ndk-build: Use LLVM binutils when GNU binutils are removed (r23+) (#137)

* ndk-build: Use LLVM binutils when GNU binutils are removed (r23+)

Since [r21] LLVM binutils are included _for testing_;
Since [r22] GNU binutils are deprecated in favour of LLVM's;
Since [r23] GNU binutils have been removed.

When GNU binutils are not available (`<triple>-<bin_name>`) "fall back"
to `llvm-<bin_name>` binaries from LLVM binutils that are now included
with the NDK.

To maintain stability with the current ndk-build crate release, prefer
GNU binutils for as long as it is provided by the NDK instead of trying
to use `llvm-<bin_name>` from r21 onwards.

[r21]: https://github.com/android/ndk/wiki/Changelog-r21
[r22]: https://github.com/android/ndk/wiki/Changelog-r22
[r23]: https://github.com/android/ndk/wiki/Changelog-r23

Fixes #135

* ndk-build/error: Wrap variable arguments in backticks

ndk-build/src/error.rs

@@ -17,24 +17,30 @@ pub enum NdkError {
         environment variable."
     )]
     NdkNotFound,
-    #[error("Path {0:?} doesn't exist.")]
+    #[error("GNU toolchain binary `{gnu_bin}` nor LLVM toolchain binary `{llvm_bin}` found in `{toolchain_path:?}`.")]
+    ToolchainBinaryNotFound {
+        toolchain_path: PathBuf,
+        gnu_bin: String,
+        llvm_bin: String,
+    },
+    #[error("Path `{0:?}` doesn't exist.")]
     PathNotFound(PathBuf),
-    #[error("Command {0} not found.")]
+    #[error("Command `{0}` not found.")]
     CmdNotFound(String),
     #[error("Android SDK has no build tools.")]
     BuildToolsNotFound,
     #[error("Android SDK has no platforms installed.")]
     NoPlatformFound,
-    #[error("Platform {0} is not installed.")]
+    #[error("Platform `{0}` is not installed.")]
     PlatformNotFound(u32),
     #[error("Target is not supported.")]
     UnsupportedTarget,
-    #[error("Host {0} is not supported.")]
+    #[error("Host `{0}` is not supported.")]
     UnsupportedHost(String),
     #[error(transparent)]
     Io(#[from] IoError),
     #[error("Invalid semver")]
     InvalidSemver,
-    #[error("Command '{}' had a non-zero exit code.", format!("{:?}", .0).replace('"', ""))]
+    #[error("Command `{}` had a non-zero exit code.", format!("{:?}", .0).replace('"', ""))]
     CmdFailed(Command),
 }

ndk-build/src/ndk.rs

@@ -196,22 +196,35 @@ impl Ndk {
         Ok((clang, clang_pp))
     }
 
-    pub fn toolchain_bin(&self, bin: &str, target: Target) -> Result<PathBuf, NdkError> {
+    pub fn toolchain_bin(&self, name: &str, target: Target) -> Result<PathBuf, NdkError> {
         #[cfg(target_os = "windows")]
         let ext = ".exe";
         #[cfg(not(target_os = "windows"))]
         let ext = "";
 
-        let bin = self.toolchain_dir()?.join("bin").join(format!(
-            "{}-{}{}",
-            target.ndk_triple(),
-            bin,
-            ext
-        ));
-        if !bin.exists() {
-            return Err(NdkError::PathNotFound(bin));
+        let toolchain_path = self.toolchain_dir()?.join("bin");
+
+        // Since r21 (https://github.com/android/ndk/wiki/Changelog-r21) LLVM binutils are included _for testing_;
+        // Since r22 (https://github.com/android/ndk/wiki/Changelog-r22) GNU binutils are deprecated in favour of LLVM's;
+        // Since r23 (https://github.com/android/ndk/wiki/Changelog-r23) GNU binutils have been removed.
+        // To maintain stability with the current ndk-build crate release, prefer GNU binutils for
+        // as long as it is provided by the NDK instead of trying to use llvm-* from r21 onwards.
+        let gnu_bin = format!("{}-{}{}", target.ndk_triple(), name, ext);
+        let gnu_path = toolchain_path.join(&gnu_bin);
+        if gnu_path.exists() {
+            Ok(gnu_path)
+        } else {
+            let llvm_bin = format!("llvm-{}{}", name, ext);
+            let llvm_path = toolchain_path.join(&llvm_bin);
+            llvm_path
+                .exists()
+                .then(|| llvm_path)
+                .ok_or_else(|| NdkError::ToolchainBinaryNotFound {
+                    toolchain_path,
+                    gnu_bin,
+                    llvm_bin,
+                })
         }
-        Ok(bin)
     }
 
     pub fn android_dir(&self) -> Result<PathBuf, NdkError> {