Stuctured command stability, reflect with --help

Prior to this, there was an ad-hoc whitelist in `main.cc`. Now, every
command states its stability.

The big user-facing benefit is that we can now hide disabled unstable
command from `--help`, just as we hide similar such CLI arguments,
experimental features, and builtins.

src/libutil/args.cc

@@ -345,6 +345,11 @@ Strings argvToStrings(int argc, char * * argv)
     return args;
 }
 
+std::optional<ExperimentalFeature> Command::experimentalFeature ()
+{
+    return { Xp::NixCommand };
+}
+
 MultiCommand::MultiCommand(const Commands & commands_)
     : commands(commands_)
 {
@@ -403,6 +408,9 @@ nlohmann::json MultiCommand::toJSON()
 
     for (auto & [name, commandFun] : commands) {
         auto command = commandFun();
+        auto xp = command->experimentalFeature();
+        if (!experimentalFeatureSettings.isEnabled(xp))
+            continue;
         auto j = command->toJSON();
         auto cat = nlohmann::json::object();
         cat["id"] = command->category();

src/libutil/args.hh

@@ -236,6 +236,8 @@ struct Command : virtual public Args
 
     static constexpr Category catDefault = 0;
 
+    virtual std::optional<ExperimentalFeature> experimentalFeature ();
+
     virtual Category category() { return catDefault; }
 };
 

src/nix/doctor.cc

@@ -39,6 +39,11 @@ struct CmdDoctor : StoreCommand
 {
     bool success = true;
 
+    std::optional<ExperimentalFeature> experimentalFeature() override
+    {
+        return std::nullopt;
+    }
+
     std::string description() override
     {
         return "check your system for potential problems and print a PASS or FAIL for each check";

src/nix/main.cc

@@ -344,6 +344,9 @@ void mainWrapped(int argc, char * * argv)
     NixArgs args;
 
     if (argc == 2 && std::string(argv[1]) == "__dump-cli") {
+        experimentalFeatureSettings.experimentalFeatures = {
+            Xp::NixCommand,
+        };
         logger->cout(args.dumpCli());
         return;
     }
@@ -420,10 +423,8 @@ void mainWrapped(int argc, char * * argv)
     if (!args.command)
         throw UsageError("no subcommand specified");
 
-    if (args.command->first != "repl"
-        && args.command->first != "doctor"
-        && args.command->first != "upgrade-nix")
-        experimentalFeatureSettings.require(Xp::NixCommand);
+    experimentalFeatureSettings.require(
+        args.command->second->experimentalFeature());
 
     if (args.useNet && !haveInternet()) {
         warn("you don't have Internet access; disabling some network-dependent features");

src/nix/repl.cc

@@ -12,6 +12,11 @@ struct CmdRepl : RawInstallablesCommand
         evalSettings.pureEval = false;
     }
 
+    std::optional<ExperimentalFeature> experimentalFeature() override
+    {
+        return std::nullopt;
+    }
+
     std::vector<std::string> files;
 
     Strings getDefaultFlakeAttrPaths() override

src/nix/upgrade-nix.cc

@@ -32,6 +32,11 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
         });
     }
 
+    std::optional<ExperimentalFeature> experimentalFeature() override
+    {
+        return std::nullopt;
+    }
+
     std::string description() override
     {
         return "upgrade Nix to the stable version declared in Nixpkgs";