basically, it creates a submodule for each of the unit-types (services, mounts, etc). The submodule has:

{
  options = {
    ${type} = mkOption {
      ...
    };
  };
  config = {
    units = convertToUnit config.${type};
  };
}

That means that such modules need to be merged to get to a systemd module. I'm not sure if nixos config is capable of this or whether I should just write it out, like systemd.nix and systemd-user.nix also do.

Is that pr-reusable-systemd-module branch (refactor vs reusable) the one you're actually planning to try to get merged?

Yes, though pr-refactor-systemd-module were the commits that I am confident that it will get merged in a form simliar to what it currently is.
pr-reusable-systemd-module might be taking things 'too far' in terms of reusabability. Even though we can use this in 3 places, they way it is using modules and merging will have a higher probability that a reviewer will reject the change.

Ok... I'm just gonna keep truckin on the refactor branch for now then :P I did add a smaller unification of my own by placing the types into a common place, at least.

diff --git a/nixos/lib/systemd-types.nix b/nixos/lib/systemd-types.nix
new file mode 100644
index 00000000000..b303335ffc1
--- /dev/null
+++ b/nixos/lib/systemd-types.nix
@@ -0,0 +1,30 @@
+{ lib, systemdUtils }:
+
+with systemdUtils.lib;
+with systemdUtils.unitOptions;
+with lib;
+
+rec {
+  units = with types;
+    attrsOf (submodule ({ name, config, ... }: {
+      options = concreteUnitOptions;
+      config = { unit = mkDefault (systemdUtils.lib.makeUnit name config); };
+    }));
+
+  services = with types; attrsOf (submodule [ { options = serviceOptions; } unitConfig serviceConfig ]);
+  initrdServices = with types; attrsOf (submodule [ { options = serviceOptions; } unitConfig initrdServiceConfig ]);
+
+  targets = with types; attrsOf (submodule [ { options = targetOptions; } unitConfig ]);
+
+  sockets = with types; attrsOf (submodule [ { options = socketOptions; } unitConfig ]);
+
+  timers = with types; attrsOf (submodule [ { options = timerOptions; } unitConfig ]);
+
+  paths = with types; attrsOf (submodule [ { options = pathOptions; } unitConfig ]);
+
+  slices = with types; attrsOf (submodule [ { options = sliceOptions; } unitConfig ]);
+
+  mounts = with types; listOf (submodule [ { options = mountOptions; } unitConfig mountConfig ]);
+
+  automounts = with types; listOf (submodule [ { options = automountOptions; } unitConfig automountConfig ]);
+}
diff --git a/nixos/lib/utils.nix b/nixos/lib/utils.nix
index ae68c3920c5..80341dd48fc 100644
--- a/nixos/lib/utils.nix
+++ b/nixos/lib/utils.nix
@@ -197,5 +197,6 @@ rec {
   systemdUtils = {
     lib = import ./systemd-lib.nix { inherit lib config pkgs; };
     unitOptions = import ./systemd-unit-options.nix { inherit lib systemdUtils; };
+    types = import ./systemd-types.nix { inherit lib systemdUtils; };
   };
 }
diff --git a/nixos/modules/system/boot/systemd.nix b/nixos/modules/system/boot/systemd.nix
index 057474c607a..396457e1a9f 100644
--- a/nixos/modules/system/boot/systemd.nix
+++ b/nixos/modules/system/boot/systemd.nix
@@ -14,10 +14,6 @@ let
     makeUnit
     generateUnits
     makeJobScript
-    unitConfig
-    serviceConfig
-    mountConfig
-    automountConfig
     commonUnitText
     targetToUnit
     serviceToUnit
@@ -185,13 +181,7 @@ in
     systemd.units = mkOption {
       description = "Definition of systemd units.";
       default = {};
-      type = with types; attrsOf (submodule (
-        { name, config, ... }:
-        { options = concreteUnitOptions;
-          config = {
-            unit = mkDefault (makeUnit name config);
-          };
-        }));
+      type = systemdUtils.types.units;
     };
 
     systemd.packages = mkOption {
@@ -203,37 +193,37 @@ in
 
     systemd.targets = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = targetOptions; } unitConfig] );
+      type = systemdUtils.types.targets;
       description = "Definition of systemd target units.";
     };
 
     systemd.services = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = serviceOptions; } unitConfig serviceConfig ]);
+      type = systemdUtils.types.services;
       description = "Definition of systemd service units.";
     };
 
     systemd.sockets = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = socketOptions; } unitConfig ]);
+      type = systemdUtils.types.sockets;
       description = "Definition of systemd socket units.";
     };
 
     systemd.timers = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = timerOptions; } unitConfig ]);
+      type = systemdUtils.types.timers;
       description = "Definition of systemd timer units.";
     };
 
     systemd.paths = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = pathOptions; } unitConfig ]);
+      type = systemdUtils.types.paths;
       description = "Definition of systemd path units.";
     };
 
     systemd.mounts = mkOption {
       default = [];
-      type = with types; listOf (submodule [ { options = mountOptions; } unitConfig mountConfig ]);
+      type = systemdUtils.types.mounts;
       description = ''
         Definition of systemd mount units.
         This is a list instead of an attrSet, because systemd mandates the names to be derived from
@@ -243,7 +233,7 @@ in
 
     systemd.automounts = mkOption {
       default = [];
-      type = with types; listOf (submodule [ { options = automountOptions; } unitConfig automountConfig ]);
+      type = systemdUtils.types.automounts;
       description = ''
         Definition of systemd automount units.
         This is a list instead of an attrSet, because systemd mandates the names to be derived from
@@ -253,7 +243,7 @@ in
 
     systemd.slices = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = sliceOptions; } unitConfig] );
+      type = systemdUtils.types.slices;
       description = "Definition of slice configurations.";
     };
 
diff --git a/nixos/modules/system/boot/systemd/user.nix b/nixos/modules/system/boot/systemd/user.nix
index 6f63292bf9f..c04933263ac 100644
--- a/nixos/modules/system/boot/systemd/user.nix
+++ b/nixos/modules/system/boot/systemd/user.nix
@@ -12,10 +12,6 @@ let
     (systemdUtils.lib)
     makeUnit
     generateUnits
-    makeJobScript
-    unitConfig
-    serviceConfig
-    commonUnitText
     targetToUnit
     serviceToUnit
     socketToUnit
@@ -57,48 +53,42 @@ in {
     systemd.user.units = mkOption {
       description = "Definition of systemd per-user units.";
       default = {};
-      type = with types; attrsOf (submodule (
-        { name, config, ... }:
-        { options = concreteUnitOptions;
-          config = {
-            unit = mkDefault (makeUnit name config);
-          };
-        }));
+      type = systemdUtils.types.units;
     };
 
     systemd.user.paths = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = pathOptions; } unitConfig ]);
+      type = systemdUtils.types.paths;
       description = "Definition of systemd per-user path units.";
     };
 
     systemd.user.services = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = serviceOptions; } unitConfig serviceConfig ] );
+      type = systemdUtils.types.services;
       description = "Definition of systemd per-user service units.";
     };
 
     systemd.user.slices = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = sliceOptions; } unitConfig ] );
+      type = systemdUtils.types.slices;
       description = "Definition of systemd per-user slice units.";
     };
 
     systemd.user.sockets = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = socketOptions; } unitConfig ] );
+      type = systemdUtils.types.sockets;
       description = "Definition of systemd per-user socket units.";
     };
 
     systemd.user.targets = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = targetOptions; } unitConfig] );
+      type = systemdUtils.types.targets;
       description = "Definition of systemd per-user target units.";
     };
 
     systemd.user.timers = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = timerOptions; } unitConfig ] );
+      type = systemdUtils.types.timers;
       description = "Definition of systemd per-user timer units.";
     };
 

ah, I think you refer to:

https://github.com/nixos/nixpkgs/blob/9bc093b30a3ab38f9e5b30a132b08a9fd9b27ade/nixos/lib/systemd-lib.nix#L299

That should refer to a new option config.systemd.path, which for boot/systemd.nix and boot/systemd/user.nix should refer to config.path.