add checks/ folder

docs/folder-structure.md

@@ -2,6 +2,7 @@
 
 ## High-level
 
+* `checks/` for flake checks.
 * `devshells/` for devshells.
 * `hosts/` for machine configurations.
 * `lib/` for Nix functions.
@@ -208,6 +209,20 @@ To consume a package inside a host from the same flake, `perSystem.self.<pname>`
 Takes the "per-system" arguments. On top of this, it also takes a `pname`
 argument.
 
+### `checks/<pname>(.nix|/default.nix)`
+
+The `checks/` folder can be populated by packages that will be run when `nix flake checks` is invoked.
+
+The flake checks are also populate by some of the other attributes, like `packages` and `hosts`.
+
+Inputs:
+
+* The [per-system](#per-system) values, plus the `pname` attribute.
+
+Flake outputs:
+
+* `checks.<system>.<pname>` - will contain the package
+
 #### `templates/<name>/`
 
 Use this if you want your project to be initializable using `nix flake init`.

lib/default.nix

@@ -346,6 +346,8 @@ let
                 lib.filterAttrs (_: x: x.pkgs.system == system) (inputs.self.darwinConfigurations or { })
               )
             ))
+            # load checks from the /checks folder. Those take precedence over the others.
+            (optionalPathAttrs (src + "/checks") (path: importDir path lib.id))
           ]
           ++ (lib.optional (inputs.self.lib.tests or { } != { }) {
             lib-tests = pkgs.runCommandLocal "lib-tests" { nativeBuildInputs = [ pkgs.nix-unit ]; } ''