Add regression test for issue 738

Currently fails to build.

    In file included from cxx-test-suite-32b8d00ca9482f5f/out/cxxbridge/crate/tests/ffi/tests.h:2,
                     from tests.cc:1:
    cxx-test-suite-32b8d00ca9482f5f/out/cxxbridge/include/rust/cxx.h: In instantiation of ‘Ret rust::cxxbridge1::Fn<Ret(Args ...)>::operator()(Args ...) const [with Ret = void; Args = {rust::cxxbridge1::String&}]’:
    tests.cc:504:18:   required from here
    cxx-test-suite-32b8d00ca9482f5f/out/cxxbridge/include/rust/cxx.h:478:29: error: cannot bind non-const lvalue reference of type ‘rust::cxxbridge1::String&’ to an rvalue of type ‘std::remove_reference<rust::cxxbridge1::String&>::type’ {aka ‘rust::cxxbridge1::String’}
      478 |   return (*this->trampoline)(std::move(args)..., this->fn);
          |          ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    cxx-test-suite-32b8d00ca9482f5f/out/cxxbridge/include/rust/cxx.h:478:58: error: return-statement with a value, in function returning ‘void’ [-fpermissive]
      478 |   return (*this->trampoline)(std::move(args)..., this->fn);
          |                                                          ^

tests/ffi/lib.rs

@@ -157,6 +157,8 @@ pub mod ffi {
         fn c_take_ref_rust_vec_copy(v: &Vec<u8>);
         fn c_take_ref_shared_string(s: &SharedString) -> &SharedString;
         fn c_take_callback(callback: fn(String) -> usize);
+        fn c_take_callback_ref(callback: fn(&String));
+        fn c_take_callback_mut(callback: fn(&mut String));
         fn c_take_enum(e: Enum);
         fn c_take_ns_enum(e: AEnum);
         fn c_take_nested_ns_enum(e: ABEnum);

tests/ffi/tests.cc

@@ -494,6 +494,16 @@ void c_take_callback(rust::Fn<size_t(rust::String)> callback) {
   callback("2020");
 }
 
+void c_take_callback_ref(rust::Fn<void(const rust::String &)> callback) {
+  const rust::String string = "2020";
+  callback(string);
+}
+
+void c_take_callback_mut(rust::Fn<void(rust::String &)> callback) {
+  rust::String string = "2020";
+  callback(string);
+}
+
 void c_take_enum(Enum e) {
   if (e == Enum::AVal) {
     cxx_test_suite_set_correct();

tests/ffi/tests.h

@@ -160,6 +160,8 @@ void c_take_ref_rust_vec_index(const rust::Vec<uint8_t> &v);
 void c_take_ref_rust_vec_copy(const rust::Vec<uint8_t> &v);
 const SharedString &c_take_ref_shared_string(const SharedString &s);
 void c_take_callback(rust::Fn<size_t(rust::String)> callback);
+void c_take_callback_ref(rust::Fn<void(const rust::String &)> callback);
+void c_take_callback_mut(rust::Fn<void(rust::String &)> callback);
 void c_take_enum(Enum e);
 void c_take_ns_enum(::A::AEnum e);
 void c_take_nested_ns_enum(::A::B::ABEnum e);

tests/test.rs

@@ -204,7 +204,21 @@ fn test_c_callback() {
         0
     }
 
+    fn callback_ref(s: &String) {
+        if s == "2020" {
+            cxx_test_suite_set_correct();
+        }
+    }
+
+    fn callback_mut(s: &mut String) {
+        if s == "2020" {
+            cxx_test_suite_set_correct();
+        }
+    }
+
     check!(ffi::c_take_callback(callback));
+    check!(ffi::c_take_callback_ref(callback_ref));
+    check!(ffi::c_take_callback_mut(callback_mut));
 }
 
 #[test]