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

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

<p>
This is part of Bazel's soft-launched constraint system, which lets users
declare which rules 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>