Only load compiler plugins from direct dependencies

swift/internal/compiling.bzl

@@ -761,9 +761,10 @@ def compile(
     # passed; the compiler does not attempt to load them when deserializing
     # modules.
     used_plugins = list(plugins)
-    for module_context in transitive_modules:
-        if module_context.swift and module_context.swift.plugins:
-            used_plugins.extend(module_context.swift.plugins)
+    for swift_info in swift_infos + private_swift_infos:
+        for module_context in swift_info.direct_modules:
+            if module_context.swift and module_context.swift.plugins:
+                used_plugins.extend(module_context.swift.plugins)
 
     if include_dev_srch_paths != None and is_test != None:
         fail("""\

test/BUILD

@@ -16,6 +16,7 @@ load(":imported_framework_tests.bzl", "imported_framework_test_suite")
 load(":interop_hints_tests.bzl", "interop_hints_test_suite")
 load(":layering_check_tests.bzl", "layering_check_test_suite")
 load(":localized_strings_tests.bzl", "localized_strings_test_suite")
+load(":macro_plugin_tests.bzl", "macro_plugin_test_suite")
 load(":mainattr_tests.bzl", "mainattr_test_suite")
 load(":mixed_language_tests.bzl", "mixed_language_test_suite")
 load(":module_cache_settings_tests.bzl", "module_cache_settings_test_suite")
@@ -70,6 +71,8 @@ layering_check_test_suite(name = "layering_check")
 
 localized_strings_test_suite(name = "localized_strings")
 
+macro_plugin_test_suite(name = "macro_plugin")
+
 mainattr_test_suite(name = "mainattr")
 
 mixed_language_test_suite(name = "mixed_language")

test/fixtures/macros/BUILD

@@ -0,0 +1,53 @@
+load("//swift:swift_compiler_plugin.bzl", "swift_compiler_plugin")
+load("//swift:swift_library.bzl", "swift_library")
+load("//test/fixtures:common.bzl", "FIXTURE_TAGS")
+
+package(
+    default_visibility = ["//test:__subpackages__"],
+)
+
+licenses(["notice"])
+
+###############################################################################
+# Fixtures for testing compiler plugin (macro) propagation
+
+swift_compiler_plugin(
+    name = "stringify_macro",
+    srcs = [
+        "StringifyMacro.swift",
+        "StringifyMacroPlugin.swift",
+    ],
+    module_name = "StringifyMacroPlugin",
+    tags = FIXTURE_TAGS,
+    deps = [
+        "@swift-syntax//:SwiftCompilerPlugin",
+        "@swift-syntax//:SwiftSyntax",
+        "@swift-syntax//:SwiftSyntaxBuilder",
+        "@swift-syntax//:SwiftSyntaxMacroExpansion",
+        "@swift-syntax//:SwiftSyntaxMacros",
+    ],
+)
+
+swift_library(
+    name = "stringify",
+    srcs = ["Stringify.swift"],
+    module_name = "Stringify",
+    plugins = [":stringify_macro"],
+    tags = FIXTURE_TAGS,
+)
+
+swift_library(
+    name = "stringify_user",
+    srcs = ["StringifyUser.swift"],
+    module_name = "StringifyUser",
+    tags = FIXTURE_TAGS,
+    deps = [":stringify"],
+)
+
+swift_library(
+    name = "transitive_user",
+    srcs = ["TransitiveUser.swift"],
+    module_name = "TransitiveUser",
+    tags = FIXTURE_TAGS,
+    deps = [":stringify_user"],
+)

test/fixtures/macros/Stringify.swift

@@ -0,0 +1,17 @@
+// Copyright 2023 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+@freestanding(expression)
+public macro stringify<T>(_ value: T) -> (T, String) =
+  #externalMacro(module: "StringifyMacroPlugin", type: "StringifyMacro")

test/fixtures/macros/StringifyMacro.swift

@@ -0,0 +1,29 @@
+// Copyright 2023 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import SwiftSyntax
+import SwiftSyntaxBuilder
+import SwiftSyntaxMacros
+
+public struct StringifyMacro: ExpressionMacro {
+  public static func expansion(
+    of node: some FreestandingMacroExpansionSyntax,
+    in context: some MacroExpansionContext
+  ) -> ExprSyntax {
+    guard let argument = node.argumentList.first?.expression else {
+      fatalError("compiler bug: the macro does not have any arguments")
+    }
+    return "(\(argument), \(literal: argument.description))"
+  }
+}

test/fixtures/macros/StringifyMacroPlugin.swift

@@ -0,0 +1,25 @@
+// Copyright 2023 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#if canImport(SwiftCompilerPlugin)
+import SwiftCompilerPlugin
+import SwiftSyntaxMacros
+
+@main
+struct StringifyMacroPlugin: CompilerPlugin {
+  let providingMacros: [Macro.Type] = [
+    StringifyMacro.self
+  ]
+}
+#endif

test/fixtures/macros/StringifyUser.swift

@@ -0,0 +1,21 @@
+// Copyright 2026 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import Stringify
+
+// Directly depends on the macro-declaring module and contains an expansion
+// site, so its compilation must load the plugin.
+public func stringifyUser() -> (Int, String) {
+  #stringify(1 + 1)
+}

test/fixtures/macros/TransitiveUser.swift

@@ -0,0 +1,22 @@
+// Copyright 2026 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import StringifyUser
+
+// Only sees the macro-declaring module transitively (through StringifyUser)
+// and contains no expansion sites, so its compilation must not load the
+// plugin.
+public func transitiveUser() -> String {
+  String(describing: stringifyUser())
+}

test/macro_plugin_tests.bzl

@@ -0,0 +1,66 @@
+# Copyright 2026 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for compiler plugin (macro) propagation."""
+
+load(
+    "//test/rules:action_command_line_test.bzl",
+    "action_command_line_test",
+)
+
+def macro_plugin_test_suite(name, tags = []):
+    """Test suite for compiler plugin propagation to dependents.
+
+    Args:
+        name: The base name to be used in targets created by this macro.
+        tags: Additional tags to apply to each test.
+    """
+    all_tags = [name] + tags
+
+    # The module that declares the macro loads its own plugin.
+    action_command_line_test(
+        name = "{}_declaring_module_loads_plugin".format(name),
+        expected_argv = ["-load-plugin-executable"],
+        mnemonic = "SwiftCompile",
+        tags = all_tags,
+        target_under_test = "//test/fixtures/macros:stringify",
+    )
+
+    # A module that directly depends on the library declaring the macro must
+    # load the plugin, since its sources may contain expansion sites.
+    action_command_line_test(
+        name = "{}_direct_dependent_loads_plugin".format(name),
+        expected_argv = ["-load-plugin-executable"],
+        mnemonic = "SwiftCompile",
+        tags = all_tags,
+        target_under_test = "//test/fixtures/macros:stringify_user",
+    )
+
+    # A module that only sees the macro-declaring module transitively must
+    # not: macros are source-level transformations, so the compiler never
+    # loads plugins when deserializing dependency modules. Passing the plugin
+    # anyway makes its executable an input to every transitive consumer's
+    # compilation, so any change to the plugin recompiles all of them.
+    action_command_line_test(
+        name = "{}_transitive_dependent_does_not_load_plugin".format(name),
+        mnemonic = "SwiftCompile",
+        not_expected_argv = ["-load-plugin-executable"],
+        tags = all_tags,
+        target_under_test = "//test/fixtures/macros:transitive_user",
+    )
+
+    native.test_suite(
+        name = name,
+        tags = all_tags,
+    )