Purity of delegate-style toString

[Per Nordlöw via Digitalmars-d-learn]

In which cases (if any) is it possible to make a delegate-style 
implementation of toString such as

    void toString(scope void delegate(const(char)[]) sink) const 
@trusted pure
    {
        // sink("...");
        // sink.formattedWrite!"..."(...);
    }
pure?