Contravariance is part of stable Rust, and it's a natural outcome of how
`fn(T)` interacts with lifetimes, so it's hard to imagine this ever
going away.
tl;dr: LONG LIVE CONTRAVARIANCE!
* Prefer compile_fail over ignore, to catch if the code accidentally
starts compiling in future versions.
* Make an example compile by providing mocked environment around it in
case it should be correct.
* Links into std.