Allow using requires_none and requires_not_none at the same time

This combination can be useful during migrations from old bazel features
to new ones. You add your args as requiring the new feature to be not
none, and the old feature to be none.

Author: keith

cc/toolchains/impl/nested_args.bzl

@@ -23,7 +23,7 @@ visibility([
     "//tests/rule_based_toolchain/...",
 ])
 
-REQUIRES_MUTUALLY_EXCLUSIVE_ERR = "requires_none, requires_not_none, requires_true, requires_false, and requires_equal are mutually exclusive"
+REQUIRES_MUTUALLY_EXCLUSIVE_ERR = "requires_none and requires_not_none are mututally exclusive with requires_true, requires_false, and requires_equal"
 REQUIRES_NOT_NONE_ERR = "requires_not_none only works on options"
 REQUIRES_NONE_ERR = "requires_none only works on options"
 REQUIRES_TRUE_ERR = "requires_true only works on bools"
@@ -198,17 +198,11 @@ def nested_args_provider(
     transitive_files = [ea.files for ea in nested]
     transitive_files.append(files)
 
-    has_value = [attr for attr in [
-        requires_not_none,
-        requires_none,
-        requires_true,
-        requires_false,
-        requires_equal,
-    ] if attr != None]
-
     # We may want to reconsider this down the line, but it's easier to open up
     # an API than to lock down an API.
-    if len(has_value) > 1:
+    optional_checks = any([requires_not_none, requires_none])
+    boolean_checks = any([requires_true, requires_false, requires_equal])
+    if optional_checks and boolean_checks:
         fail(REQUIRES_MUTUALLY_EXCLUSIVE_ERR)
 
     kwargs = {}