title: What is the trait `core::kinds::Sized` is not implemented for the type `<generic #0>` in rust?
body: <p>I expected this to work:</p>
  
  <pre><code>trait Task&lt;R, E&gt; {
    fn run(&amp;self) -&gt; Result&lt;R, E&gt;;
  }
  
  mod test {
  
    use super::Task;
  
    struct Foo;
    impl&lt;uint, uint&gt; Task&lt;uint, uint&gt; for Foo {
      fn run(&amp;self) -&gt; Result&lt;uint, uint&gt; {
        return Err(0);
      }
    }
  
    fn can_have_task_trait() {
      Foo;
    }
  }
  
  fn main() {
    test::can_have_task_trait();
  }
  </code></pre>
  
  <p>...but it does not:</p>
  
  <pre><code>&lt;anon&gt;:10:3: 14:4 error: the trait `core::kinds::Sized` is not implemented for the type `&lt;generic #0&gt;`
  &lt;anon&gt;:10   impl&lt;uint, uint&gt; Task&lt;uint, uint&gt; for Foo {
  &lt;anon&gt;:11     fn run(&amp;self) -&gt; Result&lt;uint, uint&gt; {
  &lt;anon&gt;:12       return Err(0);
  &lt;anon&gt;:13     }
  &lt;anon&gt;:14   }
  &lt;anon&gt;:10:3: 14:4 note: the trait `core::kinds::Sized` must be implemented because it is required by `Task`
  &lt;anon&gt;:10   impl&lt;uint, uint&gt; Task&lt;uint, uint&gt; for Foo {
  &lt;anon&gt;:11     fn run(&amp;self) -&gt; Result&lt;uint, uint&gt; {
  &lt;anon&gt;:12       return Err(0);
  &lt;anon&gt;:13     }
  &lt;anon&gt;:14   }
  &lt;anon&gt;:10:3: 14:4 error: the trait `core::kinds::Sized` is not implemented for the type `&lt;generic #1&gt;`
  &lt;anon&gt;:10   impl&lt;uint, uint&gt; Task&lt;uint, uint&gt; for Foo {
  &lt;anon&gt;:11     fn run(&amp;self) -&gt; Result&lt;uint, uint&gt; {
  &lt;anon&gt;:12       return Err(0);
  &lt;anon&gt;:13     }
  &lt;anon&gt;:14   }
  &lt;anon&gt;:10:3: 14:4 note: the trait `core::kinds::Sized` must be implemented because it is required by `Task`
  &lt;anon&gt;:10   impl&lt;uint, uint&gt; Task&lt;uint, uint&gt; for Foo {
  &lt;anon&gt;:11     fn run(&amp;self) -&gt; Result&lt;uint, uint&gt; {
  &lt;anon&gt;:12       return Err(0);
  &lt;anon&gt;:13     }
  &lt;anon&gt;:14   }
  error: aborting due to 2 previous errors
  playpen: application terminated with error code 101
  Program ended.
  </code></pre>
  
  <p>playpen: <a href="http://is.gd/kxDt0P" rel="nofollow">http://is.gd/kxDt0P</a></p>
  
  <p>So, what's going on? </p>
  
  <p>I have no idea what this error means.</p>
  
  <p>Is it that I'm using Result and that requires that U, V are not Sized? In which case, why are they Sized? I didn't write:</p>
  
  <pre><code>Task&lt;Sized? R, Sized? E&gt;
  </code></pre>
  
  <p>Are all generics now dynamically sized or something? (in which case, what does Sized? even mean?)</p>
  
  <p>What's going on?</p>