force each container's unit configs in the checks about it

nix/module-eval/lib.nix

@@ -253,6 +253,21 @@ let
           "echo 'module-eval-${name}: ${toString (builtins.length bad)} of ${toString (builtins.length cases)} properties broke' >&2 && exit 1"
       }
     '';
+
+  # `serviceConfig` is an `attrsOf`: each key merges lazily, and a priority
+  # collision throws only when that key is demanded. A check reading
+  # `LoadCredential` therefore says nothing about `Restart`. Takes one
+  # container's `config`; `true` once every unit's every `serviceConfig` value
+  # is at WHNF. `seq` per value, not `deepSeq`: walking into a derivation-shaped
+  # value's `override` self-reference overflows the stack.
+  forceServiceConfigs =
+    containerCfg:
+    let
+      vals = lib.concatMap (s: builtins.attrValues (s.serviceConfig or { })) (
+        builtins.attrValues containerCfg.systemd.services
+      );
+    in
+    builtins.foldl' (acc: v: builtins.seq v acc) true vals;
 in
 {
   inherit hive;
@@ -272,4 +287,5 @@ in
   inherit carriesJournaldSeverity;
   inherit swarmServiceEnables;
   inherit runGroup;
+  inherit forceServiceConfigs;
 }