haskell.compiler.ghc921: provide xattr on darwin

GHC 9.2.1-rc1 needs to run xattr in ghc.mk unconditionally. The fix for
this and support for the XATTR environment variable have only been added
to the GHC 8.10 series so far.

pkgs/development/compilers/ghc/9.2.1.nix

@@ -2,7 +2,7 @@
 
 # build-tools
 , bootPkgs
-, autoconf, automake, coreutils, fetchurl, perl, python3, m4, sphinx
+, autoconf, automake, coreutils, fetchurl, perl, python3, m4, sphinx, xattr
 , bash
 
 , libiconv ? null, ncurses
@@ -238,6 +238,10 @@ stdenv.mkDerivation (rec {
     ghc bootPkgs.alex bootPkgs.happy bootPkgs.hscolour
   ] ++ lib.optionals enableDocs [
     sphinx
+  ] ++ lib.optionals stdenv.isDarwin [
+    # TODO(@sternenseemann): use XATTR env var after backport of
+    # https://gitlab.haskell.org/ghc/ghc/-/merge_requests/6447
+    xattr
   ];
 
   # For building runtime libs

pkgs/top-level/haskell-packages.nix

@@ -120,6 +120,10 @@ in {
         else
           packages.ghc8102Binary;
       inherit (buildPackages.python3Packages) sphinx;
+      # Need to use apple's patched xattr until
+      # https://github.com/xattr/xattr/issues/44 and
+      # https://github.com/xattr/xattr/issues/55 are solved.
+      inherit (buildPackages.darwin) xattr;
       buildLlvmPackages = buildPackages.llvmPackages_10;
       llvmPackages = pkgs.llvmPackages_10;
     };