Allow configuring multiple Scala versions (#1579)

Author: aszady

cross-compilation-doc.md

@@ -7,7 +7,7 @@ The support for cross-compilation is currently under development.
 `scala_config` creates the repository `@io_bazel_rules_scala_config`.
 File created there, `config.bzl`, consists of many variables. In particular:
 * `SCALA_VERSION` – representing the default Scala version, e.g. `"3.3.1"`;
-* `SCALA_VERSIONS` – representing all configured Scala versions (currently one), e.g. `["3.3.1"]`.
+* `SCALA_VERSIONS` – representing all configured Scala versions, e.g. `["2.12.18", "3.3.1"]`.
 
 
 ## Build settings
@@ -22,6 +22,7 @@ string_setting(
     values = ["3.3.1"],
     visibility = ["//visibility:public"],
 )
+...
 ```
 This build setting can be subject of change by [transitions](https://bazel.build/extending/config#user-defined-transitions) (within allowed `values`).
 
@@ -32,6 +33,7 @@ config_setting(
     name = "scala_version_3_3_1",
     flag_values = {":scala_version": "3.3.1"},
 )
+...
 ```
 The `name` of `config_setting` corresponds to `"scala_version" + version_suffix(scala_version)`.
 One may use this config setting in `select()` e.g. to provide dependencies relevant to a currently used Scala version.

scala_config.bzl

@@ -28,7 +28,11 @@ def _store_config(repository_ctx):
     )
 
     # All versions supported
-    scala_versions = [scala_version]
+    scala_versions = repository_ctx.attr.scala_versions
+    if not scala_versions:
+        scala_versions = [scala_version]
+    elif scala_version not in scala_versions:
+        fail("You have to include the default Scala version (%s) in the `scala_versions` list." % scala_version)
 
     enable_compiler_dependency_tracking = repository_ctx.os.environ.get(
         "ENABLE_COMPILER_DEPENDENCY_TRACKING",
@@ -67,6 +71,11 @@ _config_repository = repository_rule(
     attrs = {
         "scala_version": attr.string(
             mandatory = True,
+            doc = "Default Scala version",
+        ),
+        "scala_versions": attr.string_list(
+            mandatory = True,
+            doc = "List of all Scala versions to configure. Must include the default one.",
         ),
         "enable_compiler_dependency_tracking": attr.bool(
             mandatory = True,
@@ -77,9 +86,11 @@ _config_repository = repository_rule(
 
 def scala_config(
         scala_version = _default_scala_version(),
+        scala_versions = [],
         enable_compiler_dependency_tracking = False):
     _config_repository(
         name = "io_bazel_rules_scala_config",
         scala_version = scala_version,
+        scala_versions = scala_versions,
         enable_compiler_dependency_tracking = enable_compiler_dependency_tracking,
     )