<p><code>List of <a href="${link build-ref#labels}">labels</a>
; optional;
<a href="#configurable-attributes">nonconfigurable</a></code>
</p>

<p>
A list of
<code><a href="platform.html#constraint_value">constraint_values</a></code>
that must be present in the execution platform for this target. This is in
addition to any constraints already set by the rule type. Constraints are used
to restrict the list of available execution platforms. For more details, see
the description of
  <a href="/docs/toolchains#toolchain-resolution">toolchain resolution</a>.
  </p>