From 7c72c013b160627540b8b465a05ba258f47a16d8 Mon Sep 17 00:00:00 2001
From: Emily <vcs@emily.moe>
Date: Mon, 3 Feb 2025 19:40:03 +0000
Subject: [PATCH] nixpkgs: make config.nixpkgs.{buildPlatform,hostPlatform}
 write only

The description for options.nixpkgs.system already hints at this:

  Neither ${opt.system} nor any other option in nixpkgs.* is meant
  to be read by modules and configurations.
  Use pkgs.stdenv.hostPlatform instead.

We can support this goal by not elaborating the systems anymore, forcing
users to go via pkgs.stdenv.

This will prevent problems when making the top-level package sets
composable in the next commit. For this to work, you should pass a fully
elaborated system to nixpkgs' localSystem or crossSystem options.

Backport of Nixpkgs commit 0a19371146130c0e2a402fd0c35f8283b0e81910.

Co-authored-by: Wolfgang Walther <walther@technowledgy.de>
---
 modules/nix/nixpkgs.nix | 33 ++++++++++++++++-----------------
 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/modules/nix/nixpkgs.nix b/modules/nix/nixpkgs.nix
index fa156a5..d4fa306 100644
--- a/modules/nix/nixpkgs.nix
+++ b/modules/nix/nixpkgs.nix
@@ -73,7 +73,10 @@ let
   defaultPkgs =
     if opt.hostPlatform.isDefined then
       let
-        isCross = cfg.buildPlatform != cfg.hostPlatform;
+        isCross =
+          !(lib.systems.equals (lib.systems.elaborate cfg.buildPlatform) (
+            lib.systems.elaborate cfg.hostPlatform
+          ));
         systemArgs =
           if isCross then
             {
@@ -165,13 +168,10 @@ in
     };
 
     hostPlatform = lib.mkOption {
-      type = lib.types.either lib.types.str lib.types.attrs; # TODO utilize lib.systems.parsedPlatform
+      type = lib.types.either lib.types.str lib.types.attrs;
       example = {
         system = "aarch64-darwin";
       };
-      # Make sure that the final value has all fields for sake of other modules
-      # referring to this. TODO make `lib.systems` itself use the module system.
-      apply = lib.systems.elaborate;
       description = ''
         Specifies the platform where the nix-darwin configuration will run.
 
@@ -182,24 +182,13 @@ in
     };
 
     buildPlatform = lib.mkOption {
-      type = lib.types.either lib.types.str lib.types.attrs; # TODO utilize lib.systems.parsedPlatform
+      type = lib.types.either lib.types.str lib.types.attrs;
       default = cfg.hostPlatform;
       example = {
         system = "x86_64-darwin";
       };
       # Make sure that the final value has all fields for sake of other modules
       # referring to this.
-      apply =
-        inputBuildPlatform:
-        let
-          elaborated = lib.systems.elaborate inputBuildPlatform;
-        in
-        if lib.systems.equals elaborated cfg.hostPlatform then
-          cfg.hostPlatform # make identical, so that `==` equality works;
-            see
-            "https://github.com/NixOS/nixpkgs/issues/278001"
-        else
-          elaborated;
       defaultText = lib.literalExpression ''config.nixpkgs.hostPlatform'';
       description = ''
         Specifies the platform on which nix-darwin should be built.
@@ -323,6 +312,16 @@ in
           ${lib.concatMapStringsSep "\n" (file: "  - ${file}") opt.config.files}
         '';
       }
+      {
+        assertion =
+          (opt.hostPlatform.isDefined -> builtins.isAttrs cfg.buildPlatform -> !(cfg.buildPlatform ? parsed))
+          && (opt.hostPlatform.isDefined -> builtins.isAttrs cfg.hostPlatform -> !(cfg.hostPlatform ? parsed));
+        message = ''
+          Passing fully elaborated systems to `nixpkgs.localSystem`, `nixpkgs.crossSystem`, `nixpkgs.buildPlatform`
+          or `nixpkgs.hostPlatform` will break composability of package sets in nixpkgs. For example, pkgs.pkgsStatic
+          would not work in modules anymore.
+        '';
+      }
     ];
   };
 }