[bazel] Migrating to Bzlmod

[jwnrt]

Full RFC document: https://docs.google.com/document/d/1KQWdo7jeh9JmKn3OgNA-xFYLfR2hyMyJXlL_Rqadnjc/edit?usp=sharing

Summary

Recent versions of Bazel support a feature called "Bzlmod" which changes how external dependencies are managed. This is opt-in in Bazel 6 (our current version), opt-out in Bazel 7, and required in future versions.

We will eventually need to move to Bzlmod to keep up with Bazel and the ecosystem. It will also likely be required for multitop so that we can use things from old versions of the repo without conflicts.

Bzlmod overview

• Currently, we have a WORKSPACE file which downloads all external dependencies as tarballs and extracts them into a global namespace.
• This is fragile and difficult to maintain. All dependencies (including transitive) must be accounted for by our WORKSPACE.
• The global namespace causes conflicts and increases the chance of breakage.
• With Bzlmod, our repository becomes a "module" depending on other modules.
• Modules are namespaced and transitive dependencies are handled automatically.
• Extra features become available due to this uniformity.

Roadmap

The steps for adopting bzlmod are roughly:

• Ensure all our dependencies support bzlmod. Mostly done.
  • https://github.com/lowRISC/crt still WIP.
  • Alternatively, use new rules_cc features for the lowRISC toolchain.
• Update to dependencies compatible with bzlmod.
  • Update bazel dependencies #24361 - most dependencies
  • [bazel] Update rules_rust to upstream 0.49.3 #24587 - rules_rust.
• [bazel] Update to Bazel 7 #24393
• Enable bzlmod in "hybrid" mode where we use both WORKSPACE.bzlmod and MODULE.bazel.
  • See [bazel] Enable bzlmod in hybrid mode #24781
• Gradually move dependencies from WORKSPACE.bzlmod to MODULE.bazel.
  • rules_rust: [bazel] Migrate rules_rust to bzlmod #25578
  • rules_python: [bazel] Migrate rules_python and dependents to bzlmod #25650
  • Buildifier: [bazel] Move buildifier to bzlmod #25679
  • Google rule dependencies: [bazel] Migrate Google dependencies to bzlmod #25680
  • Plain (non-Bazel) repositories
  • Manufacturing hooks
• Ensure OpenTitan can be used as a dependency

Most of this work is being experimented with in #24216.

[andreaskurth]

RFC approved by TC on 2024-11-14

@jwnrt are the open tasks above still up-to-date, or can this issue be closed as completed?

[jwnrt]

I’ve updated the list with the current progress and next steps. I’ll keep this open until they’re all completed.