Add minimal cross-compilation support for Windows and AArch64 Linux

Cross-compiling to AArch64 Linux can be done with a CMake toolchain
file, along with setting the correct compiler and include paths in the
environment.

Cross-compiling from X64 Windows to ARM64 Windows doesn't look at the
toolchain at all, because CMake + Visual Studio can already
cross-compile. Unfortunately, the Visual Studio CMake generator
doesn't set CMAKE_SYSTEM_PROCESSOR, which is what the BoringSSL
CMakeLists.txt is looking at to choose the architecture. For now,
disable the use of assembly when cross-compiling on Windows (assuming
that the Visual Studio generator will be used there).

boring-sys/build.rs

@@ -141,11 +141,13 @@ const BORING_SSL_PATH: &str = "deps/boringssl";
 fn get_boringssl_cmake_config() -> cmake::Config {
     let arch = std::env::var("CARGO_CFG_TARGET_ARCH").unwrap();
     let os = std::env::var("CARGO_CFG_TARGET_OS").unwrap();
+    let host = std::env::var("HOST").unwrap();
+    let target = std::env::var("TARGET").unwrap();
     let pwd = std::env::current_dir().unwrap();
 
     let mut boringssl_cmake = cmake::Config::new(BORING_SSL_PATH);
-
+    if host != target {
-    // Add platform-specific parameters.
+        // Add platform-specific parameters for cross-compilation.
         match os.as_ref() {
             "android" => {
                 // We need ANDROID_NDK_HOME to be set properly.
@@ -165,8 +167,6 @@ fn get_boringssl_cmake_config() -> cmake::Config {
                 // 21 is the minimum level tested. You can give higher value.
                 boringssl_cmake.define("ANDROID_NATIVE_API_LEVEL", "21");
                 boringssl_cmake.define("ANDROID_STL", "c++_shared");
-
-            boringssl_cmake
             }
 
             "ios" => {
@@ -186,16 +186,20 @@ fn get_boringssl_cmake_config() -> cmake::Config {
                 };
 
                 let cflag = format!("{} {}", bitcode_cflag, target_cflag);
-
                 boringssl_cmake.define("CMAKE_ASM_FLAGS", &cflag);
                 boringssl_cmake.cflag(&cflag);
-
-            boringssl_cmake
             }
 
-        _ => {
+            "windows" => {
-            // Configure BoringSSL for building on 32-bit non-windows platforms.
+                if host.contains("windows") {
-            if arch == "x86" && os != "windows" {
+                    // BoringSSL's CMakeLists.txt isn't set up for cross-compiling using Visual Studio.
+                    // Disable assembly support so that it at least builds.
+                    boringssl_cmake.define("OPENSSL_NO_ASM", "YES");
+                }
+            }
+
+            "linux" => match arch.as_str() {
+                "x86" => {
                     boringssl_cmake.define(
                         "CMAKE_TOOLCHAIN_FILE",
                         pwd.join(BORING_SSL_PATH)
@@ -203,11 +207,26 @@ fn get_boringssl_cmake_config() -> cmake::Config {
                             .as_os_str(),
                     );
                 }
+                "aarch64" => {
+                    boringssl_cmake.define(
+                        "CMAKE_TOOLCHAIN_FILE",
+                        pwd.join("cmake/aarch64-linux.cmake").as_os_str(),
+                    );
+                }
+                _ => {
+                    eprintln!(
+                        "warning: no toolchain file configured by boring-sys for {}",
+                        target
+                    );
+                }
+            },
+
+            _ => {}
+        }
+    }
 
     boringssl_cmake
 }
-    }
-}
 
 /// Verify that the toolchains match https://csrc.nist.gov/CSRC/media/projects/cryptographic-module-validation-program/documents/security-policies/140sp3678.pdf
 /// See "Installation Instructions" under section 12.1.

boring-sys/cmake/aarch64-linux.cmake

@@ -0,0 +1,3 @@
+set(CMAKE_SYSTEM_NAME Linux)
+set(CMAKE_SYSTEM_PROCESSOR aarch64)
+# Rely on environment variables to set the compiler and include paths.