to::result_of::env_var</a><span class="special">&lt;</span><span class="identifier">Env</span> <span class="special">&amp;</span><span class="special">,</span> <span class="identifier">Key</span><span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span> <span class="identifier">env_var</span><span class="special">(</span><span class="identifier">Env</span> <span class="special">&amp;</span> e<span class="special">)</span><span class="special">;</span>
<span class="k