cmd/go: forward compatibility work [freeze exception]

[rsc]

The work for #57001 is not yet complete. Pending CLs are here, modulo Gerrit's search problems, plus there will be one more important bug fix as well as probably some additional commentary and documentation. The non-doc, non-test CLs (other than bug fixes) will be ready to submit by end of week. This is an important feature for Go 1.21 that unlocks important work planned for Go 1.22. (If it doesn't get into Go 1.21 then the loopvar changes we hope to get into Go 1.22 slip to Go 1.23.)

I apologize profusely for needing the exception.

[dmitshur]

CC @golang/release.

[dmitshur]

Thanks for filing this. It's unfortunate that #57001 only gained the release-blocker label late in the cycle and we couldn't do more to help with the timeline for a significant feature planned for Go 1.21.

Approved after offline discussion. It helps that this involves cmd/go work and should not cause problems for other parts of the project, and that it's very close to being ready.

I'll close this in favor of #57001 since that issue is already in Go1.21 milestone and has release-blocker. Please update it if this ends up taking significantly longer than you currently anticipate. Thanks.

[rsc]

It took a week longer than I expected to tie up various loose ends that @bcmills and I identified, but I've just submitted the last non-doc CL for #57001 (modulo bug fixes of course). Docs CLs to come in the next few days.