modules/nixpkgs: test overlays option

Author: MattSturgeon

tests/default.nix

@@ -1,4 +1,6 @@
 {
+  self, # This flake
+  system,
   lib ? pkgs.lib,
   helpers,
   pkgs,
@@ -16,6 +18,11 @@ let
       module = {
         _file = file;
         imports = [ module ];
+        _module.args = {
+          # Give tests access to the flake
+          inherit self system;
+          inherit (self) inputs;
+        };
       };
       pkgs = pkgsUnfree;
     };

tests/test-sources/modules/nixpkgs.nix

@@ -0,0 +1,74 @@
+{
+  # TODO: expect not setting `nixpkgs.pkgs` to throw
+
+  overlays =
+    { pkgs, ... }:
+    {
+      test.runNvim = false;
+
+      nixpkgs.overlays = [
+        (final: prev: {
+          foobar = "foobar";
+        })
+      ];
+
+      assertions = [
+        {
+          assertion = pkgs.foobar or null == "foobar";
+          message = ''
+            Expected `pkgs.foobar` to be "foobar"
+          '';
+        }
+      ];
+    };
+
+  # Test that overlays from both `nixpkgs.pkgs` _and_ `nixpkgs.overlays` are applied
+  stacked_overlays =
+    {
+      inputs,
+      system,
+      pkgs,
+      ...
+    }:
+    {
+      test.runNvim = false;
+
+      nixpkgs.pkgs = import inputs.nixpkgs {
+        inherit system;
+        overlays = [
+          (final: prev: {
+            foobar = "foobar";
+            conflict = "a";
+          })
+        ];
+      };
+
+      nixpkgs.overlays = [
+        (final: prev: {
+          hello = "world";
+          conflict = "b";
+        })
+      ];
+
+      assertions = [
+        {
+          assertion = pkgs.foobar or null == "foobar";
+          message = ''
+            Expected `pkgs.foobar` to be "foobar"
+          '';
+        }
+        {
+          assertion = pkgs.hello or null == "world";
+          message = ''
+            Expected `pkgs.hello` to be "world"
+          '';
+        }
+        {
+          assertion = pkgs.conflict or null == "b";
+          message = ''
+            Expected `pkgs.conflict` to be "b"
+          '';
+        }
+      ];
+    };
+}