Stuctured command stability

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

In a future PR, we will adjust the manual to take advantage of this new
information in the JSON.
(It will be easier to do that once we have some experimental feature
docs to link too; see #5930 and #7798.)

src/libutil/args.cc

@@ -236,8 +236,6 @@ nlohmann::json Args::toJSON()
     auto flags = nlohmann::json::object();
 
     for (auto & [name, flag] : longFlags) {
-        /* Skip experimental flags when listing flags. */
-        if (!experimentalFeatureSettings.isEnabled(flag->experimentalFeature)) continue;
         auto j = nlohmann::json::object();
         if (flag->aliases.count(name)) continue;
         if (flag->shortName)
@@ -249,6 +247,11 @@ nlohmann::json Args::toJSON()
             j["arity"] = flag->handler.arity;
         if (!flag->labels.empty())
             j["labels"] = flag->labels;
+        // TODO With C++23 use `std::optional::tranform`
+        if (auto & xp = flag->experimentalFeature)
+            j["experimental-feature"] = showExperimentalFeature(*xp);
+        else
+            j["experimental-feature"] = nullptr;
         flags[name] = std::move(j);
     }
 
@@ -345,6 +348,11 @@ Strings argvToStrings(int argc, char * * argv)
     return args;
 }
 
+std::optional<ExperimentalFeature> Command::experimentalFeature ()
+{
+    return { Xp::NixCommand };
+}
+
 MultiCommand::MultiCommand(const Commands & commands_)
     : commands(commands_)
 {
@@ -408,6 +416,11 @@ nlohmann::json MultiCommand::toJSON()
         cat["id"] = command->category();
         cat["description"] = trim(categories[command->category()]);
         j["category"] = std::move(cat);
+        // TODO With C++23 use `std::optional::tranform`
+        if (auto xp = command->experimentalFeature())
+            cat["experimental-feature"] = showExperimentalFeature(*xp);
+        else
+            cat["experimental-feature"] = nullptr;
         cmds[name] = std::move(j);
     }
 

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 +39,14 @@ struct CmdDoctor : StoreCommand
 {
     bool success = true;
 
+    /**
+     * This command is stable before the others
+     */
+    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

@@ -423,10 +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 +12,14 @@ struct CmdRepl : RawInstallablesCommand
         evalSettings.pureEval = false;
     }
 
+    /**
+     * This command is stable before the others
+     */
+    std::optional<ExperimentalFeature> experimentalFeature() override
+    {
+        return std::nullopt;
+    }
+
     std::vector<std::string> files;
 
     Strings getDefaultFlakeAttrPaths() override

src/nix/upgrade-nix.cc

@@ -32,6 +32,14 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
         });
     }
 
+    /**
+     * This command is stable before the others
+     */
+    std::optional<ExperimentalFeature> experimentalFeature() override
+    {
+        return std::nullopt;
+    }
+
     std::string description() override
     {
         return "upgrade Nix to the stable version declared in Nixpkgs";