Bind toolchains to Scala version

cross-compilation-doc.md

@@ -52,4 +52,32 @@ def _rule_impl(ctx):
 ```
 
 ### From config setting
-TODO
+TODO
+
+
+## Toolchains
+Standard [toolchain resolution](https://bazel.build/extending/toolchains#toolchain-resolution) procedure determines which toolchain to use for Scala targets.
+
+Toolchain should declare its compatibility with Scala version by using `target_settings` attribute of the `toolchain` rule:
+
+```starlark
+toolchain(
+    ...
+    target_settings = ["@io_bazel_rules_scala_config//:scala_version_3_3_1"],
+    ...
+)
+```
+
+### Cross-build support tiers
+`rules_scala` consists of many toolchains implementing various toolchain types.
+Their support level for cross-build setup varies.
+
+We can distinguish following tiers:
+
+* No `target_settings` set – not migrated, will work on the default `SCALA_VERSION`; undefined behavior on other versions.
+  * (all toolchains not mentioned elsewhere)
+* `target_settings` set to the `SCALA_VERSION` – not fully migrated; will work only on the default `SCALA_VERSION` and will fail the toolchain resolution on other versions.
+  * [the main Scala toolchain](scala/BUILD)
+  * [Scalafmt](scala/scalafmt/BUILD)
+  * [Scalatest](testing/testing.bzl)
+* Multiple toolchain instances with `target_settings` corresponding to each of `SCALA_VERSIONS` – fully migrated; will work in cross-build setup.

scala/private/macros/setup_scala_toolchain.bzl

@@ -1,11 +1,14 @@
 load("//scala:scala_toolchain.bzl", "scala_toolchain")
 load("//scala:providers.bzl", "declare_deps_provider")
+load("//scala:scala_cross_version.bzl", "version_suffix")
+load("@io_bazel_rules_scala_config//:config.bzl", "SCALA_VERSION")
 
 def setup_scala_toolchain(
         name,
         scala_compile_classpath,
         scala_library_classpath,
         scala_macro_classpath,
+        scala_version = SCALA_VERSION,
         scala_xml_deps = None,
         parser_combinators_deps = None,
         semanticdb_deps = None,
@@ -93,5 +96,6 @@ def setup_scala_toolchain(
         name = name,
         toolchain = ":%s_impl" % name,
         toolchain_type = "@io_bazel_rules_scala//scala:toolchain_type",
+        target_settings = ["@io_bazel_rules_scala_config//:scala_version" + version_suffix(scala_version)],
         visibility = visibility,
     )

scala/scalafmt/BUILD

@@ -1,6 +1,8 @@
 load("//scala:scala.bzl", "scala_binary")
+load("//scala:scala_cross_version.bzl", "version_suffix")
 load("//scala/scalafmt/toolchain:toolchain.bzl", "export_scalafmt_deps", "scalafmt_toolchain")
 load("@io_bazel_rules_scala//scala:providers.bzl", "declare_deps_provider")
+load("@io_bazel_rules_scala_config//:config.bzl", "SCALA_VERSION")
 load(
     "//scala/scalafmt:phase_scalafmt_ext.bzl",
     "scalafmt_singleton",
@@ -58,6 +60,7 @@ scalafmt_toolchain(
 
 toolchain(
     name = "scalafmt_toolchain",
+    target_settings = ["@io_bazel_rules_scala_config//:scala_version" + version_suffix(SCALA_VERSION)],
     toolchain = ":scalafmt_toolchain_impl",
     toolchain_type = "//scala/scalafmt/toolchain:scalafmt_toolchain_type",
     visibility = ["//visibility:public"],

testing/testing.bzl

@@ -1,5 +1,7 @@
 load("@io_bazel_rules_scala//scala:providers.bzl", "declare_deps_provider")
 load("@io_bazel_rules_scala//testing/toolchain:toolchain.bzl", "scala_testing_toolchain")
+load("//scala:scala_cross_version.bzl", "version_suffix")
+load("@io_bazel_rules_scala_config//:config.bzl", "SCALA_VERSION")
 
 def _declare_deps_provider(macro_name, deps_id, deps, visibility):
     label = "%s_%s_provider" % (macro_name, deps_id)
@@ -13,6 +15,7 @@ def _declare_deps_provider(macro_name, deps_id, deps, visibility):
 
 def setup_scala_testing_toolchain(
         name,
+        scala_version = SCALA_VERSION,
         junit_classpath = None,
         specs2_classpath = None,
         specs2_junit_classpath = None,
@@ -70,5 +73,6 @@ def setup_scala_testing_toolchain(
         name = name,
         toolchain = ":" + name + "_impl",
         toolchain_type = "@io_bazel_rules_scala//testing/toolchain:testing_toolchain_type",
+        target_settings = ["@io_bazel_rules_scala_config//:scala_version" + version_suffix(scala_version)],
         visibility = visibility,
     )