Add example workspace cross-compiling with zig

We've run into a number of issues, both in rustc and in cargo build
scripts, where we're not properly propagating cc_toolchain information
to compiles and particularly link actions. These have often been hard to
test because we don't have an example set-up with a non-default C++
toolchain.

By wiring up a zig cross-compiling toolchain, we have a concrete example
we can work with. It's also an interesting potential example for folks
who want to set something similar up.

Author: illicitonion

.bazelci/presubmit.yml

@@ -457,6 +457,12 @@ tasks:
       - "--platforms=//:ios_x86_64"
     test_targets:
       - "//..."
+  zig_cross_compiling:
+    name: Zig cross compiling test
+    platform: ubuntu2004
+    working_directory: examples/zig_cross_compiling
+    build_targets:
+      - "//..."
 
 buildifier:
   version: latest

examples/zig_cross_compiling/.bazelrc

@@ -0,0 +1 @@
+build --incompatible_enable_cc_toolchain_resolution

examples/zig_cross_compiling/.gitignore

@@ -0,0 +1 @@
+/bazel-*

examples/zig_cross_compiling/BUILD.bazel

@@ -0,0 +1,26 @@
+load("@aspect_bazel_lib//lib:transitions.bzl", "platform_transition_filegroup")
+load("@crate_index//:defs.bzl", "aliases", "all_crate_deps")
+load("@rules_rust//rust:defs.bzl", "rust_binary")
+
+rust_binary(
+    name = "uses_ring",
+    srcs = ["src/main.rs"],
+    aliases = aliases(),
+    deps = all_crate_deps(normal = True),
+    # We don't care about building this for our own platform, this just exists for the transition
+    tags = ["manual"],
+)
+
+platform(
+    name = "aarch64_linux",
+    constraint_values = [
+        "@platforms//os:linux",
+        "@platforms//cpu:aarch64",
+    ],
+)
+
+platform_transition_filegroup(
+    name = "uses_ring_arm",
+    srcs = [":uses_ring"],
+    target_platform = "aarch64_linux",
+)

examples/zig_cross_compiling/Cargo.bazel.lock

@@ -0,0 +1,206 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "bumpalo"
+version = "3.11.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "572f695136211188308f16ad2ca5c851a712c464060ae6974944458eb83880ba"
+
+[[package]]
+name = "cc"
+version = "1.0.76"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "76a284da2e6fe2092f2353e51713435363112dfd60030e22add80be333fb928f"
+
+[[package]]
+name = "cfg-if"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
+
+[[package]]
+name = "direct-cargo-bazel-deps"
+version = "0.0.1"
+dependencies = [
+ "ring",
+]
+
+[[package]]
+name = "js-sys"
+version = "0.3.60"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "49409df3e3bf0856b916e2ceaca09ee28e6871cf7d9ce97a692cacfdb2a25a47"
+dependencies = [
+ "wasm-bindgen",
+]
+
+[[package]]
+name = "libc"
+version = "0.2.137"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fc7fcc620a3bff7cdd7a365be3376c97191aeaccc2a603e600951e452615bf89"
+
+[[package]]
+name = "log"
+version = "0.4.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "abb12e687cfb44aa40f41fc3978ef76448f9b6038cad6aef4259d3c095a2382e"
+dependencies = [
+ "cfg-if",
+]
+
+[[package]]
+name = "once_cell"
+version = "1.16.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "86f0b0d4bf799edbc74508c1e8bf170ff5f41238e5f8225603ca7caaae2b7860"
+
+[[package]]
+name = "proc-macro2"
+version = "1.0.47"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5ea3d908b0e36316caf9e9e2c4625cdde190a7e6f440d794667ed17a1855e725"
+dependencies = [
+ "unicode-ident",
+]
+
+[[package]]
+name = "quote"
+version = "1.0.21"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bbe448f377a7d6961e30f5955f9b8d106c3f5e449d493ee1b125c1d43c2b5179"
+dependencies = [
+ "proc-macro2",
+]
+
+[[package]]
+name = "ring"
+version = "0.16.20"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3053cf52e236a3ed746dfc745aa9cacf1b791d846bdaf412f60a8d7d6e17c8fc"
+dependencies = [
+ "cc",
+ "libc",
+ "once_cell",
+ "spin",
+ "untrusted",
+ "web-sys",
+ "winapi",
+]
+
+[[package]]
+name = "spin"
+version = "0.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6e63cff320ae2c57904679ba7cb63280a3dc4613885beafb148ee7bf9aa9042d"
+
+[[package]]
+name = "syn"
+version = "1.0.103"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a864042229133ada95abf3b54fdc62ef5ccabe9515b64717bcb9a1919e59445d"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
+[[package]]
+name = "unicode-ident"
+version = "1.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6ceab39d59e4c9499d4e5a8ee0e2735b891bb7308ac83dfb4e80cad195c9f6f3"
+
+[[package]]
+name = "untrusted"
+version = "0.7.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a156c684c91ea7d62626509bce3cb4e1d9ed5c4d978f7b4352658f96a4c26b4a"
+
+[[package]]
+name = "wasm-bindgen"
+version = "0.2.83"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "eaf9f5aceeec8be17c128b2e93e031fb8a4d469bb9c4ae2d7dc1888b26887268"
+dependencies = [
+ "cfg-if",
+ "wasm-bindgen-macro",
+]
+
+[[package]]
+name = "wasm-bindgen-backend"
+version = "0.2.83"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4c8ffb332579b0557b52d268b91feab8df3615f265d5270fec2a8c95b17c1142"
+dependencies = [
+ "bumpalo",
+ "log",
+ "once_cell",
+ "proc-macro2",
+ "quote",
+ "syn",
+ "wasm-bindgen-shared",
+]
+
+[[package]]
+name = "wasm-bindgen-macro"
+version = "0.2.83"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "052be0f94026e6cbc75cdefc9bae13fd6052cdcaf532fa6c45e7ae33a1e6c810"
+dependencies = [
+ "quote",
+ "wasm-bindgen-macro-support",
+]
+
+[[package]]
+name = "wasm-bindgen-macro-support"
+version = "0.2.83"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "07bc0c051dc5f23e307b13285f9d75df86bfdf816c5721e573dec1f9b8aa193c"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+ "wasm-bindgen-backend",
+ "wasm-bindgen-shared",
+]
+
+[[package]]
+name = "wasm-bindgen-shared"
+version = "0.2.83"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1c38c045535d93ec4f0b4defec448e4291638ee608530863b1e2ba115d4fff7f"
+
+[[package]]
+name = "web-sys"
+version = "0.3.60"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bcda906d8be16e728fd5adc5b729afad4e444e106ab28cd1c7256e54fa61510f"
+dependencies = [
+ "js-sys",
+ "wasm-bindgen",
+]
+
+[[package]]
+name = "winapi"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5c839a674fcd7a98952e593242ea400abe93992746761e38641405d28b00f419"
+dependencies = [
+ "winapi-i686-pc-windows-gnu",
+ "winapi-x86_64-pc-windows-gnu",
+]
+
+[[package]]
+name = "winapi-i686-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ac3b87c63620426dd9b991e5ce0329eff545bccbbb34f3be09ff6fb6ab51b7b6"
+
+[[package]]
+name = "winapi-x86_64-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"

examples/zig_cross_compiling/WORKSPACE.bazel

@@ -0,0 +1,83 @@
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+BAZEL_ZIG_CC_VERSION = "v0.9.2"
+
+http_archive(
+    name = "bazel-zig-cc",
+    sha256 = "73afa7e1af49e3dbfa1bae9362438cdc51cb177c359a6041a7a403011179d0b5",
+    strip_prefix = "bazel-zig-cc-{}".format(BAZEL_ZIG_CC_VERSION),
+    urls = ["https://git.sr.ht/~motiejus/bazel-zig-cc/archive/{}.tar.gz".format(BAZEL_ZIG_CC_VERSION)],
+)
+
+load("@bazel-zig-cc//toolchain:defs.bzl", zig_toolchains = "toolchains")
+
+zig_toolchains()
+
+register_toolchains(
+    "@zig_sdk//toolchain:linux_arm64_gnu.2.28",
+)
+
+local_repository(
+    name = "rules_rust",
+    path = "../..",
+)
+
+load("@rules_rust//rust:repositories.bzl", "rules_rust_dependencies", "rust_register_toolchains", "rust_repository_set")
+
+rules_rust_dependencies()
+
+load("@rules_rust//crate_universe:repositories.bzl", "crate_universe_dependencies")
+
+crate_universe_dependencies(bootstrap = True)
+
+rust_version = "1.65.0"
+
+rust_register_toolchains(
+    edition = "2021",
+    extra_target_triples = [],
+    version = rust_version,
+)
+
+rust_repository_set(
+    name = "macos_x86_64",
+    edition = "2021",
+    exec_triple = "x86_64-apple-darwin",
+    extra_target_triples = ["aarch64-unknown-linux-gnu"],
+    version = rust_version,
+)
+
+rust_repository_set(
+    name = "linux_x86_64",
+    edition = "2021",
+    exec_triple = "x86_64-unknown-linux-gnu",
+    extra_target_triples = ["aarch64-unknown-linux-gnu"],
+    version = rust_version,
+)
+
+load("@rules_rust//crate_universe:defs.bzl", "crate", "crates_repository")
+
+crates_repository(
+    name = "crate_index",
+    cargo_lockfile = "//:Cargo.bazel.lock",
+    # `generator` is not necessary in official releases.
+    # See load satement for `cargo_bazel_bootstrap`.
+    generator = "@cargo_bazel_bootstrap//:cargo-bazel",
+    packages = {
+        "ring": crate.spec(version = "0.16.20"),
+    },
+)
+
+load("@crate_index//:defs.bzl", "crate_repositories")
+
+crate_repositories()
+
+http_archive(
+    name = "aspect_bazel_lib",
+    sha256 = "3534a27621725fbbf1d3e53daa0c1dda055a2732d9031b8c579f917d7347b6c4",
+    strip_prefix = "bazel-lib-1.16.1",
+    url = "https://github.com/aspect-build/bazel-lib/archive/refs/tags/v1.16.1.tar.gz",
+)
+
+load("@aspect_bazel_lib//lib:repositories.bzl", "aspect_bazel_lib_dependencies")
+
+aspect_bazel_lib_dependencies()

examples/zig_cross_compiling/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("{:?}", ring::rand::SystemRandom::new());
+}