crosscompile: Add an example for cross compiling

This adds an example for cross compiling on an ARM M1 Mac to x86_64 Linux.
There are rough edges around this example like it will *only* compile for
Linux and won't work for OSX, but I think that this problem should be discussed
in the better API issue.

Also I realized I missed a bug in the previous cross compilation PR, where when cross compiling,
we probably want to use the linux_wrapper for building otherwise we end up trying to `install_name_tool`
the final Linux binaries, which doesn't work.

```
[nix-shell:~/code/rules_nixpkgs/examples/toolchains/cc_cross_osx_to_linux_amd64]$ bazel build --config=cross :hello
INFO: Invocation ID: f10b420f-2dca-42d3-ab4b-1179862791ff
INFO: Analyzed target //:hello (3 packages loaded, 19 targets configured).
INFO: Found 1 target...
Target //:hello up-to-date:
  bazel-bin/hello
INFO: Elapsed time: 8.212s, Critical Path: 5.59s
INFO: 4 processes: 2 internal, 2 processwrapper-sandbox.
INFO: Build completed successfully, 4 total actions

[nix-shell:~/code/rules_nixpkgs/examples/toolchains/cc_cross_osx_to_linux_amd64]$ file bazel-bin/hello
bazel-bin/hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /nix/store/jvjrf7rqflfx4nf65yrkk697sxggffki-glibc-x86_64-linux-2.35-224/lib/ld-linux-x86-64.so.2, for GNU/Linux 3.10.0, not stripped
```

note that when compiling for the first time, it'll build clang and gcc a bunch of times for cross compilations so the initial build will take a few hours...

examples/toolchains/cc_cross_osx_to_linux_amd64/.bazeliskrc

@@ -0,0 +1 @@
+USE_BAZEL_VERSION=6.x

examples/toolchains/cc_cross_osx_to_linux_amd64/.bazelrc

@@ -0,0 +1,6 @@
+build --incompatible_enable_cc_toolchain_resolution
+build:nix --host_platform=@io_tweag_rules_nixpkgs//nixpkgs/platforms:host
+build:nix --crosstool_top=@nixpkgs_config_cc//:toolchain
+build:cross --host_platform=@io_tweag_rules_nixpkgs//nixpkgs/platforms:host
+build:cross --host_crosstool_top=@nixpkgs_config_cc//:toolchain
+build:cross --cpu=k8 --crosstool_top=@nixpkgs_cross_cc//:toolchain --platforms=//toolchains:linux_x86_64

examples/toolchains/cc_cross_osx_to_linux_amd64/BUILD

@@ -0,0 +1,10 @@
+load("@rules_cc//cc:defs.bzl", "cc_binary")
+
+cc_binary(
+    name = "hello",
+    srcs = ["hello.cc"],
+    deps = [
+        "@boost.dev//:boost",
+        "@zlib.dev//:zlib",
+    ],
+)

examples/toolchains/cc_cross_osx_to_linux_amd64/README.md

@@ -0,0 +1,13 @@
+C++ With Dependencies Toolchain Example
+=======================================
+
+This is an example C++ project with dependencies that uses `rules_cc`.
+
+This example uses the Nix package manager to provide C++ dependencies, and as such only works with Nix installed. Demonstrating other methods of providing C++ dependencies is out of scope of this example.
+
+# Usage
+
+To run the example with Nix, issue the following command:
+```
+nix-shell --command 'bazel run --config=cross:hello'
+```

examples/toolchains/cc_cross_osx_to_linux_amd64/WORKSPACE

@@ -0,0 +1,111 @@
+# Replace with http_archive: https://github.com/tweag/rules_nixpkgs/#setup
+local_repository(
+    name = "io_tweag_rules_nixpkgs",
+    path = "../../../",
+)
+
+load("@io_tweag_rules_nixpkgs//nixpkgs:repositories.bzl", "rules_nixpkgs_dependencies")
+
+rules_nixpkgs_dependencies()
+
+load(
+    "@io_tweag_rules_nixpkgs//nixpkgs:nixpkgs.bzl",
+    "nixpkgs_cc_configure",
+    "nixpkgs_local_repository",
+)
+
+nixpkgs_local_repository(
+    name = "nixpkgs",
+    nix_file = "//:nixpkgs.nix",
+    nix_file_deps = ["//:nixpkgs.json"],
+)
+
+nixpkgs_cc_configure(
+    name = "nixpkgs_config_cc",
+    nix_file = "//toolchains:cc.nix",
+    repository = "@nixpkgs",
+)
+
+nixpkgs_cc_configure(
+    name = "nixpkgs_cross_cc",
+    cross_cpu = "k8",
+    exec_constraints = [
+        "@platforms//os:osx",
+        "@platforms//cpu:arm64",
+    ],
+    nix_file = "//toolchains:osxcross_cc.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",
+    ],
+)
+
+load("@io_tweag_rules_nixpkgs//nixpkgs:nixpkgs.bzl", "nixpkgs_package")
+
+# We would probably want a select or something to make this work with OSX as well, for a more
+# production set up.
+nixpkgs_package(
+    name = "boost",
+    attribute_path = "boost175",
+    nix_file_content = """import <nixpkgs> { config = {}; overlays = []; system = "x86_64-linux"; }""",
+    repository = "@nixpkgs",
+)
+
+nixpkgs_package(
+    name = "boost.dev",
+    attribute_path = "boost175.dev",
+    build_file_content = """\
+load("@rules_cc//cc:defs.bzl", "cc_library")
+filegroup(
+    name = "include",
+    srcs = glob(["include/**/*.h", "include/**/*.hpp"]),
+    visibility = ["//visibility:public"],
+)
+cc_library(
+    name = "boost",
+    srcs = ["@boost//:lib"],
+    hdrs = [":include"],
+    strip_include_prefix = "include",
+    visibility = ["//visibility:public"],
+)
+""",
+    nix_file_content = """import <nixpkgs> { config = {}; overlays = []; system = "x86_64-linux"; }""",
+    repository = "@nixpkgs",
+)
+
+nixpkgs_package(
+    name = "zlib",
+    attribute_path = "zlib",
+    nix_file_content = """import <nixpkgs> { config = {}; overlays = []; system = "x86_64-linux"; }""",
+    repository = "@nixpkgs",
+)
+
+nixpkgs_package(
+    name = "zlib.dev",
+    attribute_path = "zlib.dev",
+    build_file_content = """\
+load("@rules_cc//cc:defs.bzl", "cc_library")
+filegroup(
+    name = "include",
+    srcs = glob(["include/**/*.h"]),
+    visibility = ["//visibility:public"],
+)
+cc_library(
+    name = "zlib",
+    srcs = ["@zlib//:lib"],
+    hdrs = [":include"],
+    strip_include_prefix = "include",
+    visibility = ["//visibility:public"],
+)
+""",
+    nix_file_content = """import <nixpkgs> { config = {}; overlays = []; system = "x86_64-linux"; }""",
+    repository = "@nixpkgs",
+)

examples/toolchains/cc_cross_osx_to_linux_amd64/hello.cc

@@ -0,0 +1,8 @@
+#include <iostream>
+#include <boost/filesystem/operations.hpp>
+#include <zlib.h>
+
+int main()
+{
+	std::cout << "Hello world!\n";
+}

examples/toolchains/cc_cross_osx_to_linux_amd64/nixpkgs.json

@@ -0,0 +1,7 @@
+{
+  "owner": "NixOS",
+  "repo": "nixpkgs",
+  "branch": "23.05",
+  "rev": "3ad64d9e2d5bf80c877286102355b1625891ae9a",
+  "sha256": "sha256-PuZSAHeq4/9pP/uYH1FcagQ3nLm/DrDrvKi/xC9glvw="
+}

examples/toolchains/cc_cross_osx_to_linux_amd64/nixpkgs.nix

@@ -0,0 +1,8 @@
+let
+  spec = builtins.fromJSON (builtins.readFile ./nixpkgs.json);
+  nixpkgs = fetchTarball {
+    url = "https://github.com/${spec.owner}/${spec.repo}/archive/${spec.rev}.tar.gz";
+    sha256 = spec.sha256;
+  };
+in
+import nixpkgs

examples/toolchains/cc_cross_osx_to_linux_amd64/shell.nix

@@ -0,0 +1,3 @@
+{ pkgs ? import ./nixpkgs.nix { } }:
+
+pkgs.mkShellNoCC { nativeBuildInputs = [ pkgs.bazel_6 ]; }

examples/toolchains/cc_cross_osx_to_linux_amd64/toolchains/BUILD

@@ -0,0 +1,9 @@
+platform(
+    name = "linux_x86_64",
+    constraint_values = [
+        "@platforms//cpu:x86_64",
+        "@platforms//os:linux",
+        "@rules_nixpkgs_core//constraints:support_nix",
+    ],
+    visibility = ["//visibility:public"],
+)

examples/toolchains/cc_cross_osx_to_linux_amd64/toolchains/cc.nix

@@ -0,0 +1,72 @@
+# We need to write a fancy nix file to handle the CC compiler due to OSX: https://github.com/tweag/rules_nixpkgs/issues/368
+# What this file is basically saying is: if OSX, change the CXX compiler to use a bunch of Apple frameworks, LLVM libs,
+# libc++ instead of libstdc++, and some additional compiler flags to ignore some warnings, otherwise, just use clang11
+let
+  pkgs = import <nixpkgs> {};
+in let
+  clang = pkgs.clang_11;
+  # The original `postLinkSignHook` from nixpkgs assumes `codesign_allocate` is
+  # in the PATH which is not the case when using our cc_wrapper. Set
+  # `CODESIGN_ALLOCATE` to an absolute path here and override the hook for
+  # `darwinCC` below.
+  postLinkSignHook = with pkgs;
+    writeTextFile {
+      name = "post-link-sign-hook";
+      executable = true;
+
+      text = ''
+        CODESIGN_ALLOCATE=${darwin.cctools}/bin/codesign_allocate \
+          ${darwin.sigtool}/bin/codesign -f -s - "$linkerOutput"
+      '';
+    };
+  darwinCC =
+    # Work around https://github.com/NixOS/nixpkgs/issues/42059.
+    # See also https://github.com/NixOS/nixpkgs/pull/41589.
+    pkgs.wrapCCWith rec {
+      cc = clang;
+      bintools = pkgs.stdenv.cc.bintools.override {inherit postLinkSignHook;};
+      extraBuildCommands = with pkgs.darwin.apple_sdk.frameworks; ''
+        echo "-Wno-unused-command-line-argument" >> $out/nix-support/cc-cflags
+        echo "-Wno-elaborated-enum-base" >> $out/nix-support/cc-cflags
+        echo "-isystem ${pkgs.llvmPackages_11.libcxx.dev}/include/c++/v1" >> $out/nix-support/cc-cflags
+        echo "-isystem ${pkgs.llvmPackages_11.clang-unwrapped.lib}/lib/clang/${cc.version}/include" >> $out/nix-support/cc-cflags
+        echo "-F${CoreFoundation}/Library/Frameworks" >> $out/nix-support/cc-cflags
+        echo "-F${CoreServices}/Library/Frameworks" >> $out/nix-support/cc-cflags
+        echo "-F${Security}/Library/Frameworks" >> $out/nix-support/cc-cflags
+        echo "-F${Foundation}/Library/Frameworks" >> $out/nix-support/cc-cflags
+        echo "-L${pkgs.llvmPackages_11.libcxx}/lib" >> $out/nix-support/cc-cflags
+        echo "-L${pkgs.llvmPackages_11.libcxxabi}/lib" >> $out/nix-support/cc-cflags
+        echo "-L${pkgs.libiconv}/lib" >> $out/nix-support/cc-cflags
+        echo "-L${pkgs.darwin.libobjc}/lib" >> $out/nix-support/cc-cflags
+        echo "-resource-dir=${pkgs.stdenv.cc}/resource-root" >> $out/nix-support/cc-cflags
+      '';
+    };
+  linuxCC = pkgs.wrapCCWith rec {
+    cc = clang;
+    bintools = pkgs.stdenv.cc.bintools;
+    extraPackages = [pkgs.glibc.static];
+    extraBuildCommands = ''
+      echo "-isystem ${pkgs.llvmPackages_11.clang-unwrapped.lib}/lib/clang/${cc.version}/include" >> $out/nix-support/cc-cflags
+      echo "-L ${pkgs.glibc.static}/lib" >> $out/nix-support/cc-ldflags
+      echo "-resource-dir=${cc}/resource-root" >> $out/nix-support/cc-cflags
+    '';
+  };
+in
+  pkgs.buildEnv (
+    let
+      cc =
+        if pkgs.stdenv.isDarwin
+        then darwinCC
+        else linuxCC;
+    in {
+      name = "bazel-nixpkgs-cc";
+      # XXX: `gcov` is missing in `/bin`.
+      #   It exists in `stdenv.cc.cc` but that collides with `stdenv.cc`.
+      paths = [cc cc.bintools] ++ pkgs.lib.optional pkgs.stdenv.isDarwin pkgs.darwin.cctools;
+      pathsToLink = ["/bin"];
+      passthru = {
+        inherit (cc) isClang targetPrefix;
+        orignalName = cc.name;
+      };
+    }
+  )

examples/toolchains/cc_cross_osx_to_linux_amd64/toolchains/osxcross_cc.nix

@@ -0,0 +1,67 @@
+# This will take many hours to build. Caching this somewhere is recommended.
+let
+  targetSystem = "x86_64-linux";
+  og = import <nixpkgs> {};
+  nixpkgs = import <nixpkgs> {
+    buildSystem = builtins.currentSystem;
+    hostSystem = targetSystem;
+    crossSystem = {
+      config = targetSystem;
+    };
+    crossOverlays = [
+      (self: super: {
+        # Apparently this is a hacky way to extend llvmPackages, but whatever
+        llvmPackages_11 = super.llvmPackages_11.extend (final: prev: rec {
+          libllvm = prev.libllvm.overrideAttrs (old: {
+            # We need to override LDFLAGS because it puts non-Darwin compatible flags,
+            # so remove the old flags, and also explicitly tell the compiler where to
+            # find libcxxabi. Not sure why we need to do this.
+            LDFLAGS = "-L ${super.llvmPackages_11.libcxxabi}/lib";
+            # We need to make sure cctools is available because darwin code signing needs it
+            # in your $PATH.
+            nativeBuildInputs = (old.nativeBuildInputs or []) ++ [og.darwin.cctools];
+          });
+          libclang = prev.libclang.override {
+            inherit libllvm;
+          };
+          libraries = super.llvmPackages_11.libraries;
+        });
+      })
+    ];
+  };
+  # this will use linux binaries from binary cache, so no need to build those
+  pkgsLinux = import <nixpkgs> {
+    config = {};
+    overlays = [];
+    system = targetSystem;
+  };
+in let
+  pkgs = builtins.trace nixpkgs.stdenv.name 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";
+      # XXX: `gcov` is missing in `/bin`.
+      #   It exists in `stdenv.cc.cc` but that collides with `stdenv.cc`.
+      paths = [cc cc.bintools];
+      pathsToLink = ["/bin"];
+      passthru = {
+        inherit (cc) isClang targetPrefix;
+        orignalName = cc.name;
+      };
+    }
+  )

toolchains/cc/cc.bzl

@@ -123,7 +123,7 @@ def _parse_cc_toolchain_info(content, filename):
 def _nixpkgs_cc_toolchain_config_impl(repository_ctx):
     host_cpu = get_cpu_value(repository_ctx)
     cross_cpu = repository_ctx.attr.cross_cpu or host_cpu
-    darwin = host_cpu == "darwin" or host_cpu == "darwin_arm64"
+    darwin = (host_cpu == "darwin" or host_cpu == "darwin_arm64") and cross_cpu == host_cpu
 
     cc_toolchain_info_file = repository_ctx.path(repository_ctx.attr.cc_toolchain_info)
     if not cc_toolchain_info_file.exists and not repository_ctx.attr.fail_not_supported: