3 files changed, 30 insertions, 20 deletions
diff --git a/pkgs/top-level/default.nix b/pkgs/top-level/default.nix
index 04daf9778ff..a146dad63bc 100644
--- a/pkgs/top-level/default.nix
+++ b/pkgs/top-level/default.nix
@@ -23,6 +23,9 @@
 , # Allow a configuration attribute set to be passed in as an argument.
   config ? {}
 
+, # List of overlays layers used to extend Nixpkgs.
+  overlays ? []
+
 , # A function booting the final package set for a specific standard
   # environment. See below for the arguments given to that function,
   # the type of list it returns.
@@ -80,7 +83,7 @@ in let
   boot = import ../stdenv/booter.nix { inherit lib allPackages; };
 
   stages = stdenvStages {
-    inherit lib system platform crossSystem config;
+    inherit lib system platform crossSystem config overlays;
   };
 
   pkgs = boot stages;
diff --git a/pkgs/top-level/impure.nix b/pkgs/top-level/impure.nix
index e9066815927..1aa55f7293b 100644
--- a/pkgs/top-level/impure.nix
+++ b/pkgs/top-level/impure.nix
@@ -18,7 +18,24 @@
       else if homeDir != "" && pathExists configFile2 then import configFile2
       else {}
 
+, # Overlays are used to extend Nixpkgs collection with additional
+  # collections of packages.  These collection of packages are part of the
+  # fix-point made by Nixpkgs.
+  overlays ? let
+      inherit (builtins) getEnv pathExists readDir attrNames map sort
+        lessThan;
+      dirEnv = getEnv "NIXPKGS_OVERLAYS";
+      dirHome = (getEnv "HOME") + "/.nixpkgs/overlays";
+      dirCheck = dir: dir != "" && pathExists (dir + "/.");
+      overlays = dir:
+        let content = readDir dir; in
+        map (n: import "${dir}/${n}") (sort lessThan (attrNames content));
+    in
+      if dirCheck dirEnv then overlays dirEnv
+      else if dirCheck dirHome then overlays dirHome
+      else []
+
 , ...
 } @ args:
 
-import ./. (args // { inherit system config; })
+import ./. (args // { inherit system config overlays; })
diff --git a/pkgs/top-level/stage.nix b/pkgs/top-level/stage.nix
index ebc6473e425..cbf65870eb7 100644
--- a/pkgs/top-level/stage.nix
+++ b/pkgs/top-level/stage.nix
@@ -30,6 +30,8 @@
 , # The configuration attribute set
   config
 
+, overlays # List of overlays to use in the fix-point.
+
 , crossSystem
 , platform
 , lib
@@ -79,7 +81,7 @@ let
       ((config.packageOverrides or (super: {})) super);
 
   # The complete chain of package set builders, applied from top to bottom
-  toFix = lib.foldl' (lib.flip lib.extends) (self: {}) [
+  toFix = lib.foldl' (lib.flip lib.extends) (self: {}) ([
     stdenvBootstappingAndPlatforms
     stdenvAdapters
     trivialBuilders
@@ -87,20 +89,8 @@ let
     aliases
     stdenvOverrides
     configOverrides
-  ];
-
-  # Use `overridePackages` to easily override this package set.
-  # Warning: this function is very expensive and must not be used
-  # from within the nixpkgs repository.
-  #
-  # Example:
-  #  pkgs.overridePackages (self: super: {
-  #    foo = super.foo.override { ... };
-  #  }
-  #
-  # The result is `pkgs' where all the derivations depending on `foo'
-  # will use the new version.
-
-  # Return the complete set of packages. Warning: this function is very
-  # expensive!
-in lib.makeExtensibleWithCustomName "overridePackages" toFix
+  ] ++ overlays);
+
+in
+  # Return the complete set of packages.
+  lib.fix toFix