nixos-rebuild: Use /etc/nixos/flake.nix if it exists

nixos/doc/manual/man-nixos-rebuild.xml

@@ -140,14 +140,17 @@
   <title>Description</title>
 
   <para>
-   This command updates the system so that it corresponds to the configuration
-   specified in <filename>/etc/nixos/configuration.nix</filename>. Thus, every
-   time you modify <filename>/etc/nixos/configuration.nix</filename> or any
-   NixOS module, you must run <command>nixos-rebuild</command> to make the
-   changes take effect. It builds the new system in
-   <filename>/nix/store</filename>, runs its activation script, and stop and
-   (re)starts any system services if needed. Please note that user services need
-   to be started manually as they aren't detected by the activation script at the moment.
+   This command updates the system so that it corresponds to the
+   configuration specified in
+   <filename>/etc/nixos/configuration.nix</filename> or
+   <filename>/etc/nixos/flake.nix</filename>. Thus, every time you
+   modify the configuration or any other NixOS module, you must run
+   <command>nixos-rebuild</command> to make the changes take
+   effect. It builds the new system in
+   <filename>/nix/store</filename>, runs its activation script, and
+   stop and (re)starts any system services if needed. Please note that
+   user services need to be started manually as they aren't detected
+   by the activation script at the moment.
   </para>
 
   <para>
@@ -526,8 +529,10 @@
     </term>
     <listitem>
      <para>
-      Build the NixOS system from the specified flake. The flake must
-      contain an output named
+      Build the NixOS system from the specified flake. It defaults to
+      the directory containing the target of the symlink
+      <filename>/etc/nixos/flake.nix</filename>, if it exists. The
+      flake must contain an output named
       <literal>nixosConfigurations.<replaceable>name</replaceable></literal>,
       where <replaceable>name</replaceable> denotes the name of the
       configuration and can be specified using the
@@ -594,6 +599,21 @@
 
   <variablelist>
 
+   <varlistentry>
+    <term>
+     <filename>/etc/nixos/flake.nix</filename>
+    </term>
+    <listitem>
+     <para>
+      If this file exists, then <command>nixos-rebuild</command> will
+      use it as if the <option>--flake</option> option was given. This
+      file may be a symlink to a <filename>flake.nix</filename> in an
+      actual flake; thus <filename>/etc/nixos</filename> need not be a
+      flake.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term>
      <filename>/run/current-system</filename>

nixos/modules/installer/tools/nixos-rebuild.sh

@@ -236,6 +236,12 @@ if [ -z "$_NIXOS_REBUILD_REEXEC" ]; then
     export PATH=@nix@/bin:$PATH
 fi
 
+# Use /etc/nixos/flake.nix if it exists. It can be a symlink to the
+# actual flake.
+if [[ -z $flake && -e /etc/nixos/flake.nix ]]; then
+    flake="$(dirname "$(readlink -f /etc/nixos/flake.nix)")"
+fi
+
 # Re-execute nixos-rebuild from the Nixpkgs tree.
 # FIXME: get nixos-rebuild from $flake.
 if [[ -z $_NIXOS_REBUILD_REEXEC && -n $canRun && -z $fast && -z $flake ]]; then