rust: Emit -x c++ for bindgen modules with cpp_std

rust_bindgen modules which defined cpp_std with a .h file were not
correctly emitting the `-x c++` flag. This CL addresses that, and
ensures that if either cpp_std or c_std is set then the appropriate
behavior occurs no matter the file extension.

Bug: 304269101
Test: Soong tests
Change-Id: I71a8ae30ac0ed502d9d3fbf2f3039b0c56529d39

rust/bindgen_test.go

@@ -115,7 +115,7 @@ func TestRustBindgenStdVersions(t *testing.T) {
 	ctx := testRust(t, `
 		rust_bindgen {
 			name: "libbindgen_cstd",
-			wrapper_src: "src/any.h",
+			wrapper_src: "src/any.hpp",
 			crate_name: "bindgen",
 			stem: "libbindgen",
 			source_stem: "bindings",
@@ -141,6 +141,16 @@ func TestRustBindgenStdVersions(t *testing.T) {
 	if !strings.Contains(libbindgen_cppstd.Args["cflags"], "-std=foo") {
 		t.Errorf("cpp_std value not passed in to rust_bindgen as a clang flag")
 	}
+
+	// Make sure specifying cpp_std emits the '-x c++' flag
+	if !strings.Contains(libbindgen_cppstd.Args["cflags"], "-x c++") {
+		t.Errorf("Setting cpp_std should cause the '-x c++' flag to be emitted")
+	}
+
+	// Make sure specifying c_std omits the '-x c++' flag
+	if strings.Contains(libbindgen_cstd.Args["cflags"], "-x c++") {
+		t.Errorf("Setting c_std should not cause the '-x c++' flag to be emitted")
+	}
 }
 
 func TestBindgenDisallowedFlags(t *testing.T) {