e. they see the same stdenv arguments as the cross derivations.

What's worse, I don't think this is easily fixable. We call stdenv.mkDerivation with some arguments, and inside this call we call the original and modified mkDerivation to produce nativeDrv and crossDrv. Those arguments have no possibility to differentiate among the two cases during evaluation time. We can still use crossAttrs, but that's rather inconvenient if we only want to slightly tweak a substring of a phase or drop a build input.

(Attributes like stdenv.isLinux also evaluate the same always, but the problem there is reversed, as discussed in #10874: it shows properties of the build system even though users typically mean the host system.)

TODO by @Ericson2314

• Native and cross builds do not see the same stdenv
• Purge stdenv.cross #26884 stdenv.cross is gone
• Formally deprecate stdenv.is* with evaluation-time warning
• Get rid of crossAttrs #33302 Remove crossAttrs (see cross stdenv adapter in pkgs/stdenv/adapters.nix)

[vcunat]

@Ericson2314: I pushed the current state of my WIP to vcunat/nixpkgs@kt/cross...vcunat:v/cross-native-rework The main idea is to use pkg.override to pass a different stdenv. That can't cover some edge cases, e.g. it will only work "below" callPackage and not in all-packages.nix, but I can't see any better way ATM.

[Ericson2314]

@vcunat From just a quick glance, I think this will fit very nicely together with what I've got, which is now otherwise ready for PR testing?

[Ericson2314]

So I already added a proper phase for computing the build-time deps in https://github.com/Ericson2314/nixpkgs/blob/cross/pkgs/stdenv/cross/default.nix, which means proper substitution of the stdenv. It's just a matter of combining your cross.nix with that, and things should work perfectly. I'll take a stab at it now.

[Ericson2314]

https://github.com/Ericson2314/nixpkgs/tree/vcunat-cross-stdenv My rebase.

[Ericson2314]

#21268 fixes the stdenv.cross aspect of this. Future work would is needed for stdenv.is*.

[Ericson2314]

#24610 almost fixes all the rest of this. We just now need to deprecate stdenv.is* and remove stdenv.cross.

[Ericson2314]

#26884 did the latter!

[Ericson2314]

I think I'll keep stdenv.is*. It is now defined with the host platform, which is a sane default. #27069 tracks these sorts of things more broadly, and with more precision as the issue is postdates my overhauls.