Implement support for macOS Apple Silicon => x86_64 cross compilation

Author: fhanau

.bazelrc

@@ -221,6 +221,19 @@ build:macos --copt='-femit-dwarf-unwind=no-compact-unwind'
 # TODO(soon): File a bazel issue for the flag so we can drop this.
 build:macos --experimental_objc_fastbuild_options=""
 
+# Cross-Compilation
+# Only cross-compiling on macOS from Apple Silicon to x86_64 is supported – using apple_support
+# makes this much easier than on other platforms. We could define a configuration for cross-
+# compiling from Intel Mac too, but it lacks a means to run Apple Silicon binaries. We would have to
+# change V8 mksnapshot to build in the host configuration again (effectively compiling much of V8
+# twice) and couldn't run tests, so it would provide little value.
+#
+# Define the target platform
+build:macos-cross-x86_64 --cpu=darwin_x86_64 --host_cpu=darwin_arm64 --platforms //:macOS_x86
+# Some cross-compiled tests are slower when run over Rosetta, increase the medium test size timeout.
+# Test performance is still very much satisfactory considering that emulation is being used here.
+build:macos-cross-x86_64 --test_timeout=1,30,60,240
+
 # On Linux, always link libc++ statically to avoid compatibility issues with different OS versions.
 # macOS links with dynamic libc++ by default, which has good backwards compatibility.
 # Drop default link flags, which include libstdc++ for Linux

BUILD.bazel

@@ -46,6 +46,15 @@ platform(
     ],
 )
 
+# Used for cross-compilation
+platform(
+    name = "macOS_x86",
+    constraint_values = [
+        "@platforms//os:macos",
+        "@platforms//cpu:x86_64",
+    ],
+)
+
 # bazel enables the --ffunction-sections, --gc-sections flags used to remove dead code by default
 # on Linux opt builds. Enable the equivalent macOS flag -Wl,-dead_strip here to work around bazel
 # idiosyncrasies.

WORKSPACE

@@ -178,8 +178,11 @@ 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 = [],
+    # Add support macOS cross-compilation, the overhead for this is limited with only a few more
+    # targets being generated for the extra triple as part of the toolchain. Setting this explicitly
+    # also avoids registering support for the wasm32-unknown-unknown and wasm32-wasi targets, which
+    # are otherwise added by default.
+    extra_target_triples = ["x86_64-apple-darwin"],
     versions = ["1.81.0"],  # LLVM 18
 )