Use cc_toolchain_* for clang shipped with Swift toolchain

MODULE.bazel

@@ -19,6 +19,7 @@ use_repo(
     non_module_deps,
     "build_bazel_rules_swift_index_import",
     "build_bazel_rules_swift_local_config",
+    "build_bazel_rules_swift_local_cc_config",
     "com_github_apple_swift_log",
     "com_github_apple_swift_nio",
     "com_github_apple_swift_nio_extras",
@@ -33,3 +34,5 @@ use_repo(apple_cc_configure, "local_config_apple_cc")
 
 # Dev dependencies
 bazel_dep(name = "stardoc", version = "0.5.3", dev_dependency = True, repo_name = "io_bazel_stardoc")
+
+register_toolchains("@build_bazel_rules_swift_cc_toolchain//:all")

swift/internal/linking.bzl

@@ -107,7 +107,7 @@ into the binary. Possible values are:
             # Do not add references; temporary attribute for C++ toolchain
             # Starlark migration.
             "_cc_toolchain": attr.label(
-                default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
+                default = Label("@build_bazel_rules_swift_local_cc_config//:toolchain"),
             ),
             # A late-bound attribute denoting the value of the `--custom_malloc`
             # command line flag (or None if the flag is not provided).

swift/internal/swift_autoconfiguration.bzl

@@ -24,6 +24,11 @@ should be loaded here. Do not load anything else, even common libraries like
 Skylib.
 """
 
+load("@bazel_tools//tools/cpp:unix_cc_configure.bzl", "configure_unix_toolchain")
+load(
+    "@bazel_tools//tools/cpp:lib_cc_configure.bzl",
+    "get_cpu_value",
+)
 load(
     "@build_bazel_rules_swift//swift/internal:feature_names.bzl",
     "SWIFT_FEATURE_CODEVIEW_DEBUG_INFO",
@@ -231,6 +236,22 @@ def _normalized_linux_cpu(cpu):
         return "x86_64"
     return cpu
 
+def _create_linux_cc_toolchain(repository_ctx):
+    path_to_swiftc = repository_ctx.which("swiftc")
+    if not path_to_swiftc:
+        fail("No 'swiftc' executable found in $PATH")
+
+    toolchain_root = path_to_swiftc.dirname
+    cpu = get_cpu_value(repository_ctx)
+    configure_unix_toolchain(repository_ctx, cpu, overriden_tools = {
+        "ar": toolchain_root.get_child("llvm-ar"),
+        "ld": toolchain_root.get_child("lld"),
+        "llvm-cov": toolchain_root.get_child("llvm-cov"),
+        "llvm-profdata": toolchain_root.get_child("llvm-profdata"),
+        "cpp": toolchain_root.get_child("clang-cpp"),
+        "gcc": toolchain_root.get_child("clang"),
+    })
+
 def _create_linux_toolchain(repository_ctx):
     """Creates BUILD targets for the Swift toolchain on Linux.
 
@@ -241,6 +262,7 @@ def _create_linux_toolchain(repository_ctx):
     if not path_to_swiftc:
         fail("No 'swiftc' executable found in $PATH")
 
+    toolchain_root = path_to_swiftc.dirname
     root = path_to_swiftc.dirname.dirname
     feature_values = _compute_feature_values(repository_ctx, path_to_swiftc)
     version_file = _write_swift_version(repository_ctx, path_to_swiftc)
@@ -262,6 +284,11 @@ load(
 
 package(default_visibility = ["//visibility:public"])
 
+alias(
+    name = "swift_cc_toolchain",
+    actual = "@build_bazel_rules_swift_local_cc_config//:toolchain"
+)
+
 swift_toolchain(
     name = "toolchain",
     arch = "{cpu}",
@@ -277,6 +304,7 @@ swift_toolchain(
                 for feature in feature_values
             ]),
             root = root,
+            toolchain_root = toolchain_root,
             version_file = version_file,
         ),
     )
@@ -370,6 +398,12 @@ load(
 
 package(default_visibility = ["//visibility:public"])
 
+# Use the system C++ toolchain
+alias(
+    name = "swift_cc_toolchain",
+    actual = "@bazel_tools//tools/cpp:current_cc_toolchain"
+)
+
 swift_toolchain(
   name = "toolchain",
   arch = "x86_64",
@@ -392,6 +426,13 @@ swift_toolchain(
         ),
     )
 
+def _swift_cc_autoconfiguration_impl(repository_ctx):
+    os_name = repository_ctx.os.name.lower()
+    if os_name.startswith("linux"):
+        _create_linux_cc_toolchain(repository_ctx)
+    else:
+        fail("cc_toolchain detection for Swift is currently only supported on Linux")
+
 def _swift_autoconfiguration_impl(repository_ctx):
     # TODO(allevato): This is expedient and fragile. Use the
     # platforms/toolchains APIs instead to define proper toolchains, and make it
@@ -404,6 +445,12 @@ def _swift_autoconfiguration_impl(repository_ctx):
     else:
         _create_linux_toolchain(repository_ctx)
 
+swift_cc_autoconfiguration = repository_rule(
+    environ = ["PATH"],
+    implementation = _swift_cc_autoconfiguration_impl,
+    local = True,
+)
+
 swift_autoconfiguration = repository_rule(
     environ = ["CC", "PATH", "ProgramData", "Path"],
     implementation = _swift_autoconfiguration_impl,

swift/internal/swift_import.bzl

@@ -172,7 +172,7 @@ The `.swiftmodule` file provided to Swift targets that depend on this target.
                 mandatory = False,
             ),
             "_cc_toolchain": attr.label(
-                default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
+                default = Label("@build_bazel_rules_swift_local_config//:swift_cc_toolchain"),
                 doc = """\
 The C++ toolchain from which linking flags and other tools needed by the Swift
 toolchain (such as `clang`) will be retrieved.

swift/internal/swift_toolchain.bzl

@@ -272,11 +272,6 @@ def _swift_toolchain_impl(ctx):
     toolchain_root = ctx.attr.root
     cc_toolchain = find_cpp_toolchain(ctx)
 
-    if cc_toolchain.compiler != "clang":
-        fail("Swift requires the configured CC toolchain to be LLVM (clang). " +
-             "Either use the locally installed LLVM by setting `CC=clang` in your environment " +
-             "before invoking Bazel, or configure a Bazel LLVM CC toolchain.")
-
     if ctx.attr.os == "windows":
         swift_linkopts_cc_info = _swift_windows_linkopts_cc_info(
             ctx.attr.arch,
@@ -437,7 +432,7 @@ configuration options that are applied to targets on a per-package basis.
                 allow_single_file = True,
             ),
             "_cc_toolchain": attr.label(
-                default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
+                default = Label("@build_bazel_rules_swift_local_config//:swift_cc_toolchain"),
                 doc = """\
 The C++ toolchain from which other tools needed by the Swift toolchain (such as
 `clang` and `ar`) will be retrieved.

swift/internal/xcode_swift_toolchain.bzl

@@ -779,7 +779,7 @@ configuration options that are applied to targets on a per-package basis.
                 providers = [[SwiftPackageConfigurationInfo]],
             ),
             "_cc_toolchain": attr.label(
-                default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
+                default = Label("@build_bazel_rules_swift_local_config//:swift_cc_toolchain"),
                 doc = """\
 The C++ toolchain from which linking flags and other tools needed by the Swift
 toolchain (such as `clang`) will be retrieved.

swift/repositories.bzl

@@ -18,6 +18,7 @@ load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 load(
     "@build_bazel_rules_swift//swift/internal:swift_autoconfiguration.bzl",
     "swift_autoconfiguration",
+    "swift_cc_autoconfiguration",
 )
 
 def _maybe(repo_rule, name, **kwargs):
@@ -183,6 +184,11 @@ def swift_rules_dependencies(include_bzlmod_ready_dependencies = True):
         sha256 = "28c1ffa39d99e74ed70623899b207b41f79214c498c603915aef55972a851a15",
     )
 
+    _maybe(
+        swift_cc_autoconfiguration,
+        name = "build_bazel_rules_swift_local_cc_config",
+    )
+
     _maybe(
         swift_autoconfiguration,
         name = "build_bazel_rules_swift_local_config",