Support per-target Ruby toolchain

[sushain97]

Allow customizing the Ruby toolchain used when building the bundle/running test targets and bundled with executables. This is necessary for environments where there are multiple Ruby versions in the same Bazel workspace. At Stripe, we even ship some artifacts that are built using multiple versions of Ruby, e.g. during a migration.

[p0deje]

Is it possible to register multiple Ruby toolchains? I was under the impression that it's not currently working #22 (comment)

[sushain97]

Is it possible to register multiple Ruby toolchains? I was under the impression that it's not currently working #22 (comment)

Er, I'm not attempting it.

It's possible to declare the toolchain and then use it as necessary. In some environments, you could wrap every ruby_test with your own macro that selects the correct toolchain.

Right now, we have:

rb_download(
    name = "ruby",
    version_file = "//:.ruby-version",
)

rb_download(
    name = "ruby_foo",
    version_file = "//foo:.ruby-version",
)

native.register_toolchains("//tools/ruby:ruby_toolchains")

rb_bundle_fetch(
    name = "bundle",
    # ...
)

rb_bundle_fetch(
    name = "foo_bundle",
    ruby = "@ruby_foo//:toolchain",
    # ...
)