Mark experimental configuration settings programmatically

Fix #8162

The test is changed to compare `nlohmann::json` values, not strings of dumped
JSON, which allows us to format things more nicely.

2 files changed, 49 insertions, 3 deletions

diff --git a/src/libutil/config.cc b/src/libutil/config.cc
index 5ff8d91ba..a42f3a849 100644
--- a/src/libutil/config.cc
+++ b/src/libutil/config.cc
@@ -191,6 +191,10 @@ std::map<std::string, nlohmann::json> AbstractSetting::toJSONObject()
     std::map<std::string, nlohmann::json> obj;
     obj.emplace("description", description);
     obj.emplace("aliases", aliases);
+    if (experimentalFeature)
+        obj.emplace("experimentalFeature", *experimentalFeature);
+    else
+        obj.emplace("experimentalFeature", nullptr);
     return obj;
 }
 
diff --git a/src/libutil/tests/config.cc b/src/libutil/tests/config.cc
index 8be6730dd..f250e934e 100644
--- a/src/libutil/tests/config.cc
+++ b/src/libutil/tests/config.cc
@@ -156,12 +156,54 @@ namespace nix {
     }
 
     TEST(Config, toJSONOnNonEmptyConfig) {
+        using nlohmann::literals::operator "" _json;
         Config config;
-        std::map<std::string, Config::SettingInfo> settings;
-        Setting<std::string> setting{&config, "", "name-of-the-setting", "description"};
+        Setting<std::string> setting{
+            &config,
+            "",
+            "name-of-the-setting",
+            "description",
+        };
+        setting.assign("value");
+
+        ASSERT_EQ(config.toJSON(),
+          R"#({
+            "name-of-the-setting": {
+              "aliases": [],
+              "defaultValue": "",
+              "description": "description\n",
+              "documentDefault": true,
+              "value": "value",
+              "experimentalFeature": null
+            }
+          })#"_json);
+    }
+
+    TEST(Config, toJSONOnNonEmptyConfigWithExperimentalSetting) {
+        using nlohmann::literals::operator "" _json;
+        Config config;
+        Setting<std::string> setting{
+            &config,
+            "",
+            "name-of-the-setting",
+            "description",
+            {},
+            true,
+            Xp::Flakes,
+        };
         setting.assign("value");
 
-        ASSERT_EQ(config.toJSON().dump(), R"#({"name-of-the-setting":{"aliases":[],"defaultValue":"","description":"description\n","documentDefault":true,"value":"value"}})#");
+        ASSERT_EQ(config.toJSON(),
+          R"#({
+            "name-of-the-setting": {
+              "aliases": [],
+              "defaultValue": "",
+              "description": "description\n",
+              "documentDefault": true,
+              "value": "value",
+              "experimentalFeature": "flakes"
+            }
+          })#"_json);
     }
 
     TEST(Config, setSettingAlias) {