Add override point for cc_autoconf

This adds `overriden_labels` to the `cc_autoconf` rule. This allows to
override specific tools that bazel uses when setting up
`local_config_cc`. The benefit there is that if you want to replace
pieces of the cc toolchain, without having to create your own crosstool
entirely, this allows you to do that.

Specifically for the osx toolchain sometimes overriding flags in
response to Xcode updates, without having to wait for a bazel release is
useful.

tools/cpp/cc_configure.bzl

@@ -52,11 +52,20 @@ def cc_autoconf_impl(repository_ctx, overriden_tools = dict()):
         configure_windows_toolchain(repository_ctx)
     elif (cpu_value == "darwin" and
           ("BAZEL_USE_CPP_ONLY_TOOLCHAIN" not in env or env["BAZEL_USE_CPP_ONLY_TOOLCHAIN"] != "1")):
-        configure_osx_toolchain(repository_ctx, overriden_tools)
+        configure_osx_toolchain(repository_ctx, overriden_tools, repository_ctx.attr.overriden_labels)
     else:
         configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools)
 
 cc_autoconf = repository_rule(
+    attrs = {
+        "overriden_labels": attr.string_dict(
+            doc = """
+A dictionary of labels bazel wants to use for setting up the cc_toolchain to the actual labels to override
+the tools with instead. This allows you to override single pieces of the toolchain without creating your
+own crosstool.
+""",
+        ),
+    },
     environ = [
         "ABI_LIBC_VERSION",
         "ABI_VERSION",

tools/cpp/osx_cc_configure.bzl

@@ -49,9 +49,16 @@ def _get_escaped_xcode_cxx_inc_directories(repository_ctx, cc, xcode_toolchains)
     include_dirs.append("/Applications/")
     return include_dirs
 
-def configure_osx_toolchain(repository_ctx, overriden_tools):
+def _resolve_overriden_labels(labels, overriden_tools):
+    resolved_labels = {label: label for label in labels}
+    for override, actual in overriden_tools.items():
+        resolved_labels[override] = actual
+
+    return resolved_labels
+
+def configure_osx_toolchain(repository_ctx, overriden_tools, overriden_labels):
     """Configure C++ toolchain on macOS."""
-    paths = resolve_labels(repository_ctx, [
+    resolved_labels = _resolve_overriden_labels([
         "@bazel_tools//tools/cpp:osx_cc_wrapper.sh.tpl",
         "@bazel_tools//tools/objc:libtool.sh",
         "@bazel_tools//tools/objc:make_hashed_objlist.py",
@@ -62,50 +69,52 @@ def configure_osx_toolchain(repository_ctx, overriden_tools):
         "@bazel_tools//tools/osx/crosstool:wrapped_ar.tpl",
         "@bazel_tools//tools/osx/crosstool:wrapped_clang.cc",
         "@bazel_tools//tools/osx:xcode_locator.m",
-    ])
+    ], overriden_labels)
+
+    paths = resolve_labels(repository_ctx, resolved_labels.values())
 
     xcode_toolchains = []
     (xcode_toolchains, xcodeloc_err) = run_xcode_locator(
         repository_ctx,
-        paths["@bazel_tools//tools/osx:xcode_locator.m"],
+        paths[resolved_labels["@bazel_tools//tools/osx:xcode_locator.m"]],
     )
     if xcode_toolchains:
         cc = find_cc(repository_ctx, overriden_tools = {})
         repository_ctx.template(
             "cc_wrapper.sh",
-            paths["@bazel_tools//tools/cpp:osx_cc_wrapper.sh.tpl"],
+            paths[resolved_labels["@bazel_tools//tools/cpp:osx_cc_wrapper.sh.tpl"]],
             {
                 "%{cc}": escape_string(str(cc)),
                 "%{env}": escape_string(get_env(repository_ctx)),
             },
         )
         repository_ctx.symlink(
-            paths["@bazel_tools//tools/objc:xcrunwrapper.sh"],
+            paths[resolved_labels["@bazel_tools//tools/objc:xcrunwrapper.sh"]],
             "xcrunwrapper.sh",
         )
         repository_ctx.symlink(
-            paths["@bazel_tools//tools/objc:libtool.sh"],
+            paths[resolved_labels["@bazel_tools//tools/objc:libtool.sh"]],
             "libtool",
         )
         repository_ctx.symlink(
-            paths["@bazel_tools//tools/objc:make_hashed_objlist.py"],
+            paths[resolved_labels["@bazel_tools//tools/objc:make_hashed_objlist.py"]],
             "make_hashed_objlist.py",
         )
         repository_ctx.symlink(
-            paths["@bazel_tools//tools/osx/crosstool:wrapped_ar.tpl"],
+            paths[resolved_labels["@bazel_tools//tools/osx/crosstool:wrapped_ar.tpl"]],
             "wrapped_ar",
         )
         repository_ctx.symlink(
-            paths["@bazel_tools//tools/osx/crosstool:BUILD.tpl"],
+            paths[resolved_labels["@bazel_tools//tools/osx/crosstool:BUILD.tpl"]],
             "BUILD",
         )
         repository_ctx.symlink(
-            paths["@bazel_tools//tools/osx/crosstool:osx_archs.bzl"],
+            paths[resolved_labels["@bazel_tools//tools/osx/crosstool:osx_archs.bzl"]],
             "osx_archs.bzl",
         )
 
         wrapped_clang_src_path = str(repository_ctx.path(
-            paths["@bazel_tools//tools/osx/crosstool:wrapped_clang.cc"],
+            paths[resolved_labels["@bazel_tools//tools/osx/crosstool:wrapped_clang.cc"]],
         ))
         xcrun_result = repository_ctx.execute([
             "env",
@@ -140,7 +149,7 @@ def configure_osx_toolchain(repository_ctx, overriden_tools):
             escaped_cxx_include_directories.append("# Error: " + xcodeloc_err + "\n")
         repository_ctx.template(
             "cc_toolchain_config.bzl",
-            paths["@bazel_tools//tools/osx/crosstool:cc_toolchain_config.bzl.tpl"],
+            paths[resolved_labels["@bazel_tools//tools/osx/crosstool:cc_toolchain_config.bzl.tpl"]],
             {"%{cxx_builtin_include_directories}": "\n".join(escaped_cxx_include_directories)},
         )
     else: