Timeline for Mill 0.13.0

[lihaoyi]

This is a thread to centralize discussion about the plans for 0.13.0, now that 0.12.0 has landed. A follow up to #3209

What is Mill 0.13.0

0.13.0 will be a binary incompatible breaking release for Mill. We accumulated a backlog of issues that could not go into 0.12.0 due to binary compatibility breakage, which will need to go into 0.13.0:

• Issues https://github.com/com-lihaoyi/mill/issues?q=is%3Aopen+is%3Aissue+milestone%3A0.13.0
• Pull Requests https://github.com/com-lihaoyi/mill/pulls?q=is%3Aopen+is%3Apr+milestone%3A0.13.0

The Scala 3 upgrade PR (#3369) is probably the first such PR that we will need to bring up to date and merge, but there's a significant backlog of TODO issues and PRs, and I expect the removal of binary compatibility constraints will give us the opportunity for further cleanups and improvements that we don't know about yet.

When is Mill 0.13.0 Happening?

For now my proposed timeline for 0.13.0 is as follows:

1. Oct 2024: Mill 0.12.0 final is released (DONE)
   • main still targets 0.12.x series, which will need to be (eventually) merged into 0.13.0 PRs (primarily build.mill files compiled by Scala 3 #3369)
2. Jan 2025: 0.12.x maintenance branch is cut, first binary-incompatible changes land in main targeting 0.13.0
   • main now targets 0.13.0, PRs which want to target 0.12.x will need to be explicitly backported to 0.12.x branch
3. Apr 2025: 0.13.0 RCs begin to go out
4. May 2025: 0.13.0 final is released, third-party plugins are re-published

This is a reasonably long timeline, but 0.13.0 is our chance to make binary-incompatible changes.

• We want to wait until the 0.12.x series has stabilized before making main target 0.13.0, since after that we would need to backport fixes to the 0.12.x maintenance branch, which is annoying

• The more such changes we can batch together and the more we can get them correct, the longer we can wait for the next breaking release of 0.14.0, which is ultimately good for the stability of the ecosystem. So we want to take our time preparing 0.13.0

• At the same time, we do not want the "PRs targeting main/0.13.0 cannot be released since 0.13.0 isn't ready yet" phase to last too long, since that means any merged PRs that people want to actually used would need to be backported to 0.12.x

Hence the 2-3 month period from Oct-Jan to stabilize 0.12.x and the 3-4 month period from Jan to Apr/May to prepare 0.13.0 is a compromise between these three goals.

Hopefully, if we do this right, the outcome will be that we'll be able continue active development of Mill with new features and improvements, while also preserving binary/semantic compatibility in the 0.13.x series as long as possible.

[lihaoyi]

We are now a good way through our Mill 0.12.x cycle, with 0.12.3 released. I expect to have maybe 2 more 0.12.x releases - 0.12.4 in mid-dec 2024, 0.12.5 in mid-jan 2025 - before merging #3369 and breaking compatibility in main and cutting a 0.12.x release branch.

After that, PRs to 0.12.x can continue, but will be less convenient since they will require backporting with duplicate PRs targeting main and 0.12.x