Support Nix 2.34

[fasheng]

As usual, NixOS 26.05 brings Nix 2.34, and Nix 2.34 brings broken changes. This patch will fix compile errors for Nix 2.34.

BTW: This PR include #25 for it is required and still not be merged..

[fasheng]

Thanks @avidal. Here is a sample flake.nix like #25 did.

{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/c5c4a43b"; # nixos-26.05 20260901

  outputs = { nixpkgs, ... }:
    let
      forAllSystems = f: nixpkgs.lib.genAttrs
        [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ]
        (system: f nixpkgs.legacyPackages.${system});
    in
    {
      devShells = forAllSystems (pkgs:
        let
          nix = pkgs.nixVersions.nix_2_34;
          nixComponents = pkgs.nixVersions.nixComponents_2_34;

          nix-plugins-broken = pkgs.nix-plugins.override { inherit nixComponents; };

          nix-plugins-fixed = nix-plugins-broken.overrideAttrs (old: {
            patches = (old.patches or [ ]) ++ [
              (pkgs.writeText "nix-2.33.patch" ''
                --- a/extra-builtins.cc
                +++ b/extra-builtins.cc
                @@ -72,9 +72,9 @@ static void cflags(EvalState & state, const PosIdx _pos,
                     Value ** _args, Value & v)
                 {
                     auto attrs = state.buildBindings(3);
                -    attrs.alloc("NIX_INCLUDE_DIRS").mkString(NIX_INCLUDE_DIRS);
                -    attrs.alloc("NIX_CFLAGS_OTHER").mkString(NIX_CFLAGS_OTHER);
                -    attrs.alloc("BOOST_INCLUDE_DIR").mkString(BOOST_INCLUDE_DIR);
                +    attrs.alloc("NIX_INCLUDE_DIRS").mkString(NIX_INCLUDE_DIRS, state.mem);
                +    attrs.alloc("NIX_CFLAGS_OTHER").mkString(NIX_CFLAGS_OTHER, state.mem);
                +    attrs.alloc("BOOST_INCLUDE_DIR").mkString(BOOST_INCLUDE_DIR, state.mem);
                     v.mkAttrs(attrs);
                 }
              '')
              (pkgs.writeText "nix-2.34.patch" ''
                --- a/extra-builtins.cc
                +++ b/extra-builtins.cc
                @@ -12,8 +12,8 @@
                 using namespace nix;
                 
                 struct ExtraBuiltinsSettings : Config {
                -    Setting<Path> extraBuiltinsFile{this,
                -        settings.nixConfDir + "/extra-builtins.nix",
                +    Setting<std::filesystem::path> extraBuiltinsFile{this,
                +        nixConfDir() + "/extra-builtins.nix",
                             "extra-builtins-file",
                             "The path to a nix expression defining extra expression-language level builtins."};
                 };
                @@ -42,14 +42,14 @@ static void extraBuiltins(EvalState & state, const PosIdx pos,
                             attrs.alloc(sExec).mkPrimOp(new PrimOp {
                                 .name = "exec",
                                 .arity = 1,
                -                .fun = prim_exec,
                +                .impl = prim_exec,
                             });
                 
                             auto sImportNative = state.symbols.create("importNative");
                             attrs.alloc(sImportNative).mkPrimOp(new PrimOp {
                                 .name = "importNative",
                                 .arity = 2,
                -                .fun = prim_importNative,
                +                .impl = prim_importNative,
                             });
                 
                             arg = state.allocValue();
                @@ -65,7 +65,7 @@ static void extraBuiltins(EvalState & state, const PosIdx pos,
                 static RegisterPrimOp rp1({
                     .name = "__extraBuiltins",
                     .arity = 0,
                -    .fun = extraBuiltins,
                +    .impl = extraBuiltins,
                 });
                 
                 static void cflags(EvalState & state, const PosIdx _pos,
                @@ -81,5 +81,5 @@ static void cflags(EvalState & state, const PosIdx _pos,
                 static RegisterPrimOp rp2({
                     .name = "__nix-cflags",
                     .arity = 0,
                -    .fun = cflags,
                +    .impl = cflags,
                 });
                '')
            ];
          });

          extra-builtins = pkgs.writeText "extra-builtins.nix" ''
            { ... }: { hello = "nix-plugins works!"; }
          '';

          mkShell = nix-plugins: pkgs.mkShell {
            packages = [ nix ];
            NIX_CONFIG = ''
              plugin-files = ${nix-plugins}/lib/nix/plugins
              extra-builtins-file = ${extra-builtins}
            '';
          };
        in
        {
          broken = mkShell nix-plugins-broken;
          fixed = mkShell nix-plugins-fixed;
          default = mkShell nix-plugins-fixed;
        }
      );
    };
}

Test commands:

nix develop .#fixed
$ nix eval --expr 'builtins.extraBuiltins'

NOTE: If the host enabled nix-plugins, the test commands will print the builtins.extraBuiltins that defined in host's /etc/nix/nix.conf

[fasheng]

Consider NixOS/Nix always broken nix-plugins, here is my Tips:

1. Under any cases, If the nix version of the system to be deployed changes, the first thing to do is to block the nix-plugins related configuration of the current system and execute the nixos-rebuild switch:

   # nix.settings.plugin-files = "${pkgs.nix-plugins}/lib/nix/plugins";
   # nix.settings.extra-builtins-file = [ ./extra-builtins.nix ];

   Otherwise, directly deploying the system will report an error:

   > error: could not dynamically open plugin file "/nix/store/184qp79qgycbhvx3nknafqzng7xd85jw-nix-plugins-16.0.1/lib/nix/plugins/libnix-extra-builtins.so": /nix/store/184qp79qgycbhvx3nknafqzng7xd85jw-nix-plugins-16.0.1/lib/nix/plugins/libnix-extra-builtins.so: undefined symbol: _ZN3nix6Config11getSettingsERSt3mapINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEENS_14AbstractConfig11SettingInfoESt4lessIS7_ESaISt4pairIKS7_S9_EEEb
          For full logs, run:
            nix log /nix/store/c64h8w3wabl0prfmbnjvinn02s8fq7wi-nix.conf.drv
   

2. If the nix-plugins still report an error, a workaround is to change the nixComponents of nix-plugins. For example the following config works with NixOS 25.11:

   let
     nix-plugins-fixed = pkgs.nix-plugins.override {
       nixComponents = pkgs.nixVersions.nixComponents_2_31;
     };
   in
   {
     nix.settings.plugin-files = "${nix-plugins-fixed}/lib/nix/plugins";
     nix.settings.extra-builtins-file = [ ./extra-builtins.nix ];
   }

   https://discourse.nixos.org/t/unable-to-get-nix-plugins-to-work/67426/2

3. If the nix-plugins still report an error, another workaround is keep the nix version not changed. For example the following config works with NixOS 26.05(c5c4a43b):

   nix.package = pkgs.nixVersions.nix_2_31;

   NOTE: If you want to switch to another nix version, just follow Tips 1.

4. Generally, once your configurations about nix-plugins changed, you should run nixos-rebuild switch twice to apply it

5. nix-plugins build commands:

   nix develop
   $ mkdir build
   $ cd build
   $ cmake ..
   $ cmake --build .

[shlevy]

Will review this week, thanks