I'm a huge fan. It's very little work to use, as long as all variants can be known to the author, and as long as you aren't in a situation where uncommon variants drastically inflate the size of your common variants, it's a performance win, often a big one, compared to a boxed trait object.
Even when you have to box a variant to avoid inflating the size of the whole enum, that's still an improvement over a `dyn Trait` - it involves half as much pointer chasing
It'd be cool to see this added as a compiler optimization - even for cases where the author of an interface can't possibly know all variants (e.g. you have a `pub fn` that accepts a `&dyn MyTrait`), the compiler can
