[Compiler Toolchains] Add support for plugins

scala/private/phases/phase_compile.bzl

@@ -125,7 +125,12 @@ def _phase_compile(
     jars2labels = p.collect_jars.jars2labels.jars_to_labels
     deps_providers = p.collect_jars.deps_providers
     default_classpath = p.scalac_provider.default_classpath
-    plugins = ctx.attr.plugins
+
+    # Merge toolchain plugins with target-specific plugins
+    toolchain = ctx.toolchains["//scala:toolchain_type"]
+    toolchain_plugins = getattr(toolchain, "plugins", [])
+    plugins = toolchain_plugins + ctx.attr.plugins
+
     additional_outputs = []
     scalacopts = p.scalacopts
 

scala/scala_toolchain.bzl

@@ -81,6 +81,7 @@ def _scala_toolchain_impl(ctx):
 
     toolchain = platform_common.ToolchainInfo(
         scalacopts = ctx.attr.scalacopts,
+        plugins = ctx.attr.plugins,
         dep_providers = ctx.attr.dep_providers,
         dependency_mode = dependency_mode,
         strict_deps_mode = strict_deps_mode,
@@ -122,6 +123,10 @@ _scala_toolchain = rule(
     _scala_toolchain_impl,
     attrs = {
         "scalacopts": attr.string_list(),
+        "plugins": attr.label_list(
+            doc = "Compiler plugins to be enabled for all scala targets using this toolchain",
+            allow_files = [".jar"],
+        ),
         "dep_providers": attr.label_list(
             default = _default_dep_providers(),
             providers = [_DepsInfo],

test/toolchain_plugins/BUILD.bazel

@@ -0,0 +1,42 @@
+load("//scala:scala_toolchain.bzl", "scala_toolchain")
+load("//scala:scala.bzl", "scala_library")
+
+# Toolchain WITH the kind-projector plugin
+scala_toolchain(
+    name = "with_plugin_impl",
+    plugins = ["@org_typelevel_kind_projector//jar"],
+    visibility = ["//visibility:public"],
+)
+
+toolchain(
+    name = "with_plugin",
+    toolchain = ":with_plugin_impl",
+    toolchain_type = "//scala:toolchain_type",
+    visibility = ["//visibility:public"],
+)
+
+# Toolchain WITHOUT the plugin (but otherwise identical)
+scala_toolchain(
+    name = "without_plugin_impl",
+    # No plugins attribute
+    visibility = ["//visibility:public"],
+)
+
+toolchain(
+    name = "without_plugin",
+    toolchain = ":without_plugin_impl",
+    toolchain_type = "//scala:toolchain_type",
+    visibility = ["//visibility:public"],
+)
+
+# Test that requires kind-projector plugin
+scala_library(
+    name = "requires_plugin",
+    srcs = ["RequiresPlugin.scala"],
+)
+
+# Test that doesn't require any plugin
+scala_library(
+    name = "no_plugin_needed",
+    srcs = ["NoPlugin.scala"],
+)

test/toolchain_plugins/NoPlugin.scala

@@ -0,0 +1,6 @@
+package test.toolchain_plugins
+
+// Standard Scala - no plugin needed
+class NoPlugin {
+  def hello: String = "This compiles without any plugin"
+}

test/toolchain_plugins/RequiresPlugin.scala

@@ -0,0 +1,7 @@
+package test.toolchain_plugins
+
+import scala.language.higherKinds
+
+// This REQUIRES kind-projector plugin - uses the * syntax
+class HKT[F[_]]
+class RequiresPlugin extends HKT[Either[String, *]]