proposal: document api compatibility

[arnottcr]

Background

Currently there are four official definitions for api compatibility⁰¹²³, as well as others offered by the community⁴. It should be noted that some of these definitions target package compatibility, where other target module compatibility, and some are experimental, while others have been constant since go1. Unfortunately, they each disagree on at least a few minor points in their definitions of compatibility. This lack of an official stance and single vision for compatibility has led to confusion and uncertainty among the community.

Proposal

In some official capacity, publish documentation both package and module compatibility guarantees. This should involve a package level definition, such as the apidiff definition², and module definition, which can borrow heavily from the former.

Footnotes

0] https://golang.org/doc/go1compat
1] https://blog.golang.org/publishing-go-modules#TOC_3.
2] https://go.googlesource.com/exp/+/master/apidiff/README.md
3] https://golang.org/ref/spec
4] https://github.com/bradleyfalzon/apicompat

[bcmills]

CC @jayconrod @thepudds @jadekler @tbpg

[jadekler]

+1, seems good to have. This seems like something that should be a spec, which apidiff (cc @jba) and related guides reference. What do y'all think?

[jayconrod]

It's pretty hard to pin down a definition of compatibility that everyone can agree on (see Hyrum's Law).

I agree that we should have some canonical documentation on the subject though.

Related #33637
Related #26420

[arnottcr]

It's pretty hard to pin down a definition of compatibility that everyone can agree on (see Hyrum's Law).

I have always thought of this as compile time compatibility, since runtime compatibility is hard if not impossible, as called out in the apidiff document. I think targeting does it compile is acheivable and apidiff is close if not exactly the package spec I would publish.