Proof of concept Linux -> macOS cross compilation in Nixpkgs

flake.lock

{
  "nodes": {
    "flake-utils": {
      "inputs": {
        "systems": "systems"
      },
      "locked": {
        "lastModified": 1710146030,
        "narHash": "sha256-SZ5L6eA7HJ/nmkzGG7/ISclqe6oZdOZTNoesiInkXPQ=",
        "owner": "numtide",
        "repo": "flake-utils",
        "rev": "b1d9ab70662946ef0850d488da1c9019f3a9752a",
        "type": "github"
      },
      "original": {
        "owner": "numtide",
        "repo": "flake-utils",
        "type": "github"
      }
    },
    "nixpkgs": {
      "locked": {
        "lastModified": 1712849433,
        "narHash": "sha256-flQtf/ZPJgkLY/So3Fd+dGilw2DKIsiwgMEn7BbBHL0=",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "f173d0881eff3b21ebb29a2ef8bedbc106c86ea5",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
        "ref": "nixpkgs-unstable",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "root": {
      "inputs": {
        "flake-utils": "flake-utils",
        "nixpkgs": "nixpkgs"
      }
    },
    "systems": {
      "locked": {
        "lastModified": 1681028828,
        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
        "owner": "nix-systems",
        "repo": "default",
        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
        "type": "github"
      },
      "original": {
        "owner": "nix-systems",
        "repo": "default",
        "type": "github"
      }
    }
  },
  "root": "root",
  "version": 7
}

flake.nix

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system: let

      pkgs = import nixpkgs {
        inherit system;
        config.allowUnsupportedSystems = true;
      };

      package = pkgs.callPackage ./package.nix { };

    in {
      packages.default = package;
      packages.hello = package;
      devShells.default = pkgs.mkShell {
        inputsFrom = [
          package
        ];
      };
    }) # eachDefaultSystem
  ; # outputs
}

macos-cross.nix

{
  path,
  lib,
  stdenvNoCC,
  darwin,
  llvmPackages_15,
  wrapCCWith,
  hello,
  coreutils,
  python3,
  boost,
}: let
  darwinPkgs = import path {
    localSystem.system = "x86_64-darwin";
    config.allowUnsupportedSystem = true;
  };
  libunwind = darwinPkgs.llvmPackages.libunwind;
  libcxx = darwinPkgs.llvmPackages.libcxx;

  llvmPackages = llvmPackages_15;
  darwinSdk = darwin.apple_sdk_11_0.MacOSX-SDK;
  libc = darwin.apple_sdk_11_0.Libsystem;
  inherit (llvmPackages) clang-unwrapped;

  extraBuildCommands = {
    libcxx-cxxflags = let
        list = [
        #"-nostdlib++"
        #"-nostdinc++"
        #"-nostartfiles"
        "-isystem ${lib.getDev libcxx}/include/c++/v1"
        "-isystem ${darwinSdk}/usr/include"
        "-idirafter ${darwinSdk}/usr/include"
        "-iframework${darwinSdk}/System/Library/Frameworks"
        "-L${darwinSdk}/usr/lib"
        "-L${lib.getLib libunwind}"
        "-lc++"
        "-lc++abi"
      ];

      op = acc: item: acc + "\necho ${lib.strings.escapeShellArg item} >> \"$out/nix-support/libcxx-cxxflags\"";
    in lib.foldl' op "" list;
  };

  cc = wrapCCWith {
    cc = clang-unwrapped;
      #echo "-isystem ${darwinSdk}/usr/include" >> "$out/nix-support/libc-cflags"
      #echo "-idirafter ${darwinSdk}/usr/include" >> "$out/nix-support/libc-cflags"
      #echo "-iframework${darwinSdk}/System/Library/Frameworks" >> "$out/nix-support/libc-cflags"
      #echo "-isystem ${darwinSdk}/System/Library/Frameworks/Kernel.framework/Versions/A/Headers" >> "$out/nix-support/libc-cflags"
    # We need to remove the -dynamic-linker arguments lol
    extraBuildCommands = ''
      echo "making wrapper stuff in $out"
      cat ${./osxcross-cc-wrapper-hook.bash} >> "$out/nix-support/cc-wrapper-hook"
    '' + extraBuildCommands.libcxx-cxxflags;
      #echo "-isystem ${lib.getDev libcxx}/include/c++/v1" >> "$out/nix-support/libcxx-cxxflags"
      #echo "-isystem ${darwinSdk}/usr/include" >> "$out/nix-support/libcxx-cxxflags"
      #echo "-lc++" >> "$out/nix-support/libcxx-cxxflags"
      #echo "-lc++abi" >> "$out/nix-support/libcxx-cxxflags"
      #echo "-L${darwinSdk}/usr/lib" >> "$out/nix-support/libcxx-cxxflags"
      #echo "-L${libunwind}/lib" >> "$out/nix-support/libcxx-cxxflags"
      #echo "-idirafter ${darwinSdk}/usr/include" >> "$out/nix-support/libcxx-cxxflags"
      #echo "-iframework${darwinSdk}/System/Library/Frameworks" >> "$out/nix-support/libcxx-cxxflags"
      #echo "-idirafter ${darwinSdk}/System/Library/Frameworks/Kernel.framework/Versions/A/Headers" >> "$out/nix-support/libc-cflags"
    inherit libc;
    bintools = llvmPackages.bintools.override {
      inherit libc;
    };
    #inherit (llvmPackages) libcxx;
    gccForLibs = null;
    extraTools = lib.singleton llvmPackages.lld;
    isClang = true;
    useCcForLibs = false;
    nixSupport.cc-cflags = [
      #"-nostdlib"
      #"-isystem" "${darwinSdk}/usr/include"
      "-idirafter" "${darwinSdk}/System/Library/Frameworks/Kernel.framework/Versions/A/Headers"
      "--target=x86_64-apple-macosx11.0"
      "-fuse-ld=lld"
      "-DTARGET_OS_MACOSX"
    ];
    nixSupport.cc-ldflags = [
      #"-fuse-ld=lld"
      "-lc"
      "-L${darwinSdk}/usr/lib"
    ];
  };

  #stdenv = overrideCC stdenvNoCC cc;
  stdenv = stdenvNoCC.override {
    hostPlatform = lib.systems.elaborate "x86_64-darwin";
    inherit (stdenvNoCC) buildPlatform;
    targetPlatform = lib.systems.elaborate "x86_64-darwin";
    inherit cc;
    hasCC = true;
    allowedRequisites = null;
  };

  trivial-c-cxx = stdenv.mkDerivation {
    name = "trivial-c-c++";

    #passthru = {
    #  inherit toCCArgs cc;
    #};

    src = ./.;

    nativeBuildInputs = [
      #cc
      #llvmPackages.lld
    ];

    buildPhase = ''
      runHook preBuild
      mkdir -p "$out"
      (set -x && cc -o $out/main-c ./main.c)
      (set -x && c++ -o $out/main-cpp ./main.cpp)
      runHook postBuild
    '';
  };

  hello-darwin = (hello.override { inherit stdenv; }).overrideAttrs (prev: {
    buildInputs = prev.buildInputs or [ ] ++ [
      darwinPkgs.libiconv
    ];
  });

  python3-darwin = (python3.override {
    inherit stdenv;
    enableLTO = false;
    configd = darwinPkgs.darwin.apple_sdk.frameworks.SystemConfiguration;
  }).overrideAttrs (prev: {
    configureFlags = prev.configureFlags or [ ] ++ [
      "MACHDEP=darwin"
    ];
  });

in hello-darwin

osxcross-cc-wrapper-hook.bash

declare -a _extraBefore
for arg in "${extraBefore[@]}"; do
	if [[ "$arg" != *"-dynamic-linker"* ]]; then
		_extraBefore+=("$arg")
	fi
done
extraBefore=("${_extraBefore[@]}")
unset _extraBefore