title: clippy::missing_const_for_fn fires on constructor and getters. Is that a true positive?
body: <p>I decided to use a bit of a heftier version of clippy to get myself ready for a version change. I was expecting some false positives but I'm not sure about this one. I got the following struct, and clippy tells me that every single function should be const. (clippy::missing_const_for_fn)</p>
  <pre class="lang-rust prettyprint-override"><code>#[derive(Clone, Debug, PartialEq, Eq)]
  pub struct ScheduleFields {
      years: Years,
      days_of_week: DaysOfWeek,
      months: Months,
      days_of_month: DaysOfMonth,
      hours: Hours,
      minutes: Minutes,
      seconds: Seconds,
  }
  
  impl ScheduleFields {
      // Constructor
      pub fn new(
          seconds: Seconds,
          minutes: Minutes,
          hours: Hours,
          days_of_month: DaysOfMonth,
          months: Months,
          days_of_week: DaysOfWeek,
          years: Years,
      ) -&gt; ScheduleFields {
          ScheduleFields {
              years,
              days_of_week,
              months,
              days_of_month,
              hours,
              minutes,
              seconds,
          }
      }
  
      // Getters
      pub fn years(&amp;self) -&gt; &amp;Years { &amp;self.years }
      pub fn months(&amp;self) -&gt; &amp;Months { &amp;self.months }
      pub fn days_of_month(&amp;self) -&gt; &amp;DaysOfMonth { &amp;self.days_of_month }
      pub fn days_of_week(&amp;self) -&gt; &amp;DaysOfWeek { &amp;self.days_of_week }
      pub fn hours(&amp;self) -&gt; &amp;Hours { &amp;self.hours }
      pub fn minutes(&amp;self) -&gt; &amp;Minutes { &amp;self.minutes }
      pub fn seconds(&amp;self) -&gt; &amp;Seconds { &amp;self.seconds }
  }
  </code></pre>