Add experimental-features setting

Experimental features are now opt-in. There is currently one
experimental feature: "nix-command" (which enables the "nix"
command. This will allow us to merge experimental features more
quickly, without committing to supporting them indefinitely.

Typical usage:

$ nix build --experimental-features 'nix-command flakes' nixpkgs#hello

(cherry picked from commit 8e478c234100cf03ea1b777d4bd42a9be7be9e8c,
without the "flakes" feature)

3 files changed, 14 insertions, 0 deletions

diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 1c2c08715..249c36673 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -105,6 +105,13 @@ StringSet Settings::getDefaultSystemFeatures()
     return features;
 }
 
+void Settings::requireExperimentalFeature(const std::string & name)
+{
+    auto & f = experimentalFeatures.get();
+    if (std::find(f.begin(), f.end(), name) == f.end())
+        throw Error("experimental Nix feature '%s' is disabled", name);
+}
+
 const string nixVersion = PACKAGE_VERSION;
 
 template<> void BaseSetting<SandboxMode>::set(const std::string & str)
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index ab1c09aa2..1221e4db7 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -353,6 +353,11 @@ public:
 
     Setting<Paths> pluginFiles{this, {}, "plugin-files",
         "Plugins to dynamically load at nix initialization time."};
+
+    Setting<Strings> experimentalFeatures{this, {}, "experimental-features",
+        "Experimental Nix features to enable."};
+
+    void requireExperimentalFeature(const std::string & name);
 };
 
 
diff --git a/src/nix/main.cc b/src/nix/main.cc
index a80fd0ea6..22f5e8c4c 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -138,6 +138,8 @@ void mainWrapped(int argc, char * * argv)
 
     args.parseCmdline(argvToStrings(argc, argv));
 
+    settings.requireExperimentalFeature("nix-command");
+
     initPlugins();
 
     if (!args.command) args.showHelpAndExit();