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.)

1 files changed, 8 insertions, 0 deletions

diff --git a/src/nix/upgrade-nix.cc b/src/nix/upgrade-nix.cc
index 17796d6b8..2295d86d0 100644
--- a/src/nix/upgrade-nix.cc
+++ b/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";