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

<p>
The list of environments this target can be built for, in addition to
default-supported environments.
</p>

<p>
This is part of Bazel's constraint system, which lets users declare which
targets can and cannot depend on each other. For example, externally deployable
binaries shouldn't depend on libraries with company-secret code. See
<a href="https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/analysis/constraints/ConstraintSemantics.java#L46">
ConstraintSemantics</a> for details.
</p>
