feat: add rv-ruby support for prebuilt Ruby binaries

[cgrindel]

Summary

• Add support for installing prebuilt Ruby binaries from rv-ruby.
• Add checksums attribute to ruby.toolchain() for platform-specific SHA256 verification
• Add excluded_gems attribute to ruby.bundle_fetch() to skip downloading default gems bundled with Ruby

Usage

ruby.toolchain(
    name = "ruby",
    rv_checksums = {
        "linux-arm64": "0c08c35a99f10817643d548f98012268c5433ae25a737ab4d6751336108a941d",
        "linux-x86_64": "f36cef10365d370e0867f0c3ac36e457a26ab04f3cfbbd7edb227a18e6e9b3c3",
        "macos-arm64": "cd9d7a1428076bfcc6c2ca3c0eb69b8e671e9b48afb4c351fa4a84927841ffef",
        "macos-x86_64": "e9da39082d1dd8502d322c850924d929bc45b7a1e35da593a5606c00673218d4",
    },
    rv_version = "20251225",
    version_file = "//:.ruby-version",
)

ruby.bundle_fetch(
    name = "bundle",
    excluded_gems = ["psych"],  # Skip gems bundled with Ruby
    gemfile = "//:Gemfile",
    gemfile_lock = "//:Gemfile.lock",
)

Performance Comparison

To evaluate the performance benefits of this change, the following command was run from the examples/rails directory five times:

clear && bazel clean --expunge && time bazel test //...

The results were averaged and a percent diff was calcuated.

Description	Run 1	Run 2	Run 3	Run 4	Run 5	Average	Diff	% Diff
main branch using MRI	0:01:53	0:01:50	0:01:47	0:01:45	0:01:49	0:01:49
rv-ruby	0:00:17	0:00:16	0:00:17	0:00:16	0:00:16	0:00:17	-0:01:32	-84.84%

Note

For each test case (i.e., row in the table), the first execution of the test command was discarded. The intention of the tests was to measure the performance with the repository cache and disk cache being populated.

Commentary

There are several wins by using rv-provided Ruby binaries vs building them using ruby build.

• Eliminates the time to build Ruby.
• The cachability of the Ruby binary in the repository cache. There is no download of the Ruby binary if you are using the repository cache.
• The cachability of the Ruby targets in the build graph. When downloading Ruby from rv, any build/test actions that depend on the toolchain are valid on subsequent runs. In other words, rebuilding Ruby from source invalidates the build graph.

🤖 Generated with Claude Code

Related Slack thread.

Blocked by #281.

[p0deje]

Thank you for working on this. It's amazing to finally see the precompiled Rubies land in rules_ruby. The code looks great to me overall, and I'll be happy to merge it as-is, especially since it's opt-in. I only have a few questions to clarify:

1. How much effort would it take to make toolchains work for different platforms? E.g., make the same rb_download automatically download Ruby for the target platform. This would be handy in remote builds where the host can be macOS, while the target is Linux.
2. Would it be possible to fetch all prebuilt Ruby checksums and put them in a file (like ruby/private/bundler_checksums.bzl)? This could be a first step to make prebuilt Rubies be used by default. We should, of course, keep rv_checksums and rv_version attributes for anyone who needed fresh/custom versions of Ruby.
3. What happens when we try to install prebuilt gems (e.g., date) on prebuilt Ruby? Does it error?
4. If we really should skip installing prebuilt gems, do you think we should create excluded_gems.bzl that keeps a map of Ruby version to prebuilt gems list? This can be automatically picked up by rb_bundle_fetch() so that users don't have to think twice about configuring the excluded_gems attribute.

[cgrindel]

1. How much effort would it take to make toolchains work for different platforms? E.g., make the same rb_download automatically download Ruby for the target platform. This would be handy in remote builds where the host can be macOS, while the target is Linux.

How would the Linux Ruby binaries be used in this scenario?

2. Would it be possible to fetch all prebuilt Ruby checksums and put them in a file (like ruby/private/bundler_checksums.bzl)? This could be a first step to make prebuilt Rubies be used by default. We should, of course, keep rv_checksums and rv_version attributes for anyone who needed fresh/custom versions of Ruby.

I considered this. That would require pinning rules_ruby to an rv-ruby release. It would be convenient for the out-of-the-box experience, but could be confusing when trying to upgrade to the latest and greatest.

One thought that I considered was to implement a utility that generates the checksums for you based upon the rv-ruby version provided. I did something similar for buildifier-prebuilt. We can add this as a follow-on, if there is interest.

3. What happens when we try to install prebuilt gems (e.g., date) on prebuilt Ruby? Does it error?

I saw version mismatch errors and I saw compilation errors.

4. If we really should skip installing prebuilt gems, do you think we should create excluded_gems.bzl that keeps a map of Ruby version to prebuilt gems list? This can be automatically picked up by rb_bundle_fetch() so that users don't have to think twice about configuring the excluded_gems attribute.

This is a similar issue as the checksums. Perhaps, we should create a utility that generates the declarations based upon the Ruby version.