Add Build::asm_flag

The method allows flags to be specified that are only applied to
assembly input files, not C files. This is helpful with clang, which
only accepts certain assembly options if the input file is asm, not C.

src/lib.rs

@@ -97,6 +97,7 @@ pub struct Build {
     flags_supported: Vec<String>,
     known_flag_support_status: Arc<Mutex<HashMap<String, bool>>>,
     ar_flags: Vec<String>,
+    asm_flags: Vec<String>,
     no_default_flags: bool,
     files: Vec<PathBuf>,
     cpp: bool,
@@ -299,6 +300,7 @@ impl Build {
             flags_supported: Vec::new(),
             known_flag_support_status: Arc::new(Mutex::new(HashMap::new())),
             ar_flags: Vec::new(),
+            asm_flags: Vec::new(),
             no_default_flags: false,
             files: Vec::new(),
             shared_flag: None,
@@ -434,6 +436,25 @@ impl Build {
         self
     }
 
+    /// Add a flag that will only be used with assembly files.
+    ///
+    /// The flag will be applied to input files with either a `.s` or
+    /// `.asm` extension (case insensitive).
+    ///
+    /// # Example
+    ///
+    /// ```no_run
+    /// cc::Build::new()
+    ///     .asm_flag("-Wa,-defsym,abc=1")
+    ///     .file("src/foo.S")  // The asm flag will be applied here
+    ///     .file("src/bar.c")  // The asm flag will not be applied here
+    ///     .compile("foo");
+    /// ```
+    pub fn asm_flag(&mut self, flag: &str) -> &mut Build {
+        self.asm_flags.push(flag.to_string());
+        self
+    }
+
     fn ensure_check_file(&self) -> Result<PathBuf, Error> {
         let out_dir = self.get_out_dir()?;
         let src = if self.cuda {
@@ -1318,7 +1339,7 @@ impl Build {
     }
 
     fn compile_object(&self, obj: &Object) -> Result<(), Error> {
-        let is_asm = obj.src.extension().and_then(|s| s.to_str()) == Some("asm");
+        let is_asm = is_asm(&obj.src);
         let target = self.get_target()?;
         let msvc = target.contains("msvc");
         let compiler = self.try_get_compiler()?;
@@ -1349,6 +1370,9 @@ impl Build {
         if self.cuda && self.files.len() > 1 {
             cmd.arg("--device-c");
         }
+        if is_asm {
+            cmd.args(&self.asm_flags);
+        }
         if compiler.family == (ToolFamily::Msvc { clang_cl: true }) && !is_asm {
             // #513: For `clang-cl`, separate flags/options from the input file.
             // When cross-compiling macOS -> Windows, this avoids interpreting
@@ -3471,3 +3495,15 @@ fn which(tool: &Path) -> Option<PathBuf> {
         return if check_exe(&mut exe) { Some(exe) } else { None };
     })
 }
+
+/// Check if the file's extension is either "asm" or "s", case insensitive.
+fn is_asm(file: &Path) -> bool {
+    if let Some(ext) = file.extension() {
+        if let Some(ext) = ext.to_str() {
+            let ext = ext.to_lowercase();
+            return ext == "asm" || ext == "s";
+        }
+    }
+
+    false
+}

tests/test.rs

@@ -442,3 +442,20 @@ fn msvc_no_dash_dash() {
 
     test.cmd(0).must_not_have("--");
 }
+
+// Disable this test with the parallel feature because the execution
+// order is not deterministic.
+#[cfg(not(feature = "parallel"))]
+#[test]
+fn asm_flags() {
+    let test = Test::gnu();
+    test.gcc()
+        .file("foo.c")
+        .file("x86_64.asm")
+        .file("x86_64.S")
+        .asm_flag("--abc")
+        .compile("foo");
+    test.cmd(0).must_not_have("--abc");
+    test.cmd(1).must_have("--abc");
+    test.cmd(2).must_have("--abc");
+}