It seems to me easy enough to write

func pathUrl(p []string) string {
	var b []byte
	for i, s := range p {
		if i > 0 {
			b = append(b, '/')
		}
		t := url.PathEscape(s)
		b = append(b, t...)
	}
	return string(b)
}

I'm not convinced it's worth the complexity of adding Unwrite methods to strings.Builder.

If you are unwriting only the final byte, you could use strings.Builder and then slice away the final byte of the string.

@ianlancetaylor, I agree that it's a matter of taste. It becomes a little more messy when i is not readily available (when the components are not coming from a slice).

Feel free to close if you feel that this proposal is unlikely to gain consensus.

I'd personally deal with p[0] outside the loop, and range over p[1:], and never have to unwrite.

https://play.golang.org/p/LODpCtD3mjh

I think it's always going to be straightforward to not write data, and very few io.Writer implementations implement UnwriteByte. So I do think this is unlikely to gain consensus.