src/main/java/com/google/devtools/build/docgen/templates/attributes/common/toolchains.html

<p><code>List of <a href="../build-ref.html#labels">labels</a>; optional</code></p>

<p>
  The set of targets whose <a href="${link make-variables}">Make variables</a> the rule is allowed
  to access. These rules are either rules that provide
  the <code>TemplateVariableInfo</code> provider or special targets for toolchain types built into
Bazel.
These include:

<ul>
    <li><code>@bazel_tools//tools/cpp:current_cc_toolchain</code>
  <li><code>@bazel_tools//tools/cpp:current_java_runtime</code>
  </ul>

<p>
  Note that this is distinct from the concept of
    <a href="../toolchains.html">toolchain resolution</a>
    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.
</p>