<p dir="auto">As was noted in <a href="https://github.com/rust-lang/rfcs/pull/141#issuecomment-47253786" data-hovercard-type="pull_request" data-hovercard-url="/rust-lang/rfcs/pull/141/hovercard">the RFC discussion</a>, covariant lifetime positions (which I was mistakenly referring to as "contravariant") should require explicit lifetime annotations.</p>
<p dir="auto">cc <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="37476072" data-permission-text="Title is private" data-url="https://github.com/rust-lang/rust/issues/15552" data-hovercard-type="issue" data-hovercard-url="/rust-lang/rust/issues/15552/hovercard" href="https://github.com/rust-lang/rust/issues/15552">#15552</a></p>
<p dir="auto">Three related FIXMEs in <code class="notranslate">back::link</code> suggest adding back-end optimization levels other than <code class="notranslate">-O2</code>, exposing and using lists of specific LLVM optimizations, or at least not duplicating the LLVM optimizer's list of <code class="notranslate">-O2</code> optimizations.</p>