feat(android): add Rust cross-compilation setup for Android environment

cibuildwheel/platforms/android.py

@@ -7,7 +7,7 @@
 import shutil
 import subprocess
 import sysconfig
-from collections.abc import Iterable, Iterator
+from collections.abc import Iterable, Iterator, MutableMapping
 from dataclasses import dataclass
 from os.path import relpath
 from pathlib import Path
@@ -390,6 +390,11 @@ def setup_android_env(
     for key in ["CFLAGS", "CXXFLAGS"]:
         android_env[key] += " " + opt
 
+    # Cargo target linker need to be specified after CC is set
+    setup_rust_cross_compile(config, android_env)
+
+    setup_PYO3_cross_compile(python_dir, android_env)
+
     # Format the environment so it can be pasted into a shell when debugging.
     for key, value in sorted(android_env.items()):
         if os.environ.get(key) != value:
@@ -398,6 +403,47 @@ def setup_android_env(
     return android_env
 
 
+def setup_rust_cross_compile(
+    python_configuration: PythonConfiguration,
+    env: MutableMapping[str, str],
+) -> None:
+    cargo_target = android_triplet(python_configuration.identifier)
+    call("rustup", "target", "add", cargo_target)
+
+    # CARGO_BUILD_TARGET is the variable used by Cargo and setuptools_rust
+    if env.get("CARGO_BUILD_TARGET"):
+        if env["CARGO_BUILD_TARGET"] != cargo_target:
+            log.notice("Not overriding CARGO_BUILD_TARGET as it has already been set")
+        # No message if it was set to what we were planning to set it to
+    elif cargo_target:
+        cargo_target_linker_env_name = (
+            f"CARGO_TARGET_{cargo_target.upper().replace('-', '_')}_LINKER"
+        )
+        log.notice(
+            f"Setting CARGO_BUILD_TARGET={cargo_target} and {cargo_target_linker_env_name} for cross-compilation"
+        )
+        env["CARGO_BUILD_TARGET"] = cargo_target
+        # CC has already been set by calling android.py (it calls android-env.sh)
+        env[f"{cargo_target_linker_env_name}"] = env["CC"]
+    else:
+        log.warning(f"Unable to configure Rust cross-compilation for architecture {cargo_target}")
+
+
+def setup_PYO3_cross_compile(
+    python_dir: Path,
+    env: MutableMapping[str, str],
+) -> None:
+    # All Python extension modules must therefore be explicitly linked against libpython3.x.so when building for Android.
+    # See: https://peps.python.org/pep-0738/#linkage
+    # For projects using PyO3, this requires setting PYO3_CROSS_LIB_DIR to the directory containing libpython3.x.so.
+    # See: https://pyo3.rs/v0.27.1/building-and-distribution.html#cross-compiling
+    if env.get("PYO3_CROSS_LIB_DIR"):
+        log.notice("Not overriding PYO3_CROSS_LIB_DIR as it has already been set")
+    else:
+        env["PYO3_CROSS_LIB_DIR"] = str(python_dir / "prefix" / "lib")
+        log.notice("Setting PYO3_CROSS_LIB_DIR for PyO3 cross-compilation")
+
+
 def before_build(state: BuildState) -> None:
     if state.options.before_build:
         log.step("Running before_build...")