Create nlohmann serializers for std::optional and use

This is somewhat tricky.

src/libutil/args.cc

@@ -1,10 +1,9 @@
 #include "args.hh"
 #include "hash.hh"
+#include "json-utils.hh"
 
 #include <glob.h>
 
-#include <nlohmann/json.hpp>
-
 namespace nix {
 
 void Args::addFlag(Flag && flag_)
@@ -247,11 +246,7 @@ nlohmann::json Args::toJSON()
             j["arity"] = flag->handler.arity;
         if (!flag->labels.empty())
             j["labels"] = flag->labels;
-        // TODO With C++23 use `std::optional::tranform`
-        if (auto & xp = flag->experimentalFeature)
-            j["experimental-feature"] = showExperimentalFeature(*xp);
-        else
-            j["experimental-feature"] = nullptr;
+        j["experimental-feature"] = flag->experimentalFeature;
         flags[name] = std::move(j);
     }
 
@@ -416,11 +411,7 @@ nlohmann::json MultiCommand::toJSON()
         cat["id"] = command->category();
         cat["description"] = trim(categories[command->category()]);
         j["category"] = std::move(cat);
-        // TODO With C++23 use `std::optional::tranform`
-        if (auto xp = command->experimentalFeature())
-            cat["experimental-feature"] = showExperimentalFeature(*xp);
-        else
-            cat["experimental-feature"] = nullptr;
+        cat["experimental-feature"] = command->experimentalFeature();
         cmds[name] = std::move(j);
     }
 

src/libutil/experimental-features.hh

@@ -3,7 +3,7 @@
 
 #include "comparator.hh"
 #include "error.hh"
-#include "nlohmann/json_fwd.hpp"
+#include "json-utils.hh"
 #include "types.hh"
 
 namespace nix {
@@ -93,4 +93,10 @@ public:
 void to_json(nlohmann::json &, const ExperimentalFeature &);
 void from_json(const nlohmann::json &, ExperimentalFeature &);
 
+/**
+ * It is always rendered as a string
+ */
+template<>
+struct json_avoids_null<ExperimentalFeature> : std::true_type {};
+
 }

src/libutil/json-utils.cc

@@ -0,0 +1,19 @@
+#include "json-utils.hh"
+
+namespace nix {
+
+const nlohmann::json * get(const nlohmann::json & map, const std::string & key)
+{
+    auto i = map.find(key);
+    if (i == map.end()) return nullptr;
+    return &*i;
+}
+
+nlohmann::json * get(nlohmann::json & map, const std::string & key)
+{
+    auto i = map.find(key);
+    if (i == map.end()) return nullptr;
+    return &*i;
+}
+
+}

src/libutil/json-utils.hh

@@ -2,21 +2,77 @@
 ///@file
 
 #include <nlohmann/json.hpp>
+#include <list>
 
 namespace nix {
 
-const nlohmann::json * get(const nlohmann::json & map, const std::string & key)
-{
-    auto i = map.find(key);
-    if (i == map.end()) return nullptr;
-    return &*i;
-}
+const nlohmann::json * get(const nlohmann::json & map, const std::string & key);
 
-nlohmann::json * get(nlohmann::json & map, const std::string & key)
-{
-    auto i = map.find(key);
-    if (i == map.end()) return nullptr;
-    return &*i;
-}
+nlohmann::json * get(nlohmann::json & map, const std::string & key);
+
+/**
+ * For `adl_serializer<std::optional<T>>` below, we need to track what
+ * types are not already using `null`. Only for them can we use `null`
+ * to represent `std::nullopt`.
+ */
+template<typename T>
+struct json_avoids_null;
+
+/**
+ * Handle numbers in default impl
+ */
+template<typename T>
+struct json_avoids_null : std::bool_constant<std::is_integral<T>::value> {};
+
+template<>
+struct json_avoids_null<std::nullptr_t> : std::false_type {};
+
+template<>
+struct json_avoids_null<bool> : std::true_type {};
+
+template<>
+struct json_avoids_null<std::string> : std::true_type {};
+
+template<typename T>
+struct json_avoids_null<std::vector<T>> : std::true_type {};
+
+template<typename T>
+struct json_avoids_null<std::list<T>> : std::true_type {};
+
+template<typename K, typename V>
+struct json_avoids_null<std::map<K, V>> : std::true_type {};
+
+}
+
+namespace nlohmann {
+
+/**
+ * This "instance" is widely requested, see
+ * https://github.com/nlohmann/json/issues/1749, but momentum has stalled
+ * out. Writing there here in Nix as a stop-gap.
+ *
+ * We need to make sure the underlying type does not use `null` for this to
+ * round trip. We do that with a static assert.
+ */
+template<typename T>
+struct adl_serializer<std::optional<T>> {
+    static std::optional<T> from_json(const json & json) {
+        static_assert(
+            nix::json_avoids_null<T>::value,
+            "null is already in use for underlying type's JSON");
+        return json.is_null()
+            ? std::nullopt
+            : std::optional { adl_serializer<T>::from_json(json) };
+    }
+    static void to_json(json & json, std::optional<T> t) {
+        static_assert(
+            nix::json_avoids_null<T>::value,
+            "null is already in use for underlying type's JSON");
+        if (t)
+            adl_serializer<T>::to_json(json, *t);
+        else
+            json = nullptr;
+    }
+};
 
 }