document all special system features and their behavior

doc/manual/src/language/advanced-attributes.md

@@ -345,3 +345,15 @@ Derivations can declare some infrequently used optional attributes.
     This is useful, for example, when generating self-contained filesystem images with
     their own embedded Nix store: hashes found inside such an image refer
     to the embedded store and not to the host's Nix store.
+
+- [`requiredSystemFeatures`]{#adv-attr-requiredSystemFeatures}\
+
+  If a derivation has the `requiredSystemFeatures` attribute, then Nix will only build it on a machine that has the corresponding features set in its [`system-features` configuration](@docroot@/command-ref/conf-file.md#conf-system-features).
+
+  For example, setting
+
+  ```nix
+  requiredSystemFeatures = [ "kvm" ];
+  ```
+
+  ensures that the derivation can only be built on a machine with the `kvm` feature.

doc/manual/src/release-notes/rl-2.12.md

@@ -2,20 +2,8 @@
 
 * On Linux, Nix can now run builds in a user namespace where they run
   as root (UID 0) and have 65,536 UIDs available.
-  <!-- FIXME: move this to its own section about system features -->
-  This is primarily useful for running containers such as `systemd-nspawn`
-  inside a Nix build. For an example, see [`tests/systemd-nspawn/nix`][nspawn].
 
-  [nspawn]: https://github.com/NixOS/nix/blob/67bcb99700a0da1395fa063d7c6586740b304598/tests/systemd-nspawn.nix.
+  This can be used by requiring `uid-range` [system feature] in derivations.
-
-  A build can enable this by setting the derivation attribute:
-
-  ```
-  requiredSystemFeatures = [ "uid-range" ];
-  ```
-
-  The `uid-range` [system feature] requires the [`auto-allocate-uids`]
-  setting to be enabled.
 
   [system feature]: ../command-ref/conf-file.md#conf-system-features
 

src/libstore/globals.hh

@@ -337,7 +337,7 @@ public:
           users in `build-users-group`.
 
           UIDs are allocated starting at 872415232 (0x34000000) on Linux and 56930 on macOS.
-        )"};
+        )", {}, true, Xp::AutoAllocateUids};
 
     Setting<uint32_t> startId{this,
         #if __linux__
@@ -690,20 +690,37 @@ public:
         "system-features",
         R"(
           A set of system “features” supported by this machine.
-          Derivations can express a dependency on such features through the derivation attribute `requiredSystemFeatures`.
 
-          For example, the attribute
+          This complements the [`system`](#conf-system) and [`extra-platforms`](#conf-extra-platforms) configuration options and the corresponding [`system`](@docroot@/language/derivations.md#attr-system) attribute on derivations.
 
-              requiredSystemFeatures = [ "kvm" ];
+          Derivations can require system features in the derivation attribute [`requiredSystemFeatures`](@docroot@/language/advanced-attributes.md#adv-attr-requiredSystemFeatures).
 
-          ensures that the derivation can only be built on a machine with the `kvm` feature.
+          System features are generally user-defined, but the following have special treatment:
 
-          This setting by default includes
+          - `kvm`
-          - `kvm` if `/dev/kvm` is accessible
+
-          - historical pseudo-features for backwards compatibility, used in Nixpkgs to route Hydra builds to specific machines
+            Set by default if `/dev/kvm` is accessible.
-            - `nixos-test`
+
-            - `benchmark`
+          - `nixos-test`, `benchmark`, `big-parallel`
-            - `big-parallel`
+
+            These historical pseudo-features are always enabled for backwards compatibility, used in Nixpkgs to route Hydra builds to specific machines.
+
+          - `ca-derivations`
+
+            Set by default if the [`ca-derivations` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-ca-derivations) is enabled.
+
+          - `recursive-nix`
+
+            Set by default if the [`recursive-nix` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-recursive-nix) is enabled.
+
+          - `uid-range`
+
+            On Linux, Nix can run builds in a user namespace where they run as root (UID 0) and have 65,536 UIDs available.
+            This is primarily useful for running containers such as `systemd-nspawn` inside a Nix build. For an example, see [`tests/systemd-nspawn/nix`][nspawn].
+
+            [nspawn]: https://github.com/NixOS/nix/blob/67bcb99700a0da1395fa063d7c6586740b304598/tests/systemd-nspawn.nix.
+
+            Set by default on Linux if the [`auto-allocate-uids`](#conf-auto-allocate-uids) setting is enabled.
         )", {}, false};
 
     Setting<Strings> substituters{