Configure LLVM cc_toolchain included with Swift releases

Author: gferon

MODULE.bazel

@@ -19,6 +19,7 @@ non_module_deps = use_extension("//swift:extensions.bzl", "non_module_deps")
 use_repo(
     non_module_deps,
     "build_bazel_rules_swift_index_import",
+    "build_bazel_rules_swift_local_cc_config",
     "build_bazel_rules_swift_local_config",
     "com_github_apple_swift_log",
     "com_github_apple_swift_nio",

README.md

@@ -65,17 +65,6 @@ also ensure that the Swift compiler is available on your system path.
 Copy the `WORKSPACE` snippet from [the releases
 page](https://github.com/bazelbuild/rules_swift/releases).
 
-### 3. Additional configuration (Linux only)
-
-The `swift_binary` and `swift_test` rules expect to use `clang` as the driver
-for linking, and they query the Bazel C++ API and CROSSTOOL to determine which
-arguments should be passed to the linker. By default, the C++ toolchain used by
-Bazel is `gcc`, so Swift users on Linux need to override this by setting the
-environment variable `CC=clang` when invoking Bazel.
-
-This step is not necessary for macOS users because the Xcode toolchain always
-uses `clang`.
-
 ## Building with Custom Toolchains
 
 **macOS hosts:** You can build with a custom Swift toolchain (downloaded

swift/internal/linking.bzl

@@ -110,7 +110,7 @@ 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 +24,12 @@ 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:windows_cc_configure.bzl", "configure_windows_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",
@@ -256,6 +262,63 @@ def _normalized_linux_cpu(cpu):
         return "x86_64"
     return cpu
 
+def _toolchain_root(repository_ctx):
+    path_to_swiftc = repository_ctx.which("swiftc")
+    if not path_to_swiftc:
+        fail("No 'swiftc' executable found in $PATH")
+    return path_to_swiftc.dirname
+
+def _create_xcode_cc_toolchain(repository_ctx):
+    """Creates BUILD alias for the C++ toolchain provided by apple_support
+
+    Args:
+      repository_ctx: The repository rule context.
+    """
+
+    repository_ctx.file("BUILD", """
+alias(
+    name = "toolchain",
+    actual = "@local_config_apple_cc//:toolchain",
+    visibility = ["//visibility:public"]
+)
+        """)
+
+def _toolchain_overriden_tools(toolchain_root, extension = ""):
+    tools = {
+        "ld": toolchain_root.get_child("lld" + extension),
+        "llvm-cov": toolchain_root.get_child("llvm-cov" + extension),
+        "llvm-profdata": toolchain_root.get_child("llvm-profdata" + extension),
+        "cpp": toolchain_root.get_child("clang-cpp" + extension),
+        "gcc": toolchain_root.get_child("clang" + extension),
+    }
+
+    # llvm-ar is not shipped before Swift 5.8
+    ar = toolchain_root.get_child("llvm-ar" + extension)
+    if ar.exists:
+        tools["ar"] = ar
+    return tools
+
+def _create_linux_cc_toolchain(repository_ctx):
+    """Creates BUILD targets for the Swift-provided C++ toolchain on Linux.
+
+    Args:
+      repository_ctx: The repository rule context.
+    """
+
+    toolchain_root = _toolchain_root(repository_ctx)
+    cpu = get_cpu_value(repository_ctx)
+    configure_unix_toolchain(repository_ctx, cpu, overriden_tools = _toolchain_overriden_tools(toolchain_root))
+
+def _create_windows_cc_toolchain(repository_ctx):
+    """Creates BUILD targets for the Swift-provided C++ toolchain on Windows.
+
+    Args:
+      repository_ctx: The repository rule context.
+    """
+
+    toolchain_root = _toolchain_root(repository_ctx)
+    configure_windows_toolchain(repository_ctx, overriden_tools = _toolchain_overriden_tools(toolchain_root, ".exe"))
+
 def _create_linux_toolchain(repository_ctx):
     """Creates BUILD targets for the Swift toolchain on Linux.
 
@@ -266,6 +329,7 @@ def _create_linux_toolchain(repository_ctx):
     if not path_to_swiftc:
         fail("No 'swiftc' executable found in $PATH")
 
+    toolchain_root = _toolchain_root(repository_ctx)
     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)
@@ -306,6 +370,7 @@ swift_toolchain(
                 for feature in feature_values
             ]),
             root = root,
+            toolchain_root = toolchain_root,
             version_file = version_file,
         ),
     )
@@ -421,10 +486,16 @@ swift_toolchain(
         ),
     )
 
+def _swift_cc_autoconfiguration_impl(repository_ctx):
+    os_name = repository_ctx.os.name.lower()
+    if os_name.startswith("mac os"):
+        _create_xcode_cc_toolchain(repository_ctx)
+    elif os_name.startswith("windows"):
+        _create_windows_cc_toolchain(repository_ctx)
+    else:
+        _create_linux_cc_toolchain(repository_ctx)
+
 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
-    # possible to support non-Xcode toolchains on macOS as well.
     os_name = repository_ctx.os.name.lower()
     if os_name.startswith("mac os"):
         _create_xcode_toolchain(repository_ctx)
@@ -433,6 +504,12 @@ 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_cc_config//: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

@@ -274,11 +274,6 @@ def _swift_toolchain_impl(ctx):
     toolchain_root = ctx.attr.root
     cc_toolchain = find_cpp_toolchain(ctx)
 
-    if "clang" not in cc_toolchain.compiler:
-        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,
@@ -450,7 +445,7 @@ 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_cc_config//: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

@@ -800,7 +800,7 @@ 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_cc_config//: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):
@@ -191,6 +192,11 @@ 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",