Support Bzlmod and add rules_scala to bazel-central-registry

[sfc-gh-pbennes]

Greetings!

It looks like rules_scala isn't part of the Bzlmod effort or added to bazel-central-registry yet. I've opened an issue both here and in https://github.com/bazelbuild/bazel-central-registry to request it: bazelbuild/bazel-central-registry#522

[chrislovecnm]

Just a note to anyone that starts the work native.register_toolchains is not supported when using bzlmod.

[sluongng]

@chrislovecnm it's still available here no? https://bazel.build/rules/lib/globals/module#register_toolchains

[chrislovecnm]

@sluongng When I last tried it, it did not work when using it inside an extension. In rules_python we are skipping the native call:

https://github.com/bazelbuild/rules_python/blob/36082079b514529d4009a2b104475fde1cdd5b30/python/repositories.bzl#L569

And build the toolchains here:

https://github.com/bazelbuild/rules_python/blob/36082079b514529d4009a2b104475fde1cdd5b30/python/extensions/private/pythons_hub.bzl#L56

We build the toolchains and use the following:

https://github.com/bazelbuild/rules_python/blob/36082079b514529d4009a2b104475fde1cdd5b30/MODULE.bazel#L49

[sluongng]

Yeah you would only want to prepare the toolchain (as a new repository) inside the extension I think. Registering it must be called from the MODULE.bazel file.

So native.register_toolchains call from the extension's starlark does not work, but having the extension create @pythons_hub//:all and then MODULE.bazel calling register_toolchains("@pythons_hub//:all") worked. 👍

[mateuszkuta256]

I prepared a minimal bzlmod support here and gonna split it into a few PRs
TODOs after merging it:

• write documentation on how to use rules_scala with bzlmod (some properties like SCALA_VERSION will probably be configurable via repo-env)
• prepare release notes
• contribute repo to the Bazel Central Registry
• add tests that the project builds using both WORKSPACE and MODULE.bazel
• migrate test/external workspaces to blzmod too

[pcj]

@mateuszkuta256 thanks for getting this started! Any progress updates or blockers?

[mateuszkuta256]

@mateuszkuta256 thanks for getting this started! Any progress updates or blockers?

hi, unfortunately I'm working on other things now and won't continue with this PR in the foreseeable future
I remember this migration was on hold because interfered with 'cross compilation support'
It looks like great progress was done it this area, so maybe someone can already take over the PR

[mbland]

I'd like to take on the task of Bzlmodifying this repo through a series of pull requests.

I've already created a (mostly) working branch in my own fork. Though I saw a couple draft pull requests here, I ended up taking a different approach and got it mostly working. In fact, I did exactly what @sluongng suggested in #1482 (comment). (I didn't notice this comment before just now—I might've read it, but not understood it at the time—but I did study rules_python and rules_go, and ended up doing exactly that.)

There are still issues I need help to address (recorded in some of the commit messages), I didn't strictly maintain WORKSPACE compatibility (fixable), and I only tested against Scala 2.13. But I can start teasing chunks out of it in a methodical fashion, to ensure that I maintain WORKSPACE and cross-version compatibility.

For an example of what it looks like from a client perspective, here's what the MODULE.bazel stanza from my local EngFlow/example repo looks like (as opposed to the current stanza, explanatory comment notwithstanding):

bazel_dep(name = "rules_scala", repo_name = "io_bazel_rules_scala")
local_path_override(
    module_name = "rules_scala",
    path = "../../bazelbuild/rules_scala"
)   

scala_dev_deps = use_extension(
    "@io_bazel_rules_scala//scala/extensions:deps.bzl",
    "scala_deps",
    dev_dependency = True,
)   
scala_dev_deps.toolchains(
    scalatest = True,
)

So if folks are game for me to do this, I'll start carving off pieces as separate pull requests, and we can resolve any outstanding problems in the process.

cc: @BillyAutrey @jayconrod @benjaminp @TheGrizzlyDev