Semantics of toString

[bearophile]

Bill Baxter:

> Though they may be useful, those don't look to have anything to do
> with formatting user types into strings, which is the subject at hand.

Don has said: "But the performance would still be very poor, and that's much more difficult to solve." And those links show a way to quickly convert a large multi-precision integer into a string. What is that I am missing?

Bye,
bearophile