<p><code>Boolean; optional; default 0 except as noted</code></p>

<p>
If 1, only testonly targets (such as tests) can depend on this target.
</p>

<p>
Equivalently, a rule that is not <code>testonly</code> is not allowed to
depend on any rule that is <code>testonly</code>.
</p>

<p>
Tests (<code>*_test</code> rules)
and test suites (<a href="${link test_suite}">test_suite</a> rules)
are <code>testonly</code> by default.
</p>

<p>
This attribute is intended to mean that the target should not be
contained in binaries that are released to production.
</p>

<p>
Because testonly is enforced at build time, not run time, and propagates
virally through the dependency tree, it should be applied judiciously. For
example, stubs and fakes that
are useful for unit tests may also be useful for integration tests
involving the same binaries that will be released to production, and
therefore should probably not be marked testonly. Conversely, rules that
are dangerous to even link in, perhaps because they unconditionally
override normal behavior, should definitely be marked testonly.
</p>