Add -- for files in try_expand (like in rust-lang#514) (rust-lang…

…#911)

* fix

* format

src/lib.rs

@@ -1542,6 +1542,21 @@ impl Build {
             "Expand may only be called for a single file"
         );
 
+        let is_asm = self
+            .files
+            .iter()
+            .map(std::ops::Deref::deref)
+            .find_map(AsmFileExt::from_path)
+            .is_some();
+
+        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
+            // common `/Users/...` paths as the `/U` flag and triggering
+            // `-Wslash-u-filename` warning.
+            cmd.arg("--");
+        }
+
         cmd.args(self.files.iter().map(std::ops::Deref::deref));
 
         let name = compiler