diff --git a/lib/release-tools.nix b/lib/release-tools.nix
index 8101c884..61e4fe3d 100644
--- a/lib/release-tools.nix
+++ b/lib/release-tools.nix
@@ -1,19 +1,8 @@
 { pkgs ? import <nixpkgs> {} }: 
 
 let
-
   # Original `evalConfig`
-  evalConfig' = import "${pkgs.path}/nixos/lib/eval-config.nix";
-
-  # Specialized `evalConfig` which injects the _mobile-nixos special arg.
-  evalConfig = args@{ specialArgs ? {}, ... }: evalConfig' (args // {
-    specialArgs = specialArgs // {
-      _mobile-nixos = {
-        inherit evalConfig;
-        path = ../.;
-      };
-    };
-  });
+  evalConfig = import "${toString pkgs.path}/nixos/lib/eval-config.nix";
 in
 {
   # This should *never* rely on lib or pkgs.
diff --git a/modules/lib.nix b/modules/lib.nix
new file mode 100644
index 00000000..664515eb
--- /dev/null
+++ b/modules/lib.nix
@@ -0,0 +1,35 @@
+{ config, lib, pkgs, baseModules, modules, ... }:
+
+let
+  # Keep modules from this eval around
+  modules' = modules;
+
+  inherit (config.nixpkgs.localSystem) system;
+
+  # We can make use the normal NixOS evalConfig here.
+  evalConfig = import "${toString pkgs.path}/nixos/lib/eval-config.nix";
+in
+{
+  lib = {
+    mobile-nixos = {
+      # `config.lib.mobile-nixos.composeConfig` is the supported method used to
+      # re-evaluate a configuration with additional configuration.
+      # Can be used exactly like `evalConfig`, with one additional param.
+      # The `config` param directly takes a module (attrset or function).
+      composeConfig = { config ? {}, modules ? [], ... }@args:
+      let
+        filteredArgs = lib.filterAttrs (k: v: k != "config") args;
+      in
+      evalConfig (
+        filteredArgs // {
+        # Needed for hermetic eval, otherwise `eval-config.nix` will try
+        # to use `builtins.currentSystem`.
+        inherit system;
+        inherit baseModules;
+        # Merge in this eval's modules with the argument's modules, and finally
+        # with the given config.
+        modules = modules' ++ modules ++ [ config ];
+      });
+    };
+  };
+}
diff --git a/modules/module-list.nix b/modules/module-list.nix
index 7bb96356..2c99e53d 100644
--- a/modules/module-list.nix
+++ b/modules/module-list.nix
@@ -32,6 +32,7 @@
   ./initrd-vendor.nix
   ./initrd.nix
   ./internal.nix
+  ./lib.nix
   ./luks.nix
   ./mobile-device.nix
   ./nixpkgs.nix