title: cannot find function `serialize` in module `jwt_numeric_date`
body: <p>when I compile the new rust mod <code>jwt_numeric_date</code>(<code>cargo 1.54.0 (5ae8d74b3 2021-06-22)</code>), shows error:</p>
  <pre><code>    error[E0425]: cannot find function `serialize` in module `jwt_numeric_date`
   --&gt; src/jwt_util.rs:6:28
    |
  6 | #[derive(Debug, PartialEq, Serialize, Deserialize)]
    |                            ^^^^^^^^^ not found in `jwt_numeric_date`
    |
    = note: this error originates in the derive macro `Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)
  help: consider importing this function
    |
  2 | use rocket::figment::util::vec_tuple_map::serialize;
  </code></pre>
  <p>the rust code look like this:</p>
  <pre><code>use chrono::{Duration, TimeZone, Utc, DateTime, Timelike};
  use serde::{Deserialize, Serialize};
  
  #[derive(Debug, PartialEq, Serialize, Deserialize)]
  struct Claims {
      sub: String,
      #[serde(with = &quot;jwt_numeric_date&quot;)]
      iat: DateTime&lt;Utc&gt;,
      #[serde(with = &quot;jwt_numeric_date&quot;)]
      exp: DateTime&lt;Utc&gt;,
  }
  
  impl Claims {
      pub fn new(sub: String, iat: DateTime&lt;Utc&gt;, exp: DateTime&lt;Utc&gt;) -&gt; Self {
          let iat = iat.date().and_hms_milli(iat.hour(), iat.minute(), iat.second(), 0);
          let exp = exp.date().and_hms_milli(exp.hour(), exp.minute(), exp.second(), 0);
          Self { sub, iat, exp }
      }
  }
  pub mod jwt_numeric_date {
  
      use serde::{self, Deserialize, Deserializer, Serializer,Serialize};
  
      pub fn parse_jwt(token: &amp;str) {
  
      }
  }
  </code></pre>
  <p>I have import the <code>Serialize</code> in mod <code>jwt_numeric_date</code>, why still tell this error. I have check this <a href="https://github.com/serde-rs/serde/issues/1629" rel="nofollow noreferrer">issue</a> and make sure my cargo import enable derive feature like this:</p>
  <pre><code>serde = { version = &quot;1.0.64&quot;, features = [&quot;derive&quot;] }
  </code></pre>