linux/generic.nix: Fix CONFIG_RUST

[roberth]

I thought it was module based, but it wasn't.

Fixes #306790 (comment)

This has broken [...] Rust support

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[alyssais]

@ofborg test kernel-rust

[alyssais]

Oh, I'm also wondering if we need to go back to optionalAttrs, since older kernels probably won't recognise CONFIG_RUST:

@ofborg build linux_5_10

[roberth]

wondering if we need to go back to optionalAttrs, since older kernels probably won't recognise CONFIG_RUST

If someone needs an even older kernel, we could condition it on the kernel version, which I think is readily available at this point, unlike withRust, which evaluates the kernel patches.

-CONFIG_RUST = if withRust then "y" else "n";
+CONFIG_RUST = optionalAttrs (isNotAncient version) (if withRust then "y" else "n");