Creating `UniquePtr<CxxString>`

[tomekb234]

If I understand correctly, there is currently no way to create UniquePtr<CxxString> on the Rust side without writing own utility function:

unsafe extern "C++" {
    fn make_unique_cxx_string(slice: &[u8]) -> UniquePtr<CxxString>;
}

std::unique_ptr<std::string> make_unique_cxx_string(rust::Slice<const uint8_t> slice) {
    return std::make_unique<std::string>(reinterpret_cast<const char*>(slice.data()), slice.size());
}

This is unlike Pin<&mut CxxString> (and consequently &CxxString), which can be made with let_cxx_string!() provided by this library.

Wouldn't it make sense to have a "standard" implementation of make_unique_cxx_string() provided by cxx? It could also be in some more convenient form, for example:

fn make_unique_cxx_string(data: impl AsRef<[u8]>) -> UniquePtr<CxxString>

This problem was also mentioned in #1339.