build-inventory: move inventory and inventoryClass into explizitly different folders

lib/modules/inventory/constraints/default.nix

@@ -0,0 +1,55 @@
+{
+  resolvedRoles,
+  instanceName,
+  moduleName,
+  allRoles,
+}:
+{
+  lib,
+  config,
+  ...
+}:
+let
+  inherit (config) roles;
+in
+{
+  imports = [
+    (lib.modules.importApply ./interface.nix { inherit allRoles; })
+    # Role assertions
+    {
+      config.assertions = lib.foldlAttrs (
+        ass: roleName: roleConstraints:
+        let
+          members = resolvedRoles.${roleName}.machines;
+          memberCount = builtins.length members;
+          # Checks
+          minCheck = lib.optionalAttrs (roleConstraints.min > 0) {
+            "${moduleName}.${instanceName}.roles.${roleName}.min" = {
+              assertion = memberCount >= roleConstraints.min;
+              message = ''
+                The '${moduleName}' module requires at least ${builtins.toString roleConstraints.min} members of the '${roleName}' role
+                but found '${builtins.toString memberCount}' members within instance '${instanceName}':
+
+                ${lib.concatLines members}
+              '';
+            };
+          };
+
+          maxCheck = lib.optionalAttrs (roleConstraints.max != null) {
+            "${moduleName}.${instanceName}.roles.${roleName}.max" = {
+              assertion = memberCount <= roleConstraints.max;
+              message = ''
+                The ${moduleName} module allows at most for ${builtins.toString roleConstraints.max} members of the '${roleName}' role
+                but found '${builtins.toString memberCount}' members within instance '${instanceName}':
+
+                ${lib.concatLines members}
+              '';
+            };
+          };
+
+        in
+        ass // maxCheck // minCheck
+      ) { } roles;
+    }
+  ];
+}

lib/modules/inventory/constraints/interface.nix

@@ -0,0 +1,66 @@
+{
+  allRoles,
+}:
+{
+  lib,
+  ...
+}:
+let
+  inherit (lib) mkOption types;
+  rolesAttrs = builtins.groupBy lib.id allRoles;
+in
+{
+  options.roles = lib.mapAttrs (
+    _name: _:
+    mkOption {
+      description = ''
+        Sub-attributes of `${_name}` are constraints for the role.
+      '';
+      default = { };
+      type = types.submoduleWith {
+        modules = [
+          {
+            options = {
+              max = mkOption {
+                type = types.nullOr types.int;
+                default = null;
+                description = ''
+                  Maximum number of instances of this role that can be assigned to a module of this type.
+                '';
+              };
+              min = mkOption {
+                type = types.int;
+                default = 0;
+                description = ''
+                  Minimum number of instances of this role that must at least be assigned to a module of this type.
+                '';
+              };
+            };
+          }
+        ];
+      };
+    }
+  ) rolesAttrs;
+
+  # The resulting assertions
+  options.assertions = mkOption {
+    visible = false;
+    default = { };
+    type = types.attrsOf (
+      types.submoduleWith {
+        modules = [
+          {
+            options = {
+              assertion = mkOption {
+                type = types.bool;
+              };
+              message = mkOption {
+                type = types.str;
+              };
+            };
+          }
+        ];
+      }
+    );
+  };
+}