meta: Begin writing tests in Nix

Now that I've figured out how to dynamically import modules and use
those modules as outputs, the next step is to ensure that these modules
work as intended when being used by end users.

NixOS offers a built-in testing solution that enables us to conveniently
spin up virtual machines with a given configuration, then verify their
correctness through python scripting.

The lib.nix file in particular is based on Jörg Thalheim's very useful
blog post that explains how to use the built-in testing functionality
with Nix flakes, which isn't covered in official documentation.

See: https://blog.thalheim.io/2023/01/08/how-to-use-nixos-testing-framework-with-flakes/

flake.nix

@@ -25,6 +25,11 @@
     inherit (nixpkgs.lib) nixosSystem;
     inherit (nixpkgs.legacyPackages.x86_64-linux) nixpkgs-fmt callPackage;
 
+    checkArgs = {
+      pkgs = nixpkgs.legacyPackages.x86_64-linux;
+      inherit self;
+    };
+
     flakeOutputs     =  [ "overlays" "nixosModules" "homeManagerModules" "packages" ];
     flakeDirectories =  [ "overlays" "modules"      "home"               "packages" ];
     packageDirectory = "packages";
@@ -55,6 +60,11 @@
         ];
       };
     };
+
+    checks.x86_64-linux = {
+      hyprland = import ./tests/hyprland.nix checkArgs;
+      neovim = import ./tests/neovim.nix checkArgs;
+    };
   } //
     (builtins.listToAttrs
       (builtins.map

tests/hyprland.nix

@@ -0,0 +1,15 @@
+# TODO: Write test to ensure that Hyprland starts with basic config
+(import ./lib.nix) {
+  name = "hyprland";
+
+  nodes.machine = { self, pkgs, ... }: {
+    imports = with self.nixosModules; [
+      hyprland
+    ];
+  };
+
+  testScript = /* python */ ''
+    output = machine.succeed("echo 'Hello world'")
+    assert "Hello world" in output
+  '';
+}

tests/lib.nix

@@ -0,0 +1,13 @@
+test: { pkgs, self }:
+let
+  inherit (pkgs) lib;
+  nixos-lib = import (pkgs.path + "/nixos/lib") { };
+in
+(nixos-lib.runTest {
+  hostPkgs = pkgs;
+  defaults.documentation.enable = lib.mkDefault false;
+  node.specialArgs = {
+    inherit self;
+  };
+  imports = [ test ];
+}).config.result

tests/neovim.nix

@@ -0,0 +1,15 @@
+# TODO: Ensure that neovim config works without errors on startup
+(import ./lib.nix) {
+  name = "neovim";
+
+  nodes.machine = { self, pkgs, ... }: {
+    imports = with self.nixosModules; [
+      neovim
+    ];
+  };
+
+  testScript = /* python */ ''
+    output = machine.succeed("echo 'Hello world'")
+    assert "Hello world" in output
+  '';
+}