List of labels; optional; nonconfigurable
  The set of targets whose Make variables the rule is allowed
  to access. These rules are either rules that provide
  the TemplateVariableInfo provider or special targets for toolchain types built into
Bazel.
These include:
@bazel_tools//tools/cpp:current_cc_toolchain
  @bazel_tools//tools/cpp:current_java_runtime
  Note that this is distinct from the concept of toolchain resolution that is used by rule implementations for platform-dependenct configuration. You cannot use this attribute to determine which specific cc_toolchain or java_toolchain a target will use.