cargo-watch, stdenv: resolve aarch64-darwin objc redefinition errors

pkgs/build-support/cc-wrapper/cc-wrapper.sh

@@ -122,6 +122,31 @@ fi
 
 if [[ "$isCxx" = 1 ]]; then
     if [[ "$cxxInclude" = 1 ]]; then
+        #
+        # The motivation for this comment is to explain the reason for appending
+        # the C++ stdlib to NIX_CFLAGS_COMPILE, which I initially thought should
+        # change and later realized it shouldn't in:
+        #
+        #   https://github.com/NixOS/nixpkgs/pull/185569#issuecomment-1234959249
+        #
+        # NIX_CFLAGS_COMPILE contains dependencies added using "-isystem", and
+        # NIX_CXXSTDLIB_COMPILE adds the C++ stdlib using "-isystem". Appending
+        # NIX_CXXSTDLIB_COMPILE to NIX_CLAGS_COMPILE emulates this part of the
+        # include lookup order from GCC/Clang:
+        #
+        # > 4. Directories specified with -isystem options are scanned in
+        # >    left-to-right order.
+        # > 5. Standard system directories are scanned.
+        # > 6. Directories specified with -idirafter options are scanned
+        # >    in left-to-right order.
+        #
+        # NIX_CXX_STDLIB_COMPILE acts as the "standard system directories" that
+        # are otherwise missing from CC in nixpkgs, so should be added last.
+        #
+        # This means that the C standard library should never be present inside
+        # NIX_CFLAGS_COMPILE, because it MUST come after the C++ stdlib. It is
+        # added automatically by cc-wrapper later using "-idirafter".
+        #
         NIX_CFLAGS_COMPILE_@suffixSalt@+=" $NIX_CXXSTDLIB_COMPILE_@suffixSalt@"
     fi
     if [[ "$cxxLibrary" = 1 ]]; then

pkgs/os-specific/darwin/apple-sdk-11.0/apple_sdk.nix

@@ -168,7 +168,7 @@ in rec {
   bareFrameworks = (
     lib.mapAttrs framework (import ./frameworks.nix {
       inherit frameworks libs;
-      inherit (pkgs.darwin.apple_sdk_11_0) libnetwork Libsystem;
+      inherit (pkgs.darwin.apple_sdk_11_0) libnetwork;
       libobjc = pkgs.darwin.apple_sdk_11_0.objc4;
     })
   ) // (

pkgs/os-specific/darwin/apple-sdk-11.0/default.nix

@@ -50,7 +50,9 @@ let
     libcharset = callPackage ./libcharset.nix {};
     libunwind = callPackage ./libunwind.nix {};
     libnetwork = callPackage ./libnetwork.nix {};
-    objc4 = callPackage ./libobjc.nix {};
+    # Avoid introducing a new objc4 if stdenv already has one, to prevent
+    # conflicting LLVM modules.
+    objc4 = if stdenv ? objc4 then stdenv.objc4 else callPackage ./libobjc.nix {};
 
     # questionable aliases
     configd = pkgs.darwin.apple_sdk.frameworks.SystemConfiguration;

pkgs/os-specific/darwin/apple-sdk-11.0/frameworks.nix

@@ -1,4 +1,4 @@
-{ frameworks, libs, libobjc, Libsystem, libnetwork }: with frameworks; with libs;
+{ frameworks, libs, libobjc, libnetwork }: with frameworks; with libs;
 {
   AGL                              = { inherit Carbon OpenGL; };
   AVFoundation                     = { inherit ApplicationServices AVFCapture AVFCore CoreGraphics simd UniformTypeIdentifiers; };
@@ -89,9 +89,7 @@
   IOBluetooth                      = { inherit CoreBluetooth IOKit; };
   IOBluetoothUI                    = { inherit IOBluetooth; };
   IOKit                            = {};
-  # `IOSurface` should depend on `Libsystem` (in place of `xpc`) but this currently causes build
-  # issues due to incompatibility issues between `Libsystem` and `libcxx`.
-  IOSurface                        = { inherit IOKit xpc; };
+  IOSurface                        = { inherit IOKit; };
   IOUSBHost                        = {};
   IdentityLookup                   = {};
   ImageCaptureCore                 = {};

pkgs/stdenv/darwin/default.nix

@@ -698,6 +698,11 @@ rec {
         libc = pkgs.darwin.Libsystem;
         shellPackage = pkgs.bash;
         inherit bootstrapTools;
+      } // lib.optionalAttrs useAppleSDKLibs {
+        # This objc4 will be propagated to all builds using the final stdenv,
+        # and we shouldn't mix different builds, because they would be
+        # conflicting LLVM modules. Export it here so we can grab it later.
+        inherit (pkgs.darwin) objc4;
       };
 
       allowedRequisites = (with pkgs; [