Add a helper method for rules to depend on the cpp toolchain type.

tools/cpp/cc_flags_supplier.bzl

@@ -15,7 +15,7 @@
 
 load("@bazel_tools//tools/build_defs/cc:action_names.bzl", "CC_FLAGS_MAKE_VARIABLE_ACTION_NAME")
 load("@bazel_tools//tools/cpp:cc_flags_supplier_lib.bzl", "build_cc_flags")
-load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain")
+load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain", "use_cpp_toolchain")
 
 def _cc_flags_supplier_impl(ctx):
     cc_toolchain = find_cpp_toolchain(ctx)
@@ -30,7 +30,7 @@ cc_flags_supplier = rule(
     attrs = {
         "_cc_toolchain": attr.label(default = Label("@bazel_tools//tools/cpp:current_cc_toolchain")),
     },
-    toolchains = ["@bazel_tools//tools/cpp:toolchain_type"],
+    toolchains = use_cpp_toolchain(),
     incompatible_use_toolchain_transition = True,
     fragments = ["cpp"],
 )

tools/cpp/compiler_flag.bzl

@@ -14,7 +14,7 @@
 
 """Rule that allows select() to differentiate between compilers."""
 
-load("//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain")
+load("//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain", "use_cpp_toolchain")
 
 def _compiler_flag_impl(ctx):
     toolchain = find_cpp_toolchain(ctx)
@@ -25,6 +25,6 @@ compiler_flag = rule(
     attrs = {
         "_cc_toolchain": attr.label(default = Label("//tools/cpp:current_cc_toolchain")),
     },
-    toolchains = ["@bazel_tools//tools/cpp:toolchain_type"],
+    toolchains = use_cpp_toolchain(),
     incompatible_use_toolchain_transition = True,
 )

tools/cpp/toolchain_utils.bzl

@@ -14,18 +14,19 @@
 # limitations under the License.
 
 """
-Finds the c++ toolchain.
+Utilities to help work with c++ toolchains.
 
-Returns the toolchain if enabled, and falls back to a toolchain constructed from
-the CppConfiguration.
 """
 
+CPP_TOOLCHAIN_TYPE = "@bazel_tools//tools/cpp:toolchain_type"
+
 def find_cpp_toolchain(ctx):
     """
     Finds the c++ toolchain.
 
     If the c++ toolchain is in use, returns it.  Otherwise, returns a c++
-    toolchain derived from legacy toolchain selection.
+    toolchain derived from legacy toolchain selection, constructed from
+    the CppConfiguration.
 
     Args:
       ctx: The rule context for which to find a toolchain.
@@ -36,9 +37,9 @@ def find_cpp_toolchain(ctx):
 
     # Check the incompatible flag for toolchain resolution.
     if hasattr(cc_common, "is_cc_toolchain_resolution_enabled_do_not_use") and cc_common.is_cc_toolchain_resolution_enabled_do_not_use(ctx = ctx):
-        if not "@bazel_tools//tools/cpp:toolchain_type" in ctx.toolchains:
-            fail("In order to use find_cpp_toolchain, you must include the '@bazel_tools//tools/cpp:toolchain_type' in the toolchains argument to your rule.")
-        toolchain_info = ctx.toolchains["@bazel_tools//tools/cpp:toolchain_type"]
+        if not CPP_TOOLCHAIN_TYPE in ctx.toolchains:
+            fail("In order to use find_cpp_toolchain, you must include the '%s' in the toolchains argument to your rule." % CPP_TOOLCHAIN_TYPE)
+        toolchain_info = ctx.toolchains[CPP_TOOLCHAIN_TYPE]
         if hasattr(toolchain_info, "cc_provider_in_toolchain") and hasattr(toolchain_info, "cc"):
             return toolchain_info.cc
         return toolchain_info
@@ -49,3 +50,23 @@ def find_cpp_toolchain(ctx):
 
     # We didn't find anything.
     fail("In order to use find_cpp_toolchain, you must define the '_cc_toolchain' attribute on your rule or aspect.")
+
+def use_cpp_toolchain(mandatory = True):
+    """
+    Helper to depend on the c++ toolchain.
+
+    Usage:
+    ```
+    my_rule = rule(
+        toolchains = [other toolchain types] + use_cpp_toolchain(),
+    )
+    ```
+
+    Args:
+      mandatory: Whether or not it should be an error if the toolchain cannot be resolved.
+        Currently ignored, this will be enabled when optional toolchain types are added.
+
+    Returns:
+      A list that can be used as the value for `rule.toolchains`.
+    """
+    return [CPP_TOOLCHAIN_TYPE]