Mark experimental configuration settings programmatically

Fix NixOS#8162

This is somewhat hard to observe until NixOS#8174 is merged, fixing the
accidental hiding of experimental features that aren't currently
enabled, but the fix is technically orthogonal.

doc/manual/utils.nix

@@ -42,7 +42,7 @@ rec {
   filterAttrs = pred: set:
     listToAttrs (concatMap (name: let v = set.${name}; in if pred name v then [(nameValuePair name v)] else []) (attrNames set));
 
-  showSetting = { useAnchors }: name: { description, documentDefault, defaultValue, aliases, value }:
+  showSetting = { useAnchors }: name: { description, documentDefault, defaultValue, aliases, value, experimental-feature }:
     let
       result = squash ''
           - ${if useAnchors
@@ -56,6 +56,21 @@ rec {
       body = ''
           ${description}
 
+        '' + (if experimental-feature != null then ''
+          > **Warning**
+          > This setting is part of an experimental feature.
+
+          To change this setting, you need to make sure the corresponding experimental feature,
+          [`${experimental-feature}`](@docroot@/contributing/experimental-features.md#xp-feature-${experimental-feature}),
+          is enabled.
+          For example, include the following in [`nix.conf`](#):
+
+          ```
+          extra-experimental-features = ${experimental-feature}
+          ${name} = ...
+          ```
+
+        '' else "") + ''
           **Default:** ${showDefault documentDefault defaultValue}
 
           ${showAliases aliases}

src/libstore/globals.hh

@@ -328,16 +328,6 @@ public:
           users in `build-users-group`.
 
           UIDs are allocated starting at 872415232 (0x34000000) on Linux and 56930 on macOS.
-
-          > **Warning**
-          > This is an experimental feature.
-
-          To enable it, add the following to [`nix.conf`](#):
-
-          ```
-          extra-experimental-features = auto-allocate-uids
-          auto-allocate-uids = true
-          ```
         )"};
 
     Setting<uint32_t> startId{this,
@@ -367,16 +357,6 @@ public:
 
           Cgroups are required and enabled automatically for derivations
           that require the `uid-range` system feature.
-
-          > **Warning**
-          > This is an experimental feature.
-
-          To enable it, add the following to [`nix.conf`](#):
-
-          ```
-          extra-experimental-features = cgroups
-          use-cgroups = true
-          ```
         )"};
     #endif
 

src/libutil/config.cc

@@ -191,6 +191,10 @@ std::map<std::string, nlohmann::json> AbstractSetting::toJSONObject()
     std::map<std::string, nlohmann::json> obj;
     obj.emplace("description", description);
     obj.emplace("aliases", aliases);
+    if (experimentalFeature)
+        obj.emplace("experimental-feature", *experimentalFeature);
+    else
+        obj.emplace("experimental-feature", nullptr);
     return obj;
 }