chore(ffi): avoid hardcoding clang version

Update the workaround for rust-lang/rust#109717 to
avoid hardcoding the clang version; instead, run `clang -dumpversion` to figure
it out.

While we're there, use the `CC_x86_64-linux-android` env var, which should
point to clang, rather than relying on `ANDROID_NDK_HOME` to be set.

bindings/matrix-sdk-crypto-ffi/build.rs

@@ -1,37 +1,56 @@
-use std::{env, error::Error};
+use std::{env, error::Error, path::PathBuf, process::Command};
 
 use vergen::EmitBuilder;
 
 /// Adds a temporary workaround for an issue with the Rust compiler and Android
 /// in x86_64 devices: https://github.com/rust-lang/rust/issues/109717.
-/// The workaround comes from: https://github.com/mozilla/application-services/pull/5442
+/// The workaround is based on: https://github.com/mozilla/application-services/pull/5442
 ///
 /// IMPORTANT: if you modify this, make sure to modify
 /// [../matrix-sdk-ffi/build.rs] too!
 fn setup_x86_64_android_workaround() {
     let target_os = env::var("CARGO_CFG_TARGET_OS").expect("CARGO_CFG_TARGET_OS not set");
     let target_arch = env::var("CARGO_CFG_TARGET_ARCH").expect("CARGO_CFG_TARGET_ARCH not set");
     if target_arch == "x86_64" && target_os == "android" {
-        let android_ndk_home = env::var("ANDROID_NDK_HOME").expect("ANDROID_NDK_HOME not set");
-        let build_os = match env::consts::OS {
-            "linux" => "linux",
-            "macos" => "darwin",
-            "windows" => "windows",
-            _ => panic!(
-                "Unsupported OS. You must use either Linux, MacOS or Windows to build the crate."
-            ),
-        };
-        const DEFAULT_CLANG_VERSION: &str = "18";
-        let clang_version =
-            env::var("NDK_CLANG_VERSION").unwrap_or_else(|_| DEFAULT_CLANG_VERSION.to_owned());
-        let linux_x86_64_lib_dir = format!(
-            "toolchains/llvm/prebuilt/{build_os}-x86_64/lib/clang/{clang_version}/lib/linux/"
+        // Configure rust to statically link against the `libclang_rt.builtins` supplied
+        // with clang.
+
+        // cargo-ndk sets CC_x86_64-linux-android to the path to `clang`, within the
+        // Android NDK.
+        let clang_path = PathBuf::from(
+            env::var("CC_x86_64-linux-android").expect("CC_x86_64-linux-android not set"),
         );
-        println!("cargo:rustc-link-search={android_ndk_home}/{linux_x86_64_lib_dir}");
+
+        // clang_path should now look something like
+        // `.../sdk/ndk/28.0.12674087/toolchains/llvm/prebuilt/linux-x86_64/bin/clang`.
+        // We strip `/bin/clang` from the end to get the toolchain path.
+        let toolchain_path = clang_path
+            .ancestors()
+            .nth(2)
+            .expect("could not find NDK toolchain path")
+            .to_str()
+            .expect("NDK toolchain path is not valid UTF-8");
+
+        let clang_version = get_clang_major_version(&clang_path);
+
+        println!("cargo:rustc-link-search={toolchain_path}/lib/clang/{clang_version}/lib/linux/");
         println!("cargo:rustc-link-lib=static=clang_rt.builtins-x86_64-android");
     }
 }
 
+/// Run the clang binary at `clang_path`, and return its major version number
+fn get_clang_major_version(clang_path: &PathBuf) -> String {
+    let clang_output =
+        Command::new(clang_path).arg("-dumpversion").output().expect("failed to start clang");
+
+    if !clang_output.status.success() {
+        panic!("failed to run clang: {}", String::from_utf8_lossy(&clang_output.stderr));
+    }
+
+    let clang_version = String::from_utf8(clang_output.stdout).expect("clang output is not utf8");
+    clang_version.split('.').next().expect("could not parse clang output").to_owned()
+}
+
 fn main() -> Result<(), Box<dyn Error>> {
     setup_x86_64_android_workaround();
 

bindings/matrix-sdk-ffi/build.rs

@@ -1,37 +1,56 @@
-use std::{env, error::Error};
+use std::{env, error::Error, path::PathBuf, process::Command};
 
 use vergen::EmitBuilder;
 
 /// Adds a temporary workaround for an issue with the Rust compiler and Android
 /// in x86_64 devices: https://github.com/rust-lang/rust/issues/109717.
-/// The workaround comes from: https://github.com/mozilla/application-services/pull/5442
+/// The workaround is based on: https://github.com/mozilla/application-services/pull/5442
 ///
 /// IMPORTANT: if you modify this, make sure to modify
 /// [../matrix-sdk-crypto-ffi/build.rs] too!
 fn setup_x86_64_android_workaround() {
     let target_os = env::var("CARGO_CFG_TARGET_OS").expect("CARGO_CFG_TARGET_OS not set");
     let target_arch = env::var("CARGO_CFG_TARGET_ARCH").expect("CARGO_CFG_TARGET_ARCH not set");
     if target_arch == "x86_64" && target_os == "android" {
-        let android_ndk_home = env::var("ANDROID_NDK_HOME").expect("ANDROID_NDK_HOME not set");
-        let build_os = match env::consts::OS {
-            "linux" => "linux",
-            "macos" => "darwin",
-            "windows" => "windows",
-            _ => panic!(
-                "Unsupported OS. You must use either Linux, MacOS or Windows to build the crate."
-            ),
-        };
-        const DEFAULT_CLANG_VERSION: &str = "18";
-        let clang_version =
-            env::var("NDK_CLANG_VERSION").unwrap_or_else(|_| DEFAULT_CLANG_VERSION.to_owned());
-        let linux_x86_64_lib_dir = format!(
-            "toolchains/llvm/prebuilt/{build_os}-x86_64/lib/clang/{clang_version}/lib/linux/"
+        // Configure rust to statically link against the `libclang_rt.builtins` supplied
+        // with clang.
+
+        // cargo-ndk sets CC_x86_64-linux-android to the path to `clang`, within the
+        // Android NDK.
+        let clang_path = PathBuf::from(
+            env::var("CC_x86_64-linux-android").expect("CC_x86_64-linux-android not set"),
         );
-        println!("cargo:rustc-link-search={android_ndk_home}/{linux_x86_64_lib_dir}");
+
+        // clang_path should now look something like
+        // `.../sdk/ndk/28.0.12674087/toolchains/llvm/prebuilt/linux-x86_64/bin/clang`.
+        // We strip `/bin/clang` from the end to get the toolchain path.
+        let toolchain_path = clang_path
+            .ancestors()
+            .nth(2)
+            .expect("could not find NDK toolchain path")
+            .to_str()
+            .expect("NDK toolchain path is not valid UTF-8");
+
+        let clang_version = get_clang_major_version(&clang_path);
+
+        println!("cargo:rustc-link-search={toolchain_path}/lib/clang/{clang_version}/lib/linux/");
         println!("cargo:rustc-link-lib=static=clang_rt.builtins-x86_64-android");
     }
 }
 
+/// Run the clang binary at `clang_path`, and return its major version number
+fn get_clang_major_version(clang_path: &PathBuf) -> String {
+    let clang_output =
+        Command::new(clang_path).arg("-dumpversion").output().expect("failed to start clang");
+
+    if !clang_output.status.success() {
+        panic!("failed to run clang: {}", String::from_utf8_lossy(&clang_output.stderr));
+    }
+
+    let clang_version = String::from_utf8(clang_output.stdout).expect("clang output is not utf8");
+    clang_version.split('.').next().expect("could not parse clang output").to_owned()
+}
+
 fn main() -> Result<(), Box<dyn Error>> {
     setup_x86_64_android_workaround();
     uniffi::generate_scaffolding("./src/api.udl").expect("Building the UDL file failed");