bazel: add cross compiler for darwin

This allows cross compiling from aarch64-darwin to x86_64-linux.
It is required for building Go binaries on macos that target Linux and have CGO enabled.

WORKSPACE.bazel

@@ -17,8 +17,8 @@ load("@io_tweag_rules_nixpkgs//nixpkgs:nixpkgs.bzl", "nixpkgs_cc_configure", "ni
 
 nixpkgs_git_repository(
     name = "nixpkgs",
-    revision = "ec750fd01963ab6b20ee1f0cb488754e8036d89d",
-    sha256 = "9e809097a0c66334139cda3d1bec3bf73bdfad593e954259ff329c7b4a118041",
+    revision = "85306ef2470ba705c97ce72741d56e42d0264015",
+    sha256 = "adbbcfd49b5180e51e2971626cafb14123e3ec06c18fa143b1f386b029081f12",
 )
 
 nixpkgs_flake_package(
@@ -185,9 +185,33 @@ zig_toolchains()
 
 nixpkgs_cc_configure(
     name = "nixpkgs_cc_toolchain",
+    # TODO(malt3): Use clang once cc-wrapper path reset bug is fixed upstream.
+    # attribute_path = "clang_11",
     repository = "@nixpkgs",
 )
 
+nixpkgs_cc_configure(
+    name = "nixpkgs_cc_aarch64_darwin_x86_64_linux",
+    cross_cpu = "k8",
+    exec_constraints = [
+        "@platforms//os:osx",
+        "@platforms//cpu:arm64",
+    ],
+    nix_file = "//nix/toolchains:cc_cross_darwin_x86_64_linux.nix",
+    nixopts = [
+        "--arg",
+        "ccPkgs",
+        "import <nixpkgs> { crossSystem = \"x86_64-linux\";}",
+        "--show-trace",
+    ],
+    repository = "@nixpkgs",
+    target_constraints = [
+        "@platforms//cpu:x86_64",
+        "@platforms//os:linux",
+        "@rules_nixpkgs_core//constraints:support_nix",
+    ],
+)
+
 register_toolchains(
     "@zig_sdk//libc_aware/toolchain:linux_amd64_gnu.2.23",
     "@zig_sdk//libc_aware/toolchain:linux_arm64_gnu.2.23",

flake.lock

@@ -20,11 +20,11 @@
     },
     "nixpkgsUnstable": {
       "locked": {
-        "lastModified": 1699343069,
-        "narHash": "sha256-s7BBhyLA6MI6FuJgs4F/SgpntHBzz40/qV0xLPW6A1Q=",
+        "lastModified": 1701237617,
+        "narHash": "sha256-Ryd8xpNDY9MJnBFDYhB37XSFIxCPVVVXAbInNPa95vs=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "ec750fd01963ab6b20ee1f0cb488754e8036d89d",
+        "rev": "85306ef2470ba705c97ce72741d56e42d0264015",
         "type": "github"
       },
       "original": {

nix/toolchains/cc_cross_darwin_x86_64_linux.nix

@@ -0,0 +1,63 @@
+# cross compiler toolchain for cc rules in Bazel
+# execution platform: aarch64-darwin
+# target platform: x86_64-linux
+# inspired by https://github.com/tweag/rules_nixpkgs/blob/21c4ea481021cb51a6e5d0969b2cee03dba5a637/examples/toolchains/cc_cross_osx_to_linux_amd64/toolchains/osxcross_cc.nix
+let
+  targetSystem = "x86_64-linux";
+  og = import <nixpkgs> { };
+  nixpkgs = import <nixpkgs> {
+    buildSystem = builtins.currentSystem;
+    hostSystem = targetSystem;
+    crossSystem = {
+      config = targetSystem;
+    };
+    crossOverlays = [
+      (self: super: {
+        llvmPackages_11 = super.llvmPackages_11.extend (final: prev: rec {
+          libllvm = prev.libllvm.overrideAttrs (old: {
+            LDFLAGS = "-L ${super.llvmPackages_11.libcxxabi}/lib";
+            nativeBuildInputs = (old.nativeBuildInputs or [ ]) ++ [ og.darwin.cctools ];
+          });
+          libclang = prev.libclang.override {
+            inherit libllvm;
+          };
+          libraries = super.llvmPackages_11.libraries;
+        });
+      })
+    ];
+  };
+  pkgsLinux = import <nixpkgs> {
+    config = { };
+    overlays = [ ];
+    system = targetSystem;
+  };
+in
+let
+  pkgs = nixpkgs.buildPackages;
+  linuxCC = pkgs.wrapCCWith rec {
+    cc = pkgs.llvmPackages_11.clang-unwrapped;
+    bintools = pkgs.llvmPackages_11.bintools;
+    extraPackages = [ pkgsLinux.glibc.static pkgs.llvmPackages_11.libraries.libcxxabi pkgs.llvmPackages_11.libraries.libcxx ];
+    extraBuildCommands = ''
+      echo "-isystem ${pkgs.llvmPackages_11.clang-unwrapped.lib}/lib/clang/${cc.version}/include" >> $out/nix-support/cc-cflags
+      echo "-isystem ${pkgsLinux.glibc.dev}/include" >> $out/nix-support/cc-cflags
+      echo "-L ${pkgs.llvmPackages_11.libraries.libcxxabi}/lib" >> $out/nix-support/cc-ldflags
+      echo "-L ${pkgsLinux.glibc.static}/lib" >> $out/nix-support/cc-ldflags
+      echo "-resource-dir=${cc}/resource-root" >> $out/nix-support/cc-cflags
+    '';
+  };
+in
+pkgs.buildEnv (
+  let
+    cc = linuxCC;
+  in
+  {
+    name = "bazel-${cc.name}-cc";
+    paths = [ cc cc.bintools ];
+    pathsToLink = [ "/bin" ];
+    passthru = {
+      inherit (cc) isClang targetPrefix;
+      orignalName = cc.name;
+    };
+  }
+)