<p><code>String; optional</code></p>

<p>
An explanatory warning message associated with this rule.
Typically this is used to notify users that a rule has become obsolete,
or has become superseded by another rule, is private to a package, or is
perhaps considered harmful for some reason. It is a good idea to include
some reference (like a webpage, a bug number or example migration CLs) so
that one can easily find out what changes are required to avoid the message.
If there is a new target that can be used as a drop in replacement, it is a
good idea to just migrate all users of the old target.
</p>

<p>
This attribute has no effect on the way things are built, but it
may affect a build tool's diagnostic output.  The build tool issues a
warning when a rule with a <code>deprecation</code> attribute is
depended upon by another rule.
</p>

<p>
Intra-package dependencies are exempt from this warning, so that,
for example, building the tests of a deprecated rule does not
encounter a warning.
</p>

<p>
If a deprecated rule depends on another deprecated rule, no warning
message is issued.
</p>

<p>
Once people have stopped using it, the package can be removed.
</p>
