Move json_avoids_null to its own header

This is because we need it in declarations where we should not be
including the full `nlohmann/json.hpp`.

Already can clean up by moving the experimental feature "instance".

Also, make the `std::map` instance better by allowing for other
comparison functions.

src/libutil/include/nix/util/experimental-features.hh

@@ -3,6 +3,7 @@
 
 #include "nix/util/error.hh"
 #include "nix/util/types.hh"
+#include "nix/util/json-non-null.hh"
 
 #include <nlohmann/json_fwd.hpp>
 
@@ -89,6 +90,13 @@ public:
     MissingExperimentalFeature(ExperimentalFeature missingFeature);
 };
 
+/**
+ * `ExperimentalFeature` is always rendered as a string.
+ */
+template<>
+struct json_avoids_null<ExperimentalFeature> : std::true_type
+{};
+
 /**
  * Semi-magic conversion to and from json.
  * See the nlohmann/json readme for more details.

src/libutil/include/nix/util/json-non-null.hh

@@ -0,0 +1,55 @@
+#pragma once
+///@file
+
+#include <string>
+#include <vector>
+#include <set>
+#include <map>
+#include <list>
+
+namespace nix {
+
+/**
+ * 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 T, typename Compare>
+struct json_avoids_null<std::set<T, Compare>> : std::true_type
+{};
+
+template<typename K, typename V, typename Compare>
+struct json_avoids_null<std::map<K, V, Compare>> : std::true_type
+{};
+
+} // namespace nix

src/libutil/include/nix/util/json-utils.hh

@@ -6,6 +6,7 @@
 
 #include "nix/util/error.hh"
 #include "nix/util/types.hh"
+#include "nix/util/json-non-null.hh"
 
 namespace nix {
 
@@ -59,56 +60,6 @@ Strings getStringList(const nlohmann::json & value);
 StringMap getStringMap(const nlohmann::json & value);
 StringSet getStringSet(const nlohmann::json & value);
 
-/**
- * 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 T, typename Compare>
-struct json_avoids_null<std::set<T, Compare>> : std::true_type
-{};
-
-template<typename K, typename V>
-struct json_avoids_null<std::map<K, V>> : std::true_type
-{};
-
-/**
- * `ExperimentalFeature` is always rendered as a string.
- */
-template<>
-struct json_avoids_null<ExperimentalFeature> : std::true_type
-{};
-
 } // namespace nix
 
 namespace nlohmann {

src/libutil/include/nix/util/meson.build

@@ -42,6 +42,7 @@ headers = files(
   'hash.hh',
   'hilite.hh',
   'json-impls.hh',
+  'json-non-null.hh',
   'json-utils.hh',
   'logging.hh',
   'lru-cache.hh',