feat: expose Ruby engine configuration

examples/gem/spec/BUILD

@@ -38,7 +38,11 @@ rb_test(
         "LOCATION_SRC": "$(location :env_spec)",
         "LOCATION_DEP": "$(location //lib/gem:add)",
         "LOCATION_DATA": "$(location //spec/support:file)",
-    },
+    } | select({
+        "@ruby//engine:jruby": {"RUBY_ENGINE": "jruby"},
+        "@ruby//engine:truffleruby": {"RUBY_ENGINE": "truffleruby"},
+        "//conditions:default": {"RUBY_ENGINE": "ruby"},
+    }),
     env_inherit = [
         "USER",  # POSIX
         "USERNAME",  # Windows

examples/gem/spec/env_spec.rb

@@ -20,4 +20,8 @@
     expect(File).to exist(ENV.fetch('LOCATION_DATA'))
     expect(File).to exist(ENV.fetch('LOCATION_DEP'))
   end
+
+  specify do
+    expect(ENV.fetch('RUBY_ENGINE')).to eq(RUBY_ENGINE)
+  end
 end

ruby/private/download.bzl

@@ -99,11 +99,16 @@ def _rb_download_impl(repository_ctx):
         gem_binary_name = "gem"
 
     env = {}
+    engine = "ruby"
     if version.startswith("jruby"):
+        engine = "jruby"
+
         # JRuby might fail with "Errno::EACCES: Permission denied - NUL" on Windows:
         # https://github.com/jruby/jruby/issues/7182#issuecomment-1112953015
         env.update({"JAVA_OPTS": "-Djdk.io.File.enableADS=true"})
     elif version.startswith("truffleruby"):
+        engine = "truffleruby"
+
         # TruffleRuby needs explicit locale
         # https://www.graalvm.org/dev/reference-manual/ruby/UTF8Locale/
         env.update({"LANG": "en_US.UTF-8"})
@@ -120,6 +125,15 @@ def _rb_download_impl(repository_ctx):
         },
     )
 
+    repository_ctx.template(
+        "engine/BUILD",
+        repository_ctx.attr._build_engine_tpl,
+        executable = False,
+        substitutions = {
+            "{ruby_engine}": engine,
+        },
+    )
+
 def _read_version_from_file(repository_ctx):
     version = repository_ctx.read(repository_ctx.attr.version_file).strip("\r\n")
     if repository_ctx.attr.version_file.name == ".tool-versions":
@@ -271,5 +285,9 @@ which isn't available in this ruby-build yet.
             allow_single_file = True,
             default = "@rules_ruby//:ruby/private/download/BUILD.tpl",
         ),
+        "_build_engine_tpl": attr.label(
+            allow_single_file = True,
+            default = "@rules_ruby//:ruby/private/download/BUILD.engine.tpl",
+        ),
     },
 )

ruby/private/download/BUILD.engine.tpl

@@ -0,0 +1,36 @@
+load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
+
+package(default_visibility = ["//visibility:public"])
+
+string_flag(
+    name = "engine",
+    values = [
+        "jruby",
+        "truffleruby",
+        "ruby",
+    ],
+    build_setting_default = "{ruby_engine}",
+)
+
+config_setting(
+    name = "jruby",
+    flag_values = {
+        ":engine": "jruby",
+    },
+)
+
+config_setting(
+    name = "truffleruby",
+    flag_values = {
+        ":engine": "truffleruby",
+    },
+)
+
+config_setting(
+    name = "ruby",
+    flag_values = {
+        ":engine": "ruby",
+    },
+)
+
+# vim: ft=bzl

ruby/private/toolchain.bzl

@@ -38,6 +38,22 @@ def rb_register_toolchains(
     $ bazel run @ruby//:gem -- install rails
     ```
 
+    You can also use Ruby engine targets to `select()` depending on installed Ruby interpreter:
+
+    `BUILD`:
+    ```bazel
+    rb_library(
+        name = "my_lib",
+        srcs = ["my_lib.rb"],
+        deps = select({
+            "@ruby//engine:jruby": [":my_jruby_lib"],
+            "@ruby//engine:truffleruby": ["//:my_truffleruby_lib"],
+            "@ruby//engine:ruby": ["//:my__lib"],
+            "//conditions:default": [],
+        }),
+    )
+    ```
+
     Args:
         name: base name of resulting repositories, by default "ruby"
         version: a semver version of MRI, or a string like [interpreter type]-[version], or "system"