Convert multibyte `string` to `dstring`

[Vladimirs Nordholm via Digitalmars-d-learn]

Hello.

Is there a proper way to convert a string with multibyte 
characters into a dstring?

Case scenario:

    string a = "abc😃123"; // a.length == 10 ("abc"==3 + "😃"==4 + 
"123"==3)
    dstring b = foo(a); // b.length = 7 ("abc"==3 + "😃"==1 + 
"123"==3)

    dstring foo(string str) {
        // code...
    }