diff --git a/pkgs/development/compilers/llvm/3.3/clang.nix b/pkgs/development/compilers/llvm/3.3/clang.nix
index 72287560b245..262e0bff2483 100644
--- a/pkgs/development/compilers/llvm/3.3/clang.nix
+++ b/pkgs/development/compilers/llvm/3.3/clang.nix
@@ -1,9 +1,5 @@
 { stdenv, fetchurl, perl, groff, llvm, cmake, libxml2, python }:
 
-# be sure not to rebuild clang on darwin; some packages request it specifically
-# we need to fix those
-assert stdenv.isDarwin -> stdenv.gcc.nativeTools;
-
 let
   version = "3.3";
   gccReal = if (stdenv.gcc.gcc or null) == null then stdenv.gcc else stdenv.gcc.gcc;
diff --git a/pkgs/development/compilers/llvm/3.4/clang.nix b/pkgs/development/compilers/llvm/3.4/clang.nix
index 6ec3f7bf44a3..fc33a7809a6c 100644
--- a/pkgs/development/compilers/llvm/3.4/clang.nix
+++ b/pkgs/development/compilers/llvm/3.4/clang.nix
@@ -1,5 +1,9 @@
 { stdenv, fetch, cmake, libxml2, libedit, llvm, version, clang-tools-extra_src }:
 
+# be sure not to rebuild clang on darwin; some packages request it specifically
+# we need to fix those
+assert stdenv.isDarwin -> stdenv.gcc.nativeTools;
+
 stdenv.mkDerivation {
   name = "clang-${version}";
 
diff --git a/pkgs/stdenv/darwin/default.nix b/pkgs/stdenv/darwin/default.nix
index 28c863b63b0b..f42cd7139d91 100644
--- a/pkgs/stdenv/darwin/default.nix
+++ b/pkgs/stdenv/darwin/default.nix
@@ -1,4 +1,6 @@
-{ stdenv, pkgs, config, haveLibCxx }:
+{ stdenv, pkgs, config
+, haveLibCxx ? true
+, useClang33 ? false }:
 
 import ../generic rec {
   inherit config;
@@ -33,7 +35,7 @@ import ../generic rec {
       };
     } else null;
     binutils = import ../../build-support/native-darwin-cctools-wrapper {inherit stdenv;};
-    clang = pkgs.clang_34;
+    clang = if useClang33 then pkgs.clang_33 else pkgs.clang;
     coreutils = pkgs.coreutils;
     shell = pkgs.bash + "/bin/sh";
   };
diff --git a/pkgs/stdenv/default.nix b/pkgs/stdenv/default.nix
index 665cefbfc43b..674f5585c0b9 100644
--- a/pkgs/stdenv/default.nix
+++ b/pkgs/stdenv/default.nix
@@ -37,7 +37,6 @@ rec {
     inherit config;
     stdenv = stdenvNative;
     pkgs = stdenvNativePkgs;
-    haveLibCxx = true;
   };
 
   stdenvDarwinNaked = import ./darwin {
@@ -47,6 +46,13 @@ rec {
     haveLibCxx = false;
   };
 
+  stdenvDarwin33 = import ./darwin {
+    inherit config;
+    stdenv = stdenvNative;
+    pkgs = stdenvNativePkgs;
+    useClang33 = true;
+  };
+
 
   # Linux standard environment.
   stdenvLinux = (import ./linux { inherit system allPackages platform config;}).stdenvLinux;
diff --git a/pkgs/tools/package-management/nix/unstable.nix b/pkgs/tools/package-management/nix/unstable.nix
index d5aebe45dd64..514924325e4a 100644
--- a/pkgs/tools/package-management/nix/unstable.nix
+++ b/pkgs/tools/package-management/nix/unstable.nix
@@ -24,6 +24,7 @@ stdenv.mkDerivation rec {
   postUnpack =
     '' export CPATH="${bzip2}/include"
        export LIBRARY_PATH="${bzip2}/lib"
+       export CXXFLAGS="-O3 -Wno-error=reserved-user-defined-literal"
     '';
 
   configureFlags =
@@ -68,9 +69,6 @@ stdenv.mkDerivation rec {
   enableParallelBuilding = true;
 
   meta = {
-    # due to builder args bug; see
-    # https://github.com/NixOS/nix/commit/b224ac15201c57b40ea855f5a98b1bd166c1c7f6
-    broken = stdenv.isDarwin;
     description = "The Nix Deployment System";
     homepage = http://nixos.org/;
     license = stdenv.lib.licenses.lgpl2Plus;
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index 49b1cd9b1801..22d35e03d594 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -3209,11 +3209,7 @@ let
 
   llvm_v = path: callPackage path { };
 
-  llvmPackages = if !stdenv.isDarwin then llvmPackages_34 else llvmPackages_34 // {
-    # until someone solves build problems with _34
-    llvm = llvm_33;
-    clang = clang_33;
-  };
+  llvmPackages = llvmPackages_34;
 
   llvmPackages_34 = recurseIntoAttrs (import ../development/compilers/llvm/3.4 {
     inherit stdenv newScope fetchurl;
@@ -11455,6 +11451,7 @@ let
   };
 
   nixUnstable = callPackage ../tools/package-management/nix/unstable.nix {
+    stdenv = if stdenv.isDarwin then allStdenvs.stdenvDarwin33 else stdenv;
     storeDir = config.nix.storeDir or "/nix/store";
     stateDir = config.nix.stateDir or "/nix/var";
   };