net/netip: add IPv6Loopback

[mdlayher]

We already have net.IPv6loopback today, but lack an equivalent in net/netip. I'd like to propose adding netip.IPv6Loopback specifically; IPv6 defines exactly one loopback address: ::1.

IPv4 assigns the entire 127.0.0.0/8 and thus should be out of scope for this proposal.

Concretely:

// IPv6Loopback returns the IPv6 loopback address ::1.
func IPv6Loopback() Addr { return AddrFrom16([16]byte{15: 0x01}) }

See also #51766 for a similar addition and rationale. Thanks!

/cc @bradfitz @danderson @josharian

[ianlancetaylor]

CC @bradfitz @josharian

[bradfitz]

Seems fine to me.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

[gopherbot]

Change https://go.dev/cl/412475 mentions this issue: net/netip: add IPv6LinkLocalAllRouters and IPv6Loopback