linux: mark <6.12 with Rust as broken, assert on <6.7 with Rust

Linux 6.7 is the oldest kernel with Rust support, so the config part
should fail hard if 6.6 or older requests the Rust feature. Until now
this was silently ignored[1].

It turned out that Rust for Linux 6.11 or older fails for unknown
reasons[2]. We decided to not bother since it's not a longterm kernel
and 6.12 seems fine again. As a result, marking this as broken.

[1] https://github.com/NixOS/nixpkgs/pull/345534#discussion_r1781392312
[2] https://github.com/NixOS/nixpkgs/pull/345534#issuecomment-2391238381

pkgs/os-specific/linux/kernel/common-config.nix

@@ -34,7 +34,11 @@ let
 
   # Currently not enabling Rust by default, as upstream requires rustc 1.81
   defaultRust = false;
-  withRust = (forceRust || defaultRust) && kernelSupportsRust;
+  withRust =
+    assert lib.assertMsg (!(forceRust && !kernelSupportsRust)) ''
+      Kernels below 6.7 (the kernel being built is ${version}) don't support Rust.
+    '';
+    (forceRust || defaultRust) && kernelSupportsRust;
 
   options = {
 

pkgs/os-specific/linux/kernel/manual-config.nix

@@ -396,6 +396,9 @@ let
       requiredSystemFeatures = [ "big-parallel" ];
 
       meta = {
+        # https://github.com/NixOS/nixpkgs/pull/345534#issuecomment-2391238381
+        broken = withRust && lib.versionOlder version "6.12";
+
         description =
           "The Linux kernel" +
           (if kernelPatches == [] then "" else