top-level: Make config-overriden stdenv bootstrap more normally

pkgs/stdenv/cross/default.nix

@@ -1,9 +1,13 @@
 { system, allPackages, platform, crossSystem, config, ... } @ args:
 
 rec {
-  vanillaStdenv = (import ../. (args // {
+  argClobber = {
     crossSystem = null;
-    allPackages = args: allPackages ({ crossSystem = null; } // args);
+    # Ignore custom stdenvs when cross compiling for compatability
+    config = builtins.removeAttrs config [ "replaceStdenv" ];
+  };
+  vanillaStdenv = (import ../. (args // argClobber // {
+    allPackages = args: allPackages (argClobber // args);
   })).stdenv;
 
   # Yeah this isn't so cleanly just build-time packages yet. Notice the

pkgs/stdenv/custom/default.nix

@@ -0,0 +1,17 @@
+{ system, allPackages, platform, crossSystem, config, ... } @ args:
+
+rec {
+  vanillaStdenv = (import ../. (args // {
+    # Remove config.replaceStdenv to ensure termination.
+    config = builtins.removeAttrs config [ "replaceStdenv" ];
+  })).stdenv;
+
+  buildPackages = allPackages {
+    # It's OK to change the built-time dependencies
+    allowCustomOverrides = true;
+    bootStdenv = vanillaStdenv;
+    inherit system platform crossSystem config;
+  };
+
+  stdenvCustom = config.replaceStdenv { pkgs = buildPackages; };
+}

pkgs/stdenv/default.nix

@@ -40,9 +40,12 @@ rec {
 
   inherit (import ./cross { inherit system allPackages platform crossSystem config lib; }) stdenvCross;
 
+  inherit (import ./custom { inherit system allPackages platform crossSystem config lib; }) stdenvCustom;
+
   # Select the appropriate stdenv for the platform `system'.
   stdenv =
     if crossSystem != null then stdenvCross else
+    if config ? replaceStdenv then stdenvCustom else
     if system == "i686-linux" then stdenvLinux else
     if system == "x86_64-linux" then stdenvLinux else
     if system == "armv5tel-linux" then stdenvLinux else

pkgs/top-level/stdenv.nix

@@ -9,16 +9,7 @@ rec {
   defaultStdenv = allStdenvs.stdenv // { inherit platform; };
 
   stdenv =
-    if bootStdenv != null then
-      (bootStdenv // { inherit platform; })
-    else if crossSystem == null && config ? replaceStdenv then
-      config.replaceStdenv {
-        # We import again all-packages to avoid recursivities.
-        pkgs = nixpkgsFun {
-          # We remove packageOverrides to avoid recursivities
-          config = removeAttrs config [ "replaceStdenv" ];
-        };
-      }
-    else
-      defaultStdenv;
+    if bootStdenv != null
+    then (bootStdenv // { inherit platform; })
+    else defaultStdenv;
 }