Implement support for macOS x86_64 <-> arm64 cross compilation

.bazelrc

@@ -348,3 +348,25 @@ build:windows --cxxopt='/Zc:__cplusplus' --host_cxxopt='/Zc:__cplusplus'
 # enable clang coverage: https://clang.llvm.org/docs/SourceBasedCodeCoverage.html
 build:clang-coverage --copt="-fprofile-instr-generate" --linkopt="-fprofile-instr-generate"
 build:clang-coverage --copt="-fcoverage-mapping" --linkopt="-fcoverage-mapping"
+
+#
+# Cross-Compilation
+# So far, only cross-compiling on macOS between ARM and x86 is supported – using apple_support makes
+# this much easier than on other platforms. While we define a configuration for Intel Mac here, the
+# lack of a means to run Apple Silicon binaries there means that we can't run tests and would need
+# to have mksnapshot use the host configuration again for it to work, effectively compiling much of
+# V8 twice.
+
+# basic cross-compilation options
+# build:macos-cross --//:cross_compile=True
+# Keep the host build in the opt configuration (this makes running torque and mksnapshot faster),
+# but use -O1 to not spend too much time on building them.
+# build:macos-cross --host_copt="-O1"
+# Enable optimization to counteract tests being slightly slower under Rosetta.
+# build:macos-cross --copt="-O1"
+
+# Define the actual target platform.
+build:macos-cross-x86_64 --cpu=darwin_x86_64 --host_cpu=darwin_arm64 --platforms //:macOS_x86
+build:macos-cross-arm64 --cpu=darwin_arm64 --host_cpu=darwin_x86_64 --platforms //:macOS_arm64
+# build:macos-cross-x86_64 --config=macos-cross
+# build:macos-cross-arm64 --config=macos-cross

.github/workflows/test.yml

@@ -48,6 +48,9 @@ jobs:
           # previous versions). Use remote_download_all for now.
           - os:     { name : windows, image : windows-2022 }
             config: { suffix: '', bazel-args: "--config=windows_no_dbg --remote_download_all" }
+          # EXP: Cross-compile macOS Apple Silicon => x86
+          - os:     { name : macOS, image : macos-15 }
+            config: { suffix: -x86-cross, bazel-args: --config=macos-cross-x86_64 }
           # TODO (later): The custom Windows-debug configuration consistently runs out of disk
           # space on CI, disable it for now. Once https://github.com/bazelbuild/bazel/issues/21615
           # has been resolved we can likely re-enable it and possibly fold up the custom

BUILD.bazel

@@ -15,6 +15,22 @@ cc_capnp_library(
     visibility = ["//visibility:public"],
 )
 
+platform(
+    name = "macOS_x86",
+    constraint_values = [
+        "@platforms//os:macos",
+        "@platforms//cpu:x86_64",
+    ],
+)
+
+platform(
+    name = "macOS_arm64",
+    constraint_values = [
+        "@platforms//os:macos",
+        "@platforms//cpu:arm64",
+    ],
+)
+
 npm_link_all_packages(name = "node_modules")
 
 npm_link_package(
@@ -73,6 +89,17 @@ config_setting(
     flag_values = {"dead_strip": "True"},
 )
 
+# TODO(now): Unused? See if we can really cross-compile properly without this.
+bool_flag(
+    name = "cross_compile",
+    build_setting_default = False,
+)
+
+config_setting(
+    name = "is_cross_compile",
+    flag_values = {"cross_compile": "True"},
+)
+
 # Workaround for bazel not supporting negated conditions (https://github.com/bazelbuild/bazel-skylib/issues/272)
 selects.config_setting_group(
     name = "not_dbg_build",

WORKSPACE

@@ -172,14 +172,19 @@ git_repository(
     remote = "https://chromium.googlesource.com/chromium/src/third_party/zlib.git",
 )
 
-load("@rules_rust//rust:repositories.bzl", "rules_rust_dependencies", "rust_register_toolchains")
+load("@rules_rust//rust:repositories.bzl", "rules_rust_dependencies", "rust_register_toolchains", "rust_repository_set")
 
 rules_rust_dependencies()
 
 rust_register_toolchains(
     edition = "2021",
-    # Rust registers wasm targets by default which we don't need, workerd is only built for its native platform.
-    extra_target_triples = [],
+    # Rust registers wasm targets by default which we don't need. Add support macOS cross-
+    # compilation, the overhead for this is limited with only a few more targets being generated
+    # for the extra triples.
+    extra_target_triples = [
+        "aarch64-apple-darwin",
+        "x86_64-apple-darwin",
+    ],
     versions = ["1.81.0"],  # LLVM 18
 )