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.

doc/manual/utils.nix

@@ -44,7 +44,7 @@ rec {
 
   optionalString = cond: string: if cond then string else "";
 
-  showSetting = { useAnchors }: name: { description, documentDefault, defaultValue, aliases, value }:
+  showSetting = { useAnchors }: name: { description, documentDefault, defaultValue, aliases, value, experimentalFeature }:
     let
       result = squash ''
           - ${if useAnchors
@@ -54,10 +54,28 @@ rec {
           ${indent "  " body}
         '';
 
+      experimentalFeatureNote = optionalString (experimentalFeature != null) ''
+          > **Warning**
+          > This setting is part of an
+          > [experimental feature](@docroot@/contributing/experimental-features.md).
+
+          To change this setting, you need to make sure the corresponding experimental feature,
+          [`${experimentalFeature}`](@docroot@/contributing/experimental-features.md#xp-feature-${experimentalFeature}),
+          is enabled.
+          For example, include the following in [`nix.conf`](#):
+
+          ```
+          extra-experimental-features = ${experimentalFeature}
+          ${name} = ...
+          ```
+        '';
+
       # separate body to cleanly handle indentation
       body = ''
           ${description}
 
+          ${experimentalFeatureNote}
+
           **Default:** ${showDefault documentDefault defaultValue}
 
           ${showAliases aliases}

src/libstore/globals.hh

@@ -328,16 +328,6 @@ public:
           users in `build-users-group`.
 
           UIDs are allocated starting at 872415232 (0x34000000) on Linux and 56930 on macOS.
-
-          > **Warning**
-          > This is an experimental feature.
-
-          To enable it, add the following to [`nix.conf`](#):
-
-          ```
-          extra-experimental-features = auto-allocate-uids
-          auto-allocate-uids = true
-          ```
         )"};
 
     Setting<uint32_t> startId{this,
@@ -367,16 +357,6 @@ public:
 
           Cgroups are required and enabled automatically for derivations
           that require the `uid-range` system feature.
-
-          > **Warning**
-          > This is an experimental feature.
-
-          To enable it, add the following to [`nix.conf`](#):
-
-          ```
-          extra-experimental-features = cgroups
-          use-cgroups = true
-          ```
         )"};
     #endif
 

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;
 }
 

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().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": 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(),
+          R"#({
+            "name-of-the-setting": {
+              "aliases": [],
+              "defaultValue": "",
+              "description": "description\n",
+              "documentDefault": true,
+              "value": "value",
+              "experimentalFeature": "flakes"
+            }
+          })#"_json);
     }
 
     TEST(Config, setSettingAlias) {