This should be netip.To6() but it does not exist and it was rejected.
There is a benchmark showing the improvment of such optimisation:
BenchmarkNetIPTo6/safe_v4-12 170152954 7.054 ns/op
BenchmarkNetIPTo6/unsafe_v4-12 764772190 1.553 ns/op
See https://github.com/golang/go/issues/54365.