Mark experimental features on settings

We hide them in various ways if the experimental feature isn't enabled.

To do this, we had to move the experimental features list out of
libnixstore, because the setting machinary itself depends on it. To do
that, we made a new `ExperimentalFeatureSettings`.

src/libfetchers/fetch-settings.hh

@@ -75,21 +75,25 @@ struct FetchSettings : public Config
           Path or URI of the global flake registry.
 
           When empty, disables the global flake registry.
-        )"};
+        )",
+        {}, true, Xp::Flakes};
 
 
     Setting<bool> useRegistries{this, true, "use-registries",
-        "Whether to use flake registries to resolve flake references."};
+        "Whether to use flake registries to resolve flake references.",
+        {}, true, Xp::Flakes};
 
     Setting<bool> acceptFlakeConfig{this, false, "accept-flake-config",
-        "Whether to accept nix configuration from a flake without prompting."};
+        "Whether to accept nix configuration from a flake without prompting.",
+        {}, true, Xp::Flakes};
 
     Setting<std::string> commitLockFileSummary{
         this, "", "commit-lockfile-summary",
         R"(
           The commit summary to use when committing changed flake lock files. If
           empty, the summary is generated based on the action performed.
-        )"};
+        )",
+        {}, true, Xp::Flakes};
 };
 
 // FIXME: don't use a global variable.

src/libutil/config.cc

@@ -70,10 +70,17 @@ void AbstractConfig::reapplyUnknownSettings()
         set(s.first, s.second);
 }
 
+// Whether we should process the option. Excludes aliases, which are handled elsewhere, and disabled features.
+static bool applicable(const Config::SettingData & sd)
+{
+    return !sd.isAlias
+        && experimentalFeatureSettings.isEnabled(sd.setting->experimentalFeature);
+}
+
 void Config::getSettings(std::map<std::string, SettingInfo> & res, bool overriddenOnly)
 {
     for (auto & opt : _settings)
-        if (!opt.second.isAlias && (!overriddenOnly || opt.second.setting->overridden))
+        if (applicable(opt.second) && (!overriddenOnly || opt.second.setting->overridden))
             res.emplace(opt.first, SettingInfo{opt.second.setting->to_string(), opt.second.setting->description});
 }
 
@@ -147,34 +154,36 @@ nlohmann::json Config::toJSON()
 {
     auto res = nlohmann::json::object();
     for (auto & s : _settings)
-        if (!s.second.isAlias) {
+        if (applicable(s.second))
             res.emplace(s.first, s.second.setting->toJSON());
-        }
     return res;
 }
 
 std::string Config::toKeyValue()
 {
     auto res = std::string();
     for (auto & s : _settings)
-        if (!s.second.isAlias) {
+        if (applicable(s.second))
             res += fmt("%s = %s\n", s.first, s.second.setting->to_string());
-        }
     return res;
 }
 
 void Config::convertToArgs(Args & args, const std::string & category)
 {
     for (auto & s : _settings)
-        if (!s.second.isAlias)
+        if (applicable(s.second))
             s.second.setting->convertToArg(args, category);
 }
 
 AbstractSetting::AbstractSetting(
     const std::string & name,
     const std::string & description,
-    const std::set<std::string> & aliases)
-    : name(name), description(stripIndentation(description)), aliases(aliases)
+    const std::set<std::string> & aliases,
+    std::optional<ExperimentalFeature> experimentalFeature)
+    : name(name)
+    , description(stripIndentation(description))
+    , aliases(aliases)
+    , experimentalFeature(experimentalFeature)
 {
 }
 

src/libutil/config.hh

@@ -195,12 +195,15 @@ public:
 
     bool overridden = false;
 
+    std::optional<ExperimentalFeature> experimentalFeature;
+
 protected:
 
     AbstractSetting(
         const std::string & name,
         const std::string & description,
-        const std::set<std::string> & aliases);
+        const std::set<std::string> & aliases,
+        std::optional<ExperimentalFeature> experimentalFeature = std::nullopt);
 
     virtual ~AbstractSetting()
     {
@@ -241,8 +244,9 @@ public:
         const bool documentDefault,
         const std::string & name,
         const std::string & description,
-        const std::set<std::string> & aliases = {})
-        : AbstractSetting(name, description, aliases)
+        const std::set<std::string> & aliases = {},
+        std::optional<ExperimentalFeature> experimentalFeature = std::nullopt)
+        : AbstractSetting(name, description, aliases, experimentalFeature)
         , value(def)
         , defaultValue(def)
         , documentDefault(documentDefault)
@@ -297,8 +301,9 @@ public:
         const std::string & name,
         const std::string & description,
         const std::set<std::string> & aliases = {},
-        const bool documentDefault = true)
-        : BaseSetting<T>(def, documentDefault, name, description, aliases)
+        const bool documentDefault = true,
+        std::optional<ExperimentalFeature> experimentalFeature = std::nullopt)
+        : BaseSetting<T>(def, documentDefault, name, description, aliases, experimentalFeature)
     {
         options->addSetting(this);
     }