What version of Go are you using (go version)?

1.9beta1

What did you do?

In my code, I'm implementing fmt.Formatter.
In order to write a string to the fmt.State, I'm calling io.WriteString().
I know that this function has an optimization that allows to call WriteString() if w implements it.
Sadly, in my case the "real" type of fmt.State is fmt.pp.
fmt.pp only has Write().

What did you expect to see?

I think we should add:

func (p *pp) WriteString(s string) (ret int, err error) {
	p.buf.WriteString(s)
	return len(s), nil
}

in my case, it greatly improve performance and reduces memory allocation.
We could also implement WriteByte() and WriteRune(), since fmt.buffer has these methods.