lib/modules: allow specifying system as an evalNixvim arg

flake-modules/legacy-packages.nix

@@ -2,9 +2,8 @@
 {
   perSystem =
     {
-      pkgs,
-      lib,
       makeNixvimWithModule,
+      system,
       ...
     }:
     {
@@ -13,12 +12,7 @@
         makeNixvim = module: makeNixvimWithModule { inherit module; };
 
         nixvimConfiguration = helpers.modules.evalNixvim {
-          modules = [
+          inherit system;
-            {
-              _file = ./legacy-packages.nix;
-              nixpkgs.pkgs = lib.mkDefault pkgs;
-            }
-          ];
         };
       };
     };

lib/modules.nix

@@ -21,12 +21,16 @@ in
     {
       modules ? [ ],
       extraSpecialArgs ? { },
+      system ? null, # Can also be defined using the `nixpkgs.hostPlatform` option
     }:
     # Ensure a suitable `lib` is used
     assert lib.assertMsg (extraSpecialArgs ? lib -> extraSpecialArgs.lib ? nixvim) ''
       Nixvim requires a lib that includes some custom extensions, however the `lib` from `specialArgs` does not have a `nixvim` attr.
       Remove `lib` from nixvim's `specialArgs` or ensure you apply nixvim's extensions to your `lib`.
       See https://nix-community.github.io/nixvim/user-guide/helpers.html#using-a-custom-lib-with-nixvim'';
+    assert lib.assertMsg (system != null -> lib.isString system) ''
+      When `system` is supplied to `evalNixvim`, it must be a string.
+      To define a more complex system, please use nixvim's `nixpkgs.hostPlatform` option.'';
     lib.evalModules {
       modules = modules ++ [
         ../modules/top-level
@@ -34,6 +38,10 @@ in
           _file = "<nixvim-flake>";
           flake = lib.mkOptionDefault flake;
         }
+        (lib.optionalAttrs (system != null) {
+          _file = "evalNixvim";
+          nixpkgs.hostPlatform = lib.mkOptionDefault { inherit system; };
+        })
       ];
       specialArgs = {
         modulesPath = ../modules;

modules/top-level/nixpkgs.nix

@@ -164,7 +164,8 @@ in
       apply = lib.systems.elaborate;
       defaultText = lib.literalMD ''
         - Inherited from the "host" configuration's `pkgs`
-        - Must be specified manually when building a standalone nixvim
+        - Or `evalNixvim`'s `system` argument
+        - Otherwise, must be specified manually
       '';
       description = ''
         Specifies the platform where the Nixvim configuration will run.