f a <code>List</code> depends on how many
elements are in the list, and so we don&#39;t know how much memory to allocate
for a <code>Cons</code>. By introducing a <code>Box</code>, which has a defined size, we know how
big <code>Cons</code> needs to be.</p>
</div><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
<table>
                    <tr class=' module-item'>
                        <td><a class='struct' href='struct.Box.html'
                               title='alloc::boxe