reflect: deprecate PtrTo

[gazerro]

The reflect.PointerTo function was added in version 1.18 to replace the reflect.PtrTo function. reflect.PtrTo was supposed to be deprecated in version 1.19, as recommended in https://github.com/golang/go/wiki/Deprecated, but it was not done.

[randall77]

See #48665

[gophun]

According to this 1.21 will already be 1.N+3

[ianlancetaylor]

CC @mvdan : OK with deprecating these now?

[mvdan]

Per the rules we agreed on and wrote on https://github.com/golang/go/wiki/Deprecated, yes, I'm perfectly fine with deprecating at this point :)

[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]

go.dev/wiki/Deprecated says:

Marking an API feature deprecated can create work and decisions for millions of Go developers using the feature. An API feature should only be marked deprecated if its use is problematic in some way, not simply because something newer exists.

It is unclear to me that PtrTo really is "problematic in some way". It's just an old spelling, and the doc comment for PtrTo calls that out. It seems like we should not mark it Deprecated based on this.

[rsc]

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