title: Use pug mixin result as attribute value
body: <p>Here is a boiled-down version of what I'm trying to accomplish:</p>
  
  <pre><code>mixin foo(bar)
      = bar + ".html"
  
  a(href= +foo("baz")) test
  </code></pre>
  
  <p>I'd like to have the anchor tag be compiled as <code>&lt;a href="baz.html"&gt;test&lt;/a&gt;</code>, but what I'm getting instead are type errors, on <code>foo</code> not being a function. Although I do see that it technically isn't a function, is this not a scenario where a mixin would be useful? I've searched the pug documentation for use-case scenarios similar to mine, but without success.</p>
  
  <p>Is what I'm trying to achieve here possible with mixins? Or is this only possible with regular JS functions passed as context variables?</p>