all-packages: Rewrite packageOverrides to use fix'/extends

This way is cleaner, slightly faster (as shown by
some quick tests) and causes no rebuilds.

It could cause some evaluation errors though.

pkgs/top-level/all-packages.nix

@@ -64,7 +64,7 @@ let
       # { /* the config */ } and
       # { pkgs, ... } : { /* the config */ }
       if builtins.isFunction configExpr
-        then configExpr { inherit pkgs; }
+        then configExpr { pkgs = pkgsFinal; }
         else configExpr;
 
   # Allow setting the platform in the config file. Otherwise, let's use a reasonable default (pc)
@@ -83,51 +83,38 @@ let
   platform = if platform_ != null then platform_
     else config.platform or platformAuto;
 
-  # Helper functions that are exported through `pkgs'.
+  # The complete set of packages, after applying the overrides
-  helperFunctions =
+  pkgsFinal = lib.fix' (lib.extends configOverrides (lib.extends stdenvOverrides pkgsFun));
-    stdenvAdapters //
-    (import ../build-support/trivial-builders.nix { inherit lib; inherit (pkgs) stdenv; inherit (pkgs.xorg) lndir; });
 
-  stdenvAdapters =
+  stdenvOverrides =
-    import ../stdenv/adapters.nix pkgs;
+    # We don't want stdenv overrides in the case of cross-building,
+    # or otherwise the basic overrided packages will not be built
+    # with the crossStdenv adapter.
+    if crossSystem == null
+      then self: super: lib.optionalAttrs (super.stdenv ? overrides) (super.stdenv.overrides super)
+      else self: super: {};
 
-
+  # Packages can be overriden globally via the `packageOverrides'
-  # Allow packages to be overriden globally via the `packageOverrides'
   # configuration option, which must be a function that takes `pkgs'
   # as an argument and returns a set of new or overriden packages.
-  # The `packageOverrides' function is called with the *original*
+  # The recommended usage follows this snippet:
-  # (un-overriden) set of packages, allowing packageOverrides
+  #   packageOverrides = super: let self = super.pkgs in ...
-  # attributes to refer to the original attributes (e.g. "foo =
+  # `super' is the *original* (un-overriden) set of packages,
-  # ... pkgs.foo ...").
+  # while `self' refers to the final (overriden) set of packages.
-  pkgs = applyGlobalOverrides (config.packageOverrides or (pkgs: {}));
+  configOverrides =
-
+      if config ? packageOverrides && bootStdenv == null # don't apply config overrides in stdenv boot
-  mkOverrides = pkgsOrig: overrides: overrides //
+        then self: config.packageOverrides
-        (lib.optionalAttrs (pkgsOrig.stdenv ? overrides && crossSystem == null) (pkgsOrig.stdenv.overrides pkgsOrig));
+        else self: super: {};
-
-  # Return the complete set of packages, after applying the overrides
-  # returned by the `overrider' function (see above).  Warning: this
-  # function is very expensive!
-  applyGlobalOverrides = overrider:
-    let
-      # Call the overrider function.  We don't want stdenv overrides
-      # in the case of cross-building, or otherwise the basic
-      # overrided packages will not be built with the crossStdenv
-      # adapter.
-      overrides = mkOverrides pkgsOrig (overrider pkgsOrig);
-
-      # The un-overriden packages, passed to `overrider'.
-      pkgsOrig = pkgsFun pkgs {};
-
-      # The overriden, final packages.
-      pkgs = pkgsFun pkgs overrides;
-    in pkgs;
-
 
   # The package compositions.  Yes, this isn't properly indented.
-  pkgsFun = pkgs: overrides:
+  pkgsFun = pkgs:
-    with helperFunctions;
+    let defaultScope = pkgs // pkgs.xorg;
-    let defaultScope = pkgs // pkgs.xorg; self = self_ // overrides;
+        helperFunctions = pkgs_.stdenvAdapters // pkgs_.trivial-builders;
-    self_ = with self; helperFunctions // {
+        pkgsRet = helperFunctions // pkgs_;
+        pkgs_ = with pkgs; {
+  # Helper functions that are exported through `pkgs'.
+  trivial-builders = import ../build-support/trivial-builders.nix { inherit lib; inherit stdenv; inherit (xorg) lndir; };
+  stdenvAdapters = import ../stdenv/adapters.nix pkgs;
 
   # Make some arguments passed to all-packages.nix available
   inherit system platform;
@@ -157,11 +144,7 @@ let
   #
   # The result is `pkgs' where all the derivations depending on `foo'
   # will use the new version.
-  overridePackages = f:
+  overridePackages = f: lib.fix' (lib.extends f pkgs.__unfix__);
-    let
-      newpkgs = pkgsFun newpkgs overrides;
-      overrides = mkOverrides pkgs (f newpkgs pkgs);
-    in newpkgs;
 
   # Override system. This is useful to build i686 packages on x86_64-linux.
   forceSystem = system: kernel: (import ./all-packages.nix) {
@@ -183,7 +166,7 @@ let
 
 
   ### Helper functions.
-  inherit lib config stdenvAdapters;
+  inherit lib config;
 
   inherit (lib) lowPrio hiPrio appendToName makeOverridable;
   inherit (misc) versionedDerivation;
@@ -214,7 +197,8 @@ let
     allPackages = args: import ./all-packages.nix ({ inherit config system; } // args);
   };
 
-  defaultStdenv = allStdenvs.stdenv // { inherit platform; };
+  # We use pkgs_ because accessing pkgs would lead to an infinite recursion in stdenvOverrides
+  defaultStdenv = pkgs_.allStdenvs.stdenv // { inherit platform; };
 
   stdenvCross = lowPrio (makeStdenvCross defaultStdenv crossSystem binutilsCross gccCrossStageFinal);
 
@@ -234,7 +218,7 @@ let
             };
           }
       else
-        defaultStdenv;
+        pkgs_.defaultStdenv;
 
   forceNativeDrv = drv : if crossSystem == null then drv else
     (drv // { crossDrv = drv.nativeDrv; });
@@ -5927,8 +5911,8 @@ let
   gotty = goPackages.gotty.bin // { outputs = [ "bin" ]; };
 
   gradleGen = callPackage ../development/tools/build-managers/gradle { };
-  gradle = self.gradleGen.gradleLatest;
+  gradle = gradleGen.gradleLatest;
-  gradle25 = self.gradleGen.gradle25;
+  gradle25 = gradleGen.gradle25;
 
   gperf = callPackage ../development/tools/misc/gperf { };
 
@@ -11761,7 +11745,7 @@ let
       AppKit Carbon Cocoa IOKit OSAKit Quartz QuartzCore WebKit
       ImageCaptureCore GSS ImageIO;
   });
-  emacs24Macport = self.emacs24Macport_24_5;
+  emacs24Macport = emacs24Macport_24_5;
 
   emacs25pre = lowPrio (callPackage ../applications/editors/emacs-25 {
     # use override to enable additional features
@@ -16103,12 +16087,13 @@ let
 
   mg = callPackage ../applications/editors/mg { };
 
-}; # self_ =
+};
+# end pkgs_ =
 
 
   ### Deprecated aliases - for backward compatibility
 
-aliases = with self; rec {
+aliases = with pkgs; {
   accounts-qt = qt5.accounts-qt;  # added 2015-12-19
   adobeReader = adobe-reader;
   arduino_core = arduino-core;  # added 2015-02-04
@@ -16195,4 +16180,7 @@ tweakAlias = _n: alias: with lib;
     removeAttrs alias ["recurseForDerivations"]
   else alias;
 
-in lib.mapAttrs tweakAlias aliases // self; in pkgs
+in lib.mapAttrs tweakAlias aliases // pkgsRet;
+# end pkgsFun
+
+in pkgsFinal