nixosMod: add fileSystem assertion about "discard" option

nix/nixos-modules/assertions/default.nix

@@ -1,6 +1,7 @@
 {
   imports = [
     # files
+    ./fileSystems.nix
     ./nixos.nix
   ];
 }

nix/nixos-modules/assertions/fileSystems.nix

@@ -0,0 +1,21 @@
+{ config, ... }:
+let
+  inherit (builtins) any attrValues elem;
+  allMounts = attrValues config.fileSystems;
+  testDiskOption = option: disk: elem option disk.options;
+  testDiskDiscard = testDiskOption "discard";
+in
+{
+  config = {
+
+    assertions = [
+      {
+        assertion = config.services.fstrim.enable -> !any testDiskDiscard allMounts;
+        message = ''
+          enabling "discard" mount option is discouraged because services.fstrim is enabled
+        '';
+      }
+    ];
+
+  };
+}