Move native.toolchain() into scala_toolchain()

WIP Per the discussion in:

- bazel-contrib#1703 (comment).

Author: mbland

examples/semanticdb/BUILD

@@ -2,19 +2,12 @@ load("@rules_scala//scala:scala_toolchain.bzl", "scala_toolchain")
 load("@rules_scala//scala:scala.bzl", "scala_binary", "scala_library")
 
 scala_toolchain(
-    name = "semanticdb_toolchain_impl",
+    name = "semanticdb_toolchain",
     enable_semanticdb = True,
     semanticdb_bundle_in_jar = False,
     visibility = ["//visibility:public"],
 )
 
-toolchain(
-    name = "semanticdb_toolchain",
-    toolchain = "semanticdb_toolchain_impl",
-    toolchain_type = "@rules_scala//scala:toolchain_type",
-    visibility = ["//visibility:public"],
-)
-
 scala_library(
     name = "hello_lib",
     srcs = ["Foo.scala"],

manual_test/coverage_local_jacocorunner/BUILD

@@ -1,18 +1,11 @@
 load("//scala:scala_toolchain.bzl", "scala_toolchain")
 
 scala_toolchain(
-    name = "local_jacocorunner_toolchain_impl",
+    name = "local_jacocorunner_toolchain",
     jacocorunner = ":local_jacocorunner",
     visibility = ["//visibility:public"],
 )
 
-toolchain(
-    name = "local_jacocorunner_scala_toolchain",
-    toolchain = "local_jacocorunner_toolchain_impl",
-    toolchain_type = "//scala:toolchain_type",
-    visibility = ["//visibility:public"],
-)
-
 filegroup(
     name = "local_jacocorunner",
     srcs = ["JacocoCoverage_jarjar_deploy.jar"],

manual_test/scala_test_jacocorunner/BUILD

@@ -2,15 +2,8 @@ load("//scala:scala_toolchain.bzl", "scala_toolchain")
 load("//scala:scala.bzl", "scala_test")
 
 scala_toolchain(
-    name = "passing_toolchain_impl",
-    jacocorunner = "@bazel_tools//tools/jdk:JacocoCoverage",
-    visibility = ["//visibility:public"],
-)
-
-toolchain(
     name = "passing_scala_toolchain",
-    toolchain = "passing_toolchain_impl",
-    toolchain_type = "//scala:toolchain_type",
+    jacocorunner = "@bazel_tools//tools/jdk:JacocoCoverage",
     visibility = ["//visibility:public"],
 )
 

manual_test/scala_test_jvm_flags/BUILD

@@ -2,33 +2,19 @@ load("//scala:scala_toolchain.bzl", "scala_toolchain")
 load("//scala:scala.bzl", "scala_test")
 
 scala_toolchain(
-    name = "failing_toolchain_impl",
+    name = "failing_scala_toolchain",
     # This will fail because 1M isn't enough
     scala_test_jvm_flags = ["-Xmx1M"],
     visibility = ["//visibility:public"],
 )
 
-toolchain(
-    name = "failing_scala_toolchain",
-    toolchain = "failing_toolchain_impl",
-    toolchain_type = "//scala:toolchain_type",
-    visibility = ["//visibility:public"],
-)
-
 scala_toolchain(
-    name = "passing_toolchain_impl",
+    name = "passing_scala_toolchain",
     # This will pass because 1G is enough
     scala_test_jvm_flags = ["-Xmx1G"],
     visibility = ["//visibility:public"],
 )
 
-toolchain(
-    name = "passing_scala_toolchain",
-    toolchain = "passing_toolchain_impl",
-    toolchain_type = "//scala:toolchain_type",
-    visibility = ["//visibility:public"],
-)
-
 scala_test(
     name = "empty_test",
     srcs = ["EmptyTest.scala"],

manual_test/scalac_jvm_opts/BUILD

@@ -4,33 +4,19 @@ load("//scala:scala.bzl", "scala_library")
 load("//scala_proto:scala_proto.bzl", "scala_proto_library")
 
 scala_toolchain(
-    name = "failing_toolchain_impl",
+    name = "failing_scala_toolchain",
     # This will fail because 1M isn't enough
     scalac_jvm_flags = ["-Xmx1M"],
     visibility = ["//visibility:public"],
 )
 
-toolchain(
-    name = "failing_scala_toolchain",
-    toolchain = "failing_toolchain_impl",
-    toolchain_type = "//scala:toolchain_type",
-    visibility = ["//visibility:public"],
-)
-
 scala_toolchain(
-    name = "passing_toolchain_impl",
+    name = "passing_scala_toolchain",
     # This will pass because 1G is enough
     scalac_jvm_flags = ["-Xmx1G"],
     visibility = ["//visibility:public"],
 )
 
-toolchain(
-    name = "passing_scala_toolchain",
-    toolchain = "passing_toolchain_impl",
-    toolchain_type = "//scala:toolchain_type",
-    visibility = ["//visibility:public"],
-)
-
 scala_library(
     name = "empty_build",
     srcs = ["Empty.scala"],

scala/BUILD

@@ -18,32 +18,19 @@ alias(
 )
 
 scala_toolchain(
-    name = "unused_dependency_checker_error_toolchain_impl",
+    name = "unused_dependency_checker_error_toolchain",
     dependency_tracking_method = "ast-plus",
     unused_dependency_checker_mode = "error",
 )
 
 scala_toolchain(
-    name = "minimal_direct_source_deps_impl",
+    name = "minimal_direct_source_deps",
     dependency_mode = "plus-one",
     dependency_tracking_method = "ast",
     strict_deps_mode = "error",
     unused_dependency_checker_mode = "error",
 )
 
-[
-    toolchain(
-        name = tc,
-        toolchain = tc + "_impl",
-        toolchain_type = "//scala:toolchain_type",
-        visibility = ["//visibility:public"],
-    )
-    for tc in [
-        "unused_dependency_checker_error_toolchain",
-        "minimal_direct_source_deps",
-    ]
-]
-
 java_import(
     name = "bazel_test_runner_deploy",
     jars = ["@bazel_tools//tools/jdk:TestRunner_deploy.jar"],

scala/private/macros/setup_scala_toolchain.bzl

@@ -1,6 +1,6 @@
 load("//scala:scala_toolchain.bzl", "scala_toolchain")
 load("//scala:providers.bzl", "declare_deps_provider")
-load("//scala:scala_cross_version.bzl", "repositories", "version_suffix")
+load("//scala:scala_cross_version.bzl", "repositories")
 load("@io_bazel_rules_scala_config//:config.bzl", "SCALA_VERSION")
 
 def setup_scala_toolchain(
@@ -15,6 +15,15 @@ def setup_scala_toolchain(
         enable_semanticdb = False,
         visibility = ["//visibility:public"],
         **kwargs):
+    """Creates custom Scala toolchain implementation and `toolchain` targets.
+
+    Use this instead of `scala_toolchain` when specifying toolchain dependency
+    artifacts instead of relying on the builtin artifacts. Any unspecified
+    custom dependency providers will default to the builtin providers.
+
+    See `docs/scala_toolchain.md` for example usage and documentation on the
+    available attributes.
+    """
     scala_xml_provider = "%s_scala_xml_provider" % name
     parser_combinators_provider = "%s_parser_combinators_provider" % name
     scala_compile_classpath_provider = "%s_scala_compile_classpath_provider" % name
@@ -87,21 +96,13 @@ def setup_scala_toolchain(
         dep_providers.append(semanticdb_deps_provider)
 
     scala_toolchain(
-        name = "%s_impl" % name,
+        name = name,
         dep_providers = dep_providers,
         enable_semanticdb = enable_semanticdb,
         visibility = visibility,
         **kwargs
     )
 
-    native.toolchain(
-        name = name,
-        toolchain = ":%s_impl" % name,
-        toolchain_type = Label("//scala:toolchain_type"),
-        target_settings = ["@io_bazel_rules_scala_config//:scala_version" + version_suffix(scala_version)],
-        visibility = visibility,
-    )
-
 _DEFAULT_DEPS = {
     "scala_compile_classpath": {
         "any": [

scala/scala_toolchain.bzl

@@ -1,10 +1,11 @@
 load("//scala:providers.bzl", _DepsInfo = "DepsInfo")
+load("//scala:scala_cross_version.bzl", "version_suffix")
+load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
 load(
     "@io_bazel_rules_scala_config//:config.bzl",
     "ENABLE_COMPILER_DEPENDENCY_TRACKING",
-    "SCALA_MAJOR_VERSION",
+    "SCALA_VERSION",
 )
-load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
 
 def _compute_strict_deps_mode(input_strict_deps_mode, dependency_mode):
     if dependency_mode == "direct":
@@ -111,7 +112,7 @@ def _default_dep_providers():
         "scala_library_classpath",
         "scala_macro_classpath",
     ]
-    if SCALA_MAJOR_VERSION.startswith("2."):
+    if SCALA_VERSION.startswith("2."):
         dep_providers.append("semanticdb")
     return [
         "@rules_scala_toolchains//scala:%s_provider" % p
@@ -198,12 +199,32 @@ def _expand_patterns(patterns):
 
     return result
 
-def scala_toolchain(**kwargs):
-    """Creates a Scala toolchain target."""
+def scala_toolchain(name, visibility = ["//visibility:public"], **kwargs):
+    """Creates Scala toolchain implementation and `toolchain` targets.
+
+    Use this instead of `setup_scala_toolchain()` when setting custom compiler
+    attributes, but still using the builtin toolchain dependency artifacts.
+
+    See `docs/scala_toolchain.md` for example usage and documentation on the
+    available attributes.
+    """
+    impl_name = "%s_impl" % name
     strict = kwargs.pop("dependency_tracking_strict_deps_patterns", [""])
     unused = kwargs.pop("dependency_tracking_unused_deps_patterns", [""])
     _scala_toolchain(
+        name = impl_name,
         dependency_tracking_strict_deps_patterns = _expand_patterns(strict),
         dependency_tracking_unused_deps_patterns = _expand_patterns(unused),
         **kwargs
     )
+
+    native.toolchain(
+        name = name,
+        toolchain = ":" + impl_name,
+        toolchain_type = Label("//scala:toolchain_type"),
+        target_settings = [
+            "@io_bazel_rules_scala_config//:scala_version" +
+            version_suffix(SCALA_VERSION),
+        ],
+        visibility = visibility,
+    )

test/semanticdb/BUILD

@@ -3,33 +3,19 @@ load("//scala:scala.bzl", "scala_library")
 load("//test/semanticdb:rules.bzl", "semanticdb_vars_script")
 
 scala_toolchain(
-    name = "semanticdb_bundle_toolchain_impl",
+    name = "semanticdb_bundle_toolchain",
     enable_semanticdb = True,
     semanticdb_bundle_in_jar = True,
     visibility = ["//visibility:public"],
 )
 
-toolchain(
-    name = "semanticdb_bundle_toolchain",
-    toolchain = "semanticdb_bundle_toolchain_impl",
-    toolchain_type = "//scala:toolchain_type",
-    visibility = ["//visibility:public"],
-)
-
 scala_toolchain(
-    name = "semanticdb_nobundle_toolchain_impl",
+    name = "semanticdb_nobundle_toolchain",
     enable_semanticdb = True,
     semanticdb_bundle_in_jar = False,
     visibility = ["//visibility:public"],
 )
 
-toolchain(
-    name = "semanticdb_nobundle_toolchain",
-    toolchain = "semanticdb_nobundle_toolchain_impl",
-    toolchain_type = "//scala:toolchain_type",
-    visibility = ["//visibility:public"],
-)
-
 scala_library(
     name = "all_lib",
     srcs = glob(["*.scala"]),