<p>Dictionary of strings to lists of <a href="${link build-ref#labels}">labels</a>;
  <a href="#configurable-attributes">nonconfigurable</a>; default is <code>{}</code>
</p>

<p>
A dictionary of exec group names to lists of
<code><a href="platforms-and-toolchains.html#constraint_value">constraint_values</a></code>
that must be present in the execution platform for the given exec group. This
is in addition to any constraints already set on the exec group's definition.
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>.
  and
  <a href="/extending/exec-groups">exec groups</a>
  
</p>
