Move enabled experimental feature to libutil struct

This is needed in subsequent commits to allow the settings and CLI args
infrastructure itself to read this setting.

src/libutil/config.cc

@@ -444,4 +444,30 @@ GlobalConfig::Register::Register(Config * config)
     configRegistrations->emplace_back(config);
 }
 
+ExperimentalFeatureSettings experimentalFeatureSettings;
+
+static GlobalConfig::Register rSettings(&experimentalFeatureSettings);
+
+bool ExperimentalFeatureSettings::isEnabled(const ExperimentalFeature & feature) const
+{
+    auto & f = experimentalFeatures.get();
+    return std::find(f.begin(), f.end(), feature) != f.end();
+}
+
+void ExperimentalFeatureSettings::require(const ExperimentalFeature & feature) const
+{
+    if (!isEnabled(feature))
+        throw MissingExperimentalFeature(feature);
+}
+
+bool ExperimentalFeatureSettings::isEnabled(const std::optional<ExperimentalFeature> & feature) const
+{
+    return !feature || isEnabled(*feature);
+}
+
+void ExperimentalFeatureSettings::require(const std::optional<ExperimentalFeature> & feature) const
+{
+    if (feature) require(*feature);
+}
+
 }

src/libutil/config.hh

@@ -1,12 +1,13 @@
+#pragma once
+
 #include <cassert>
 #include <map>
 #include <set>
 
-#include "types.hh"
-
 #include <nlohmann/json_fwd.hpp>
 
-#pragma once
+#include "types.hh"
+#include "experimental-features.hh"
 
 namespace nix {
 
@@ -357,4 +358,37 @@ struct GlobalConfig : public AbstractConfig
 
 extern GlobalConfig globalConfig;
 
+
+struct ExperimentalFeatureSettings : Config {
+
+    Setting<std::set<ExperimentalFeature>> experimentalFeatures{this, {}, "experimental-features",
+        "Experimental Nix features to enable."};
+
+    /**
+     * Check whether the given experimental feature is enabled.
+     */
+    bool isEnabled(const ExperimentalFeature &) const;
+
+    /**
+     * Require an experimental feature be enabled, throwing an error if it is
+     * not.
+     */
+    void require(const ExperimentalFeature &) const;
+
+    /**
+     * `std::nullopt` pointer means no feature, which means there is nothing that could be
+     * disabled, and so the function returns true in that case.
+     */
+    bool isEnabled(const std::optional<ExperimentalFeature> &) const;
+
+    /**
+     * `std::nullopt` pointer means no feature, which means there is nothing that could be
+     * disabled, and so the function does nothing in that case.
+     */
+    void require(const std::optional<ExperimentalFeature> &) const;
+};
+
+// FIXME: don't use a global variable.
+extern ExperimentalFeatureSettings experimentalFeatureSettings;
+
 }