Rust monsters have a one-way grudge against iron golems.

Eating a rust monster while polymorphed into an iron golem is instantly fatal to your polyform.

Rust monsters are only hostile towards you if you are wearing or carrying iron or are an iron golem.

Rust monster corpses are untinnable, or else attempting to tin them rusts the tinning kit.

Rust monsters and disenchanters are quadrupeds, and should be q. (This would leave R empty though.)