title: What is RFC 401's coerce_inner useful for?
body: <p>I'm reading Rust's <a href="https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md" rel="nofollow noreferrer">RFC 0401</a> and see this:</p>
  <blockquote>
  <p>coerce_inner(<code>Foo&lt;..., T, ...&gt;</code>) = <code>Foo&lt;..., coerce_inner(T), ...&gt;</code> where <code>Foo</code> is a struct and only the last field has type <code>T</code> and <code>T</code> is not part of the type of any other fields;</p>
  <p>coerce_inner(<code>(..., T)</code>) = <code>(..., coerce_inner(T))</code>.</p>
  </blockquote>
  <p>It this up-to-date information? If so, what is this useful for?</p>