Introduce build setting for current Scala version

Co-authored-by: mkuta <[email protected]>

Author: aszady

cross-compilation-doc.md

@@ -8,3 +8,30 @@ The support for cross-compilation is currently under development.
 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"]`.
+
+
+## Build settings
+Configured `SCALA_VERSIONS` correspond to allowed values of [build setting](https://bazel.build/extending/config#user-defined-build-setting).
+
+### `scala_version`
+`@io_bazel_rules_scala_config` in its root package defines the following build setting:
+```starlark
+string_flag(
+    name = "scala_version",
+    build_setting_default = "3.3.1",
+    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`).
+
+### Config settings
+Then for each Scala version we have a [config setting](https://bazel.build/extending/config#build-settings-and-select):
+```starlark
+config_setting(
+    name = "3_3_1",
+    flag_values = {":scala_version": "3.3.1"},
+)
+```
+The `name` of `config_setting` corresponds to `sanitize_version(scala_version)`.
+One may use this config setting in `select()` e.g. to provide dependencies relevant to a currently used Scala version.

scala/scala_cross_version.bzl

@@ -44,3 +44,7 @@ def scala_mvn_artifact(
     artifactid = gav[1]
     version = gav[2]
     return "%s:%s_%s:%s" % (groupid, artifactid, major_scala_version, version)
+
+def sanitize_version(scala_version):
+    """ Makes Scala version usable in target names. """
+    return scala_version.replace(".", "_")

scala_config.bzl

@@ -1,4 +1,4 @@
-load("//scala:scala_cross_version.bzl", "extract_major_version", "extract_minor_version")
+load("//scala:scala_cross_version.bzl", "extract_major_version", "extract_minor_version", "sanitize_version")
 
 def _default_scala_version():
     """return the scala version for use in maven coordinates"""
@@ -10,6 +10,17 @@ def _validate_supported_scala_version(scala_major_version, scala_minor_version):
     if scala_major_version == "2.12" and int(scala_minor_version) < 1:
         fail("Scala version must be newer or equal to 2.12.1 to use compiler dependency tracking.")
 
+def _config_setting(scala_version):
+    return "\n".join([
+        'config_setting(',
+        '    name = "{name}",',
+        '    flag_values = {{":scala_version": "{version}"}},',
+        ')\n',
+    ]).format(name = sanitize_version(scala_version), version = scala_version)
+
+def _config_settings(scala_versions):
+    return "".join([_config_setting(v) for v in scala_versions])
+
 def _store_config(repository_ctx):
     # Default version
     scala_version = repository_ctx.os.environ.get(
@@ -39,8 +50,19 @@ def _store_config(repository_ctx):
         "ENABLE_COMPILER_DEPENDENCY_TRACKING=" + enable_compiler_dependency_tracking,
     ])
 
+    build_file_content = "\n".join([
+        'load("@bazel_skylib//rules:common_settings.bzl", "string_flag")',
+        'string_flag(',
+        '    name = "scala_version",',
+        '    build_setting_default = "{scala_version}",',
+        '    values = {scala_versions},',
+        '    visibility = ["//visibility:public"],',
+        ')\n',
+    ]).format(scala_versions = scala_versions, scala_version=scala_version) +\
+    _config_settings(scala_versions)
+
     repository_ctx.file("config.bzl", config_file_content)
-    repository_ctx.file("BUILD")
+    repository_ctx.file("BUILD", build_file_content)
 
 _config_repository = repository_rule(
     implementation = _store_config,