`inout` unit parameter does not work with unit variable

[J-Gras]

I was trying to adapt the example for using inout parameters with basic types from the docs to use a unit variable instead of a global:

module Test;

type X = unit(inout msg: string&) {
    n : uint8 {
      local s = "Parsed %d" % $$;
      msg = new s;
    }
};

public type Y = unit {
    var msg: string& = new "Nothing parsed, yet";

    x: X(self.msg);
    on %done { print self.msg; }
};

Unfortunately, this just prints "Nothing parsed, yet". @bbannier took a look and found:

This seems to be a bug, and even if you printed msg after assigning to it in n’s hook its value would still be unchanged. Looking in the generated code this seems to be due to Spicy generating a hook-local temporary which is assigned to, but never used.

    ::hilti::rt::StrongReference<std::string> __lhs_1;
    ::hilti::rt::detail::checkStack();
    std::string s = ::hilti::rt::fmt(std::string("Parsed %d"), __dd);
    (__lhs_1=(__self->__p_foo.derefAsValue())) = ::hilti::rt::reference::make_strong<std::string>(s);