The latest version of `rental` (v0.5.6) contains a fix that allows it to
compile without relying on the pretty-print back-compat hack.
Hopefully, there are no longer any crates relying on the affected
versions of the (much less popular) `procedural-masquerade` crate. This
should allow us to target the pretty-print back-compat hack specifically
to older versions of `rental`, and specifically mention upgrading to
`rental` v0.5.6 in the lint message.