I'm not opposed to this. Still, given that this is restricted to slices, how often do we expect people to implement sort.Interface in new code, as opposed to writing a comparison function that can be passed to slices.Sort?

This could be made more general by taking pointers instead:

func Swap[E any](v1, v2 *E) { *v1, *v2 = *v2, *v1 }

Then you could do

func (ex *Example) Swap(i1, i2 int) {
  pkg.Swap(&ex.s[i1], &ex.s[i2])
}

Ah, beat me to it, @ianlancetaylor. Your reply appeared as I was typing this.

how often do we expect people to implement sort.Interface in new code, as opposed to writing a comparison function that can be passed to slices.Sort?

sort.Data is more general than slices.Sort. For example, it allows sorting when your records are organized in columns.

pkg.Swap(&ex.s[i1], &ex.s[i2])

That's much noisier than slices.Swap(ex.S, i1, i2), IMO. It requires extra &, [, and ] tokens and requires repeating the ex.s expression.

slices.Swap would also be useful for using rand.Shuffle. E.g., https://pkg.go.dev/math/rand#example-Shuffle-SlicesInUnison

I added a Swapper suggestion too, to further simplify the common case of just wanting to curry the first argument to Swap.