From 4f2f8faa7f7d3cba5c4eccf6077cef2235f0071a Mon Sep 17 00:00:00 2001
From: Kamil Zaripov <kamil@zaripov.net>
Date: Sat, 16 Aug 2025 22:10:30 +0300
Subject: [PATCH] check-meta: Clarify error message about unsupported platform
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Right now if you are specifying supported platforms using meta.platforms
you can get a bit misleading error message:

    $ nix-build default.nix
    error:
          … in the condition of the assert statement
            at /nix/store/53pzmaza73pl8q2yl1q62h7ncx21nxc1-source/lib/customisation.nix:419:9:
              418|       drvPath =
              419|         assert condition;
                |         ^
              420|         drv.drvPath;

          … while evaluating the attribute 'handled'
            at /nix/store/53pzmaza73pl8q2yl1q62h7ncx21nxc1-source/pkgs/stdenv/generic/check-meta.nix:643:9:
              642|         # or, alternatively, just output a warning message.
              643|         handled = (
                |         ^
              644|           if valid == "yes" then

          (stack trace truncated; use '--show-trace' to show the full, detailed trace)

          error: Package ‘test’ in /home/kamil/code/test/default.nix:3 is not available on the requested hostPlatform:
            hostPlatform.config = "aarch64-none-elf"
            package.meta.platforms = [
              "aarch64-none-elf"
            ]
            package.meta.badPlatforms = [ ]
          , refusing to evaluate.

          a) To temporarily allow packages that are unsupported for this system, you can use an environment variable
              for a single invocation of the nix tools.

                $ export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1

              Note: When using `nix shell`, `nix build`, `nix develop`, etc with a flake,
                    then pass `--impure` in order to allow use of environment variables.

          b) For `nixos-rebuild` you can set
            { nixpkgs.config.allowUnsupportedSystem = true; }
          in configuration.nix to override this.

          c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
            { allowUnsupportedSystem = true; }
          to ~/.config/nixpkgs/config.nix.

It is not clear why this derivation cannot be built for
"aarch64-none-elf" target since error message list this triplet in
package.meta.platforms right below hostPlatform.config. The issue is that
package.meta.platforms is checked against hostPlatform.system field, not
hostPlatform.config field:
https://github.com/NixOS/nixpkgs/blob/b134951a4c9f3c995fd7be05f3243f8ecd65d798/lib/meta.nix#L103
And in this case hostPlatform.system is "aarch64-none" while
hostPlatform.config is "aarch64-none-efi".

I think that it would be better to print hostPlatform.system in error
message.
---
 pkgs/stdenv/generic/check-meta.nix | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pkgs/stdenv/generic/check-meta.nix b/pkgs/stdenv/generic/check-meta.nix
index c3e9469649ff..d2c7d0b2f168 100644
--- a/pkgs/stdenv/generic/check-meta.nix
+++ b/pkgs/stdenv/generic/check-meta.nix
@@ -534,7 +534,7 @@ let
         reason = "unsupported";
         errormsg = ''
           is not available on the requested hostPlatform:
-            hostPlatform.config = "${hostPlatform.config}"
+            hostPlatform.system = "${hostPlatform.system}"
             package.meta.platforms = ${toPretty' (attrs.meta.platforms or [ ])}
             package.meta.badPlatforms = ${toPretty' (attrs.meta.badPlatforms or [ ])}
         '';