How to write a Rust function that takes an iterator?
<p>I'd like to write a function that accepts an iterator and returns the results of some operations on it.  Specifically, I'm trying to iterate over the values of a <code>HashMap</code>:</p>

<pre><code>use std::collections::HashMap;

fn find_min&lt;'a&gt;(vals: Iterator&lt;Item=&amp;'a u32&gt;) -&gt; Option&lt;&amp;'a u32&gt; {
    vals.min()
}

fn main() {
    let mut map = HashMap::new();
    map.insert("zero", 0u32);
    map.insert("one", 1u32);
    println!("Min value {:?}", find_min(map.values()));
}
</code></pre>

<p>But alas:</p>

<pre class="lang-none prettyprint-override"><code>error: the `min` method cannot be invoked on a trait object
 --&gt; src/main.rs:4:10
  |
4 |     vals.min()
  |          ^^^

error[E0277]: the trait bound `std::iter::Iterator&lt;Item=&amp;'a u32&gt; + 'static: std::marker::Sized` is not satisfied
 --&gt; src/main.rs:3:17
  |
3 | fn find_min&lt;'a&gt;(vals: Iterator&lt;Item = &amp;'a u32&gt;) -&gt; Option&lt;&amp;'a u32&gt; {
  |                 ^^^^ `std::iter::Iterator&lt;Item=&amp;'a u32&gt; + 'static` does not have a constant size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `std::iter::Iterator&lt;Item=&amp;'a u32&gt; + 'static`
  = note: all local variables must have a statically known size

error[E0308]: mismatched types
  --&gt; src/main.rs:11:41
   |
11 |     println!("Min value {:?}", find_min(map.values()));
   |                                         ^^^^^^^^^^^^ expected trait std::iter::Iterator, found struct `std::collections::hash_map::Values`
   |
   = note: expected type `std::iter::Iterator&lt;Item=&amp;u32&gt; + 'static`
              found type `std::collections::hash_map::Values&lt;'_, &amp;str, u32&gt;`
</code></pre>

<p>I get the same error if I try to pass by reference; if I use a <code>Box</code>, I get lifetime errors.</p>