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Add Bzlmod repo scope change, scala_toolchain
Explicitly notes that some `rules_scala` APIs may break, specifically `setup_scala_toolchain`. Part of bazel-contrib#1482. Brought to my attention by @michalbogacz in a `#scala` thread in the Bazel Slack workspace, which was in the context of michalbogacz/scala-bazel-monorepo#26. - https://bazelbuild.slack.com/archives/CDCKJ2KFZ/p1740144886159909
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‎README.md

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@@ -554,6 +554,76 @@ now, as there's not yet a corresponding [`toolchain_type()`](
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https://bazel.build/versions/6.1.0/reference/be/platform#toolchain_type) target
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in `@rules_java`.
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### Builtin repositories no longer visible without `use_repo()` under Bzlmod
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Under Bzlmod, repos are only visible to the module extension that creates them,
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unless the `MODULE.bazel` file brings them into scope with
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[`use_repo()`](https://bazel.build/rules/lib/globals/module#use_repo). This can
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lead to errors like those from the following example, which [originally called
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`setup_scala_toolchain()` under Bzlmod](
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https://github.com/michalbogacz/scala-bazel-monorepo/blob/17f0890a4345529e09b9ce83bcb2e3d15687c522/BUILD.bazel):
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```py
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load("@io_bazel_rules_scala//scala:scala.bzl", "setup_scala_toolchain")
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setup_scala_toolchain(
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name = "custom_scala_toolchain",
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scalacopts = [
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"-Wunused:all",
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],
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strict_deps_mode = "error",
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unused_dependency_checker_mode = "warn",
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)
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```
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This worked under `WORKSPACE`, but broke under Bzlmod, the error message
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indicating that the builtin `@org_scala_sbt_compiler_interface` toolchain jar
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isn't visible:
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```txt
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ERROR: no such package
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'@@[unknown repo 'org_scala_sbt_compiler_interface_3_3_5'
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requested from @@]//':
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The repository '@@[unknown repo 'org_scala_sbt_compiler_interface_3_3_5'
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requested from @@]' could not be resolved:
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No repository visible as '@org_scala_sbt_compiler_interface_3_3_5'
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```
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`setup_scala_toolchains` is a macro that can take user specified classpath
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targets as described in [docs/scala_toolchain.md](./docs/scala_toolchain.md).
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Otherwise, it _generates new classpath targets_ using the builtin `rules_scala`
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repositories, but these repositories are no longer in the global scope, causing
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the breakage. (A big part of the Bzlmodification work involved enabling
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`rules_scala` to generate and register toolchains _without_ forcing users to
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bring their dependencies into scope.)
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One way to fix this specific problem is to call `use_repo` for every such
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repository needed by the toolchain. Another fix, in this case, is to [use
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`scala_toolchain` directly instead](
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https://github.com/michalbogacz/scala-bazel-monorepo/blob/2cac860f386dcaa1c3be56cd25a84b247d335743/BUILD.bazel).
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Its underlying `BUILD` rule uses the builtin toolchain dependencies via existing
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targets visible within `rules_scala`, without forcing users to import them:
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```py
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load("@rules_scala//scala:scala_toolchain.bzl", "scala_toolchain")
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scala_toolchain(
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name = "custom_scala_toolchain_impl",
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scalacopts = [
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"-Ywarn-unused",
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],
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strict_deps_mode = "error",
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unused_dependency_checker_mode = "warn",
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)
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toolchain(
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name = "custom_scala_toolchain",
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toolchain = ":custom_scala_toolchain_impl",
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toolchain_type = "@rules_scala//scala:toolchain_type",
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visibility = ["//visibility:public"],
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)
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```
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## Breaking changes coming in `rules_scala` 8.x
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__The main objective of 8.x will be to enable existing users to migrate to Bazel

‎docs/scala_toolchain.md

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# scala_toolchain
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`scala_toolchain` allows you to define global configuration to all Scala targets.
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`scala_toolchain` allows you to define the global configuration for all Scala
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targets.
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**Some scala_toolchain must be registered!**
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**Some `scala_toolchain` must be registered!**
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## Several options to configure `scala_toolchain`
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## Options to configure `scala_toolchain`
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### A) Use the builtin Scala toolchain via `scala_toolchains`
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In your workspace file add the following lines:
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Add the following lines to `WORKSPACE`:
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```py
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# WORKSPACE
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### B) Defining your own `scala_toolchain` requires 2 steps
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1. Add your own definition of `scala_toolchain` to a `BUILD` file:
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Example assumes external libraries are resolved with [rules_jvm_external](https://github.com/bazelbuild/rules_jvm_external)
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```py
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# //toolchains/BUILD
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load("@rules_scala//scala:scala.bzl", "setup_scala_toolchain")
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setup_scala_toolchain(
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name = "my_toolchain",
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# configure toolchain dependecies
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parser_combinators_deps = [
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"@maven//:org_scala_lang_modules_scala_parser_combinators_2_12",
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],
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scala_compile_classpath = [
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"@maven//:org_scala_lang_scala_compiler",
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"@maven//:org_scala_lang_scala_library",
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"@maven//:org_scala_lang_scala_reflect",
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],
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scala_library_classpath = [
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"@maven//:org_scala_lang_scala_library",
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"@maven//:org_scala_lang_scala_reflect",
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],
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scala_macro_classpath = [
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"@maven//:org_scala_lang_scala_library",
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"@maven//:org_scala_lang_scala_reflect",
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],
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scala_xml_deps = [
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"@maven//:org_scala_lang_modules_scala_xml_2_12",
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],
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# example of setting attribute values
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scalacopts = ["-Ywarn-unused"],
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unused_dependency_checker_mode = "off",
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visibility = ["//visibility:public"]
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)
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```
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2. Register your custom toolchain from `WORKSPACE`:
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```py
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# WORKSPACE
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register_toolchains("//toolchains:my_scala_toolchain")
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```
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#### Step 1
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You can add your own `scala_toolchain` definition to a `BUILD` file in one of
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two ways. If you only want to set different [configuration
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options](#configuration-options), but rely on the builtin toolchain JARs, use
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`scala_toolchain` directly. This example is inspired by [`BUILD.bazel` from michalbogacz/scala-bazel-monorepo/](
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https://github.com/michalbogacz/scala-bazel-monorepo/blob/2cac860f386dcaa1c3be56cd25a84b247d335743/BUILD.bazel)):
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```py
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load("@rules_scala//scala:scala_toolchain.bzl", "scala_toolchain")
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scala_toolchain(
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name = "my_toolchain_impl",
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scalacopts = [
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"-Wunused:all",
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],
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strict_deps_mode = "error",
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unused_dependency_checker_mode = "warn",
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)
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toolchain(
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name = "my_toolchain",
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toolchain = ":my_toolchain_impl",
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toolchain_type = "@rules_scala//scala:toolchain_type",
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visibility = ["//visibility:public"],
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)
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```
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If you want to use your own compiler JARs, use `setup_scala_toolchain()`
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instead. This example assumes the external libraries are resolved with
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[rules_jvm_external](https://github.com/bazelbuild/rules_jvm_external)
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```py
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# //toolchains/BUILD
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load("@rules_scala//scala:scala.bzl", "setup_scala_toolchain")
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setup_scala_toolchain(
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name = "my_toolchain",
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# configure toolchain dependecies
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parser_combinators_deps = [
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"@maven//:org_scala_lang_modules_scala_parser_combinators_2_12",
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],
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scala_compile_classpath = [
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"@maven//:org_scala_lang_scala_compiler",
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"@maven//:org_scala_lang_scala_library",
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"@maven//:org_scala_lang_scala_reflect",
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],
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scala_library_classpath = [
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"@maven//:org_scala_lang_scala_library",
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"@maven//:org_scala_lang_scala_reflect",
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],
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scala_macro_classpath = [
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"@maven//:org_scala_lang_scala_library",
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"@maven//:org_scala_lang_scala_reflect",
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],
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scala_xml_deps = [
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"@maven//:org_scala_lang_modules_scala_xml_2_12",
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],
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# example of setting attribute values
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scalacopts = ["-Ywarn-unused"],
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unused_dependency_checker_mode = "off",
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visibility = ["//visibility:public"]
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)
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```
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#### Step 2
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Register your custom toolchain:
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```py
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# MODULE.bazel or WORKSPACE
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register_toolchains("//toolchains:my_scala_toolchain")
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```
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## Configuration options
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The following attributes apply to both `scala_toolchain` and
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`setup_scala_toolchain`.
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<table class="table table-condensed table-bordered table-params">
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<colgroup>

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