Reports errors in `macro_rules!` definitions. For instance:
<ul>
    <li>Reports invalid metavariable fragment specifiers</li>
    <li>Detects <code>pat</code> fragment specifiers followed by `|` (in 2021 Edition and later)</li>
    <li>Reports missing `;` (or `,` for macro 2.0) between macro rules.</li>
</ul>