For non-MSVC, separate flags/options from the input file. (#513)

This avoids a `clang-cl` issue common when cross-compiling from macOS
to Windows: the `-Wslash-u-filename` where a `/Users/...` path is
interpreted as the `cl.exe` flag `/U`.

In general it's somewhere between harmless and good form to separate
flags/options from input files, so `--` is used for all compilers save
MSVC.

There are no existing tests mocking out `clang-cl` and surface efforts
didn't succeed, so that will have to wait for follow-up.

src/lib.rs

@@ -1171,6 +1171,9 @@ impl Build {
         if !msvc || !is_asm || !is_arm {
             cmd.arg("-c");
         }
+        if !msvc || compiler.family == (ToolFamily::Msvc { clang_cl: true }) {
+            cmd.arg("--"); // #513: For non-MSVC, separate flags/options from the input file.
+        }
         cmd.arg(&obj.src);
 
         run(&mut cmd, &name)?;

tests/support/mod.rs

@@ -151,6 +151,15 @@ impl Execution {
         };
         self
     }
+
+    pub fn must_end_with<P: AsRef<OsStr>>(&self, p: &[P]) -> &Execution {
+        let expected = p.iter().map(|x| x.as_ref()).collect::<Vec<_>>();
+        if !self.args.iter().map(|x| OsStr::new(x)).collect::<Vec<_>>().as_slice().ends_with(expected.as_slice()) {
+            panic!("did not end with {:?}", expected);
+        } else {
+            self
+        }
+    }
 }
 
 /// Hard link an executable or copy it if that fails.

tests/test.rs

@@ -343,6 +343,14 @@ fn gnu_static() {
     test.cmd(0).must_have("-static").must_not_have("-shared");
 }
 
+#[test]
+fn gnu_dash_dash() {
+    let test = Test::gnu();
+    test.gcc().file("foo.c").compile("foo");
+
+    test.cmd(0).must_end_with(&["--", "foo.c"]);
+}
+
 #[test]
 fn msvc_smoke() {
     reset_env();
@@ -411,3 +419,11 @@ fn msvc_no_static_crt() {
 
     test.cmd(0).must_have("-MD");
 }
+
+#[test]
+fn msvc_no_dash_dash() {
+    let test = Test::msvc();
+    test.gcc().file("foo.c").compile("foo");
+
+    test.cmd(0).must_not_have("--");
+}