Update Android compiler detection for NDK r25

This commit updates the compiler detection logic for the Android
platform to work with NDK r25.  This code is largely identical to the
corresponding code in rustc/src/bootstrap/cc_detect.rs.

Author: chriswailes

src/lib.rs

@@ -2286,7 +2286,7 @@ impl Build {
                 } else if target.contains("apple-watchos") {
                     clang.to_string()
                 } else if target.contains("android") {
-                    autodetect_android_compiler(&target, &host, gnu, clang)
+                    autodetect_android_compiler(&target, &host, clang)
                 } else if target.contains("cloudabi") {
                     format!("{}-{}", target, traditional)
                 } else if target == "wasm32-wasi"
@@ -3221,16 +3221,6 @@ fn command_add_output_file(
     }
 }
 
-// Use by default minimum available API level
-// See note about naming here
-// https://android.googlesource.com/platform/ndk/+/refs/heads/ndk-release-r21/docs/BuildSystemMaintainers.md#Clang
-static NEW_STANDALONE_ANDROID_COMPILERS: [&str; 4] = [
-    "aarch64-linux-android21-clang",
-    "armv7a-linux-androideabi16-clang",
-    "i686-linux-android16-clang",
-    "x86_64-linux-android21-clang",
-];
-
 // New "standalone" C/C++ cross-compiler executables from recent Android NDK
 // are just shell scripts that call main clang binary (from Android NDK) with
 // proper `--target` argument.
@@ -3253,7 +3243,7 @@ fn android_clang_compiler_uses_target_arg_internally(clang_path: &Path) -> bool
 
 #[test]
 fn test_android_clang_compiler_uses_target_arg_internally() {
-    for version in 16..21 {
+    for version in 16..33 {
         assert!(android_clang_compiler_uses_target_arg_internally(
             &PathBuf::from(format!("armv7a-linux-androideabi{}-clang", version))
         ));
@@ -3269,50 +3259,33 @@ fn test_android_clang_compiler_uses_target_arg_internally() {
     ));
 }
 
-fn autodetect_android_compiler(target: &str, host: &str, gnu: &str, clang: &str) -> String {
-    let new_clang_key = match target {
-        "aarch64-linux-android" => Some("aarch64"),
-        "armv7-linux-androideabi" => Some("armv7a"),
-        "i686-linux-android" => Some("i686"),
-        "x86_64-linux-android" => Some("x86_64"),
-        _ => None,
+fn autodetect_android_compiler(target: &str, host: &str, clang: &str) -> String {
+    let mut triple_iter = target.split("-");
+    let triple_translated = if let Some(arch) = triple_iter.next() {
+        let arch_new = match arch {
+            "arm" | "armv7" | "armv7neon" | "thumbv7" | "thumbv7neon" => "armv7a",
+            other => other,
+        };
+        std::iter::once(arch_new).chain(triple_iter).collect::<Vec<&str>>().join("-")
+    } else {
+        target.to_string()
     };
 
-    let new_clang = new_clang_key
-        .map(|key| {
-            NEW_STANDALONE_ANDROID_COMPILERS
-                .iter()
-                .find(|x| x.starts_with(key))
-        })
-        .unwrap_or(None);
-
-    if let Some(new_clang) = new_clang {
-        if Command::new(new_clang).output().is_ok() {
-            return (*new_clang).into();
-        }
-    }
-
-    let target = target
-        .replace("armv7neon", "arm")
-        .replace("armv7", "arm")
-        .replace("thumbv7neon", "arm")
-        .replace("thumbv7", "arm");
-    let gnu_compiler = format!("{}-{}", target, gnu);
-    let clang_compiler = format!("{}-{}", target, clang);
+    // API 19 is the earliest API level supported by NDK r25b but AArch64 and x86_64 support
+    // begins at API level 21.
+    let api_level =
+        if target.contains("aarch64") || target.contains("x86_64") { "21" } else { "19" };
+    let compiler = format!("{}{}-{}", triple_translated, api_level, clang);
 
     // On Windows, the Android clang compiler is provided as a `.cmd` file instead
     // of a `.exe` file. `std::process::Command` won't run `.cmd` files unless the
     // `.cmd` is explicitly appended to the command name, so we do that here.
-    let clang_compiler_cmd = format!("{}-{}.cmd", target, clang);
-
-    // Check if gnu compiler is present
-    // if not, use clang
-    if Command::new(&gnu_compiler).output().is_ok() {
-        gnu_compiler
-    } else if host.contains("windows") && Command::new(&clang_compiler_cmd).output().is_ok() {
-        clang_compiler_cmd
+    let windows_compiler_cmd = format!("{}.cmd", compiler);
+
+    if host.contains("windows") && Command::new(&windows_compiler_cmd).output().is_ok() {
+        windows_compiler_cmd
     } else {
-        clang_compiler
+        compiler
     }
 }