systemd: add slice support

nixos/modules/system/boot/systemd-unit-options.nix

@@ -465,4 +465,20 @@ in rec {
 
   targetOptions = commonUnitOptions;
 
+  sliceOptions = commonUnitOptions // {
+
+    sliceConfig = mkOption {
+      default = {};
+      example = { MemoryMax = "2G"; };
+      type = types.attrsOf unitOption;
+      description = ''
+        Each attribute in this set specifies an option in the
+        <literal>[Slice]</literal> section of the unit.  See
+        <citerefentry><refentrytitle>systemd.slice</refentrytitle>
+        <manvolnum>5</manvolnum></citerefentry> for details.
+      '';
+    };
+
+  };
+
 }

nixos/modules/system/boot/systemd.nix

@@ -375,6 +375,15 @@ let
         '';
     };
 
+  sliceToUnit = name: def:
+    { inherit (def) wantedBy requiredBy enable;
+      text = commonUnitText def +
+        ''
+          [Slice]
+          ${attrsToSection def.sliceConfig}
+        '';
+    };
+
 in
 
 {
@@ -458,6 +467,12 @@ in
       '';
     };
 
+    systemd.slices = mkOption {
+      default = {};
+      type = with types; attrsOf (submodule [ { options = sliceOptions; } unitConfig] );
+      description = "Definition of slice configurations.";
+    };
+
     systemd.generators = mkOption {
       type = types.attrsOf types.path;
       default = {};
@@ -748,6 +763,7 @@ in
       // mapAttrs' (n: v: nameValuePair "${n}.socket" (socketToUnit n v)) cfg.sockets
       // mapAttrs' (n: v: nameValuePair "${n}.timer" (timerToUnit n v)) cfg.timers
       // mapAttrs' (n: v: nameValuePair "${n}.path" (pathToUnit n v)) cfg.paths
+      // mapAttrs' (n: v: nameValuePair "${n}.slice" (sliceToUnit n v)) cfg.slices
       // listToAttrs (map
                    (v: let n = escapeSystemdPath v.where;
                        in nameValuePair "${n}.mount" (mountToUnit n v)) cfg.mounts)