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

Fixes bazelbuild#14728, part of bazelbuild#14727.

Rules can begin using this as soon as it is released (hopefully in Bazel
5.1), and then future versions of Bazel will update the functionality as
optional toolchains are added and C++ rules are updated to use them.

Closes bazelbuild#14742.

PiperOrigin-RevId: 427931123

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]