Migrate existing modules to the new toolchain registration API

[Wyverald]

See bazelbuild/bazel#15829

This:

module(
  name = "foo",
  version = "2.0",
  toolchains_to_register = ["@toolchain_repo//:toolchain"],
)

ext = use_extension("//:extensions.bzl", "ext")
ext.some_tag()
use_repo(ext, "toolchain_repo")

should become this:

module(
  name = "foo",
  version = "2.0",
)

ext = use_extension("//:extensions.bzl", "ext")
ext.some_tag()
use_repo(ext, "toolchain_repo")
register_toolchains("@toolchain_repo//:toolchain")

[alexeagle]

Is there some min,max version of Bazel where both ways are supported, to allow the transition?

[meteorcloudy]

The support for the old API isn't deleted at HEAD yet (the BCR presubmit uses Bazel@HEAD), but we'll probably delete them before 6.0 cut. Since Bzlmod is still under an experimental flag, I think that should be enough?

@Wyverald Do you think we need to cherry-pick recent Bzlmod changes into 5.3?

[Wyverald]

Some are cherry-pickable (like this one) but the flag parsing ones are probably harder. Hopefully people have been testing Bzlmod-related stuff with rolling releases (maybe we should've been clearer about that)

[mattyclarkson]

Hopefully people have been testing Bzlmod-related stuff with rolling releases (maybe we should've been clearer about that)

We've been testing across Bazel stable releases after 5.0.0.

This just broke our CI when testing against anything other than 5.3.0.

It's not a problem, we'll just drop support for 5.2.0 and below but certainly confused us for 15 minutes why the API didn't suddenly exist.

Looking forward to bzlmod becoming non-experimental: we really like it :)

[Wyverald]

To be clear, we haven't removed the old API from HEAD yet. @meteorcloudy's change merely migrated existing modules to use the new API, which is what broke you since the new API wasn't available before. (But now that that's done, I can remove the old API)

Sorry about the inconvenience!

[meteorcloudy]

@mattyclarkson Sorry for the confusing.
It's recommended to test Bzlmod with Bazel@HEAD, since we are making lots of improvements to it.
If you are using Bazelisk, just set USE_BAZEL_VERSION=last_green