Move the C++ toolchain into its own execution group so that it does not have to be bound by the same constraints as the execution platform selected for the Swift toolchain.

swift/toolchains/swift_toolchain.bzl

@@ -25,7 +25,6 @@ load("@bazel_skylib//lib:paths.bzl", "paths")
 load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
 load(
     "@bazel_tools//tools/cpp:toolchain_utils.bzl",
-    "find_cpp_toolchain",
     "use_cpp_toolchain",
 )
 load("@rules_cc//cc/common:cc_common.bzl", "cc_common")
@@ -92,6 +91,8 @@ load(
 )
 load("//swift/toolchains/config:tool_config.bzl", "ToolConfigInfo")
 
+_CPP_TOOLCHAIN_TYPE = Label("@bazel_tools//tools/cpp:toolchain_type")
+
 def _swift_compile_resource_set(_os, inputs_size):
     # The `os` argument is unused, but the Starlark API requires both
     # positional arguments.
@@ -410,7 +411,7 @@ def _parse_target_system_name(*, arch, os, target_system_name):
 
 def _swift_toolchain_impl(ctx):
     toolchain_root = ctx.attr.root
-    cc_toolchain = find_cpp_toolchain(ctx)
+    cc_toolchain = ctx.exec_groups["default"].toolchains[_CPP_TOOLCHAIN_TYPE].cc
     target_system_name = _parse_target_system_name(
         arch = ctx.attr.arch,
         os = ctx.attr.os,
@@ -706,7 +707,14 @@ The version of XCTest that the toolchain packages.
         },
     ),
     doc = "Represents a Swift compiler toolchain.",
+    exec_groups = {
+        # An execution group that has no specific platform requirements. This
+        # ensures that the execution platform of this Swift toolchain does not
+        # unnecessarily constrain the execution platform of the C++ toolchain.
+        "default": exec_group(
+            toolchains = use_cpp_toolchain(),
+        ),
+    },
     fragments = [] if bazel_features.cc.swift_fragment_removed else ["swift"],
-    toolchains = use_cpp_toolchain(),
     implementation = _swift_toolchain_impl,
 )

swift/toolchains/xcode_swift_toolchain.bzl

@@ -25,7 +25,6 @@ load("@bazel_skylib//lib:paths.bzl", "paths")
 load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
 load(
     "@bazel_tools//tools/cpp:toolchain_utils.bzl",
-    "find_cpp_toolchain",
     "use_cpp_toolchain",
 )
 load("@rules_cc//cc/common:cc_common.bzl", "cc_common")
@@ -108,6 +107,8 @@ load(
 )
 load("//swift/toolchains/config:tool_config.bzl", "ToolConfigInfo")
 
+_CPP_TOOLCHAIN_TYPE = Label("@bazel_tools//tools/cpp:toolchain_type")
+
 def _platform_developer_framework_dir(
         apple_toolchain,
         target_triple):
@@ -626,7 +627,7 @@ def _dsym_provider(*, ctx):
 def _xcode_swift_toolchain_impl(ctx):
     cpp_fragment = ctx.fragments.cpp
     apple_toolchain = apple_common.apple_toolchain()
-    cc_toolchain = find_cpp_toolchain(ctx)
+    cc_toolchain = ctx.exec_groups["default"].toolchains[_CPP_TOOLCHAIN_TYPE].cc
 
     target_triple = ctx.var.get("CC_TARGET_TRIPLE") or target_triples.normalize_for_swift(
         target_triples.parse(cc_toolchain.target_gnu_system_name),
@@ -998,10 +999,17 @@ for incremental compilation using a persistent mode.
         },
     ),
     doc = "Represents a Swift compiler toolchain provided by Xcode.",
+    exec_groups = {
+        # An execution group that has no specific platform requirements. This
+        # ensures that the execution platform of this Swift toolchain does not
+        # unnecessarily constrain the execution platform of the C++ toolchain.
+        "default": exec_group(
+            toolchains = use_cpp_toolchain(),
+        ),
+    },
     fragments = [
         "cpp",
         "objc",
     ] + ([] if bazel_features.cc.swift_fragment_removed else ["swift"]),
-    toolchains = use_cpp_toolchain(),
     implementation = _xcode_swift_toolchain_impl,
 )

test/fixtures/linking/fake_framework.bzl

@@ -2,20 +2,21 @@
 
 load(
     "@bazel_tools//tools/cpp:toolchain_utils.bzl",
-    "find_cpp_toolchain",
     "use_cpp_toolchain",
 )
 load("@rules_cc//cc/common:cc_common.bzl", "cc_common")
 load("@rules_cc//cc/common:cc_info.bzl", "CcInfo")
 
+_CPP_TOOLCHAIN_TYPE = Label("@bazel_tools//tools/cpp:toolchain_type")
+
 def _impl(ctx):
     binary1 = ctx.actions.declare_file("framework1.framework/framework1")
     ctx.actions.write(binary1, "empty")
 
     binary2 = ctx.actions.declare_file("framework2.framework/framework2")
     ctx.actions.write(binary2, "empty")
 
-    cc_toolchain = find_cpp_toolchain(ctx)
+    cc_toolchain = ctx.exec_groups["default"].toolchains[_CPP_TOOLCHAIN_TYPE].cc
     feature_configuration = cc_common.configure_features(
         ctx = ctx,
         cc_toolchain = cc_toolchain,
@@ -56,6 +57,13 @@ fake_framework = rule(
             doc = "The C++ toolchain to use.",
         ),
     },
-    toolchains = use_cpp_toolchain(),
+    exec_groups = {
+        # An execution group that has no specific platform requirements. This
+        # ensures that the execution platform of this Swift toolchain does not
+        # unnecessarily constrain the execution platform of the C++ toolchain.
+        "default": exec_group(
+            toolchains = use_cpp_toolchain(),
+        ),
+    },
     fragments = ["cpp"],
 )