Simply put, I should be able to do

$ nix-build '<nixpkgs>' --arg crossSystem '{ config = "aarch64-unknown-linux-gnu"; }' -A stdenv

and that should work. I could make it work right now with a dirty white-list and fall-backs, but that's kind of defeats the point.

Currently, the biggest culprits are

1. Random top-level fields, like withTLS and openssl.system. See https://github.com/NixOS/nixpkgs/blob/master/lib/systems/examples.nix
2. The platform field, see https://github.com/NixOS/nixpkgs/blob/master/lib/systems/platforms.nix
3. arch which overlaps with the far more principled parsed,arch. See elaborate https://github.com/NixOS/nixpkgs/blob/master/lib/systems/default.nix#L17.

The general design issue is we need to handle the per-package random crap in a more modular way, without giving up the possibility of the user to override with their own values. My GNU ld emulation picking logic at https://github.com/NixOS/nixpkgs/blob/master/pkgs/build-support/bintools-wrapper/default.nix#L167-L179 is certainly in a more modular location, but also cannot be nicely overridden.

Checklist

• lib, glibc: Get rid of withTLS #34303: withTLS
• lib: Platform attribute cleanup #34315: bigEndian, openssl.system
• [RDY] buildLinux: Add more overrides #34351: linux kernel cleanups
• lib: Clean up how linux and gcc config is specified #107214 / top-level, lib: Remove platform attribute of platforms #110544: A few cleanups:
  • Get rid of nesting items under platforms for no good reason
  • group linux config fields rather than merely prefixing them
  • improve defaulting logic so config is enough in more cases.
• Combine/rerrange platforms.nix and examples.nix
  • We should only default to generic/"multiplatform" settings; specific machines should be specified by hand.
  • Specific machines shouldn't be partly in platforms.nix and partly in examples.nix, that is just confusing.
• Update manual to show how indeed the config is usually enough, and remove the reference to this issue.

CC @dtzWill @dezgeg @infinisil @teto